Elisa MOLINARI - personale UniMoRe

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Elisa MOLINARI

Professore Ordinario
Dipartimento di Scienze Fisiche, Informatiche e Matematiche sede ex-Fisica

Pubblicazioni

2023 - Effect of uniaxial strain on the excitonic properties of monolayer C3N: A symmetry-based analysis [Articolo su rivista]
Zanfrognini, M; Spallanzani, N; Bonacci, M; Molinari, E; Ruini, A; Caldas, Mj; Ferretti, A; Varsano, D
abstract

In recent years, the application of mechanical stress has become a widespread experimental method to tune the electronic and optical properties of two-dimensional (2D) materials. In this work, we investigate the impact of uniaxial tensile strain along zigzag and armchair directions on the excitonic properties of graphene-like C3N, a single-layer indirect-gap material with relevant mechanical and optical properties. To do that, we develop a tight -binding Bethe-Salpeter equation framework based on a Wannier-function description of the frontier bands of the system, and use it to compute both dark and bright excitons of C3N for different applied strain configurations. Then, we use this model approach to classify excitons of pristine and strained C3N according to the crystal symmetry and to explain the appearance of bright excitons with intense optical anisotropy in strained C3N, even at small strains. Finally, the effect of strain on the exciton dispersion at small center-of-mass momenta is discussed, with special focus on the implications for 2D linear-nonanalytic dispersions.

2022 - Anomalous non-equilibrium response in black phosphorus to sub-gap mid-infrared excitation [Articolo su rivista]
Montanaro, A.; Giusti, F.; Zanfrognini, M.; Di Pietro, P.; Glerean, F.; Jarc, G.; Rigoni, E. M.; Mathengattil, S. Y.; Varsano, D.; Rontani, M.; Perucchi, A.; Molinari, E.; Fausti, D.
abstract

The competition between the electron-hole Coulomb attraction and the 3D dielectric screening dictates the optical properties of layered semiconductors. In low-dimensional materials, the equilibrium dielectric environment can be significantly altered by the ultrafast excitation of photo-carriers, leading to renormalized band gap and exciton binding energies. Recently, black phosphorus emerged as a 2D material with strongly layer-dependent electronic properties. Here, we resolve the response of bulk black phosphorus to mid-infrared pulses tuned across the band gap. We find that, while above-gap excitation leads to a broadband light-induced transparency, sub-gap pulses drive an anomalous response, peaked at the single-layer exciton resonance. With the support of DFT calculations, we tentatively ascribe this experimental evidence to a non-adiabatic modification of the screening environment. Our work heralds the non-adiabatic optical manipulation of the electronic properties of 2D materials, which is of great relevance for the engineering of versatile van der Waals materials.

2022 - Anomalous screening in narrow-gap carbon nanotubes [Articolo su rivista]
Sesti, G.; Varsano, D.; Molinari, E.; Rontani, M.
abstract

The screening of Coulomb interaction controls many-body physics in carbon nanotubes, as it tunes the range and strength of the force that acts on charge carriers and binds electron-hole pairs into excitons. In doped tubes, the effective Coulomb interaction drives the competition between Luttinger liquid and Wigner crystal, whereas in undoped narrow-gap tubes it dictates the Mott or excitonic nature of the correlated insulator observed at low temperature. Here, by computing the dielectric function of selected narrow- and zero-gap tubes from first principles, we show that the standard effective-mass model of screening systematically underestimates the interaction strength at long wavelength, hence missing the binding of low-energy excitons. The reason is that the model critically lacks the full three-dimensional topology of the tube, being adapted from graphene theory. As ab inito calculations are limited to small tubes, we develop a two-band model dielectric function based on the plane-wave expansion of Bloch states and the exact truncated Coulomb cutoff technique. We demonstrate that our - computationally cheap - approach provides the correct screening for narrow-gap tubes of any size and chirality. A striking result is that the screened interaction remains long-ranged even in gapless tubes, as an effect of the microscopic local fields generated by the electrons moving on the curved tube surface. As an application, we show that the effective electron-electron force that is felt at distances relevant to quantum transport experiments is super Coulombic.

2022 - Band structure modulation by methoxy-functionalization of graphene nanoribbons [Articolo su rivista]
Götz, Alicia; Wang, Xiao-Ye; Ruini, Alice; Zheng, Wenhao; Soltani, Paniz; Graf, Robert; Tries, Alexander; Li, Juan; Palma, Carlos-Andres; Molinari, Elisa; Hansen, Michael Ryan; Wang, Hai I.; Prezzi, Deborah; Müllen, Klaus; Narita, Akimitsu
abstract

Graphene nanoribbons (GNRs) are considered as potential candidates for next-generation electronic materials, and chemical functionalization can be an efficient method to modulate their electronic properties. This work presents a solution synthesis of methoxy-substituted GNRs through the Diels-Alder polymerization of a tetraphenylcyclopentadienone-based monomer bearing four methoxy groups, followed by oxidative cyclodehydrogenation. The methoxy-functionalization of the GNRs was unambiguously validated by FTIR and solid-state NMR analyses. Moreover, theoretical studies by ab initio calculations predicted both charge redistribution and structural distortion induced by the methoxy substitution, revealing reduction of both the bandgap and of the effective mass of charge carriers. Employing THz spectroscopy, we found that methoxy-substitution at the edges enhanced the photoconductivity of GNRs by a factor of similar to 25%, primarily due to the reduced charge effective mass.

2022 - Evidence for equilibrium exciton condensation in monolayer WTe2 [Articolo su rivista]
Sun, B.; Zhao, W.; Palomaki, T.; Fei, Z.; Runburg, E.; Malinowski, P.; Huang, X.; Cenker, J.; Cui, Y. -T.; Chu, J. -H.; Xu, X.; Ataei, S. S.; Varsano, D.; Palummo, M.; Molinari, E.; Rontani, M.; Cobden, D. H.
abstract

We present evidence that the two-dimensional bulk of monolayer WTe2 contains electrons and holes bound by Coulomb attraction—excitons—that spontaneously form in thermal equilibrium. On cooling from room temperature to 100 K, the conductivity develops a V-shaped dependence on electrostatic doping, while the chemical potential develops a step at the neutral point. These features are much sharper than is possible in an independent-electron picture, but they can be accounted for if electrons and holes interact strongly and are paired in equilibrium. Our calculations from first principles show that the exciton binding energy is larger than 100 meV and the radius as small as 4 nm, explaining their formation at high temperature and doping levels. Below 100 K, more strongly insulating behaviour is seen, suggesting that a charge-ordered state forms. The observed absence of charge density waves in this state is surprising within an excitonic insulator picture, but we show that it can be explained by the symmetries of the exciton wavefunction. Therefore, in addition to being a topological insulator, monolayer WTe2 exhibits strong correlations over a wide temperature range.

2022 - Excitonic effects in graphene-like C3N [Articolo su rivista]
Bonacci, Miki; Zanfrognini, Matteo; Molinari, Elisa; Ruini, Alice; Caldas, Marilia J.; Ferretti, Andrea; Varsano, Daniele
abstract

2022 - Gap Opening in Double-Sided Highly Hydrogenated Free-Standing Graphene [Articolo su rivista]
Betti, M. G.; Placidi, E.; Izzo, C.; Blundo, E.; Polimeni, A.; Sbroscia, M.; Avila, J.; Dudin, P.; Hu, K.; Ito, Y.; Prezzi, D.; Bonacci, M.; Molinari, E.; Mariani, C.
abstract

Conversion of free-standing graphene into pure graphane-where each C atom is sp3bound to a hydrogen atom-has not been achieved so far, in spite of numerous experimental attempts. Here, we obtain an unprecedented level of hydrogenation (≈90% of sp3bonds) by exposing fully free-standing nanoporous samples-constituted by a single to a few veils of smoothly rippled graphene-to atomic hydrogen in ultrahigh vacuum. Such a controlled hydrogenation of high-quality and high-specific-area samples converts the original conductive graphene into a wide gap semiconductor, with the valence band maximum (VBM) ∼3.5 eV below the Fermi level, as monitored by photoemission spectromicroscopy and confirmed by theoretical predictions. In fact, the calculated band structure unequivocally identifies the achievement of a stable, double-sided fully hydrogenated configuration, with gap opening and no trace of πstates, in excellent agreement with the experimental results.

2022 - Graphene decoupling through oxygen intercalation on Gr/Co and Gr/Co/Ir interfaces [Articolo su rivista]
Leon, Da; Ferretti, A; Varsano, D; Molinari, E; Cardoso, C
abstract

We perform a density functional theory study of the effects of oxygen adsorption on the structural and electronic properties of Gr/Co(0001) and Gr/Co/Ir(111) interfaces. In both interfaces, the graphene-Co distance increases with increasing O concentration. The oxygen intercalation effectively decreases the electronic interaction, preventing the hybridization of graphene states with Cod orbitals, hence (partly) restoring the typical Dirac cone of pristine graphene. In the case of graphene/Co 1ML/Ir(111), which presents a moire pattern, the interplay between the O distribution and the continuous change of the graphene-Co registry can be used to tune graphene corrugation and electronic properties. The computed electronic properties are in very good agreement with previously reported angle-resolved photoemission spectroscopy and photoemission electron microscopy measurements for Gr/Co(0001).

2022 - Unveiling Vibronic Coupling within the Q-bands of a Freebase Porphyrin [Relazione in Atti di Convegno]
Petropoulos, V.; Russo, M.; Rukin, P.; Quintela, F.; Moretti, L.; Moore, A.; Moore, T.; Gust, D.; Prezzi, D.; Scholes, G.; Molinari, E.; Troiani, F.; Cerullo, G.; Rozzi, C.; Maiuri, M.
abstract

Multidimensional spectroscopies unveil the presence of vibronic coupling within the Q-states in a free-base porphyrin. High-frequency coupling and tuning modes drive the ultrafast internal conversion and track the excited state structural evolution.

2021 - Evidence of ideal excitonic insulator in bulk MoS2 under pressure [Articolo su rivista]
Samaneh Ataei, S.; Varsano, D.; Molinari, E.; Rontani, M.
abstract

Spontaneous condensation of excitons is a long-sought phenomenon analogous to the condensation of Cooper pairs in a superconductor. It is expected to occur in a semiconductor at thermodynamic equilibrium if the binding energy of the excitons—electron (e) and hole (h) pairs interacting by Coulomb force—overcomes the band gap, giving rise to a new phase: the “excitonic insulator” (EI). Transition metal dichalcogenides are excellent candidates for the EI realization because of reduced Coulomb screening, and indeed a structural phase transition was observed in few-layer systems. However, previous work could not disentangle to which extent the origin of the transition was in the formation of bound excitons or in the softening of a phonon. Here we focus on bulk MoS2 and demonstrate theoretically that at high pressure it is prone to the condensation of genuine excitons of finite momentum, whereas the phonon dispersion remains regular. Starting from first-principles many-body perturbation theory, we also predict that the self-consistent electronic charge density of the EI sustains an out-of-plane permanent electric dipole moment with an antiferroelectric texture in the layer plane: At the onset of the EI phase, those optical phonons that share the exciton momentum provide a unique Raman fingerprint for the EI formation. Finally, we identify such fingerprint in a Raman feature that was previously observed experimentally, thus providing direct spectroscopic confirmation of an ideal excitonic insulator phase in bulk MoS2 above 30 GPa.

2021 - Frequency dependence in made simple using a multipole approximation [Articolo su rivista]
Leon, D. A.; Cardoso, C.; Chiarotti, T.; Varsano, D.; Molinari, E.; Ferretti, A.
abstract

In the approximation, the screened interaction is a nonlocal and dynamical potential that usually has a complex frequency dependence. A full description of such a dependence is possible but often computationally demanding. For this reason, it is still common practice to approximate using a plasmon pole (PP) model. Such an approach, however, may deliver an accuracy limited by its simplistic description of the frequency dependence of the polarizability, i.e., of . In this work, we explore a multipole approach (MPA) and develop an effective representation of the frequency dependence of . We show that an appropriate sampling of the polarizability in the frequency complex plane and a multipole interpolation can lead to a level of accuracy comparable with full-frequency methods at a much lower computational cost. Moreover, both accuracy and cost are controllable by the number of poles used in MPA. Eventually, we validate the MPA approach in selected prototype systems, showing that full-frequency quality results can be obtained with a limited number of poles.

2021 - Roadmap on bio-nano-photonics [Articolo su rivista]
Herkert, E.; Slesiona, N.; Recchia, M. E.; Deckert, T.; Garcia-Parajo, M. F.; Fantuzzi, E. M.; Pruccoli, A.; Ragupathy, I. C.; Gudavicius, D.; Rigneault, H.; Majer, J.; Zumbusch, A.; Munger, E.; Brasselet, S.; Jones, A. T.; Watson, P.; Boppart, S. A.; Singh, V.; Borkar, S.; Quintela Rodriguez, F. E.; Langbein, W.; Petropoulos, V.; van Hulst, N. F.; Maiuri, M.; Cerullo, G.; Brida, D.; Troiani, F.; Rozzi, C. A.; Molinari, E.; Vengris, M.; Borri, P.
abstract

In the quest to decipher the chain of life from molecules to cells, the biological and biophysical questions being asked increasingly demand techniques that are capable of identifying specific biomolecules in their native environment, and can measure biomolecular interactions quantitatively, at the smallest possible scale in space and time, without perturbing the system under observation. The interaction of light with biomolecules offers a wealth of phenomena and tools that can be exploited to drive this progress. This Roadmap is written collectively by prominent researchers and encompasses selected aspects of bio-nano-photonics, spanning from the development of optical micro/nano-spectroscopy technologies for quantitative bioimaging and biosensing to the fundamental understanding of light–matter interaction phenomena with biomolecules at the nanoscale. It will be of interest to a wide cross-disciplinary audience in the physical sciences and life sciences.

2020 - A monolayer transition-metal dichalcogenide as a topological excitonic insulator [Articolo su rivista]
Varsano, D.; Palummo, M.; Molinari, E.; Rontani, M.
abstract

Monolayer transition-metal dichalcogenides in the T′ phase could enable the realization of the quantum spin Hall effect1 at room temperature, because they exhibit a prominent spin–orbit gap between inverted bands in the bulk2,3. Here we show that the binding energy of electron–hole pairs excited through this gap is larger than the gap itself in the paradigmatic case of monolayer T′ MoS2, which we investigate from first principles using many-body perturbation theory4. This paradoxical result hints at the instability of the T′ phase in the presence of spontaneous generation of excitons, and we predict that it will give rise to a reconstructed ‘excitonic insulator’ ground state5–7. Importantly, we show that in this monolayer system, topological and excitonic order cooperatively enhance the bulk gap by breaking the crystal inversion symmetry, in contrast to the case of bilayers8–16 where the frustration between the two orders is relieved by breaking time reversal symmetry13,15,16. The excitonic topological insulator is distinct from the bare topological phase because it lifts the band spin degeneracy, which results in circular dichroism. A moderate biaxial strain applied to the system leads to two additional excitonic phases, different in their topological character but both ferroelectric17,18 as an effect of electron–electron interaction.

2020 - Adsorption and Motion of Single Molecular Motors on TiO2(110) [Articolo su rivista]
Jacobson, P.; Prezzi, D.; Liu, D.; Schied, M.; Tour, J. M.; Corni, S.; Calzolari, A.; Molinari, E.; Grill, L.
abstract

Motorized molecules where an external stimulus leads to controlled motion can perform work on the atomic scale. In Feringa-type motors, controlled motion is initiated by ultraviolet light that triggers a sequence of isomerization and helical inversion steps leading to the unidirectional rotation of the motor. Studying motor molecules on solid surfaces is advantageous because molecules can be studied in real space with scanning probe microscopy, surface features act as a spatial reference, and motion can be activated by pulses from the scanning probe tip. However, commonly used metal substrates have drawbacks, notably the quenching of excited molecular states by surface conduction electrons. An alternate approach is to deposit molecular motors on semiconducting substrates, thereby removing a potential path for quenching. Here we present results on the adsorption configurations and demonstrate the motion of unidirectional Feringa molecular motors adsorbed on the wide band gap semiconductor rutile TiO2(110).

2020 - CMAS corrosion of YSZ thermal barrier coatings obtained by different thermal spray processes [Articolo su rivista]
Morelli, S.; Testa, V.; Bolelli, G.; Ligabue, O.; Molinari, E.; Antolotti, N.; Lusvarghi, L.
abstract

Degradation of yttria-stabilized zirconia (YSZ) layers by molten CaO-MgO-Al2O3-SiO2 (CMAS)-based deposits is an important failure mode of thermal barrier coating (TBC) systems in modern gas turbines. The present work aimed to understand how the chemical purity and microstructure of plasma-sprayed YSZ layers affect their response to CMAS corrosion. To this end, isothermal corrosion tests (1 h at 1250 °C) were performed on four different kinds of YSZ coatings: atmospheric plasma-sprayed (APS) layers obtained from standard- and high-purity feedstock powders, a dense – vertically cracked (DVC) layer, and a suspension plasma sprayed (SPS) one. Characterization of corroded and non-corroded samples by FEG-SEM, EBSD and micro-Raman spectroscopy techniques reveals that, whilst all YSZ samples suffered grain-boundary corrosion by molten CMAS, its extent could vary considerably. High chemical purity limits the extent of grain-boundary dissolution by molten CMAS, whereas high porosity and/or fine crystalline grain structure lead to more severe degradation.

2020 - Intermolecular conical intersections in molecular aggregates [Articolo su rivista]
De Sio, A.; Sommer, E.; Nguyen, X. T.; Gross, L.; Popovic, D.; Nebgen, B. T.; Fernandez-Alberti, S.; Pittalis, S.; Rozzi, C. A.; Molinari, E.; Mena-Osteritz, E.; Bauerle, P.; Frauenheim, T.; Tretiak, S.; Lienau, C.
abstract

Conical intersections (CoIns) of multidimensional potential energy surfaces are ubiquitous in nature and control pathways and yields of many photo-initiated intramolecular processes. Such topologies can be potentially involved in the energy transport in aggregated molecules or polymers but are yet to be uncovered. Here, using ultrafast two-dimensional electronic spectroscopy (2DES), we reveal the existence of intermolecular CoIns in molecular aggregates relevant for photovoltaics. Ultrafast, sub-10-fs 2DES tracks the coherent motion of a vibrational wave packet on an optically bright state and its abrupt transition into a dark state via a CoIn after only 40 fs. Non-adiabatic dynamics simulations identify an intermolecular CoIn as the source of these unusual dynamics. Our results indicate that intermolecular CoIns may effectively steer energy pathways in functional nanostructures for optoelectronics.

2020 - Intrinsic edge excitons in two-dimensional MoS2 [Articolo su rivista]
D'Amico, P.; Gibertini, M.; Prezzi, D.; Varsano, D.; Ferretti, A.; Marzari, N.; Molinari, E.
abstract

Using accurate first-principles calculations based on many-body perturbation theory, we predict that two-dimensional MoS2 hosts edge excitons with universal character, intrinsic to the existence of edges and lying well below the onset of bulk features. These excitons are largely insensitive to edge terminations or orientation, persisting even in the presence of metallic screening at zigzag edges, with large binding energies of ∼0.4 eV. Additional excitons can also emerge in ultranarrow ribbons or as a function of the chemical nature of the termination. The chemical, structural, and electronic similarities with Se- or W-based transition-metal dichalcogenides suggest that these optical features could be common in this class of materials.

2020 - Vibrational signature of the graphene nanoribbon edge structure from high-resolution electron energy-loss spectroscopy [Articolo su rivista]
Cavani, N.; De Corato, M.; Ruini, A.; Prezzi, D.; Molinari, E.; Lodi Rizzini, A.; Rosi, A.; Biagi, R.; Corradini, V.; Wang, X. -Y.; Feng, X.; Narita, A.; Mullen, K.; De Renzi, V.
abstract

Bottom-up approaches exploiting on-surface synthesis reactions allow atomic-scale precision in the fabrication of graphene nanoribbons (GNRs); this is essential for their technological applications since their unique electronic and optical properties are largely controlled by the specific edge structure. By means of a combined experimental-theoretical investigation of some prototype GNRs, we show here that high-resolution electron energy-loss spectroscopy (HREELS) can be successfully employed to fingerprint the details of the GNR edge structure. In particular, we demonstrate how the features of HREEL vibrational spectra-mainly dictated by edge CH out-of-plane modes-are unambiguously related to the GNR edge structure. Moreover, we single out those modes which are localized at the GNR termini and show how their relative intensity can be related to the average GNR length.

2019 - Electronic and optical properties of doped TiO2 by many-body perturbation theory [Articolo su rivista]
Atambo, M. O.; Varsano, D.; Ferretti, A.; Ataei, S. S.; Caldas, M. J.; Molinari, E.; Selloni, A.
abstract

Doping is one of the most common strategies for improving the photocatalytic and solar energy conversion properties of TiO2, hence an accurate theoretical description of the electronic and optical properties of doped TiO2 is of both scientific and practical interest. In this work we use many-body perturbation theory techniques to investigate two typical n-type dopants, niobium and hydrogen, in TiO2 rutile. Using the GW approximation to determine band edges and defect energy levels, and the Bethe-Salpeter equation for the calculation of the absorption spectra, we find that the defect energy levels form nondispersive bands lying ∼2.2 eV above the top of the corresponding valence bands (∼0.9 eV below the conduction bands of the pristine material). The defect states are also responsible for the appearance of low-energy absorption peaks that enhance the solar spectrum absorption of rutile. The spatial distributions of the excitonic wave functions associated with these low-energy excitations are very different for the two dopants, suggesting a larger mobility of photoexcited electrons in Nb-TiO2.

2019 - Multiwavelength Raman spectroscopy of ultranarrow nanoribbons made by solution-mediated bottom-up approach [Articolo su rivista]
Rizzo, D.; Prezzi, D.; Ruini, A.; Nagyte, V.; Keerthi, A.; Narita, A.; Beser, U.; Xu, F.; Mai, Y.; Feng, X.; Mullen, K.; Molinari, E.; Casiraghi, C.
abstract

Here we present a combined experimental and theoretical study of graphene nanoribbons (GNRs), where detailed multiwavelength Raman measurements are integrated by accurate ab initio simulations. Our study covers several ultranarrow GNRs, obtained by means of solution-based bottom-up synthetic approach, allowing to rationalize the effect of edge morphology, position and type of functional groups, as well as the length on the GNR Raman spectrum. We show that the low-energy region, especially in the presence of bulky functional groups, is populated by several modes, and a single radial breathinglike mode cannot be identified. In the Raman optical region, we find that, except for the fully brominated case, all GNRs functionalized at the edges with different side groups show a characteristic dispersion of the D peak (8-22 cm-1/eV). This has been attributed to the internal degrees of freedom of these functional groups, which act as dispersion-activating defects. The G peak shows small to negligible dispersion in most of the cases, with larger values only in the presence of poor control of the edge functionalization, exceeding the values reported for highly defective graphene. In conclusion, we have shown that the characteristic dispersion of the G and D peaks offers further insight into the GNR structure and functionalization, by making Raman spectroscopy an important tool for the characterization of GNRs.

2019 - Tailoring optical properties and stimulated emission in nanostructured polythiophene [Articolo su rivista]
Portone, A.; Ganzer, L.; Branchi, F.; Ramos, R.; Caldas, M. J.; Pisignano, D.; Molinari, E.; Cerullo, G.; Persano, L.; Prezzi, D.; Virgili, T.
abstract

Polythiophenes are the most widely utilized semiconducting polymers in organic electronics, but they are scarcely exploited in photonics due to their high photo-induced absorption caused by interchain polaron pairs, which prevents the establishment of a window of net optical gain. Here we study the photophysics of poly(3-hexylthiophene) configured with different degrees of supramolecular ordering, spin-coated thin films and templated nanowires, and find marked differences in their optical properties. Transient absorption measurements evidence a partially-polarized stimulated emission band in the nanowire samples, in contrast with the photo-induced absorption band observed in spin-coated thin films. In combination with theoretical modeling, our experimental results reveal the origin of the primary photoexcitations dominating the dynamics for different supramolecular ordering, with singlet excitons in the nanostructured samples superseding the presence of polaron pairs, which are present in the disordered films. Our approach demonstrates a viable strategy to direct optical properties through structural control, and the observation of optical gain opens the possibility to the use of polythiophene nanostructures as building blocks of organic optical amplifiers and active photonic devices.

2018 - Bandgap Engineering of Graphene Nanoribbons by Control over Structural Distortion [Articolo su rivista]
Hu, Yunbin; Xie, Peng; DE CORATO, Marzio; Ruini, Alice; Zhao, Shen; Meggendorfer, Felix; Straasø, Lasse Arnt; Rondin, Loic; Simon, Patrick; Li, Juan; Finley, Jonathan J.; Hansen, Michael Ryan; Lauret, Jean-Sébastien; Molinari, Elisa; Feng, Xinliang; Barth, Johannes V.; Palma, Carlos-Andres; Prezzi, Deborah; Müllen, Klaus; Narita, Akimitsu
abstract

Among organic electronic materials, graphene nanoribbons (GNRs) offer extraordinary versatility as next-generation semiconducting materials for nanoelectronics and optoelectronics due to their tunable properties, including charge-carrier mobility, optical absorption, and electronic bandgap, which are uniquely defined by their chemical structures. Although planar GNRs have been predominantly considered until now, nonplanarity can be an additional parameter to modulate their properties without changing the aromatic core. Herein, we report theoretical and experimental studies on two GNR structures with "cove"-type edges, having an identical aromatic core but with alkyl side chains at different peripheral positions. The theoretical results indicate that installment of alkyl chains at the innermost positions of the "cove"-type edges can "bend" the peripheral rings of the GNR through steric repulsion between aromatic protons and the introduced alkyl chains. This structural distortion is theoretically predicted to reduce the bandgap by up to 0.27 eV, which is corroborated by experimental comparison of thus synthesized planar and nonplanar GNRs through UV-vis-near-infrared absorption and photoluminescence excitation spectroscopy. Our results extend the possibility of engineering GNR properties, adding subtle structural distortion as a distinct and potentially highly versatile parameter.

2018 - Interaction-Driven Giant Orbital Magnetic Moments in Carbon Nanotubes [Articolo su rivista]
Island, J. O.; Ostermann, M.; Aspitarte, L.; Minot, E. D.; Varsano, D.; Molinari, E.; Rontani, M.; Steele, G. A.
abstract

Carbon nanotubes continue to be model systems for studies of confinement and interactions. This is particularly true in the case of so-called "ultraclean" carbon nanotube devices offering the study of quantum dots with extremely low disorder. The quality of such systems, however, has increasingly revealed glaring discrepancies between experiment and theory. Here, we address the outstanding anomaly of exceptionally large orbital magnetic moments in carbon nanotube quantum dots. We perform low temperature magnetotransport measurements of the orbital magnetic moment and find it is up to 7 times larger than expected from the conventional semiclassical model. Moreover, the magnitude of the magnetic moment monotonically drops with the addition of each electron to the quantum dot directly contradicting the widely accepted shell filling picture of single-particle levels. We carry out quasiparticle calculations, both from first principles and within the effective-mass approximation, and find the giant magnetic moments can only be captured by considering a self-energy correction to the electronic band structure due to electron-electron interactions.

2017 - Carbon nanotubes as excitonic insulators [Articolo su rivista]
Varsano, Daniele; Sorella, Sandro; Sangalli, Davide; Barborini, Matteo; Corni, Stefano; Molinari, Elisa; Rontani, Massimo
abstract

Fifty years ago Walter Kohn speculated that a zero-gap semiconductor might be unstable against the spontaneous generation of excitons-electron-hole pairs bound together by Coulomb attraction. The reconstructed ground state would then open a gap breaking the symmetry of the underlying lattice, a genuine consequence of electronic correlations. Here we show that this excitonic insulator is realized in zero-gap carbon nanotubes by performing first-principles calculations through many-body perturbation theory as well as quantum Monte Carlo. The excitonic order modulates the charge between the two carbon sublattices opening an experimentally observable gap, which scales as the inverse of the tube radius and weakly depends on the axial magnetic field. Our findings call into question the Luttinger liquid paradigm for nanotubes and provide tests to experimentally discriminate between excitonic and Mott insulators.

2017 - How to Identify Plasmons from the Optical Response of Nanostructures [Articolo su rivista]
Zhang, Runmin; Bursi, Luca; Cox, Joel D.; Cui, Yao; Krauter, Caroline M.; Alabastri, Alessandro; Manjavacas, Alejandro; Calzolari, Arrigo; Corni, Stefano; Molinari, Elisa; Carter, Emily A.; García De Abajo, F. Javier; Zhang, Hui; Nordlander, Peter
abstract

A promising trend in plasmonics involves shrinking the size of plasmon-supporting structures down to a few nanometers, thus enabling control over light-matter interaction at extreme-subwavelength scales. In this limit, quantum mechanical effects, such as nonlocal screening and size quantization, strongly affect the plasmonic response, rendering it substantially different from classical predictions. For very small clusters and molecules, collective plasmonic modes are hard to distinguish from other excitations such as single-electron transitions. Using rigorous quantum mechanical computational techniques for a wide variety of physical systems, we describe how an optical resonance of a nanostructure can be classified as either plasmonic or nonplasmonic. More precisely, we define a universal metric for such classification, the generalized plasmonicity index (GPI), which can be straightforwardly implemented in any computational electronic-structure method or classical electromagnetic approach to discriminate plasmons from single-particle excitations and photonic modes. Using the GPI, we investigate the plasmonicity of optical resonances in a wide range of systems including: the emergence of plasmonic behavior in small jellium spheres as the size and the number of electrons increase; atomic-scale metallic clusters as a function of the number of atoms; and nanostructured graphene as a function of size and doping down to the molecular plasmons in polycyclic aromatic hydrocarbons. Our study provides a rigorous foundation for the further development of ultrasmall nanostructures based on molecular plasmonics.

2016 - Coherent ultrafast polaron pair formation in a conjugated polymer at room temperature [Relazione in Atti di Convegno]
De Sio, A; Sommer, E; Troiani, F; Rehault, J; Maiuri, M; Molinari, E; Cerullo, G; Lienau, C
abstract

Ultrafast 2D optical spectroscopy with 10-fs-time-resolution, complemented by density matrix simulations, show that coherent vibronic coupling accelerates polaron-pair formation and results in strongly correlated exciton and polaron-pair dynamics in polythiophene thin films at room temperature.

2016 - Coherent vibronic coupling in a conjugated polymer at room temperature [Relazione in Atti di Convegno]
De Sio, Antonietta; Sommer, Ephraim; Troiani, Filippo; Maiuri, Margherita; Rehault, Julien; Lim, James; Huelga, Susana F.; Plenio, Martin B.; Molinari, Elisa; Cerullo, Giulio; Lienau, Christoph
abstract

Two-dimensional electronic spectroscopy with sub-10-fs time resolution shows that coherent vibronic coupling promotes charge delocalization and results in long-lasting coherent oscillatory dynamics of strongly coupled excitons and polaron-pairs in a conjugated polymer thin film at room temperature.

2016 - Exciton-exciton annihilation and biexciton stimulated emission in graphene nanoribbons [Articolo su rivista]
Soavi, Giancarlo; Dal Conte, Stefano; Manzoni, Cristian; Viola, Daniele; Narita, Akimitsu; Hu, Yunbin; Feng, Xinliang; Hohenester, Ulrich; Molinari, Elisa; Prezzi, Deborah; Müllen, Klaus; Cerullo, Giulio
abstract

Graphene nanoribbons display extraordinary optical properties due to one-dimensional quantum-confinement, such as width-dependent bandgap and strong electron-hole interactions, responsible for the formation of excitons with extremely high binding energies. Here we use femtosecond transient absorption spectroscopy to explore the ultrafast optical properties of ultranarrow, structurally well-defined graphene nanoribbons as a function of the excitation fluence, and the impact of enhanced Coulomb interaction on their excited states dynamics. We show that in the high-excitation regime biexcitons are formed by nonlinear exciton-exciton annihilation, and that they radiatively recombine via stimulated emission. We obtain a biexciton binding energy of â ‰250 meV, in very good agreement with theoretical results from quantum Monte Carlo simulations. These observations pave the way for the application of graphene nanoribbons in photonics and optoelectronics.

2016 - Quantifying the Plasmonic Character of Optical Excitations in Nanostructures [Articolo su rivista]
Bursi, Luca; Calzolari, Arrigo; Corni, Stefano; Molinari, Elisa
abstract

The microscopic definition of plasmons in nanosystems is a tremendous challenge. Any sharp distinction of the excitation nature (nonplasmonic vs plasmonic) becomes blurred at the nanoscale, where quantum effects become important. Here we introduce the concept of plasmonicity index, i.e., a direct measure of the plasmonic character of the optical excitations in nanosystems. Its definition is based on a rigorous theoretical derivation, which leads to the physically sound result that the plasmonicity index is related to the capability of enhancing locally an applied electromagnetic radiation. The proposed expression is general and can be applied to any finite system. We show its usefulness in modeling metallic nanoparticles, prototypical C-based molecules, and paradigmatic hybrid systems, starting from first-principles calculations, based on (TD)DFT. Our results represent a step forward in the fundamental understanding of what a plasmon is in nanometer-sized particles and molecular systems.

2016 - Raman Fingerprints of Atomically Precise Graphene Nanoribbons [Articolo su rivista]
Verzhbitskiy, Ivan A.; De Corato, Marzio; Ruini, Alice; Molinari, Elisa; Narita, Akimitsu; Hu, Yunbin; Schwab, Matthias G.; Bruna, Matteo; Yoon, Duhee; Milana, Silvia; Feng, Xinliang; MÃ¼llen, Klaus; Ferrari, Andrea C.; Casiraghi, Cinzia; Prezzi, Deborah
abstract

Bottom-up approaches allow the production of ultranarrow and atomically precise graphene nanoribbons (GNRs) with electronic and optical properties controlled by the specific atomic structure. Combining Raman spectroscopy and ab initio simulations, we show that GNR width, edge geometry, and functional groups all influence their Raman spectra. The low-energy spectral region below 1000 cm-1is particularly sensitive to edge morphology and functionalization, while the D peak dispersion can be used to uniquely fingerprint the presence of GNRs and differentiates them from other sp2carbon nanostructures.

2016 - Tracking the coherent generation of polaron pairs in conjugated polymers [Articolo su rivista]
De Sio, Antonietta; Troiani, Filippo; Maiuri, Margherita; Réhault, Julien; Sommer, Ephraim; Lim, James; Huelga, Susana F.; Plenio, Martin B.; Rozzi, Carlo Andrea; Cerullo, Giulio; Molinari, Elisa; Lienau, Christoph
abstract

The optical excitation of organic semiconductors not only generates charge-neutral electron-hole pairs (excitons), but also charge-separated polaron pairs with high yield. The microscopic mechanisms underlying this charge separation have been debated for many years. Here we use ultrafast two-dimensional electronic spectroscopy to study the dynamics of polaron pair formation in a prototypical polymer thin film on a sub-20-fs time scale. We observe multi-period peak oscillations persisting for up to about 1 ps as distinct signatures of vibronic quantum coherence at room temperature. The measured two-dimensional spectra show pronounced peak splittings revealing that the elementary optical excitations of this polymer are hybridized exciton-polaron-pairs, strongly coupled to a dominant underdamped vibrational mode. Coherent vibronic coupling induces ultrafast polaron pair formation, accelerates the charge separation dynamics and makes it insensitive to disorder. These findings open up new perspectives for tailoring light-to-current conversion in organic materials.

2015 - Coherent ultrafast charge transfer in an organic photovoltaic blend [Relazione in Atti di Convegno]
de Sio, Antonietta; Falke, Sarah M.; Rozzi, Carlo Andrea; Brida, Daniele; Maiuri, Margherita; Amato, Michele; Sommer, Ephraim; Rubio, Angel; Cerullo, Giulio; Molinari, Elisa; Lienau, Christoph
abstract

Combining high-time resolution pump-probe spectroscopy and timedependent density functional theory calculations, we show that coherent vibroniccoupling is of key importance in triggering charge transfer in a technologically relevant organic photovoltaic blend.

2015 - First-principles comparative study on the interlayer adhesion and shear strength of transition-metal dichalcogenides and graphene [Articolo su rivista]
Levita, Giacomo; Molinari, Elisa; Polcar, Tomas; Righi, Maria Clelia
abstract

Due to their layered structure, graphene and transition-metal dichalcogenides (TMDs) are easily sheared along the basal planes. Despite a growing attention towards their use as solid lubricants, so far no head-to-head comparison has been carried out. By means of ab initio modeling of a bilayer sliding motion, we show that graphene is characterized by a shallower potential energy landscape while more similarities are attained when considering the sliding forces; we propose that the calculated interfacial ideal shear strengths afford the most accurate information on the intrinsic sliding capability of layered materials. We also investigate the effect of an applied uniaxial load: in graphene, this introduces a limited increase in the sliding barrier while in TMDs it has a substantially different impact on the possible polytypes. The polytype presenting a parallel orientation of the layers (R0) bears more similarities to graphene while that with antiparallel orientation (R180) shows deep changes in the potential energy landscape and consequently a sharper increase of its sliding barrier.

2015 - Probing coherent ultrafast exciton dissociation in a polymer: fullerene photovoltaic absorber [Relazione in Atti di Convegno]
De Sio, A; Sommer, E; Maiuri, M; Rehault, J; Rozzi, Ca; Molinari, E; Cerullo, ANTONIO GERARDO; Lienau, C
abstract

Combining high-time resolution optical spectroscopy and time-dependent density functional theory calculations, we provide strong evidence for the role of vibronic coupling in driving the initial steps of the current photogeneration in an organic photovoltaic system.

2015 - Surface-Assisted Reactions toward Formation of Graphene Nanoribbons on Au(110) Surface [Articolo su rivista]
Massimi, Lorenzo; Ourdjini, Oualid; Lafferentz, Leif; Koch, Matthias; Grill, Leonhard; Cavaliere, Emanuele; Gavioli, Luca; Cardoso, Claudia; Prezzi, Deborah; Molinari, Elisa; Ferretti, Andrea; Mariani, Carlo; Betti, Maria Grazia
abstract

Scanning tunneling microscopy and X-ray spectroscopy measurements are combined to first-principles simulations to investigate the formation of graphene nanoribbons (GNRs) on Au(110), as based on the surface-mediated reaction of 10,10′-dibromo-9,9′-bianthracene (DBBA) molecules. At variance with Au(111), two different pathways are identified for the GNR self-assembly on Au(110), as controlled by both the adsorption temperature and the surface coverage of the DBBA molecular precursors. Room-temperature DBBA deposition on Au(110) leads to the same reaction steps obtained on Au(111), even though with lower activation temperatures. For DBBA deposition at 470 K, the cyclodehydrogenation of the precursors preceds their polymerization, and the GNR formation is fostered by increasing the surface coverage. While the initial stages of the reaction are found to crucially determine the final configuration and orientation of the GNRs, the molecular diffusion is found to limit in both cases the formation of high-density long-range ordered GNRs. Overall, the direct comparison between the Au(110) and Au(111) surfaces unveils the delicate interplay among the different factors driving the growth of GNRs.

2014 - Ab Initio Simulation of Optical Limiting: The Case of Metal-Free Phthalocyanine [Articolo su rivista]
Cocchi, C; Prezzi, D; Ruini, Alice; Molinari, Elisa; Rozzi, Ca
abstract

We present a fully ab initio, nonperturbative description of the optical limiting properties of a metal-free phthalocyanine by simulating the effects of a broadband electric field of increasing intensity. The results confirm reverse saturable absorption as the leading mechanism for optical limiting phenomena in this system and reveal that a number of dipole-forbidden excitations are populated by excited-state absorption at more intense external fields. The excellent agreement with the experimental data supports our approach as a powerful tool to predict optical limiting in view of applications.

2014 - Anisotropy and Size Effects on the Optical Spectra of Polycyclic Aromatic Hydrocarbons [Articolo su rivista]
Cocchi, Caterina; Prezzi, Deborah; Ruini, Alice; Caldas, Marilia J; Molinari, Elisa
abstract

The electronic and optical properties of polycydic aromatic hydrocarbons (PAHs) present a strong dependence on their size and geometry. We tackle this issue by analyzing the spectral features of two prototypical classes of PM-Is, belonging to D-6h and D-2h symmetry point groups and related to coronene as multifunctional seed. While the size variation induces an overall red shift of the spectra and a redistribution of the oscillator strength between the main peaks, a lower molecular symmetry is responsible for the appearance of new optical features. Along with broken molecular orbital degeneracies, optical peaks split and dark states are activated in the low-energy part of the spectrum. Supported by a systematic analysis of the composition and the character of the optical transitions, our results contribute in shedding light to the mechanisms responsible for spectral modifications in the visible and near UV absorption bands of medium-size PAHs.

2014 - Coherent ultrafast charge transfer in an organic photovoltaic blend [Relazione in Atti di Convegno]
De Sio, A.; Falke, S. M.; Rozzi, C. A.; Brida, D.; Maiuri, M.; Amato, M.; Sommer, E.; Rubio, A.; Cerullo, G.; Molinari, E.; Lienau, C.
abstract

2014 - Coherent ultrafast charge transfer in an organic photovoltaic blend [Articolo su rivista]
Falke, Sarah Maria; Rozzi, Carlo Andrea; Brida, Daniele; Maiuri, Margherita; Amato, Michele; Sommer, Ephraim; De Sio, Antonietta; Rubio, Angel; Cerullo, Giulio; Molinari, Elisa; Lienau, Christoph
abstract

Blends of conjugated polymers and fullerene derivatives are prototype systems for organic photovoltaic devices. The primary charge-generation mechanism involves a light-induced ultrafast electron transfer from the light-absorbing and electron-donating polymer to the fullerene electron acceptor. Here, we elucidate the initial quantum dynamics of this process. Experimentally, we observed coherent vibrational motion of the fullerene moiety after impulsive optical excitation of the polymer donor. Comparison with first-principle theoretical simulations evidences coherent electron transfer between donor and acceptor and oscillations of the transferred charge with a 25-femtosecond period matching that of the observed vibrational modes. Our results show that coherent vibronic coupling between electronic and nuclear degrees of freedom is of key importance in triggering charge delocalization and transfer in a noncovalently bound reference system.

2014 - Coherent ultrafast exciton dissociation in polymer: Fullerene thin films [Abstract in Atti di Convegno]
De Sio, A.; Sommer, E.; Maiuri, M.; Rehault, J.; Rozzi, C. A.; Molinari, E.; Cerullo, G.; Lienau, C.
abstract

2014 - Exciton-dominated optical response of ultra-narrow graphene nanoribbons [Articolo su rivista]
Denk, Richard; Hohage, Michael; Zeppenfeld, Peter; Cai, Jinming; Pignedoli, Carlo A; Söde, Hajo; Fasel, Roman; Feng, Xinliang; Müllen, Klaus; Wang, Shudong; Prezzi, Deborah; Ferretti, Andrea; Ruini, Alice; Molinari, Elisa; Ruffieux, Pascal
abstract

Narrow graphene nanoribbons exhibit substantial electronic bandgaps and optical properties fundamentally different from those of graphene. Unlike graphene--which shows a wavelength-independent absorbance for visible light--the electronic bandgap, and therefore the optical response, of graphene nanoribbons changes with ribbon width. Here we report on the optical properties of armchair graphene nanoribbons of width N=7 grown on metal surfaces. Reflectance difference spectroscopy in combination with ab initio calculations show that ultranarrow graphene nanoribbons have fully anisotropic optical properties dominated by excitonic effects that sensitively depend on the exact atomic structure. For N=7 armchair graphene nanoribbons, the optical response is dominated by absorption features at 2.1, 2.3 and 4.2 eV, in excellent agreement with ab initio calculations, which also reveal an absorbance of more than twice the one of graphene for linearly polarized light in the visible range of wavelengths.

2014 - Light-Induced Field Enhancement in Nanoscale Systems from First-Principles: The Case of Polyacenes [Articolo su rivista]
Bursi, Luca; Calzolari, Arrigo; Corni, Stefano; Molinari, Elisa
abstract

Using first-principles calculations we studied the electric field enhancement in polyacene molecules upon illumination. These molecules can be seen as a specific class of C-based (i.e., graphene-derived) nanostructures, recently proposed as alternative materials for plasmonics. We demonstrate that optical transitions may generate oscillating dipolar response charge, giving rise to an induced electric field near the molecule, which thus acts as a plasmon-like nanoantenna. While the field amplification in the vicinity of single acenes is rather small and decreases when the size of the system is increased, it may be selectively enhanced in the case of acenes assemblies. This paves the way for the design of more complex C-based architectures explicitly conceived to improve the amplification factor.

2014 - Optical Properties of Bilayer Graphene Nanoflakes [Articolo su rivista]
DE CORATO, Marzio; Cocchi, Caterina; Prezzi, Deborah; Caldas Marilia, J; Molinari, Elisa; Ruini, Alice
abstract

The optical properties of coupled graphene nanoflakes are investigated theoretically within the framework of HartreeFock based semiempirical methods, with the aim of unraveling the role of pi pi interactions. Two different types of pi-stacking are considered, obtained either by coupling two identical flakes with different relative displacement or by coupling flakes having different width or edge functionalization, i.e., with different electronic gap or ionization potential. Our results indicate that a systematic red shift and broadening of lowest excitations occur: an overall widening of the optical absorption range can therefore be expected in an ensemble of flakes. However, the coupling prevents a strong enhancement of the absorption intensity. In the case of a heterogeneous ensemble of flakes, the possibility of introducing low-energy excitations with considerable charge transfer character is also demonstrated by properly exploiting the chemical edge functionalization.

2014 - Probing the mechanism for graphene nanoribbon formation on gold surfaces through X-ray spectroscopy [Articolo su rivista]
Batra, Arunabh; Cvetko, Dean; Kladnik, Gregor; Adak, Olgun; Cardoso, Claudia; Ferretti, Andrea; Prezzi, Deborah; Molinari, Elisa; Morgante, Alberto; Venkataraman, Latha
abstract

We studied the formation of graphene nanoribbons (GNRs) via the self-assembly of 10,10'-dibromo-9,9'-bianthryl precursor molecules on gold surfaces with different synchrotron spectroscopies. Through X-ray photoemission spectroscopy core-level shifts, we followed each step of the synthetic process, and could show that the Br-C bonds of the precursors cleave at temperatures as low as 100 degrees C on both Au(111) and Au(110). We established that the resulting radicals bind to Au, forming Au-C and Au-Br bonds. We show that the polymerization of the precursors follows Br desorption from Au, suggesting that the presence of halogens is the limiting factor in this step. Finally, with angle-resolved ultraviolet photoemission spectroscopy and density functional theory we show that the GNR/Au interaction results in an upshift of the Shockley surface state of Au(111) by similar to 0.14 eV, together with an increased electron effective mass.

2014 - Sliding Properties of MoS2 Layers: Load and Interlayer Orientation Effects [Articolo su rivista]
Levita, G.; Cavaleiro, A.; Molinari, Elisa; Polcar, T.; Righi, Maria Clelia
abstract

Among the members of the transition metal dichalcogenides (TMD) family, molybdenum disulfide has the most consolidated application outcomes in tribological fields. However, despite the growing usage as nanostructured solid lubricant due to its lamellar structure, little is known about the atomistic interactions taking place at the interface between two MoS2 sliding layers, especially at high loads. By means of ab initio modeling of the static potential energy surface and charge distribution analysis, we demonstrate how electrostatic interactions, negligible in comparison with van der Waals and Pauli contributions at zero load, progressively affect the sliding motion at increasing loads. As such, they discriminate the relative stability and the frictional behavior of bilayers where the two monolayers defining the interface have a different relative orientation. In particular, for antiparallel sliding layers we observed a load-induced increase of both the depth of the minima and the height of the energy barriers compared to parallel ones, which may have important consequences for the fabrication of more efficient ultralow friction devices at the nanoscale.

2013 - Concavity Effects on the Optical Properties of Aromatic Hydrocarbons [Articolo su rivista]
Caterina, Cocchi; Deborah, Prezzi; Ruini, Alice; Marilia J., Caldas; Annalisa, Fasolino; Molinari, Elisa
abstract

We study the modifications on the ground and excited state properties of polycydic aromatic hydrocarbons (PAHs), induced by the variation of concavity and pi-connectivity. Inspired by experimentally feasible systems, we study three series of PAHs, from H-saturated graphene flakes to geodesic buckybowls, related to the formation of fullerene C-60 and C-50-carbon nanotube caps. Working within the framework of quantum chemistry semiempirical methods AM1 and ZINDO/S, we find that the interplay between concavity and pi-connectivity shifts the bright optical lines to higher energies and introduces symmetry-forbidden dark excitations at low energy. A generally good agreement with the available experimental data supports our results, which can be viewed as the basis for designing optical properties of novel curved aromatic molecules.

2013 - Frictional properties of multilayer graphene by ab initio and classical molecular dynamics calculations [Abstract in Atti di Convegno]
Reguzzoni, M.; Fasolino, A.; Molinari, E.; Righi, M. C.
abstract

2013 - Quantum coherence controls the charge separation in a prototypical artificial light harvesting system [Relazione in Atti di Convegno]
Falke, S. M.; Rozzi, C. A.; Spallanzani, N.; Rubio, A.; Molinari, E.; Brida, D.; Maiuri, M.; Cerullo, G.; Schramm, H.; Christoffers, J.; Lienau, C.
abstract

In artificial light harvesting systems the conversion of light into charges or chemical energy happens on the femtosecond time scale and is thought to involve the incoherent jump of an electron from the optical absorber to an electron acceptor. Here we investigate the primary process of electronic charge transfer dynamics in a carotene-porphyrin-fullerene triad, a prototypical elementary component for an artificial light harvesting system combining coherent femtosecond spectroscopy and first-principles quantum dynamics simulations. Our experimental and theoretical results provide strong evidence that the driving mechanism of the photoinduced current generation cycle is a quantum-correlated wavelike motion of electrons and nuclei on a timescale of few tens of femtoseconds. We furthermore highlight the fundamental role played by the interface between the light-absorbing chromophore and the charge acceptor in triggering the coherent wavelike electron-hole splitting. © 2013 IEEE.

Ultrafast spectroscopy and quantum-dynamics simulations of an artificial supramolecular light-harvesting system - a supramolecular triad - provide strong evidence that the quantum-correlated wavelike motion of electrons and nuclei on a timescale of few tens of femtoseconds governs the ultrafast electronic charge transfer. © Owned by the authors, published by EDP Sciences, 2013.

2013 - Quantum coherence controls the charge separation in a prototypical artificial light-harvesting system [Articolo su rivista]
C. A., Rozzi; S. M., Falke; Spallanzani, Nicola; A., Rubio; Molinari, Elisa; D., Brida; M., Maiuri; G., Cerullo; H., Schramm; J., Christoffers; C., Lienau
abstract

The efficient conversion of light into electricity or chemical fuels is a fundamental challenge. In artificial photosynthetic and photovoltaic devices, this conversion is generally thought to happen on ultrafast, femto-to-picosecond timescales and to involve an incoherent electron transfer process. In some biological systems, however, there is growing evidence that the coherent motion of electronic wavepackets is an essential primary step, raising questions about the role of quantum coherence in artificial devices. Here we investigate the primary charge-transfer process in a supramolecular triad, a prototypical artificial reaction centre. Combining high time-resolution femtosecond spectroscopy and time-dependent density functional theory, we provide compelling evidence that the driving mechanism of the photoinduced current generation cycle is a correlated wavelike motion of electrons and nuclei on a timescale of few tens of femtoseconds. We highlight the fundamental role of the interface between chromophore and charge acceptor in triggering the coherent wavelike electron-hole splitting.

2012 - Electronic Structure of Atomically Precise Graphene NanoribbonsACS Nano, 2012, 6 (8), pp 6930–6935Publication Date (Web): August 1, 2012 (Article)DOI: [Articolo su rivista]
P., Ruffieux; J., Cai; N. C., Plumb; L., Patthey; Prezzi, Deborah; Ferretti, Andrea; Molinari, Elisa; X., Feng; K., Müllen; C. A., Pignedoli; R., Fasel
abstract

Some of the most intriguing properties of graphene are predicted for speciﬁcally designed nanostructures such as nanoribbons. Functionalities far beyond those known from extended graphene systems include electronic band gap variations related to quantum conﬁnement and edge eﬀects, as well as localized spin-polarized edge states for speciﬁc edge geometries. The inability to produce graphene nanostructures with the needed precision, however, has so far hampered the veriﬁcation of the predicted electronic properties. Here, we report on the electronic band gap anddispersion of the occupied electronic bands of atomically precise graphene nanoribbons fabricated via on-surface synthesis. Angle-resolved photoelectron spectroscopy and scanning tunnelingspectroscopy data from armchair graphene nanoribbons of width N = 7 supported on Au(111) reveal a band gap of 2.3 eV, an eﬀective mass of 0.21 m0 at the top of the valence band, and anenergy-dependent charge carrier velocity reaching 8.2 105 m/s in the linear part of the valence band. These results are in quantitative agreement with theoretical predictions that include image charge corrections accounting for screening by the metal substrate and conﬁrm the importance of electron-electron interactions in graphene nanoribbons

2012 - Electronics and Optics of Graphene Nanoflakes: Edge Functionalization and Structural Distortions [Articolo su rivista]
Cocchi, Caterina; Prezzi, Deborah; Ruini, Alice; M. J., Caldas; Molinari, Elisa
abstract

The effects of edge covalent functionalization on the structural, electronic, and optical properties of elongated armchair graphene nanoflakes (AGNFs) are analyzed in detail for a wide range of terminations, within the framework of Hartree–Fock-based semiempirical methods. The chemical features of the functional groups, their distribution, and the resulting system symmetry are identified as the key factors that determine the modification of strutural and optoelectronic features. While the electronic gap is always reduced in the presence of substituents, functionalization-induced distortions contribute to the observed lowering by about 35–55%. This effect is paired with a red shift of the first optical peak, corresponding to about 75% of the total optical gap reduction. Further, the functionalization pattern and the specific features of the edge–substituent bond are found to influence the strength and the character of the low-energy excitations. All of these effects are discussed for flakes of different widths, representing the three families of AGNFs.

2012 - Friction by Shear Deformations in Multilayer Graphene [Articolo su rivista]
Reguzzoni, Marco; Fasolino, Annalisa; Molinari, Elisa; Righi, Maria Clelia
abstract

We present accurate calculations of friction in graphene films in configurations simulating the presence of an anchoring substrate. We find that a slider induces both out-of-plane and shear deformations, which increase with the thickness of the supported film. We elucidate the new frictional mechanism connected to shear layer motions, which is minimal for systems with the smallest number of layers

2012 - Optical Excitations and Field Enhancement in Short Graphene Nanoribbons [Articolo su rivista]
Cocchi, Caterina; Prezzi, Deborah; Ruini, Alice; Benassi, Enrico; Marilia J., Caldas; Stefano, Corni; Molinari, Elisa
abstract

Abstract: The optical excitations of elongated graphene nanoflakes of finite length are investigated theoretically through quantum chemistry semiempirical approaches. The spectra and the resulting dipole fields are analyzed, accounting in full atomistic details for quantum confinement effects, which are crucial in the nanoscale regime. We find that the optical spectra of these nanostructures are dominated at low energy by excitations with strong intensity, comprised of characteristic coherent combinations of a few single-particle transitions with comparable weight. They give rise to stationary collective oscillations of the photoexcited carrier density extending throughout the flake and to a strong dipole and field enhancement. This behavior is robust with respect to width and length variations, thus ensuring tunability in a large frequency range. The implications for nanoantennas and other nanoplasmonic applications are discussed for realistic geometries.

2012 - Potential energy surface for graphene on graphene: Ab initio derivation, analytical description, and microscopic interpretation [Articolo su rivista]
Reguzzoni, Marco; Fasolino, Annalisa; Molinari, Elisa; Righi, Maria Clelia
abstract

We derive an analytical expression that describes the interaction energy between two graphene layers identically oriented as a function of the relative lateral and vertical positions, in excellent agreement with first principles calculations. Thanks to its formal simplicity, the proposed model allows for an immediate interpretation of the interactions, in particular of the potential corrugation. This last quantity plays a crucial role in determining the intrinsic resistance to interlayer sliding and its increase upon compression influences the frictional behavior under load. We show that, for these weakly adherent layers, the corrugation possesses the same nature and z dependence of Pauli repulsion. We investigate the microscopic origin of these phenomena by analyzing the electronic charge distribution: We observe a pressure-induced charge transfer from the interlayer region toward the near-layer regions, with a much more consistent depletion of charge occurring for the AA stacking than for the AB stacking of the two layers. DOI:10.1103/PhysRevB.86.245434

2012 - Quantum coherence controls the charge separation in a prototypical artificial light harvesting system [Relazione in Atti di Convegno]
Falke, S. M.; Rozzi, C. A.; Spallanzani, N.; Rubio, A.; Molinari, E.; Brida, D.; Maiuri, M.; Cerullo, G.; Schramm, H.; Christoffers, J.; Lienau, C.
abstract

Ultrafast spectroscopy and quantum-dynamics simulations of an artificial supramolecular light-harvesting system give strong evidence that the quantum-correlated wavelike motion of electrons and nuclei governs the ultrafast electronic charge transfer. © 2012 OSA.

2011 - Designing All-Graphene Nanojunctions by Covalent Functionalization [Articolo su rivista]
C., Cocchi; Ruini, Alice; D., Prezzi; M. J., Caldas; Molinari, Elisa
abstract

We investigated theoretically the effect of covalent edge functionalization, with organic functional groups, on the electronic properties of graphene nanostructures and nanojunctions. Our analysis shows that functionalization can be designed to tune electron affinities and ionization potentials of graphene flakes, and to control the energy alignment of frontier orbitals in nanometer-wide graphene junctions. The stability of the proposed mechanism is discussed with respect to the functional groups, their number as well as the width of graphene nanostructures. The results of our work indicate that different level alignments can be obtained and engineered in order to realize stable all-graphene nanodevices.

2011 - Optical Properties and Charge-Transfer Excitations in Edge-Functionalized All-Graphene Nanojunctions [Articolo su rivista]
C., Cocchi; D., Prezzi; Ruini, Alice; M. J., Caldas; Molinari, Elisa
abstract

We investigate the optical properties of edge-fiinctionalized graphene nanosystems, focusing on the formation of junctions and charge-transfer excitons. We consider a class of graphene structures that combine the main electronic features of graphene with the wide tunability of large polycyclic aromatic hydrocarbons. By investigating prototypical ribbon-like systems, we show that, upon convenient choice of functional groups, low-energy excitations with remarkable charge-transfer character and large oscillator strength are obtained. These properties can be further modulated through an appropriate width variation, thus spanning a wide range in the low-energy region of the UV-vis spectra. Our results are relevant in view of designing all-graphene optoelectronic nanodevices, which take advantage of the versatility of molecular functionalization, together with the stability and the electronic properties of graphene nanostructures.

2011 - Quantum dot states and optical excitations of edge-modulated graphene nanoribbons [Articolo su rivista]
D., Prezzi; D., Varsano; Ruini, Alice; Molinari, Elisa
abstract

We investigate from first principles the electronic and optical properties of edge-modulated armchair graphene nanoribbons, including both quasiparticle corrections and excitonic effects. Exploiting the oscillating behavior of the ribbon energy gap, we show that minimal width-modulations are sufficient to obtain confinement of both electrons and holes, thus forming optically active quantum dots with unique properties, such as the coexistence of dotlike and extended excitations and the fine tunability of optical spectra, with great potential for optoelectronic applications.

2011 - SiO(2) in density functional theory and beyond [Articolo su rivista]
L., Martin Samos; G., Bussi; Ruini, Alice; Molinari, Elisa; M. J., Caldas
abstract

We present the first-principle electronic structure calculation on an amorphous material including many-body corrections within the GW approximation. We show that the inclusion of the local field effects in the exchange-correlation potential is crucial to quantitatively describe amorphous systems and defect states. We show that the mobility gap of amorphous silica coincides with the band gap of quartz, contrary to the traditional picture and the densityfunctional theory results.

2011 - SiO2 in Density Functional Theory and Beyond [Capitolo/Saggio]
Martin-Samos, L.; Bussi, G.; Ruini, A.; Molinari, E.; Caldas, M. J.
abstract

2010 - Protein-surface interactions: challenging experiments and computations [Articolo su rivista]
O., Cohavi; S., Corni; DE RIENZO, Francesca; DI FELICE, Rosa; K. E., Gottschalk; M., Hoefling; D., Kokh; Molinari, Elisa; G., Schreiber; A., Vaskevich; R. C., Wade
abstract

Protein-surface interactions are fundamental in natural processes, and have great potential for applications ranging from nanotechnology to medicine. A recent workshop highlighted the current achievements and the main challenges in the field. Copyright (C) 2009 John Wiley & Sons, Ltd.

2010 - Spin-transport selectivity upon Co adsorption on antiferromagnetic graphene nanoribbons [Articolo su rivista]
Cocchi, Caterina; Prezzi, Deborah; Calzolari, Arrigo; Molinari, Elisa
abstract

We investigate from first principles the electronic and transport properties of zigzag graphene nanoribbons in the presence of Co adatoms. Comparing different adsorption sites across the width, we find that the Co-C coupling is rather sensitive to the local environment. While a net spin polarization appears in all cases, the spin filtering effect is significantly enhanced when the Co adatom is at the edge, where the adsorption energy is maximized and a partial suppression of edge-associated transport channels occurs. We also probe the magnetic interaction in the nonbonding regime, for Co-graphene nanoribbon (GNR) distances ranging from adsorption to - typical configurations. Our results indicate that Co-GNR coupling is still appreciable in an intermediate range, whereas it becomes vanishingly small in the limit of - distances.

2010 - Unraveling effects of disorder on the electronic structure of SiO2 from first principles [Articolo su rivista]
L., Martin Samos; Bussi, Giovanni; Ruini, Alice; Molinari, Elisa; M. J., Caldas
abstract

Unraveling effects of disorder on the electronic structure of SiO2 from first principles

2009 - Computational challenges about nanostructures and biosystems at surfaces [Articolo su rivista]
DI FELICE, Rosa; S., Corni; Molinari, Elisa
abstract

The interaction between nano-objects and extended solid surface's is of paramount importance in the context of nanoscience and nanotechnology. It plays a role both in investigating the properties of nanoscale materials and in exploiting such properties. We overview selected examples inspired by experimental facts. We focus on bio-nano cases such as the adsorpion of small polypeptides and nucleotides on metal surfaces, including the role of an aqueous environment. We highlight evidences that call for interpretation, computational approaches based on density functional theory and molecular dynamics that are currently available to face the open problems, and desired developments that, may increase the potentialities of theory to guide and interpret technology progress.

2009 - Electronic and magnetic states in core multishell nanowires: Edge localization, Landau levels and Aharonov-Bohm oscillations [Relazione in Atti di Convegno]
Ferrari, Giulio; Cuoghi, Gianpaolo; Bertoni, Andrea; Goldoni, Guido; Molinari, Elisa
abstract

We study the electronic states of hexagonal core multishell semiconductor nanowires, including the effect of magnetic fields. We find that the two dimensional electron states formed at the interface between different layers are mostly localized at the six edges of the hexagonal prism, and behave as a set of quasi-1D quantum channels. They can be manipulated by magnetic fields either parallel or perpendicular to the wire axis. These results can be rationalized in terms of Aharonov-Bohm oscillations or Landau level formation. We also show that inter-channel coupling and magnetic behavior is influenced by the geometric details of the nanowires. Â© 2009 IOP Publishing Ltd.

2009 - GoIP: An Atomistic Force-Field to Describe the Interaction of Proteins With Au(111) Surfaces in Water [Articolo su rivista]
Iori, Francesco; DI FELICE, Rosa; Molinari, Elisa; S., Corni
abstract

A classical atomistic force field to describe the interaction of proteins with gold (111) surfaces in explicit water has been devised. The force field is specifically designed to be easily usable in most common bio-oriented molecular dynamics codes, such as GROMACS and NAMD. Its parametrization is based on quantum mechanical (density functional theory [DFT] and second order Moller-Plesset perturbation theory [MP2]) calculations and experimental data oil the adsorption of small molecules on gold. In particular, a systematic DFT survey of the interaction between Au(111) and the natural amino acid side chains has been performed to single out chemisorption effects. Van der Waals parameters have been instead fitted to experimental desorption energy data of linear alkanes and were also studied via MP2 calculations. Finally, gold polarization (image charge effects) is taken into account by a recently proposed procedure (Iori, F.; Corni, S. J Comp Chem 2008, 29 1656). Preliminary validation results of GoIP on an independent test set of small molecules show the good performances of the force field. (C) Wiley Periodicals, Inc. J Comput Chem 30: 1465-1476, 2009

2009 - Magnetic States in Prismatic Core Multishell Nanowires [Articolo su rivista]
Ferrari, Giulio; Goldoni, Guido; Bertoni, Andrea; G., Cuoghi; Molinari, Elisa
abstract

We study the electronic states of core multishell semiconductor nanowires, including the effect of strong magnetic fields. We show that the multishell overgrowth of a free-standing nanowire, together with the prismatic symmetry of the substrate, may induce quantum confinement of carriers in a set of quasi-1D quantum channels corresponding to the nanowire edges. Localization and interchannel tunnel coupling are controlled by the curvature at the edges and the diameter of the underlying nanowire. We also show that a magnetic field may induce either Aharonov-Bohm oscillations of the energy levels in the axial configuration, or a dimensional transition of the quantum states from quasi-1D to Landau levels for fields normal to the axis. Explicit predictions are given for nanostructures based on GaAs, InAs, and InGaN with different symmetries.

2009 - Probing collective modes of correlated states of few electrons in semiconductor quantum dots [Articolo su rivista]
S., Kalliakos; Rontani, Massimo; V., Pellegrini; A., Pinczuk; A., Shinga; C. P., Garcia; Goldoni, Guido; Molinari, Elisa; L. N., Pfeiffer; K. W., West
abstract

Low-lying collective excitations above highly correlated ground states of few interacting electrons confined in GaAs semiconductor quantum dots are probed by resonant inelastic light scattering. We highlight that separate studies of the changes in the spin and charge degrees of freedom offer unique access to the fundamental interactions. The case of quantum dots with four electrons is found to be determined by a competition between triplet and singlet ground states that is uncovered in the rich light scattering spectra of spin excitations. These light scattering results are described within a configuration-interaction framework that captures the role of electron correlation with quantitative accuracy. Recent light scattering results that reveal the impact of anisotropic confining potentials in laterally coupled quantum dots are also reviewed. In these studies, inelastic light scattering methods emerge as powerful probes of collective phenomena and spin configurations in quantum dots with few electrons. (C) 2009 Elsevier Ltd. All rights reserved.

2008 - A molecular state of correlated electrons in a quantum dot [Articolo su rivista]
S., Kalliakos; M., Rontani; V., Pellegrini; C. P., Garcia; A., Pinczuk; Goldoni, Guido; Molinari, Elisa; L. N., Pfeiffer; AND K. W., West
abstract

Correlation among particles in finite quantum systems leads to complex behaviour and novel states of matter. One remarkable example is predicted to occur in a semiconductor quantum dot(1-3), where at vanishing electron density the Coulomb interaction between electrons rigidly fixes their relative positions as those of the nuclei in a molecule(4-14). In this limit, the neutral few-body excitations are roto-vibrations, which have either rigid-rotor or relative-motion character(15). In the weak correlation regime, on the contrary, the Coriolis force mixes rotational and vibrational motions. Here, we report evidence for roto-vibrationalmodes of an electron molecular state at densities for which electron localization is not yet fully achieved. We probe these collective modes by using inelastic light scattering(16-18) in quantum dots containing four electrons(19). Spectra of low-lying excitations associated with changes of the relative-motion wavefunction-the analogues of the vibration modes of a conventional molecule-do not depend on the rotational state represented by the total angular momentum. Theoretical simulations by the configuration-interaction method(20) are in agreement with the observed roto-vibrational modes and indicate that such molecular excitations develop at the onset of short-range correlation.

2008 - Carrier states on cylindrical 2DEGs in a magnetic field [Articolo su rivista]
G., Ferrari; Bertoni, Andrea; Goldoni, Guido; Molinari, Elisa
abstract

We compute carrier states on a cylindrical 2DEG under the influence of a magnetic field perpendicular to the tube axis. The field and the topology of the cylindrical surface have been included in the Schrodinger equation, that has been solved exactly. The results show that carrier states can be driven from a 2D regime, to a quasi-ID regime by transverse magnetic field. In the case of a spatially modulated magnetic field, the carriers can localise to quasi-OD states. (c) 2007 Elsevier B.V. All rights reserved.

2008 - Competitive chemisorption of bifunctional carboxylic acids on H : Si(100): A first-principles study [Articolo su rivista]
CUCINOTTA C., S; Ruini, Alice; Molinari, Elisa; PIGNEDOLI C., A; Catellani, A; Caldas, M. J.
abstract

We investigate competitive chemisorption processes of bifunctional alpha-carboxy omega-alkenes and omega-alkynes on fully hydrogenated H:Si(100), using first-principles density functional theory, in extended surface simulations. We study the structural properties and quantify the energetics and activation barriers, analyzing the reaction paths. Our results reveal that, if the plain, unactivated chemisorption reaction is always achieved through high barriers, once realized the configurations are very stable, ensuring robustness and reliability of the functionalized interface. We identify the conditions where disordered configurations are more likely to arise, with both functionalities offered at the free surface. For all stable configurations, a thorough analysis of the electronic properties and the extent of hybridization in the functionalized interface allows us to identify promising candidates for applications in molecular electronics.

2008 - Correlated states and spin transitions in nanofabricated AlGaAs/GaAs few-electron quantum dots probed by inelastic light scattering [Articolo su rivista]
S., Kalliacos; C. P., Garcia; V., Pellegrini; A., Pinczuk; B. S., Dennis; L. N., Pfeiffer; K. W., West; Rontani, Massimo; Goldoni, Guido; Molinari, Elisa
abstract

Spin transitions and correlated few-electron states are investigated by resonant inelastic light scattering in dilute arrays of GaAs/AlGaAs modulation-doped quantum dots (QDs) fabricated by electron-beam lithography and low impact reactive-ion etching. We focus on QDs with four electrons. We show that at moderate magnetic fields, the ground state is a singlet with total spin S = 0. A rich spectrum of distinct spin and charge inter-shell excitations is found, which cannot be described by a mean-field Hartree-Fock framework based on the quantum description of Fock-Darwin energy levels. Instead, the experimental results are well modeled by numerical evaluations within a full configuration interaction approach that highlights the impact of correlation effects in this configuration. This work demonstrates that the sensitivity reached by resonant inelastic light scattering enables the study of few-electron effects in QDs formed by state-of-the-art nanofabrication processes under the extreme conditions of low temperatures and high magnetic fields. (C) 2007 Elsevier B.V. All rights reserved.

2008 - Cylindrical two-dimensional electron gas in a transverse magnetic field [Articolo su rivista]
G., Ferrari; Bertoni, Andrea; Goldoni, Guido; Molinari, Elisa
abstract

We compute the single-particle states of a two-dimensional (2D) electron gas confined to the surface of a cylinder immersed in a magnetic field. The envelope-function equation is solved exactly for both a homogeneous and a periodically modulated magnetic field perpendicular to the cylinder axis. The nature and energy dispersion of the quantum states reflects the interplay between different length scales, namely, the cylinder diameter, the magnetic length, and, possibly, the wavelength of the field modulation. We show that a transverse homogeneous magnetic field drives carrier states from a quasi-2D (cylindrical) regime to a quasi-one-dimensional regime where carriers form channels along the cylinder surface. Furthermore, a magnetic field which is periodically modulated along the cylinder axis may confine the carriers to tunnel-coupled stripes, rings, and dots on the cylinder surface depending on the ratio between the field periodicity and the cylinder radius. Results in different regimes are traced to either incipient Landau-level formation or Aharonov-Bohm behavior.

2008 - Exact biexciton binding energy in carbon nanotubes using a quantum Monte Carlo approach [Articolo su rivista]
Kammerlander, David; Prezzi, Deborah; Goldoni, Guido; Molinari, Elisa; U., Hohenester
abstract

We performed quantum Monte Carlo (QMC) calculations for a model system of excitons and biexcitons in carbon nanotubes (CNTs) and compared the results with those of a variational approach [T.G. Pedersen, K. Pedersen, H.D. Cornean, P. Duclos, Nano Lett. 5 (2005) 291]. Due to their geometric properties, the shape of a hollow cylinder, CNTs can be treated as 2D objects. With decreasing diameter one expects them even to exhibit quasi-1D properties. In the present study the biexciton in its ground state is found to be more strongly bound than estimated before. Biexcitonic complexes are predicted to remain stable for all diameters even at room temperature. The binding energy grows significantly with decreasing diameter, showing indeed a transition from a quasi-2D system to a quasi-ID system. (c) 2007 Elsevier B.V. All rights reserved.

2008 - Optical properties of graphene nanoribbons: The role of many-body effects [Articolo su rivista]
Prezzi, Deborah; Varsano, Daniele; Ruini, Alice; Marini, Andrea; Molinari, Elisa
abstract

We investigate from first principles the optoelectronic properties of nanometer-sized armchair graphene nanoribbons (GNRs). We show that many-body effects are essential to correctly describe both energy gaps and optical response. As a signature of the confined geometry, we observe strongly bound excitons dominating the optical spectra, with a clear family-dependent binding energy. Our results demonstrate that GNRs constitute one-dimensional nanostructures whose absorption and luminescence performance can be controlled by changing both family and edge termination.

2008 - Oxygen-mediated electron transport through hybrid silicon-organic interfaces [Articolo su rivista]
Bonferroni, Benedetta; Ferretti, Andrea; Calzolari, Arrigo; Ruini, Alice; Caldas, Marilia J.; Molinari, Elisa
abstract

We investigate from first principles the electronic and transport properties of hybrid organic/silicon interfaces of relevance to molecular electronics. We focus on conjugated molecules bonded to hydrogenated Si through hydroxyl or thiol groups. The electronic structure of the systems is addressed within density functional theory, and the electron transport across the interface is directly evaluated within the Landauer approach. The microscopic effects of molecule-substrate bonding on the transport efficiency are explicitly analyzed, and the oxygen-bonded interface is identified as a candidate system when preferential hole transfer is needed.

2008 - Publisher's Note: Optical properties of graphene nanoribbons: The role of many-body effects (Physical Review B - Condensed Matter and Materials Physics (2008) 77, (041404)) [Articolo su rivista]
Prezzi, D.; Varsano, D.; Ruini, A.; Marini, A.; Molinari, E.
abstract

2008 - Spin relaxation due to spin-orbit coupling in multi-electron quantum dots [Articolo su rivista]
J. I., Climente; A., Bertoni; Goldoni, Guido; M., Rontani; Molinari, Elisa
abstract

We show that the number of electrons confined in a semiconductor quantum dot has a strong influence over the Rashba and Dresselhaus spin-orbit (SO) admixture. This can be exploited to improve the lifetime of spin excitations, as compared to the usual one- and two-electron devices. The physical mechanisms reducing SO admixture are discussed, and numerical results for realistic weakly confined GaAs/AlGaAs dots are reported. (c) 2007 Elsevier B.V. AD rights reserved.

2007 - Biexciton Stability in Carbon Nanotubes [Articolo su rivista]
Kammerlander, David; Prezzi, Deborah; Goldoni, Guido; Molinari, Elisa; AND U., Hohenester
abstract

We have applied the quantum Monte Carlo method and tight-binding modeling to calculate the binding energy of biexcitons in semiconductor carbon nanotubes for a wide range of diameters and chiralities. For typical nanotube diameters we find that biexciton binding energies are much larger than previously predicted from variational methods, which easily brings the biexciton binding energy above the room temperature threshold.

2007 - Correlation Effects in Wave Function Mapping of Molecular Beam Epitaxy Grown Quantum Dots [Articolo su rivista]
Giuseppe, Maruccio; Martin, Janson; Andreas, Schramm; Christian, Meyer; Tomohiro, Matsui; Christian, Heyn; Wolfgang, Hansen; Roland, Wiesendanger; Rontani, Massimo; Molinari, Elisa
abstract

We investigate correlation effects in the regime of a few electrons in uncapped InAs quantum dots by tunneling spectroscopy and wave function (WF) mapping at high tunneling currents where electron-electron interactions become relevant. Four clearly resolved states are found, whose approximate symmetries are roughly s and p, in order of increasing energy. Because the major axes of the p-like states coincide, the WF sequence is inconsistent with the imaging of independent-electron orbitals. The results are explained in terms of many-body tunneling theory, by comparing measured maps with those calculated by taking correlation effects into account.

2007 - Directionality of acoustic phonon emission in weakly-confined semiconductor quantum dots [Articolo su rivista]
Climente, J. I.; Bertoni, Andrea; Goldoni, Guido; Molinari, Elisa
abstract

The direction of propagation of acoustic phonons emitted by electron relaxation in weakly confined, parabolic quantum dots charged with one or two electrons is studied theoretically. The emission angle strongly depends on the energy of the phonon, the dominant electron-phonon scattering mechanism (deformation potential or piezoelectric field), and the orbital symmetries of the initial and final electron states. This leads to different behaviors for phonons emitted by electrons relaxing between levels of single and coupled quantum dots. Our results establish the basis to control the direction of propagation of phonon modes triggered by transitions in quantum dot systems.

2007 - Effect of electron-electron interaction on the phonon-mediated spin relaxation in quantum dots [Articolo su rivista]
J. I., Climente; Bertoni, Andrea; Goldoni, Guido; Rontani, Massimo; Molinari, Elisa
abstract

We estimate the spin relaxation rate due to spin-orbit coupling and acoustic phonon scattering in weakly confined quantum dots with up to five interacting electrons. The full configuration interaction approach is used to account for the interelectron repulsion, and Rashba and Dresselhaus spin-orbit couplings are exactly diagonalized. We show that electron-electron interaction strongly affects spin-orbit admixture in the sample. Consequently, relaxation rates strongly depend on the number of carriers confined in the dot. We identify the mechanisms which may lead to improved spin stability in few electron (> 2) quantum dots as compared to the usual one and two electron devices. Finally, we discuss recent experiments on triplet-singlet transitions in GaAs dots subject to external magnetic fields. Our simulations are in good agreement with the experimental findings, and support the interpretation of the observed spin relaxation as being due to spin-orbit coupling assisted by acoustic phonon emission.

2007 - Imaging correlated wave functions of few-electron quantum dots: Theory and scanning tunneling spectroscopy experiments [Articolo su rivista]
Rontani, M.; Molinari, E.; Maruccio, G.; Janson, M.; Schramm, A.; Meyer, C.; Matsui, T.; Heyn, C.; Hansen, W.; Wiesendanger, R.
abstract

We show both theoretically and experimentally that scanning tunneling spectroscopy (STS) images of semiconductor quantum dots may display clear signatures of electron-electron correlation. We apply many-body tunneling theory to a realistic model, which fully takes into account correlation effects and dot anisotropy. Comparing measured STS images of freestanding InAs quantum dots with those calculated by the full configuration interaction method, we explain the wave-function sequence in terms of images of one- and two-electron states. The STS map corresponding to double charging is significantly distorted by electron correlation with respect to the noninteracting case. © 2007 American Institute of Physics.

2007 - Imaging correlated wave functions of few-electron quantum dots: theory and STS experiments [Articolo su rivista]
Rontani, Massimo; Molinari, Elisa; Mariuccio, G; Janson, M; Schramm, A; Meyer, C; Matsui, T; Heyn, C; Hansen, W; AND WIESENDANGER, R.
abstract

2007 - Magnetic field dependence of triplet-singlet relaxation in quantum dots with spin-orbit coupling [Articolo su rivista]
Climente, Ji; Bertoni, A; Goldoni, Guido; Rontani, M; Molinari, Elisa
abstract

We estimate the triplet-singlet relaxation time due to spin-orbit coupling assisted by phonon emission in weakly confined quantum dots. Calculations are performed taking into account Coulomb and spin-orbit interactions exactly within the full configuration interaction method, and Fermi golden rule. Our results for two and four electrons show that different triplet-singlet relaxation trends observed in recent experiments under magnetic fields can be understood within a unified theoretical description, as the result of the competition between spin-orbit coupling and phonon emission efficiency. Moreover, we show that properly designed QD structures may give access to very long-lived triplet states as well as to selective population of the triplet Zeeman sublevels.

2007 - Mixing of electronic states in pentacene adsorption on copper [Articolo su rivista]
Ferretti, Andrea; C., Baldacchini; Calzolari, Arrigo; DI FELICE, Rosa; Ruini, Alice; Molinari, Elisa; M. G., Betti
abstract

By combining experimental and theoretical approaches, we study the adsorption of pentacene on copper as a model for the coupling between aromatic molecules and metal surfaces. Our results for the interface electronic structure are not compatible with a purely physisorption picture, which is conventionally employed for such systems. Nay, we demonstrate electronic mixing between molecular orbitals and metal electronic states

2007 - Optical near-field mapping of bright and dark quantum dot states [Relazione in Atti di Convegno]
Hohenester, U.; Goldoni, G.; Molinari, E.
abstract

We theoretically investigate scanning nearfield optical microscopy (SNOM) of semiconductor quantum dots. In the nearfield regime bright and dark excitonic states become mixed, opening new channels for the coupling to the electromagnetic field. As a consequence, ultra-narrow luminescence lines appear in the spectra, corresponding to very long lived excitonic states. © 2007 American Institute of Physics.

2007 - Optical properties of one-dimensional graphene polymers: the case of polyphenanthrene [Articolo su rivista]
Prezzi, Deborah; D., Varsano; Ruini, Alice; A., Marini; Molinari, Elisa
abstract

We investigate from first principles the effect of many-body corrections on the optoelectronic properties of polyphenanthrene (PPh), a prototype system for carbon-based ladder polymers and I D nanographenes with cis-polyene edges. We show that the inclusion of many-body effects is essential to correctly describe both quasiparticle bandstructure and optical response. Consistently with the reduced dimensionality of the system, the inclusion of electron-hole interaction leads to strongly bound excitons which dominate the spectra. A complete characterization of the low-energy excitonic states is carried out, together with their optical activity. In particular, we find a dark exciton below the first optically active one, which is expected to crucially affect the luminescence efficiency. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

2007 - Probing spin states in AlGaAs/GaAs few-electron quantum dots by inelastic light scattering [Relazione in Atti di Convegno]
Kalliakos, S.; Pascual Garcia, C.; Pellegrini, V.; Pinczuk, A.; Dennis, B. S.; Pfeiffer, L. N.; West, K. W.; Rontani, M.; Goldoni, G.; Molinari, E.
abstract

Resonant inelastic light scattering provides a venue for the study of spin transitions and interactions in quantum dots (QDs) with few electrons. Here we present the observation of inter-shell excitations in GaAs/AlGaAs nanofabricated QDs containing four electrons. The ground state with four electrons dictated by Hund's rule is a triplet with total spin S = 1 so that peculiar to this configuration there are two monopole inter-shell spin modes with changes ΔS = ±1 that connect the triplet state to the excited states with S = 0 and S = 2, respectively. We present light scattering spectra that offer evidence of these excitations. The light scattering spectra are interpreted by numerical evaluations within a full configuration interaction approach. © 2007 American Institute of Physics.

2007 - Symmetry lowering of pentacene molecular states interacting with a Cu surface [Articolo su rivista]
Baldacchini, C.; Mariani, C.; Betti, M. G.; Vobornik, I.; Fujii, J.; Annese, E.; Rossi, G.; Ferretti, A.; Calzolari, A.; Di Felice, R.; Ruini, A.; Molinari, E.
abstract

Pentacene adsorbed on the Cu(119) vicinal surface forms long-range ordered chain structures. Photoemission spectroscopy measurements and ab initio density functional theory simulations provide consistent evidences that pentacene molecular orbitals mix with the copper bands, giving rise to interaction states localized at the interface. Angular-resolved and polarization dependent photoemission spectroscopy shows that most of the pentacene derived intensity is strongly dichroic. The symmetry of the molecular states of the free pentacene molecules is reduced upon adsorption on Cu(119), as a consequence of the molecule-metal interaction. Theoretical results show a redistribution of the charge density in π molecular states close to the Fermi level, consistent with the photoemission intensities (density of states) and polarization dependence (orbital symmetry).

2007 - Water-mediated electron transfer between protein redox centers [Articolo su rivista]
MIGLIORE, Agostino; Corni, S; DI FELICE, ROSA; MOLINARI, Elisa
abstract

Recent experimental and theoretical investigations show that water molecules between or near redox partners can significantly affect their electron-transfer (ET) properties. Here we study the effects of intervening water molecules on the electron self-exchange reaction of azurin (Az), by performing a conformational sampling on the water medium and by using a newly developed ab initio method to calculate transfer integrals between molecular redox sites. We show that the insertion of water molecules at the interface between the copper active sites of Az dimers slightly increases the overall ET rate, while some favorable water conformations can considerably enhance the ET kinetics. These features are traced back to the interplay of two competing factors: the electrostatic interaction between the water and protein subsystems (mainly opposing the ET process for the water arrangements drawn from MD simulations) and the effectiveness of water in mediating ET coupling pathways. Such an interplay provides a physical basis for the found absence of correlation between the electronic couplings derived through ab initio electronic structure calculations and the related quantities obtained through the Empirical Pathways (EP) method. In fact, the latter does not account for electrostatic effects on the transfer integrals. Thus, we conclude that the water-mediated electron tunneling is not controlled by the geometry of a single physical pathway. We discuss the results in terms of the interplay between different ET pathways controlled by the conformational changes of one of the water molecules via its electrostatic influence. Finally, we examine the dynamical effects of the interfacial water and check the validity of the Condon approximation.

2006 - Controlling spin phases of few electrons in artificial molecules by magnetic fields [Articolo su rivista]
Rontani, M; Goldoni, Guido; Bellucci, D; Molinari, Elisa
abstract

We investigate the phase diagram of realistic vertically coupled quantum dots under a magnetic field of arbitrary strength and direction. The ground state wave function of four electrons is determined by means of the full configuration interaction method, able to accurately treat electron-electron correlation, which can dominate at experimentally attainable fields. (C) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

2006 - Correlation effects in quantum dot wave function imaging [Articolo su rivista]
Rontani, Massimo; Molinari, Elisa
abstract

We demonstrate that in semiconductor quantum dots wave functions, its imaged by local tunneling spectroscopies like STM, show characteristic signatures of electron-electron Coulomb correlation. We predict that such images correspond to quasi-particle wave functions which cannot be computed by standard mean-field techniques in the strongly correlated regime. From the configuration-interaction solution of the few-particle problem for prototype dots, we find that quasi-particle wave function images may display signatures of Wigner crystallization.

2006 - Effect of the Coulomb interaction on the electron relaxation of weakly-confined quantum dot systems using the full configuration interaction approach [Articolo su rivista]
J. I., Climente; Bertoni, Andrea; Rontani, Massimo; Goldoni, Guido; Molinari, Elisa
abstract

We study acoustic-phonon-induced relaxation of charge excitations in single and tunnel-coupled quantum dots containing few confined interacting electrons. The full configuration interaction approach is used to account for the electron-electron repulsion. Electron-phonon interaction is accounted for through both deformation potential and piezoelectric field mechanisms. We show that electronic correlations generally reduce intradot and interdot transition rates with respect to corresponding single-electron transitions, but this effect is lessened by external magnetic fields. On the other hand, piezoelectric field scattering is found to become the dominant relaxation mechanism as the number of confined electrons increases. Previous proposals to strongly suppress electron-phonon coupling in properly designed single-electron quantum dots are shown to hold also in multielectron devices. Our results indicate that few-electron orbital degrees of freedom are more stable than single-electron ones

2006 - Excitons in carbon nanotubes [Articolo su rivista]
Maultzsch, J; Pomraenke, R; Reich, S; Chang, E; Prezzi, D; Ruini, Alice; Molinari, Elisa; Strano, Ms; Thomsen, C; Lienau, C.
abstract

We present two-photon excitation luminescence experiments on carbon nanotubes which show the excitonic origin of the optical excitations. The two-photon allowed exciton state, which has even parity under rotation about the U-axis, is roughly 300 meV above the one-photon active, odd-parity state. This indicates exciton binding energies on the order of 400 meV for nanotubes with diameters around 8 A. Ab-initio calculations of the exciton wavefunctions and energies are in good agreement with our experimental results, confirming the predictions on the symmetry of the exciton states. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

2006 - First-principles DFT calculations of Electron Transfer rates in Azurin Dimers [Articolo su rivista]
Migliore, Agostino; S., Corni; DI FELICE, Rosa; Molinari, Elisa
abstract

We have conceived and implemented a new method to calculate transfer integrals between molecular sites, which exploits few quantities derived from density-functional theory electronic structure computations and does not require the knowledge of the exact transition state coordinate. The method uses a complete multielectron scheme, thus including electronic relaxation effects. Moreover, it makes no use of empirical parameters. The computed electronic couplings can then be combined with estimates of the reorganization energy to evaluate electron-transfer rates that are measured in kinetic experiments: the latter are the basis to interpret electron-transfer mechanisms. We have applied our approach to the study of the electron self-exchange reaction of azurin, an electron-transfer protein belonging to the family of cupredoxins. The transfer integral estimates provided by the proposed method have been compared with those resulting from other computational techniques, from empirical models, and with available experimental data.

2006 - First-principles investigation of functionalization-defects on silicon surfaces [Articolo su rivista]
C. S., Cucinotta; B., Bonferroni; Ferretti, Andrea; Ruini, Alice; M. J., Caldas; Molinari, Elisa
abstract

We present a theoretical study of chemisorption of CH C-CH2-COOH molecules on the H:Si(100) surface. We perform simulations for different chemisorbed configurations, attained by reactions through the alkyne tail. We use the periodic slab approximation for the extended surface, within ab initio density functional theory, and analyse results from several different approaches. We conclude that structures composed of single Si-C bridges are very stable.. while a previously proposed structure, with a double Si-C-Si bridge, should be metastable on the flat surface, and introduce electron and hole traps in the Si band gap. (c) 2006 Elsevier B.V. All rights reserved.

2006 - Optical excitations of quasi-one-dimensional systems: carbon nanotubes versus polymers and semiconductor wires [Articolo su rivista]
Prezzi, Deborah; Molinari, Elisa
abstract

We review the main characteristics of optical excitations of semiconductor nanotubes, as obtained from accurate ab-initio theories and model calculations as well as experimental evidence, and discuss them in light of the previous understanding of other quasi-one-dimensional semiconducting systems. We point out striking similarities of nanotubes with III-V quantum wires and conjugated polymers, especially (i) the clear excitonic nature of absorption, very far from the single-particle behaviour; (ii) its manifestations in optical spectra, where excitonic peaks are accompanied by a strong intensity reduction at the onset of the free-particle continuum; (iii) the strategies that allow experimental access to exciton binding energies. The recent theoretical and experimental evidence obtained on semiconducting single-walled nanotubes converges quantitatively to a picture of strongly bound excitons (about 0.3-1.0 eV for nanotubes with 0.4-1.0 nm diameter). We discuss its implications and list a few open issues of relevance to fundamental understanding and optoelectronic applications. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

2006 - Phonon-induced electron relaxation in correlated quantum dots [Relazione in Atti di Convegno]
J. I., Climente; A., Bertoni; M., Rontani; Goldoni, Guido; Molinari, Elisa
abstract

We study the electron momentum relaxation in single and vertically coupled quantum dots due to coupling with acoustic phonons. We consider both the single-electron and the few-electron regimes, using a Full Configuration Interaction approach to account for the electron-electron repulsion. Our findings suggest that electronic correlations tend to reduce intradot and interdot transition rates. We also show that external magnetic fields acting on properly designed quantum dots may strongly suppress electron-phonon coupling

2006 - Phonon-induced electron relaxation in weakly confined single and coupled quantum dots [Articolo su rivista]
Climente, Ji; Bertoni, A; Goldoni, Guido; Molinari, Elisa
abstract

We investigate charge relaxation rates due to acoustic phonons in weakly confined quantum dot systems, including both deformation potential and piezoelectric field interactions. Single-electron excited states lifetimes are calculated for single and coupled quantum dot structures, both in homonuclear and heteronuclear devices. Piezoelectric field scattering is shown to be the dominant relaxation mechanism in many experimentally relevant situations. On the other hand, we show that appropriate structure design allows to minimize separately deformation potential and piezolectric field interactions, and may bring electron lifetimes in the range of microseconds.

2006 - Quantum phases of correlated electrons in artificial molecules under magnetic fields [Articolo su rivista]
Bellucci, Devis; Rontani, Massimo; Goldoni, Guido; Molinari, Elisa
abstract

We investigate the stability of few-electron quantum phases in vertically coupled quantum dots under a magnetic field of arbitrary strength and direction. The orbital and spin stability diagrams of realistic devices containing up to five electrons, from strong to weak interdot coupling, is determined. Correlation effects and realistic sample geometries are fully taken into account within the full configuration interaction method. In general, the magnetic field drives the system into a strongly correlated regime by modulating the single-particle gaps. In coupled quantum dots different components of the field, either parallel or perpendicular to the tunneling direction, affect single-dot orbitals and tunneling energy, respectively. Therefore the stability of the quantum phases is related to different correlation mechanisms, depending on the field direction. Comparison of exact diagonalization results with simple models allows one to identify the specific role of correlations.

2006 - Response to "Comment on 'Field-controlled suppression of phonon-induced transitions in coupled quantum dots' [Appl. Phys. Lett. 88, 4729 (2006)]" [Articolo su rivista]
Bertoni, A; Rontani, M; Goldoni, Guido; Troiani, F; Molinari, Elisa
abstract

Risposta ad un Comment

2006 - Spin excitations in few-electrons AlGaAs/GaAs quantum dots probed by inelastic light scattering [Articolo su rivista]
C. P., Garcia; V., Pellegrini; A., Pinczuk; Rontani, Massimo; Goldoni, Guido; Molinari, Elisa; B. S., Dennis; L. N., Pfeiffer; K. W., West
abstract

We present recent studies of electronic excitations in nanofabricated AlGaAs/GaAs semiconductor quantum dots (QDs) by resonant inelastic light scattering. The resonant light scattering spectra are dominated by excitations from parity-allowed inter-shell transitions between Fock-Darwin levels. In QDs With very few electrons the resonant spectra are characterized by distinct charge and spin excitations that reveal the strong impact of both exchange and correlation effects. A sharp inter-shell spin excitation of the triplet spin QD state with four electrons is identified. (c) 2006 Elsevier B.V. All rights reserved.

2006 - Surface nano-patterning through styrene adsorption on Si(100) [Articolo su rivista]
A., Calzolari; Ruini, Alice; Mj, Caldas; Molinari, Elisa
abstract

We present an ab initio study of the electronic properties of styrene molecules adsorbed on the dimerized Si(100) surface, ranging from the single molecule to the full monolayer (ML). The adsorption mechanism primarily involves the vinyl group via a [2+2] cycloaddition process that leads to the formation of covalent SiC bonds and a local surface derelaxation, while it leaves the phenyl group almost unperturbed. The investigation of the functionalized surface as a function of the coverage (e.g., 0.5–1 ML) and of the substrate reconstruction reveals two major effects. The first results from Si dimer-vinyl interaction and concerns the controlled variation of the energy band gap of the interface. The second is associated to phenyl-phenyl interactions, which give rise to a regular pattern of electronic wires at surface, stemming from the π-π coupling. These findings suggest a rationale for tailoring the nanopatterning of the surface in a controlled way.

2006 - Two-photon photoluminescence and exciton binding energies in single-walled carbon nanotubes [Articolo su rivista]
R., Pomraenke; J., Maultzsch; S., Reich; E., Chang; D., Prezzi; Ruini, Alice; Molinari, Elisa; M. S., Strano; C., Thomsen; C., Lienau
abstract

We compare experimental one- and two-photon luminescence excitation spectra of single-walled carbon nanotubes at room temperature to ab initio calculations. The experimental spectra reveal a Rydberg-like series of excitonic states. The energy splitting between these states is a clear fingerprint of excitonic correlations in carbon nanotubes. From those spectra, we derive exciton binding energies of 0.3-0.4 eV for nanotubes with diameters between 6.8 angstrom and 9.0 angstrom. These energies are in quantitative agreement with our nanotubes with diameters between 6.8 angstrom and 9.0 angstrom. These energies are in quantitative agreement with our theoretical calculations, which predict the symmetries of the relevant excitonic wave functions and indicate that a low-lying optically dark excitonic state may be responsible for the low luminescence quantum yields in nanotubes.

2006 - Two-photon photoluminescence and exciton binding in single-walled carbon nanotubes: Experiment and theory [Relazione in Atti di Convegno]
Pomraenke, R.; Lienau, C.; Maultzsch, J.; Reich, S.; Chang, E.; Prezzi, D.; Ruini, A.; Molinari, E.; Strano, M. S.; Thomsen, C.
abstract

One- and two-photon luminescence excitation spectra, showing a series of excitonic states, are compared to ab-initio calculations to unravel binding energies, symmetries and spatial extent of excitonic wavefunctions in single-walled carbon nanotubes. © 2005 Optical Society of America.

2006 - Water Effects on Electron Transfer in Azurin Dimers [Articolo su rivista]
MIGLIORE, Agostino; S., CORNI; DI FELICE, ROSA; MOLINARI, Elisa
abstract

Recent experimental and theoretical analyses indicate that water molecules between or near redox partners can significantly affect their electron-transfer (ET) properties. Here, we study the effects of intervening water molecules on the electron self-exchange reaction of azurin (Az) by using a newly developed ab-initio method to calculate transfer integrals between molecular sites. We show that the insertion of water molecules in the gap between the copper active sites of Az dimers slows down the exponential decay of the ET rates with the copper-to-copper distance. Depending on the distance between the redox sites, water can enhance or suppress the electron-transfer kinetics. We show that this behavior can be ascribed to the simultaneous action of two competing effects: the electrostatic interaction of water with the protein subsystem and its ability to mediate ET coupling pathways

2005 - A symmetrized-basis approach to excitons in carbon nanotubes [Relazione in Atti di Convegno]
G., Bussi; E., Chang; Ruini, Alice; Molinari, Elisa
abstract

We calculate from first-principles the optical spectrum of a (4,2) single-wall carbon nanotube including quasi-particle corrections and excitonic effects. We expand every quantity on a special basis sets which completely exploits the symmetries of the nanotube, allowing calculations for tubes with a long unit cell. The results indicate that the excitonic effects are crucial and a strong peak in the absorption spectrum is predicted at 2.2 eV This value is compared with experimental results, with excellent agreement.

2005 - Ab-initio study of excitonic effects in conventional and organic semiconductors [Articolo su rivista]
Hummer, K; Ambrosch Draxl, C; Bussi, G; Ruini, Alice; Caldas, Mj; Molinari, Elisa; Laskowski, R; Christensen, Ne
abstract

The excitonic effects on the optical absorption properties of organic semiconductors as well as gallium nitride are studied from first-principles. The Coulomb interaction between the electron and the hole is accounted for by solving the two-particle Bethe-Salpeter Equation. In the organic semiconductors the exciton binding energies strongly depend on the molecular size, the crystalline packing, as well as the polarization direction of the incoming light. We show that the electron-hole interaction can lead to strongly bound excitons with binding energies of the order of 1 eV or to a mere redistribution of oscillator strength. In several cases, the screening is efficient enough such that free charge carriers govern the optical absorption process. In the inorganic counterparts the sensitivity of the exciton binding energy is tested against the structural parameters and the screening of the electron-hole Coulomb interaction.

2005 - Biexcitons in artificial molecules with in-plane magnetic field [Articolo su rivista]
Bellucci, D.; Troiani, Filippo; Goldoni, Guido; Molinari, Elisa
abstract

We theoretically investigate the effect of a magnetic field perpendicular to the tunneling direction on the ground-state properties of biexcitons in coupled quantum dots. The single-particle states are computed by numerically solving the 3D Scrodinger equation. The biexciton states are obtained by means of a configuration-interaction approach, which fully accounts for the intra- and inter-dot Coulomb correlations. We show that the biexciton ground state undergoes nontrivial transitions as a function of the applied magnetic field, which can be traced back to unexpected carrier localizations. (C) 2004 Elsevier B.V. All rights reserved.

2005 - Collective properties of electrons and holes in coupled quantum dots [Relazione in Atti di Convegno]
Goldoni, Guido; Troiani, F.; Rontani, M.; Bellucci, D.; Molinari, Elisa
abstract

We discuss the properties of few electrons and electron-hole pairs confined in coupled semiconductor quantum dots, with emphasis on correlation effects and the role of tunneling. We shall discuss, in particular, exact diagonalization results for biexciton binding energy, electron-hole localization, magnetic-field induced Wigner molecules, and spin ordering.

2005 - Dark-state luminescence of macroatoms at the near field [Articolo su rivista]
U., Hohenester; Goldoni, Guido; Molinari, Elisa
abstract

We theoretically analyze the optical near-field response of a semiconductor macroatom induced by local monolayer fluctuations in the thickness of a semiconductor quantum well, where the large active volume results in a strong enhancement of the light-matter coupling. We find that in the near-field regime bright and dark excitonic states become mixed, opening new channels for the coupling to the electromagnetic field. As a consequence, ultranarrow luminescence lines appear in the simulated two-photon experiments, corresponding to very long lived excitonic states, which undergo Stark shift and Rabi splitting at relatively small field intensities.

2005 - Evidence of correlation in spin excitations of few-electron quantum dots [Articolo su rivista]
Garcia, Cp; Pellegrini, V; Pinczuk, A; Rontani, M; Goldoni, Guido; Molinari, Elisa; Dennis, Bs; Pfeiffer, Ln; West, Kw
abstract

We report inelastic light scattering measurements of spin and charge excitations in nanofabricated AlGaAs/GaAs quantum dots with few electrons. A narrow spin excitation peak is observed and assigned to the intershell triplet-to-singlet monopole mode of dots with four electrons. Configuration-interaction theory provides precise quantitative interpretations that uncover large correlation effects that are comparable to exchange Coulomb interactions.

2005 - Exciton binding energies in carbon nanotubes from two-photon photoluminescence [Articolo su rivista]
J., Maultszsch; R., Pomraenke; S., Reich; E., Chang; D., Prezzi; Ruini, Alice; Molinari, Elisa; Ms, Strano; C., Thomsen; C., Lienau
abstract

Excitonic effects in the linear and nonlinear optical properties of single-walled carbon nanotubes are manifested by photoluminescence excitation experiments and ab initio calculations. One- and two-photon spectra showed a series of exciton states; their energy splitting is the fingerprint of excitonic interactions in carbon nanotubes. By ab initio calculations we determine the energies, wave functions, and symmetries of the excitonic states. Combining experiment and theory we find binding energies of 0.3–0.4 eV for nanotubes with diameters between 6.8 and 9.0 Å.

2005 - First-principle theory of correlated transport through nano-junctions [Articolo su rivista]
Ferretti, Andrea; Calzolari, Arrigo; DI FELICE, Rosa; Manghi, Franca; M. J., Caldas; M., BUONGIORNO NARDELLI; Molinari, Elisa
abstract

We report the inclusion of electron-electron correlation in the calculation of transport properties within an ab initio scheme. A key step is the reformulation of Landauer's approach in terms of an effective transmittance for the interacting electron system. We apply this framework to analyze the effect of shortrange interactions on Pt atomic wires and discuss the coherent and incoherent correction to the mean-field approach.

2005 - First-principles approach for the calculation of optical properties of one-dimensional systems with helical symmetry: The case of carbon nanotubes [Articolo su rivista]
E., Chang; G., Bussi; Ruini, Alice; Molinari, Elisa
abstract

We present a recently developed ab initio method based on many-body perturbation theory to calculate the optical absorption spectrum of one-dimensional systems with helical symmetry. Our scheme involves a local, symmetrized basis set which allows for the calculation of large systems otherwise prohibitive in the standard plane-wave approach. It also affords an understanding of the symmetry character of the single-particle states and the excitonic wave functions, which has the advantage of determining in a precise way the selection rules related to the optical transitions of the system in question. We apply our method to single-wall carbon nanotubes of type (4,2) and present the calculated self-energy corrections, absorption spectra, and excitonic states; we find that GW corrections are substantial and excitonic effects strongly affect the optical properties.

2005 - Imaging quasi-particle wavefunctions in quantum dots in via tunneling spectroscopy [Articolo su rivista]
Rontani, Massimo; Molinari, Elisa
abstract

We show that in quantum dots the physical quantities probed by local tunneling spectroscopies-namely, the quasiparticle wave functions of interacting electrons-can considerably deviate from their single-particle counterparts as an effect of Coulomb correlation. From the exact solution of the few-particle Hamiltonian for prototype dots, we find that such deviations are crucial to predict wave function images at low electron densities or high magnetic fields.

2005 - Interacting electrons in artificial molecules with magnetic field of arbitrary direction [Articolo su rivista]
Bellucci, D.; Troiani, Filippo; Goldoni, Guido; Molinari, Elisa
abstract

We theoretically investigate the mechanisms for the magnetic field-induced singlet-triplet transition of two electrons in vertically coupled quantum dots, as a function of the field strength and direction. Our numerical approach is based on a real-space description of single-particle states in realistic samples and exact diagonalization of carrier-carrier Coulomb interaction. The three-electron system is also discussed, and we show that the magnetic field drives non-trivial transitions in the ground and excited states. (C) 2004 Elsevier B.V. All rights reserved.

2005 - Light-emitting polymers: a first-principles analysis of singlet-exciton harvesting in PPV [Relazione in Atti di Convegno]
M. J., Caldas; G., Bussi; Ruini, Alice; Molinari, Elisa
abstract

We study poly(para-phenylene-vinylene) PPV in a pi-stacked crystalline con guration pound, through ab initio density functional techniques for the electronic structure and optical properties. We End that interchain interactions, while maintaining the quasi-1D characteristics of the lowest singlet and triplet excitons, introduces other bound excitons that should favor intersystem crossing and enhance the singlet exciton yield.

2005 - Magnetic-field-dependent optical properties and interdot correlations in coupled quantum dots [Articolo su rivista]
Bellucci, D; Troiani, F; Goldoni, Guido; Molinari, Elisa
abstract

We theoretically investigate the properties of neutral and charged excitons in vertically coupled quantum dots, as a function of the in-plane magnetic field. The single-particle states are computed by numerically solving the 3D effective-mass equation, while the neutral- and charged-exciton states are obtained by means of a configuration interaction approach. We show that the field determines an enhancement of the interdot correlations, resulting in unexpected carrier localization. The field effect on the excitonic binding energies is also discussed, and is shown to strongly depend on the charging. (c) 2004 Elsevier B.V. All rights reserved.

2005 - Quantum interferences in the Raman cross section for the radial breathing mode in metallic carbon nanotubes [Articolo su rivista]
Bussi, Giovanni; J., Menendez; J., Ren; M., Canonico; Molinari, Elisa
abstract

The line shapes of the Raman excitation profiles for radial breathing modes in carbon nanotubes are shown to be strongly affected by interference effects that arise whenever strong optical transitions are separated by a small energy. This is the case in metallic zigzag and chiral tubes, where one-dimensional singularities in the electronic joint density of states are split due to the trigonal warping of the electronic band structure of a two-dimensional graphene. It is shown that the proper modeling of these interferences is crucial for the identification of the (n,m) indices using Raman spectroscopy.

2005 - Reduced electron relaxation rate in multi-electron quantum dots [Articolo su rivista]
A., Bertoni; M., Rontani; Goldoni, Guido; Molinari, Elisa
abstract

We use a configuration-interaction approach and the Fermi golden rule to investigate electron-phonon interaction in multielectron quantum dots. Lifetimes are computed in the low-density, highly correlated regime. We report numerical evidence that electron-electron interaction generally leads to reduced decay rates of excited electronic states in weakly confined quantum dots, where carrier relaxation is dominated by the interaction with longitudinal acoustic phonons.

2005 - Role of the electronic properties of azurin active site in the electron-transfer process [Articolo su rivista]
Corni, S; DE RIENZO, Francesca; DI FELICE, Rosa; Molinari, Elisa
abstract

Electron transfer proteins, such as azurin (a blue copper protein), are promising candidates for the implementation of biomolecular nanoelectronic devices. To understand the details of electron transfer in redox active azurin molecules, we performed plane-wave pseudo-potential density functional theory (DFT) calculations of the protein active site in the two possible oxidation states Cu(I) and Cu(II). The ab initio results are used to discuss how the electronic spectrum and wavefunctions may mediate the shuttling of electrons through the copper ion. We find that the Cu-ligand hybridization is very similar in the two charge states of the metal center, but the energy spectrum changes substantially. This result might indicate important effects of electronic correlations in the redox activity and consequent electron transfer through the Cu site.

2005 - Saturated carboxylic acids on silicon: a first-principles study [Relazione in Atti di Convegno]
C. S., Cucinotta; Ruini, Alice; M. J., Caldas; Molinari, Elisa
abstract

We present a first-principles calculation of the energetics of different possible dissociative chemisorption reactions leading to the attachment of organic acids with a functional carboxylic group to a hydrogenated silicon surface. Our study allows us to understand the role of oxygen atoms in the stable anchoring of the organic layer to the surface.

2005 - Suppression of acoustic-phonon-induced electron transitions in coupled quantum dots [Articolo su rivista]
Bertoni, A; Rontani, M; Goldoni, Guido; Troiani, F; Molinari, Elisa
abstract

We calculate the longitudinal-acoustic phonon scattering rate for a vertical double quantum dot (DQD) system and show that a strong modulation of the single-electron excited states lifetime can be induced by an external magnetic field. The results are obtained for typical realistic devices using a Fermi golden rule approach and a three-dimensional description of the electronic quantum states. The DQDs considered are characterized by a weak lateral confinement and the longitudinal-phonon scattering represents the dominant source of decoherence, its tunable suppression can be a valuable tool for an experimental measure of electron-states lifetimes and serve as a signature for coherent delocalization of electrons in the DQD.

2005 - Towards protein field-effect transistors: report and model of a prototype [Articolo su rivista]
G., Maruccio; A., Biasco; P., Visconti; A., Bramanti; P. P., Pompa; F., Calabi; R., Cingolani; R., Rinaldi; S., Corni; DI FELICE, Rosa; Molinari, Elisa
abstract

A protein field-effect transistor (Pro-FET) based on the blue-copper protein azurins (see Figure) and operating at room temperature and ambient pressure is demonstrated. The transfer characteristics of the Pro-FET exhibit a pronounced resonance due to the switch from behaving as a n-metal oxide semiconductor FET (n-MOSFET) to a p-MOSFET. Carrier transport through the device is explained in terms of an equilibrium between the two possible oxidation states of the redox site (Cu1+ and Cu2+).

2004 - Ab initio study of transport parameters in polymer crystals [Articolo su rivista]
Ferretti, A; Ruini, Alice; Bussi, G; Molinari, Elisa; Caldas, Mj
abstract

Transfer integrals (TI's) are essential parameters in the calculation of electron transport both in coherent and incoherent regimes. We show that TI's for polymer crystals can be obtained from first principles, starting from plane-wave density-functional calculations of the electronic structure in the local-density approximation, and propose methods at different levels of approximation. We demonstrate that special choices of single-chain states can be used very effectively as building blocks for the crystal electronic structure, thus allowing a deeper insight into the transport properties of molecular crystals. We apply this approach to polymer systems of great interest to molecular electronics, such as poly-para-phenylene-vinylene and polythiophene in different crystal packing morphologies, and show that it offers a very powerful tool to understand and design the impact of intermolecular interactions on conduction of organic crystals.

2004 - Ab-initio study of chemisorption reactions for carboxylic acids on hydrogenated silicon surfaces [Articolo su rivista]
C. S., Cucinotta; Ruini, Alice; Molinari, Elisa; M. J., Caldas
abstract

We study chemisorbed configurations of C3H6O2 on the extended H:Si(100) surface, through first-principles density-functional calculations in a supercell approach. We demonstrate that oxygen-bonded organic monolayers on this silicon substrate is thermodynamically very stable, and comparing several Si-O-C and Si-C linked configurations, we find that the doubly-O-bonded configuration is favored and should lead to ordered SAMs. We find, moreover, that the Si-O-C bridge in this case does not block charge transfer from surface to molecule.

2004 - Competing mechanisms for singlet-triplet transition in artificial molecules [Articolo su rivista]
Bellucci, D.; Rontani, Massimo; Troiani, Filippo; Goldoni, Guido; Molinari, Elisa
abstract

We study the magnetic field induced singlet/triplet transition for two electrons in vertically-coupled quantum dots by exact diagonalization of the Coulomb interaction. We identify the different mechanisms occurring in the transition, involving either in-plane correlations or localization in opposite dots, depending on the field direction. Therefore, both spin and orbital degrees of freedom can be manipulated by field strength and direction. The phase diagram of realistic devices is determined.

2004 - Electron channels in biomolecular nanowires [Articolo su rivista]
Calzolari, Arrigo; DI FELICE, Rosa; Molinari, Elisa; Garbesi, Patrizia
abstract

We report a first-principle study of the electronic and conduction properties of a quadruple-helix guanine wire (G4 wire), a DNA derivative, with inner potassium ions. The analysis of the electronic structure highlights the presence of energy manifolds that are equivalent to the bands of (semi)conducting materials and reveals the formation of extended electron channels available for charge transport along the wire. The specific metal-nucleobase interactions affect the electronic properties at the Fermi level, leading the wire to behave as an intrinsically p-doped system.

2004 - Electronic properties of guanine-based nanowires [Articolo su rivista]
Calzolari, Arrigo; DI FELICE, Rosa; Molinari, Elisa
abstract

We present a first-principles study of the electronic and conduction properties of a few classes of nanowires composed of guanine (G) molecules, self-assembled in different geometries. We first analyze the effect of the vertical pi-pi interaction in model G-stack columns. Then, we exploit the results obtained from those models to interpret the features of realistically stacked and hydrogen-bonded aggregates, namely the guanine quadruple helices and the planar ribbons. With respect to natural DNA, the different structures drastically affect the bonding pattern among the bases, introducing novel features in the electronic properties of the systems. These supramolecular G-aggregates, alternative to DNA, are expected to show interesting properties for molecular electronics applications.

2004 - Excitons in carbon nanotubes: an ab initio symmetry-based approach [Articolo su rivista]
E., Chang; G., Bussi; RUINI, Alice; MOLINARI, Elisa
abstract

The optical absorption spectrum of the carbon (4,2) nanotube is computed using an ab initio many-body approach which takes into account excitonic effects. We develop a new method involving a local basis set which is symmetric with respect to the screw-symmetry of the tube. Such a method has the advantages of scaling faster than plane-wave methods and allowing for a precise determination of the symmetry character of the single-particle states, two-particle excitations, and selection rules. The binding energy of the lowest, optically active states is approximately 0.8 eV. The corresponding exciton wave functions are delocalized along the circumference of the tube and localized in the direction of the tube axis.

2004 - Field-controlled suppression of phonon-induced transitions in coupled quantum dots [Articolo su rivista]
Bertoni, Andrea; Rontani, Massimo; Goldoni, Guido; Troiani, Filippo; Molinari, Elisa
abstract

We suggest that order-of-magnitude reduction of the longitudinal-acoustic phonon scattering rate, the dominant decoherence mechanism in quantum dots, can be achieved in coupled structures by the application of an external electric or magnetic field. Modulation of the scattering rate is traced to the relation between the wavelength of the emitted phonon and the length scale of delocalized electron wave functions. Explicit calculations for realistic devices, performed with a Fermi golden rule approach and a fully three-dimensional description of the electronic quantum states, show that the lifetime of specific states can achieve tens of microseconds. Our findings extend the feasibility basis of many proposals for quantum gates based on coupled quantum dots. (C) 2004 American Institute of Physics.

2004 - High-finesse optical quantum gates for electron spins in artificial molecules [Articolo su rivista]
Troiani, Filippo; Hohenester, U; Molinari, Elisa
abstract

We propose the storage of the quantum information (qubit) in the spin of an excess electron in two vertically coupled quantum dots and its all-optical manipulation through ps-laser pulses. The auxiliary levels of the artificial molecule allow the implementation of the logical gates solely by means of stimulated Raman adiabatic passages, and the consequent suppression of the environment losses. The mapping of the spin states onto the orbital degrees of freedom results in the switch on of the dipole-dipole interaction between neighbouring qubits and thus allows the conditional dynamics (two-qubit gates) to be performed.

2004 - Molecular phases in coupled quantum dots [Articolo su rivista]
M., Rontani; S., Amaha; K., Muraki; Manghi, Franca; Molinari, Elisa; S., Tarucha; D. G., Austing
abstract

We present excitation energy spectra of few-electron vertically coupled quantum dots for strong and intermediate interdot coupling. By applying a magnetic field, we induce ground state transitions and identify the corresponding quantum numbers by comparison with few-body calculations. In addition to atomiclike states, we find novel "molecularlike" phases. The isospin index characterizes the nature of the bond of the artificial molecule and this we control. Like spin in a single quantum dot, transitions in isospin leading to full polarization are observed with increasing magnetic field.

2004 - Neutral and charged electron-hole complexes in artificial molecules: quantum transitions induced by the in-plane magnetic field [Articolo su rivista]
Bellucci, D.; Troiani, Filippo; Goldoni, Guido; Molinari, Elisa
abstract

We theoretically investigate the properties of neutral and charged excitons and of the biexciton in vertically coupled quantum dots, as a function of the in-plane magnetic field B-parallel to. The main effect of the field consists in the suppression of the bonding-antibonding splitting, and in the resulting enhancement of the interdot correlations. As a consequence, the excitons form with the additional carrier a bound or an unbound complex depending on the sign of the charging, whereas the biexciton undergoes a transition between different quantum states with increasing B-parallel to. The discussed behaviors and transitions show up in the field dependence of experimentally accessible quantities, such as the charged-exciton and biexciton binding energies.

2004 - On the electronic structure analysis for one redox-active molecule [Articolo su rivista]
Corni, S; DI FELICE, Rosa; Molinari, Elisa
abstract

A theoretical method is proposed to circumvent the orbital relaxation problem (variations in the orbital composition when the electron number in the system changes) in the analysis of the calculated electronic structure of a redox-active molecule. The method is based on a convenient partition of the electron-transfer integral: one contribution depends on the coupling of the molecule with the external redox partner; the other is an intrinsic feature of the molecule under study and takes into account the orbital relaxation upon reduction/oxidation. The method is applied to the electronic structure analysis of the active site of the electron-transfer protein azurin.

2004 - Optical near-field mapping of excitons and biexcitons in naturally occurring semiconductor quantum dots [Articolo su rivista]
U., Hohenester; Goldoni, Guido; Molinari, Elisa
abstract

We calculate the near-field optical spectra of excitons and biexcitons in semiconductor quantum dots naturally occurring at interface fluctuations in GaAs-based quantum wells, using a nonlocal description of the response function to a spatially modulated electromagnetic field. The relative intensity of the lowest, far-field forbidden excitonic states is predicted; the spatial extension of the ground biexciton state is found, in agreement with recently published experiments.

2004 - Relationship between structural and optoelectronic properties in semiconducting polymers [Articolo su rivista]
Ruini, Alice; Ferretti, A.; Bussi, G.; Molinari, Elisa; Caldas, M. J.
abstract

The optoelectronic behaviour of poly(para)phenylenevinylene is explored through solid-state ab initio approaches, which are appropriate for extended crystalline systems, and allow one to access transfer integrals and excitonic spectra. The microscopic interpretation of our results highlights the impact of interchain coupling on both transport and emissive properties of semiconducting polymer crystals.

2004 - Spin-spin interaction in artificial molecules with in-plane magnetic field [Articolo su rivista]
Bellucci, D; Rontania, M; Goldoni, Guido; Troiani, F; Molinari, Elisa
abstract

We investigate theoretically the spin-spin interaction of two-electrons in vertically coupled QDs as a function of the angle between magnetic field and growth axis. Our numerical approach is based on a real-space description of single-particle states in realistic samples and exact diagonalization of carrier-carrier Coulomb interaction. In particular, the effect of the in-plane field component on tunneling and, therefore, spin-spin interaction will be discussed; the singlet-triplet phase diagram as a function of the field strength and direction is drawn.

2003 - A DFT Study of Cysteine Adsorption on Au(111) [Articolo su rivista]
DI FELICE, Rosa; A., Selloni; Molinari, Elisa
abstract

The adsorption of the cysteine amino acid (H-SCbetaH2-CalphaH-NH2-COOH) on the (111) surface of gold is studied by means of periodic density functional calculations. Results for different adsorption sites and molecular configurations show that chemisorption involving S(thiolate)-Au bonds on Au(111) is favored by starting with either cysteine or cystine gas-phase molecular precursors. In the most stable adsorption configuration, the sulfur headgroup sits at the bridge site between two surface An atoms, and the S-C-beta bond is tilted by 57degrees with respect to the surface normal, whereas the in-plane orientation of the molecular backbone plays a secondary role. The analysis of the electronic properties shows that the hybridization of the p-like S states with the d-like Au states produces both bonding and antibonding occupied orbitals, and the process is well described by a model for the interaction of localized orbitals with narrow-band dispersive electron states. The bonding orbitals well below the Fermi level contribute to the strong chemisorption of cysteine on gold. The calculated sulfur-projected density of states allows us to locate the cysteine molecular orbitals with respect to the system Fermi level, which gives a measure of the injection barrier at the molecule/electrode junction.

2003 - Charge transport and radiative recombination in polythiophene crystals: a first-principles study [Articolo su rivista]
Ruini, A.; Bussi, G.; Ferretti, A.; Caldas, M. J.; Molinari, E.
abstract

We investigate two phases of polythiophene crystals by means of first-principles calculations, focusing on the effect of the different structure on charge transport parameters and luminescence quantum yield. The resulting microscopic interpretation highlights the impact of solid-state interchain coupling on both transport and emissive properties of semiconducting polymer crystals.

2003 - Electronic properties of polymer crystals: The effect of interchain interactions [Articolo su rivista]
Ruini, Alice; Molinari, Elisa; Caldas, Marilia J.; Ferretti, Andrea
abstract

We present a theoretical study of the transport parameters in a prototype conjugated-polymer, poly-para-phenylenevinylene, in two different possible crystalline packings. Our analysis is performed through density-functional electronic structure calculations, and allows one to obtain the fundamental parameters describing charge transport. The transfer integrals are found to be a crucial quantity to appreciate the effects of crystalline aggregation on conduction properties: our results indicate that interchain interactions can be viewed as a tunable parameter for the design of efficient electronic devices based on organic materials.

2003 - Electronic rectification in protein devices [Articolo su rivista]
Rinaldi, R; Biasco, A; Maruccio, G; Arima, V; Visconti, P; Cingolani, R; Facci, P; DE RIENZO, Francesca; DI FELICE, Rosa; Molinari, Elisa; Verbeet, Mp; Canters, Gw
abstract

We show that the electron-transfer protein azurin can be used to fabricate biomolecular rectifiers exploiting its native redox properties, chemisorption capability and electrostatic features. The devices consist of a protein layer interconnecting nanoscale electrodes fabricated by electron beam lithography. They exhibit a rectification ratio as large as 500 at 10 V, and operate at room temperature and in air.

2003 - Few-electron liquid and solid phases in artificial molecules at high magnetic field [Capitolo/Saggio]
M., Rontani; Goldoni, Guido; Molinari, Elisa
abstract

Coupled semiconductor quantum dots form artificial molecules where relevant energy scales controlling the interacting ground state can be easily tuned. By applying an external magnetic field it is possible to drive the system from a weak to a strong correlation regime where eventually electrons localize in space in an ordered manner reminiscent of the two-dimensional Wigner crystal. We explore the phase diagram of such "Wigner molecules" analyzing the angular correlation function obtained by the Configuration Interaction solution of the full interacting Hamiltonian. Focus is on the role of tunneling in stabilizing different ground states.

2003 - High-Finesse Optical Quantum Gates for Electron Spins in Artificial Molecules [Articolo su rivista]
Troiani, Filippo; U., Hohenester; Molinari, Elisa
abstract

A doped semiconductor double-quantum-dot molecule is proposed as a qubit realization. The quantum information is encoded in the electron spin, thus benefiting from the long relevant decoherence times; the enhanced flexibility of the molecular structure allows one to map the spin degrees of freedom onto the orbital ones and vice versa and opens the possibility for high-finesse (conditional and unconditional) quantum gates by means of stimulated Raman adiabatic passages.

2003 - Molecular phases in vertically coupled quantum dot molecules [Relazione in Atti di Convegno]
S., Amaha; M., Rontani; Manghi, Franca; Molinari, Elisa; D. G., Austing; K., Ono; S., Tarucha
abstract

2003 - Optics and transport in conjugated polymer crystals: Interchain interaction effects [Relazione in Atti di Convegno]
G., Bussi; A., Ferretti; Ruini, Alice; M. J., Caldas; Molinari, Elisa
abstract

We investigate the fundamental properties of conjugated-polymer semiconductors from the novel viewpoint of solid-state ab initio approaches, that are appropriate for extended and crystalline systems. The impact of interchain interactions on optics and transport of these materials is analyzed by developing computational schemes for transfer integrals and exciton states. We focus on a prototype polymer of great interest for optoelectronics, poly-para-phenylenevinylene (PPV), and compare different solid-state packings, where the character of interactions ranges from quasi-one-dimensional to quasi-three-dimensional. Interchain coupling is found to control light emission and charge conduction, and can thus be used as a tunable parameter for the design of devices based on organic materials.

2002 - Ab initio study of model guanine assemblies: The role of pi-pi coupling and band transport [Articolo su rivista]
DI FELICE, Rosa; Calzolari, Arrigo; Molinari, Elisa; Garbesi, A.
abstract

Several assemblies of guanine molecules are investigated by means of first-principles calculations. Such structures include stacked and hydrogen-bonded dimers, as well as vertical columns and planar ribbons, respectively, obtained by periodically replicating the dimers. Our results are in good agreement with experimental data for isolated molecules, isolated dimers, and periodic ribbons. For stacked dimers and columns, the stability is affected by the relative charge distribution of the pi orbitals in adjacent guanine molecules. pi-pi coupling in some stacked columns induces dispersive energy bands, while no dispersion is identified in the planar ribbons along the connections of hydrogen bonds. The implications for different materials comprised of guanine aggregates are discussed. The band structure of dispersive configurations may justify a contribution of band transport (Bloch type) in the conduction mechanism of deoxyguanosine fibres, while in DNA-like configurations band transport should be negligible.

2002 - Biomolecular electronic devices based on self-organized deoxyguanosine nanocrystals [Articolo su rivista]
Rinaldi, R; Branca, E; Cingolani, R; DI FELICE, Rosa; Calzolari, Arrigo; Molinari, Elisa; Masiero, S; Spada, G; Gottarelli, G; Garbesi, A.
abstract

We report on a new class of hybrid electronic devices based on a DNA nucleoside (deoxyguanosine lipophilic derivative) whose assembled polymeric ribbons interconnect a submicron metallic gate. The device exhibits large conductivity at room temperature, rectifying behavior and strong current-voltage hysteresis. The transport mechanism through the molecules is investigated by comparing films with different self-assembling morphology. We found that the main transport mechanism is connected to pi-pi interactions between guanosine molecules and to the formation of a strong dipole along ribbons, consistently with the results of our first-principles calculations.

2002 - Dominance of Charged Excitons in Single Quantum Dot Photoluminescence Spectra [Articolo su rivista]
M., Lomascolo; A., Vergine; T. K., Johal; R., Rinaldi; A., Passaseo; R., Cingolani; S., Patanè; M., Lombardi; M., Allegroni; Troiani, Filippo; Molinari, Elisa
abstract

Single InxGa1-xAs/GaAs quantum dot photoluminescence spectra, obtained by low-temperature near-field scanning optical microscopy, are compared with theoretically derived optical spectra. The spectra show shell filling as well as few-particle fine structure associated with neutral and charged multiexcitons, in good agreement with the many-body calculations. There appears to be a greater tendency to charged-exciton formation, which is discussed in terms of the high diffusivity of photogenerated electrons.

2002 - Electron-Hole Localization in Coupled Quantum Dots [Articolo su rivista]
Troiani, Filippo; U., Hohenester; Molinari, Elisa
abstract

We theoretically investigate correlated electron-hole states in vertically coupled quantum dots. Employing a prototypical double-dot confinement and a configuration-interaction description for the electron-hole states, it is shown that the few-particle ground state undergoes transitions between different quantum states as a function of the interdot distance, resulting in unexpected spatial correlations among carriers and in electron-hole localization. Such transitions provide direct manifestations of inter- and intradot correlations, which can be directly monitored in experiments.

2002 - G-Quartet Biomolecular NanoWires [Articolo su rivista]
Calzolari, Arrigo; DI FELICE, Rosa; Molinari, Elisa; A., Garbesi
abstract

We present a first-principle investigation of quadruple helix nanowires, consisting of stacked planar hydrogen-bonded guanine tetramers. Our results show that long wires form and are stable in potassium-rich conditions. We present their electronic band structure and discuss the interpretation in terms of effective wide-band-gap semiconductors. The microscopic structural and electronic properties of the guanine quadruple helices make them suitable candidates for molecular nanoelectronics. (C) 2002 American Institute of Physics.

2002 - Interchain interaction and Davydov splitting in polythiophene crystals: An ab initio approach [Articolo su rivista]
Bussi, Giovanni; Ruini, Alice; Molinari, Elisa; Caldas, Marilia J.; Puschnig, Peter; Ambrosch-Draxl, Claudia
abstract

The crystal-induced energy splitting of the lowest excitonic state in polymer crystals, the so-called Davydov splitting Delta, is calculated with a first-principles density-matrix scheme. We show that different crystalline arrangements lead to significant variations in Delta, from below to above the thermal energy k(B)T at room temperature, with relevant implications on the luminescence efficiency. This is one more piece of evidence supporting the fact that control of interchain interactions and solid-state packing is essential for the design of efficient optical devices. (C) 2002 American Institute of Physics.

2002 - Optical properties of organic materials: from single molecules to solid state [Capitolo/Saggio]
Ruini, Alice; Mj, Caldas; G., Bussi; A., Ferretti; Bm, Silva; Goldoni, Guido; Molinari, Elisa
abstract

Light-matter interactions in organic conjugated materials have always been of great interest due to their rich abosorption and emission acitivity, and their versatility in, or near, the visible range. The complexity of the molecular structure itself, particularly when assembled in solid-state films or crystals, has precluded theoretical srudies based on realistic models. We here present a study of the behavior of organic structures composed of thienyl units (SC4H2), in two different assemblages: finite oligothiophenes, including one functionalised unit (O2SC4H2), and in infinite polythiophene chains, assembled three-dimenasionally in the realistic herringbone crystalline packing. To do that, we use two different approaches to obtain the excited states of the system, each appropriate to the system under study. We find that subtle symmetry properties are ,in both cases, responsible for dramatic effects in emissive efficiency.

2002 - Optically detected single-electron charging in a quantum dot [Articolo su rivista]
Zrenner, A; Findeis, F; Baier, M; Bichler, M; Abstreiter, G; Hohenester, U; Molinari, Elisa
abstract

By means of photoluminescence spectroscopy we compare the bias-dependent emissions of single-quantum dots which are embedded in two differently designed photodiode structures. Controlled single-electron charging allows to identify neutral, single- and double-charged excitons in the optical spectra of both samples. At high magnetic fields, one Zeeman component of the single-charged exciton is found to be quenched, which is attributed to the competing effects of tunnelling and spin-flip processes. The strength of the tunnelling coupling between quantum dot and back-contact was found to have a strong influence on the observed spectral features-in particular, the parallel appearance of emission lines resulting from the radiative decay of differently charged quantum dot states is suppressed in case of strong tunnelling interaction. (C) 2002 Elsevier Science B.V. All rights reserved.

2002 - Raman Signatures of Classical and Quantum Phases in Coupled Dots: a Theoretical Prediction [Articolo su rivista]
Rontani, Massimo; Goldoni, Guido; Manghi, Franca; Molinari, Elisa
abstract

We study electron molecules in realistic vertically coupled quantum dots in a strong magnetic field. Computing the energy spectrum, pair correlation functions, and dynamical form factor as a function of inter-dot coupling via diagonalization of the many-body Hamiltonian, we identify structural transitions between different phases, some of which do not have a classical counterpart. The calculated Raman cross-section shows how such phases can be experimentally singled out.

2002 - Self-assembled guanine ribbons as wide-bandgap semiconductors [Articolo su rivista]
Calzolari, Arrigo; DI FELICE, Rosa; Molinari, Elisa; Garbesi, A.
abstract

We present a first principle study about the stability and the electronic properties of a new biomolecular solid-state material, obtained by the self-assembling of guanine (G) molecules. We consider hydrogen-bonded planar ribbons in isolated and stacked configurations. These aggregates present electronic properties similar to inorganic wide-band gap semiconductors. The formation of Bloch-type orbitals is observed along the stacking direction, white it is negligible in the ribbon plane. Global band-like conduction may be affected by a dipole-Field which spontaneously arises along the ribbon axis. Our results indicate that G-ribbon assemblies are promising materials for biomolecular nanodevices, consistently with recent experimental results. (C) 2002 Elsevier Science B.V. All rights reserved.

2002 - Self-induced transparency in semiconductor quantum dots [Articolo su rivista]
Panzarini, G; Hohenester, U; Molinari, Elisa
abstract

We present a theoretical analysis of self-induced transparency in a sample of inhomogeneously broadened semiconductor quantum dots. A general theoretical framework accounting for mutual light-matter interactions in the presence of single and biexciton transitions is presented. Numerical results demonstrate that intense light pulses can propagate in realistic state-of-the-art dot samples without suffering strong losses.

2002 - Solid State Effects on Exciton States and Optical Properties of PPV [Articolo su rivista]
Ruini, Alice; Bussi, Giovanni; Molinari, Elisa; Caldas, Marilia J
abstract

We perform ab initio calculations of optical properties for a typical semiconductor conjugated polymer, poly-para-phenylenevinylene, in both isolated chain and crystalline packing. In order to obtain results for excitonic energies and real-space wave functions we explicitly include electron-hole interaction within the density-matrix formalism. We find that the details of crystalline arrangement crucially affect the optical properties, leading to a richer exciton structure and opening nonradiative decay channels. This has implications for the optical activity and optoelectronic applications of polymer films

2002 - Solid-State Molecular Rectifier Based on Self-Organized Metalloproteins [Articolo su rivista]
R., Rinaldi; A., Biasco; G., Maruccio; R., Cingolani; D., Alliata; L., Andolfi; P., Facci; DE RIENZO, Francesca; DI FELICE, Rosa; Molinari, Elisa
abstract

A protein-based solid-state electronic device that operates in air is reported; it is fabricated by interconnecting an azurin monolayer immobilized on SiO2 with two gold nanoelectrodes. The operating mechanism is ascribed to resonant tunneling through the redox active center, combined with macroscopic polarization of the oriented self-organized protein layer. The Figure shows the molecular electrostatic potential of the oxidized protein.

2002 - Spin quantum beats in semiconductor microcavities in the strong coupling regime [Articolo su rivista]
Panzarini, G.; Molinari, Elisa
abstract

We have observed polariton spin quantum beats under transverse magnetic field and circular polarization excitation (Voigt configuration) by time-resolved photoluminescence spectroscopy. The temporal behaviour of the beats strongly differs depending on whether the excitation is resonant or not with the lower polariton branch. As in bare quantum wells, we show that this is the consequence of the hole spin instability under non-resonant excitation. In both cases, we evidence a dependence on the cavity detuning δ of the electron effective Landé g-factor (geff). This is attributed to strong coupling regime, where the bare electron Landé g-factor is depends on the polariton exciton component. All these characteristics are well described by a simplified model, which takes into account the transfer dynamics between two classes of polaritons.

2002 - The effect of dielectric polarization-induced surface states on many-body configurations in a quantum dot [Articolo su rivista]
Orlandi, A; Goldoni, Guido; Manghi, Franca; Molinari, Elisa
abstract

We compute the exact lowest energy states and addition spectra for N interacting carriers in semiconductor quantum dots (QDs) embedded in low-dielectric-constant matrices. In particular we focus on the transition between the low dielectric mismatch regime, where single-particle states are confined inside the QD, and the large mismatch regime, where surface localized states may be induced by the dielectric potential outside the QD. In this transition, many-body ground-state reconstructions occur which give rise to strong deviations from the regular shell-filling behaviour dictated by Hand's rules. Our calculated addition spectra indicate that such effects could be monitored in transport experiments.

2002 - Weak and strong coupling for quantum boxes in pillar microcavities [Articolo su rivista]
Panzarini, G; Molinari, Elisa
abstract

The interaction of a quantum box with the vacuum field inside a semiconductor pillar microcavity is theoretically investigated by means of a master equation approach accounting also for incoherent relaxation processes. We show that both exciton and biexciton states bound to monolayer fluctuations in narrow quantum wells can be in strong coupling. Finally, we calculate the luminescence spectrum and we explain its structures in terms of transitions between dressed states.

2001 - Ab-Initio Study of Model Guanosine Assemblies: the Role of pi-pi Coupling and Band Transport [Articolo su rivista]
DI FELICE, Rosa; Calzolari, Arrigo; Molinari, Elisa; A., Garbesi
abstract

2001 - Ab-initio study of Coulomb-correlated optical properties in conjugated polymers [Articolo su rivista]
Ruini, Alice; F., Rossi; U., Hohenester; Molinari, Elisa; R., Capaz; M. J., Caldas
abstract

The spatial extension and binding energy of excitons in semiconducting conjugated polymers are still the subject of a great debate. We address this problem through first-principles calculations (within DFT-LDA, plane-waves and ab-initio pseudopotentials), which allow to include electron-hole correlation effects in a fully three-dimensional approach through the density-matrix formalism. We show results for the correlated optical spectrum and the exciton wavefunctions of single-chain poly(para)phenylene-vinylene (PPV), that support the picture of a strongly bound anisotropic exciton localized over similar to 4-5 monomers.

2001 - Correlation effects in strain-induced quantum dots [Articolo su rivista]
Rinaldi, R; Devittorio, M; Cingolani, R; Hohenester, U; Molinari, Elisa; Lipsanen, H; Tulkki, J; Ahopelto, J; Uchida, K; Miura, N; Arakawa, Y.
abstract

We report on Coulomb correlation effects in the luminescence of strain-induced quantum dots. In single dots, under low power excitation, we observe the rising of sharp lines associated to the formation of excitonic molecules. In the grand-ensemble, in magnetic fields up to 45 T, we observe Darwin-Fock stales of the dots to merge into a unique Landau level, with a considerable reduction in the total diamagnetic shift due to the enhanced electron-hole correlation caused by the increased degeneracy of the slate.

2001 - Local absorption spectra of single and coupled semiconductor quantum dots [Articolo su rivista]
Simserides, Cd; Hohenester, U; Goldoni, Guido; Molinari, Elisa
abstract

We study theoretically the local absorption spectra of single and double semiconductor quantum dots (QDs). in the linear regime. The three-dimensional confinement leads to an enhancement of the Coulomb correlations, while the spectra depend crucially on the size of the 'local' probe. We show that because of such Coulomb correlations the intensity of certain optical peaks as a function of the resolution can exhibit an unexpected non-monotonic behavior for spatial resolutions comparable with the excitonic Bohr radius. We finally discuss the optical near-field properties of coupled QDs for different coupling strengths. (C) 2001 Elsevier Science B.V. All rights reserved.

2001 - Local optical absorption by confined excitons in single and coupled quantum dots [Articolo su rivista]
Simserides, Cd; Hohenester, U; Goldoni, Guido; Molinari, Elisa
abstract

We investigate optical near-field spectra of single and coupled semiconductor quantum dots. An enhanced role for the Coulomb correlations is predicted, and it is shown that the spectra depend crucially on the spatial resolution of the local probe. The intensity of certain optical peaks as a function of the resolution exhibits an unexpected non-monotonic behavior, which is identified as a fingerprint of Coulomb interactions in zero-dimensional nanostructures.

2001 - Observation of charged few-particle states in the optical spectra of single semiconductor quantum dots [Articolo su rivista]
Ducommun, Y; Hartmann, A; Kapon, E; Hohenester, U; Molinari, Elisa
abstract

A detailed discussion of the optical properties of single n-type modulation-doped semiconductor quantum dots is presented. We use the photo-depletion/back-hopping mechanism to optically control the number of surplus electrons in the dot. Comparison of the experimental data with calculated luminescence spectra, obtained within a direct-diagonalization approach for calculating electron-hole states, allows the identification of luminescence from the decay of up to fivefold charged excitons. Finally we study the influence of fluctuating local electric fields, produced by ionized impurities in the surrounding of the dots, on the optical selection rules and on the linewidth of the emission peaks.

2001 - Optical excitations of a self-assembled artificial ion [Articolo su rivista]
F., Findeis; M., Baier; A., Zrenner; M., Bichler; G., Abstreiter; U., Hohenester; Molinari, Elisa
abstract

By use of magnetophotoluminescence spectroscopy, we demonstrate bias-controlled single-electron charging of a single quantum dot. Neutral, single, and double charged excitons are identified in the optical spectra. At high magnetic fields one Zeeman component of the single charged exciton is found to be quenched, which is attributed to the competing effects of tunneling and spin-flip processes. Our experimental data are in good agreement with theoretical model calculations for situations where the spatial extent of the hole wave functions is smaller as compared to the electron wave functions

2001 - Quantum phases in artificial molecules [Articolo su rivista]
Rontani, Massimo; Troiani, Filippo; Hohenester, U; Molinari, Elisa
abstract

The few-particle state of carriers confined in a quantum dot is controlled by the balance between their kinetic energy and their Coulomb correlation. In coupled quantum dots, both can be tuned by varying the inter-dot tunneling and interactions. Using a theoretical approach based on the diagonalization of the exact Hamiltonian, we show that the transitions between different quantum phases can be induced through the inter-dot coupling both for a system of few electrons (or holes) and for aggregates of electrons and holes. We discuss their manifestations, in addition energy spectra (accessible through capacitance or transport experiments) and optical spectra. (C) 2001 Elsevier Science Ltd. All rights reserved.

2001 - Quantum-information processing in semiconductor quantum dots [Articolo su rivista]
Troiani, Filippo; Hohenester, U; Molinari, Elisa
abstract

We propose an all-optical implementation of quantum-information processing in semiconductor quantum dots, where electron-hole excitations (excitons) serve ve as the computational degrees of freedom (qubits). The strong dot confinement leads to a strong renormalization of excitonic states, which, in analogy to NMR:based implementations of quantum-information processing, can be exploited for performing conditional and unconditional qubit operations.

2001 - Single-electron charging in quantum dots with large dielectric mismatch [Articolo su rivista]
A., Orlandi; M., Rontani; Goldoni, Guido; Manghi, Franca; Molinari, Elisa
abstract

Semiconductor quantum dots characterized by a strong dielectric mismatch with their environment arestudied theoretically through direct diagonalization of the many-body Hamiltonian. The enhancement of theelectron-electron Coulomb interaction, arising from polarization effects, is found to induce a strong increase inaddition energies with increasing dielectric mismatch. For large dielectric mismatch, the excited many-bodystates can undergo reconstructions as the dot is filled with carriers even in the absence of external magneticfields.

2001 - Strong coupling and dressed states of an interface island in a pillar semiconductor microcavity [Articolo su rivista]
Panzarini, G; Molinari, Elisa
abstract

We present a theoretical scheme to investigate the coupled states of a zero-dimensional semiconductor structure and the confined modes of a pillar microcavity, taking into account interacting exciton states and both coherent and incoherent dynamical processes. For quantum-well monolayer fluctuations we find that the frequency of the cavity modes can be tuned in such a way that both exciton and biexciton states are in the strong coupling regime. The calculated luminescence spectra are interpreted in terms of transitions between the dressed states of the composite radiation-matter system.

2001 - Tailoring of Light Emission Properties of Functionalised Oligothiophenes [Articolo su rivista]
M. J., Caldas; E., Pettenati; Goldoni, Guido; Molinari, Elisa
abstract

We investigate theoretically the light emission properties of short oligothiophenes containing a thienyl-S,S-dioxide moiety, which have recently been shown to exhibit strong photoluminescence efficiency and tunability. We find that the dioxide substitution tends to increase the torsion angle between rings and strongly affects the states at the optical band edges, in a way depending on the position of the substituted ring. We have identified a low-energy transition which is optically active only when the dioxide unit is inserted between two thiophene rings, as a result of increased inter-ring torsional angle also in the excited state. With the dioxide unit in a terminal position, planarity is favored in the excited state, and the transition is optically inactive. For short oligomers, this can lead to nonradiative decay quenching the luminescence. (C) 2001 American Institute of Physics.

2000 - Coherent population transfer in coupled semiconductor quantum dots [Articolo su rivista]
Hohenester, U; Troiani, Filippo; Molinari, Elisa; Panzarini, G; Macchiavello, C.
abstract

We propose a solid-state implementation of stimulated Raman adiabatic passage in two coupled semiconductor quantum dots. Proper combination of two pulsed laser fields allows the coherent carrier transfer between the two nanostructures without suffering significant losses due to environment coupling. By use of a general solution scheme for the carrier states in the double-dot structure, we identify the pertinent dot and laser parameters. (C) 2000 American Institute of Physics. [S0003-6951(00)05338-9].

2000 - Effects of few-particle interaction on the atomiclike levels of a single strain-induced quantum dot [Articolo su rivista]
Rinaldi, R; Antonaci, S; Devittorio, M; Cingolani, R; Hohenester, U; Molinari, Elisa; Lipsanen, H; Tulkki, J.
abstract

We investigate the effects of few-particle population of a single strain-induced quantum dot by optical excitation. The low-power photoluminescence spectra consist of sharp lines with energy separation of a few meV, associated to the formation of excitonic molecules in the single dot. With increasing photoexcitation intensity, the population of higher states is observed; however, we also observe a clear intensity dependence of the transition energies, inconsistent with a simple filling of exciton levels. Based on a theoretical model for interacting electron-hole pairs in the dot, we obtain good agreement with experiment and show that exciton-exciton interactions control the spectral changes as the number of pairs is increased.

2000 - Enhancement of Coulomb interactions in semiconductor nanostructures by dielectric confinement [Articolo su rivista]
Goldoni, Guido; F., Rossi; A., Orlandi; M., Rontani; Manghi, Franca; Molinari, Elisa
abstract

We present a theoretical analysis of the effect of dielectric confinement on the Coulomb interaction in dielectrically modulated quantum structures. We discuss the implications of the strong enhancement of the electron-hole and electron-electron coupling for two specific examples: (i) GaAs-based quantum wires with remote oxide barriers, where combined quantum and dielectric confinements are predicted to lead to room temperature exciton binding, and (ii) semiconductor quantum dots in colloidal environments, when the addition spectra are predicted to depend on the dielectric mismatch. (C) 2000 Elsevier Science B.V. All rights reserved.

2000 - Excitonic and biexcitonic states in semiconductor quantum dots [Articolo su rivista]
Hohenester, U; Molinari, Elisa
abstract

We analyze excitonic and biexcitonic effects in the coherent optical spectra of semiconductor quantum dots using a density-matrix approach, that explicitly accounts for exciton-exciton interactions. A consistent description of additional peaks appearing at higher photoexcitation density is given.

2000 - Exploiting exciton-exciton interactions in semiconductor quantum dots for quantum-information processing [Articolo su rivista]
Troiani, Filippo; Hohenester, U; Molinari, Elisa
abstract

We propose an all-optical implementation of quantum-information processing in semiconductor quantum dots, when electron-hole excitations (excitons) serve as the computational degrees of freedom (qubits). We show that the strong dot confinement leads to an overall enhancement of Coulomb correlations and to a strong renormalization of the excitonic states, which can be exploited for performing conditional and unconditional qubit operations.

2000 - Few-particle effects in semiconductor quantum dots: Observation of multicharged excitons [Articolo su rivista]
Hartmann, A; Ducommun, Y; Kapon, E; Hohenester, U; Molinari, Elisa
abstract

We investigate experimentally and theoretically few-particle effects in the optical spectra of single quantum dots (QDs). Photodepletion of the QD together with the slow hopping transport of impurity-bound electrons back to the QD are employed to efficiently control the number of electrons present in the QD. By investigating structurally identical QDs, we show that the spectral evolutions observed can be attributed to intrinsic, multi-particle-related effects, as opposed to extrinsic QD-impurity environment-related interactions. From our theoretical calculations we identify the distinct transitions related to excitons and excitons charged with up to five additional electrons, as well as neutral and charged biexcitons.

2000 - Local absorption spectra of artificial atoms and molecules [Articolo su rivista]
Simserides, C.; Hohenester, U.; Goldoni, Guido; Molinari, Elisa
abstract

We investigate theoretically the spatial dependence of the linear absorption spectra of single and coupledsemiconductor quantum dots, where the strong three-dimensional quantum confinement leads to an overallenhancement of Coulomb interaction and, in turn, to a pronounced renormalization of the excitonic properties.We show that—because of such Coulomb correlations and the spatial interference of the exciton wavefunctions—unexpected spectral features appear whose intensity depends on spatial resolution in a highlynonmonotonic way when the spatial resolution is comparable with the excitonic Bohr radius. We finallydiscuss how the optical near-field properties of double quantum dots are affected by their coupling.

2000 - Local optical spectroscopy of semiconductor nanostructures in the linear regime [Articolo su rivista]
Mauritz, O; Goldoni, Guido; Molinari, Elisa; Rossi, F.
abstract

We present a theoretical approach to calculate the local absorption spectrum of excitons confined in a semiconductor nanostructure. Using the density-matrix formalism, we derive a microscopic expression for the nonlocal susceptibility, both in the linear and nonlinear regimes, which includes a three-dimensional description of electronic quantum states and their Coulomb interaction. The knowledge of the nonlocal susceptibility allows us to calculate a properly defined local absorbed power, which depends on the electromagnetic field distribution. We report on explicit calculations of the local linear response of excitons confined in single and coupled T-shaped quantum wires with realistic geometry and composition. We show that significant interference effects in the interacting electron-hole wave function induce new features in the space-resolved optical spectra, particularly in coupled nanostructures. When the spatial extension of the electromagnetic held is comparable to the exciton Bohr radius, Coulomb effects on the local spectra must be taken into account for a correct assignment of the observed features.

2000 - Nanoscale compositional fluctuations in multiple InGaAs/GaAs quantum wires [Articolo su rivista]
M., Catalano; A., Taurino; M., Lomascolo; L., Vasanelli; M., De Giorgi; A., Passaseo; R., Rinaldi; R., Cingolani; O., Mauritz; Goldoni, Guido; F., Rossi; Molinari, Elisa; P., Crozier
abstract

An accurate analysis of nanoscale compositional fluctuations in InGaAs/GaAs quantum wires grown by metalorganic chemical vapor deposition on V-grooved substrates was performed by means of high-spatial-resolution transmission electron microscopy techniques. Small In fluctuations (2%-3% excess indium), spatially localized over approximately 5 nm, were detected and related to changes in the photoluminescence and photoluminescence excitation spectra. (C) 2000 American Institute of Physics. [S0021-8979(00)07905-6].

2000 - Optical spectra of single quantum dots: Influence of impurities and few-particle effects [Articolo su rivista]
Hartmann, A; Ducommun, Y; Kapon, E; Hohenester, U; Simserides, C; Molinari, Elisa
abstract

The evolution of photoluminescence (PL) spectra of single GaAs/AlGaAs quantum dots (QD) is studied as a function of laser excitation power and temperature. At very low powers, where multi-exciton occupation of the QD can be excluded, an unexpected and pronounced spectral evolution is observed (large energy shifts and appearance of multiple emission lines). A similar evolution is observed at low excitation powers with increasing,temperature. A model, taking into account the influence of the shallow, residual impurities in the environment of each QD, explains the observed spectral evolutions in terms of photo-depletion of the QD and hopping of impurity-bound carriers back into the QD. Theoretical calculations of the PL due to N electrons + 1 hole (Ne + 1h) QD states allow us to attribute the approximate to 2 meV spaced lines in the experimental spectra to the different charge states Ne + 1h, (N - 1) e + 1h,... of the QD.

2000 - Role of Coulomb correlations in the optical spectra of semiconductor quantum dots [Articolo su rivista]
Hohenester, U; Molinari, Elisa
abstract

We present a consistent theoretical description of few-particle effects in the optical spectra of semiconductor quantum dots, based on a direct-diagonalization approach. We show that, because of the strong Coulomb interaction among electrons and holes, each configuration of the confined few-particle system leads to its characteristic signature in the optical spectra. We discuss quantitative predictions and comparison with experiments for both absorption and luminescence.

2000 - Theoretical analysis of the optical spectra of InxGa1-xN quantum dots in InyGa1-yN layers [Articolo su rivista]
Hohenester, U; DI FELICE, Rosa; Molinari, Elisa; Rossi, F.
abstract

A theoretical investigation of the optical properties of InxGa1-N-x quantum dots in InyGa1-y N layers is presented. We compute the single-particle energies and wave functions for different dot dimension and indium composition in the alloys, and use them to predict optical properties. The results allow us to identify signatures of quantum confinement characterizing the optical spectra. (C) 2000 Elsevier Science B.V. All rights reserved.

1999 - Coulomb Correlation in the Addition Spectra of Single and Coupled Quantum Dots [Relazione in Atti di Convegno]
M., Rontani; F., Rossi; Manghi, Franca; Molinari, Elisa
abstract

Quantum dots

1999 - Coulomb correlation effects in semiconductor quantum dots: The role of dimensionality [Articolo su rivista]
Rontani, M; Rossi, F; Manghi, Franca; Molinari, Elisa
abstract

We study the energy spectra of small three-dimensional (3D) and two-dimensional (2D) semiconductor quantum dots through different theoretical approaches (single-site Hubbard and Hartree-Fock Hamiltonians); in the smallest dots we also compare with exact results. We find that purely 2D models often lead to an inadequate description of the Coulomb interaction existing in realistic Structures, as a consequence: of the overestimated carrier localization. We show that the dimensionality of the dots has a crucial impact on (i) the accuracy of the predicted addition spectra, and (ii) the range of validity of approximate theoretical schemes. When applied to realistic 3D geometries, the latter are found to be much more accurate than in the corresponding 2D cases for a large class of quantum dots; the single-site Hubbard Hamiltonian is shown to provide a very effective and accurate scheme to describe quantum dot spectra, leading to good agreement with experiments. [S0163-1829(99)10211-X].

1999 - Excitonic and biexcitonic effects in the coherent optical response of semiconductor quantum dots [Articolo su rivista]
Hohenester, U; Rossi, F; Molinari, Elisa
abstract

We analyze few-particle effects in the coherent optical spectra of semiconductor quantum dots using a density-matrix approach that explicitly accounts for exciton-exciton interactions. A consistent description of additional peaks appearing at high photoexcitation density is given. (C) 1999 Elsevier Science B.V. All rights reserved.

1999 - Few-Particle Effects in the Optical Spectra of Quantum Dots [Articolo su rivista]
U., Hohenester; F., Rossi; Molinari, Elisa
abstract

We analyze few-particle effects in the optical spectra of semiconductor quantum dots using a density-matrix approach that explicitly accounts for exciton–exciton, as well as exciton–carrier interactions. We give a consistent description of additional peaks appearing at high photoexcitation density, and predict that a strong polarization dependence should be characteristic of few-particle features.

1999 - Local optical spectroscopy in quantum confined systems: a theoretical description [Articolo su rivista]
Mauritz, O.; Goldoni, Guido; Rossi, F.; Molinari, Elisa
abstract

length scale comparable with the extension of the relevant quantum states. A general formulation,including Coulomb correlation, is derived within the density-matrix formalism and applied to theprototypical case of coupled quantum wires. The results show that excitonic effects may have acrucial impact on the local absorption, with implications for the spatial resolution and the interpretationof near-field optical spectra

1999 - Multiple quantum phases in artificial double-dot molecules [Articolo su rivista]
M., Rontani; F., Rossi; Manghi, Franca; Molinari, Elisa
abstract

We study coupled semiconductor quantum dots theoretically through a generalized Hubbard approach, where intra- and inter-dot Coulomb correlation, as well as tunneling effects are described on the basis of realistic electron wavefunctions. We find that the ground-state configuration of vertically coupled double dots undergoes non-trivial quantum transitions as a function of the inter-dot distance d; at intermediate values of d we predict a new phase that should be observable in the addition spectra and in the magnetization changes. (C) 1999 Elsevier Science Ltd. All rights reserved.

1999 - Optical spectra of nitride quantum dots: Quantum confinement and electron-hole coupling [Articolo su rivista]
Hohenester, U; DI FELICE, Rosa; Molinari, Elisa; Rossi, F.
abstract

We calculate the optical properties of nitride-based quantum dots by taking into account quantum confinement as well as electron-hole interaction. We analyze model structures simulating InxGa1-xN dots in InyGa1-yN layers with different alloy compositions. We discuss the trends with the dot size and show that quantum confined excitations exist for a broad range of sizes down to the smallest observed dots. Our results allow us to identify the strong role of Coulomb correlations in the optical spectra and to predict a strong influence of photoexcitation power on selection rules in polarized samples. The signature of quantum confinement can be utilized for a critical interpretation of measured optical spectra. (C) 1999 American Institute of Physics. [S0003-6951(99)03448-8].

1999 - Strong exciton binding in hybrid GaAs-based nanostructures [Articolo su rivista]
Goldoni, Guido; Rossi, F; Molinari, Elisa
abstract

We propose a new type of hybrid systems formed by conventional semi-conductor nanostructures with the addition of remote insulating layers, where electron-hole interaction is enhanced by combining quantum and dielectric confinement over different length scales. Due to the polarization charges induced by dielectric mismatch at the semiconductor/insulator interfaces, the exciton binding energy can be strongly enhanced. By a novel theoretical scheme, we show that, for realistic structures based on conventional III-V quantum wires, such remote dielectric confinement allows exciton binding at room temperature. (C) 1999 Elsevier Science B.V. All rights reserved.

1999 - Theory of addition spectra in double quantum dots: Single-particle tunneling vs Coulomb interactions [Articolo su rivista]
M., Rontani; F., Rossi; Manghi, Franca; Molinari, Elisa
abstract

We study coupled semiconductor quantum dots theoretically using a generalized Hubbard approach, where intra- and inter-dot Coulomb correlation, as well as tunneling effects are described on the basis of realistic electron wavefunctions. We find that the ground-state configuration of vertically coupled double dots undergoes non-trivial quantum transitions as the inter-dot distance d changes; at intermediate values of d we predict a new phase that should be observable in the addition spectra and in the magnetization changes.

1999 - Theory of excitonic confinement in semiconductor quantum wires [Articolo su rivista]
Rossi, F; Goldoni, Guido; Mauritz, O; Molinari, Elisa
abstract

We review our theoretical approach to the optical response of low-dimensional semiconductor structures. The method is based on the density-matrix formalism and can treat low-density (excitonic) and high-density (gain) regimes on the same footing while retaining the full complexity of realistic nanostructures. We discuss in particular its generalization for studying the combined effects of dielectric and quantum confinement. as well as novel developments aimed at the analysis of local absorption spectra. We examine the main effects of electron-hole Coulomb correlation on the optical spectra of semiconductor quantum wires, where it determines the suppression of band-edge singularities and the peculiar scaling properties of excitonic binding and non-linearities. On the basis of our recent results on different types of nanostructure, we present a critical discussion of possible strategies for tailoring electron-hole Coulomb interaction, and predicting its influence on near-field spectra.

1998 - Addition energies in semiconductor quantum dots: Role of electron-electron interaction [Articolo su rivista]
M., Rontani; F., Rossi; Manghi, Franca; Molinari, Elisa
abstract

We show that the addition spectra of semiconductor quantum dots in the presence of magnetic field can be studied through a theoretical scheme that allows an accurate and practical treatment of the single-particle states and electron-electron interaction up to large numbers of electrons. The calculated addition spectra exhibit the typical structures of Hund-like shell filling, and account for recent experimental findings. A full three-dimensional description of Coulomb interaction is found to be essential for predicting the conductance characteristics of few-electron semiconductor structures.

1998 - Engineering the strain field for the control of quantum confinement: An analytical model for arbitrary shape nanostructures [Articolo su rivista]
Mazzer, M; De Giorgi, M; Cingolani, R; Porello, G; Rossi, F; Molinari, Elisa
abstract

We describe an analytical method to calculate the strain field and the corresponding band gap modulation induced in a quantum well by a surface stressor of arbitrary shape. In this way, it is possible to engineer the confinement potential of different strained nanostructures based on patterned heterojunctions. Band gap modulations up to 130-140 meV are predicted for suitably designed II-VI/III-V and III-V/III-V heterostructures. (C) 1998 American Institute of Physics. [S0021-8979(98)02219-1].

1998 - Exciton formation and relaxation in GaAs epilayers [Articolo su rivista]
Gurioli, M; Borri, P; Colocci, M; Gulia, M; Rossi, F; Molinari, Elisa; Selbmann, Pe; Lugli, P.
abstract

Exciton formation and relaxation in GaAs bulk epilayers have been studied by means of time-resolved photoluminescence techniques. It is found that the time evolution of the free exciton luminescence, nonresonantly excited at low temperature and low intensity, is extremely slow, with a rise time of the order of 1 ns and a decay time of several ns. Simulations based on Monte Carlo solution of the set of coupled Boltzmann-like equations for free carriers and excitons show a nice agreement with the experimental data, and suggest a dominant role played by acoustic phonons in the exciton relaxation. [S0163-1829(98)53044-5].

1998 - Magneto-acoustic phonon antiresonances in Wannier-Stark superlattices [Articolo su rivista]
Nogaret, A; Eaves, L; Main, Pc; Henini, M; Maude, Dk; Portal, Jc; Molinari, Elisa; Beaumont, Sp
abstract

We have measured the hopping current of electrons magnetically confined in a 1D superlattice. We find that phonon assisted hopping between Wannier-Stark states vanishes at hopping energies where the momentum of an LA phonon mode satisfies the condition of Bragg reflection in the superlattice. This effect proves that, even in the presence of an external electric field, the electron states of a crystal conserve their periodicity in k-space. This periodicity reveals a fundamental property of Wannier's effective Hamiltonian. (C) 1998 Elsevier Science Ltd. All rights reserved.

1998 - Strong exciton binding in quantum structures through remote dielectric confinement [Articolo su rivista]
Goldoni, Guido; F., Rossi; Molinari, Elisa
abstract

We propose-a new type of hybrid systems formed by conventional semiconductor nanostructures with the addition of remote insulating layers,where the electron-hole interaction is enhanced by combining quantum and dielectric confinement over different length scales. Because of the polarization charges induced by the dielectric mismatch at the semicondcutor/insulator interfaces, we show that the exciton binding energy can be more than doubled. For conventional m-V quantum wires such remote dielectric confinement allows exciton binding at room temperature.

1997 - Band structure and optical anisotropy in V-shaped and T-shaped semiconductor quantum wires [Articolo su rivista]
Goldoni, Guido; Rossi, F.; Molinari, Elisa; Fasolino, Annalisa
abstract

We present a theoretical investigation of the electronic and optical properties of V- and T-shaped quantum wires. Valence-band mixing as well as realistic sample geometries are fully included through an accurate and efficient approach that is described here in detail. We investigate the resulting valence-band structure, which shows some significant peculiarities, such as an anomalously large spin splitting in the lowest heavy-hole subband of T-shaped wires. For both classes of wires we obtain good agreement between calculated optical absorption and recent experimental spectra, and we demonstrate that the analysis of optical anisotropy can be used as an effective tool to extract information on valence states, which is usually very difficult to obtain otherwise.

1997 - Coulomb-correlation effects on the non-linear optical properties of realistic quantum wires [Articolo su rivista]
Rossi, F; Molinari, Elisa
abstract

We review recent results on the linear and non-linear optical response of realistic quantum-wire structures. Our theoretical approach is based on a set of generalized semiconductor Bloch equations, and allows a full three-dimensional multisubband description of Coulomb correlation for any shape of the confinement profile, thus permitting a direct comparison with experiments for available state-of-the-art wire structures. Our results show that electron-hole Coulomb correlation removes the one-dimensional band-edge singularities from the absorption spectra, whose shape results to be heavily modified with respect to the ideal free-particle case over the whole range of photoexcited carrier densities.

1997 - Excitonic effects in quantum wires [Articolo su rivista]
Goldoni, Guido; Rossi, F; Molinari, Elisa
abstract

We review the effects of Coulomb correlation on the linear and nonlinear optical properties of semiconductor quantum wires, with emphasis on recent results for the bound excitonic states. Our theoretical approach is based on generalized semiconductor Bloch equations, and allows full three-dimensional multisubband description of electron-hole correlation for arbitrary confinement profiles. In particular, we consider V- and T-shaped structures for which significant experimental advances were obtained recently. Above band gap, a very general result obtained by this approach is that electron-hole Coulomb correlation removes the inverse-square-root single-particle singularity in the optical spectra at band edge, in agreement with previous reports from purely one-dimensional models. Strong correlation effects on transitions in the continuum are found to persist also at high densities of photoexcited carriers. Below band gap, we find that the same potential- (Coulomb) to kinetic-energy ratio holds for quite different wire cross-sections and compositions. As a consequence, we identify a shape-and barrier-independent parameter that governs a universal scaling law for exciton binding energy with size. Previous indications that the shape of the wire cross-section may have important effects on exciton binding are discussed in the light of the present results.

1997 - Optical emission from small Si particles [Articolo su rivista]
Baierle, R. J.; Caldas, M. J.; Molinari, Elisa; Ossicini, Stefano
abstract

The optical properties of hydrogen-saturated Si clusters are studied theoretically through an approach that can treat consistently both small molecules and bulk crystals and fully includes electron-electron correlation. We find that optical spectra in the ground state cannot explain the observed luminescence of porous Si for the small particle sizes consistent with structural data. However, the same clusters in their excited state relax to locally distorted equilibrium configurations, giving rise to new transitions involving localized states that lower the emission threshold. These results allow to reconcile absorption and luminescence experiments with no need to invoke extrinsic effects.

1997 - Optically- induced defects in Si-H nanoparticles [Relazione in Atti di Convegno]
Mj, Caldas; Rj, Baierle; Molinari, Elisa; Ossicini, Stefano
abstract

We study the behavior of perfectly hydrogenated Si nanoparticles, under optical excitation. We use semiempirical selfconsistent techniques within the Hartree-Fock formalism, specially parametrized to reproduce key properties of the crystalline environment for the Si atom. We find that the optical absortion spectra of particles ranging in size from 29 to 71 Si atoms evidence both quantum confinement and relaxation effects, with optical gaps in the green-blue region. These particles show however strength for luminescence at much lower energy, related to localised surface defects created by gap-light excitation. These defects involve two surface Si atoms and a H-atom, in a Si-H-Si bridge configuration, and we propose they should be the active luminescent centers in porous Si.

1997 - Phonon-assisted exciton formation and relaxation in GaAs/AlxGa1-xAs quantum wells [Articolo su rivista]
Gulia, M; Rossi, F; Molinari, Elisa; Selbmann, Pe; Lugli, P.
abstract

A microscopic analysis of exciton formation and relaxation in photoexcited quantum wells is presented. The theoretical approach is based on a Monte Carlo simulation of the coupled free-carrier and exciton dynamics, and includes various mechanisms contributing to exciton formation and relaxation. Our investigation clarifies the ori,ain of excitonic luminescence in time-resolved experiments. In particular, we address the problem of the relative efficiencies of exciton formation assisted by either LO phonons or acoustic phonons, respectively.

1997 - Phonon-assisted exciton formation and relaxation: Bulk and two-dimensional systems [Articolo su rivista]
Gulia, M; Rossi, F; Molinari, Elisa; Selbmann, Pe; Compagnone, F; Lugli, P.
abstract

A microscopic analysis of exciton formation and relaxation in photoexcited quantum wells is presented. The theoretical approach is based on a Monte Carlo simulation of the coupled free-carrier and exciton dynamics and includes various mechanisms contributing to the formation of excitons and to their subsequent energy relaxation. Our investigation clarifies the effects of the exciton relaxation due to AC-phonon scattering in the building up of excitonic luminescence in time-resolved experiments. The role of carrier confinement is discussed for wells of different width.

1997 - Quantum interference in nanometric devices: Ballistic transport across arrays of T-shaped quantum wires [Articolo su rivista]
Goldoni, Guido; F., Rossi; Molinari, Elisa
abstract

We propose that recently realized T-shaped semiconductor quantum wires (T wires) could be exploited as three-terminal quantum interference devices. T wires are formed by intersecting two quantum wells (QWs). By use of a scattering matrix approach and Landauer-Buttiker theory, we calculate the conductance for ballistic transport in the parent QWs and across the wire region as a function of the injection energy. We show that different conductance profiles can be selected by tailoring the widths of the QWs and/or by combining more wires on the scale of the Fermi wavelength. Finally, we discuss the possibility of obtaining spin-dependent conductance of ballistic holes in the same structures.

1997 - Shape-independent scaling of excitonic confinement in realistic quantum wires [Articolo su rivista]
Rossi, F.; Goldoni, Guido; Molinari, Elisa
abstract

The scaling of exciton binding energy in semiconductor quantum wires is investigated theoreticallythrough a nonvariational, fully three-dimensional approach for a wide set of realistic state-of-the-artstructures. We find that in the strong confinement limit the same potential-to-kinetic energy ratio holdsfor quite different wire cross sections and compositions. As a consequence, a universal (shape- andcomposition-independent) parameter can be identified that governs the scaling of the binding energywith size. Previous indications that the shape of the wire cross section may have important effects onexciton binding are discussed in the light of the present results

1996 - Coulomb-induced suppression of band-edge singularities in the optical spectra of realistic quantum-wire structures [Articolo su rivista]
F., Rossi; Molinari, Elisa
abstract

The linear and nonlinear optical properties of realistic quantum wires are studied through a theoretical approach based on a set of generalized semiconductor Bloch equations. Our scheme allows a full three-dimensional multisubband description of electron-hole correlation for any confinement profile, thus permitting a direct comparison with experiments for available quantum-wire structures. Our results show that electron-hole Coulomb correlation removes the one-dimensional band-edge singularities from the absorption spectra, whose shape is heavily modified with respect to the ideal free-carrier single-subband case over the whole density range.

1996 - Coupled free-carrier and exciton relaxation in optically excited semiconductors [Articolo su rivista]
Selbmann, Pe; Gulia, M; Rossi, F; Molinari, Elisa; Lugli, P.
abstract

The energy relaxation of coupled free-carrier and exciton populations in semiconductors after low-density ultrafast optical excitation is studied through a kinetic approach. The set of semiclassical Boltzmann equations, usually written for electron and hole populations only, is complemented by an additional equation for the exciton distribution. The equations are coupled by reaction terms describing phonon-mediated exciton binding and dissociation. All the other relevant scattering mechanisms, such as carrier-carrier, carrier-phonon, and exciton-phonon interactions, are also included. The resulting system of rate equations in reciprocal space is solved by an extended ensemble Monte Carlo method. As a first application, we show results for the dynamics of bulk GaAs in the range from 1 to similar to 200 ps after photoexcitation. The build-up of an exciton population and its sensitivity to the excitation conditions are discussed in detail. As a consequence of the pronounced energy dependence of the LO-phonon-assisted transition probabilities between free-pair states and excitons, it is found that the efficiency of the exciton-formation process and the temporal evolution of the resulting population are sensitive to the excitation energy. We discuss the effects on luminescence experiments.

1996 - InAs/GaSb(001) valence-band offset: Independence of interface composition and strain [Articolo su rivista]
B., Montanari; M., Peressi; S., Baroni; Molinari, Elisa
abstract

The InAs/GaSb(001) valence-band offset is calculated for the two inequivalent GaAs-like and InSb-like interfaces and found to coincide to within approximate to 30 meV. This result is rationalized and generalized to arbitrary composition profiles and induced strain by using a simple model, based on the linear response theory, which is validated by a number of accurate first-principles calculations for intermixed interfaces.

1996 - Linear and nonlinear optical properties of realistic quantum-wire structures: The dominant role of Coulomb correlation [Articolo su rivista]
Rossi, F; Molinari, Elisa
abstract

A systematic analysis of the linear and nonlinear optical properties of realistic quantum wires is presented. The proposed theoretical approach, based on a set of generalized semiconductor Bloch equations, provides a full three-dimensional multisubband description of carrier-carrier correlation for any profile of the confinement potential, thus allowing a direct comparison with experiments on available structures, In agreement with previous investigations based on simplified one-dimensional models, our analysis shows that. also for realistic quantum-wire structures, electron-hole Coulomb correlation completely removes the one-dimensional bandedge singularities from the linear-absorption spectra. Moreover. we find that this effect is present also at high densities (corresponding to gain regimes) and contributes significantly in suppressing the ideal sharp features of the free-carrier density of states. The multisubband nature of available state-of-the-art structures is found to play a dominant role in determining the overall spectral shape in the whale density range.

1996 - Optical absorption of small hydrogenated Si particles [Articolo su rivista]
BAIERLE R., J; CALDAS M., J; Molinari, Elisa; Ossicini, Stefano
abstract

Ab-initio calculations

1996 - Thermal ionization of excitons in V-shaped quantum wires [Articolo su rivista]
Rinaldi, R; Giugno, Pv; Cingolani, R; Rossi, F; Molinari, Elisa; Marti, U; Reinhart, Fk
abstract

The exciton-to-free-carrier transition in GaAs and InxGa1-xAs V-shaped quantum wires is revealed by means of temperature-dependent magnetoluminescence experiments. The experimental results are in excellent agreement with the diamagnetic shift obtained from a solution of the full two-dimensional Schrodinger equation for electrons and holes including magnetic-field and excitonic effects. In the GaAs wires, the exciton-to-free-carrier transition is found to occur at temperature consistent with the exciton binding energies. In the InxGa1-xAs wires the diamagnetic shift of the luminescence is found to be free-carrier-like, independent of temperature, due to the weakening of the exciton binding energy induced by the internal piezoelectric field.

1996 - V-grooved quantum wires as prototypes of 1D-systems: Single particle properties and correlation effects [Articolo su rivista]
Rossi, F; Molinari, Elisa; Rinaldi, R; Cingolani, R.
abstract

We present a combined theoretical and experimental study of the electronic and excitonic properties of GaAs-based V-grooved quantum wires as well as their dependence on external magnetic fields. We propose a numerical approach which allows us to compute electron and hole states, as well as their mutual correlation effects for realistic wire geometries. Our predictions for the optical spectra and their dependence on magnetic field are compared with several spectroscopical observations on samples grown by MBE on holographic patterns. The agreement is excellent, thus demonstrating the one-dimensional nature of the ground state exciton and the excitonic nature of the first-subband transitions in the presence of a magnetic field.

1996 - Valence band spectroscopy in V-grooved quantum wires [Articolo su rivista]
Goldoni, Guido; F., Rossi; Molinari, Elisa; A., Fasolino; R., Rinaldi; R., Cingolani
abstract

We present a combined theoretical and experimental study of the anisotropy in the optical absorption of V-shaped quantum wires. By means of realistic band structure calculations for these structures, we show that detailed information on the heavy- and light-hole states can be singled out from the anisotropy spectra independently of the electron confinement, thus allowing accurate valence band spectroscopy.

1995 - Anharmonic phonon lifetimes in semiconductors from density-functional perturbation theory [Articolo su rivista]
A., Debernardi; S., Baroni; Molinari, Elisa
abstract

The anharmonic lifetimes of zone-center optical phonons in C, Si, and Ge are calculated along with their temperature and pressure dependences, using third-order density-functional perturbation theory. Our basic ingredients are by-products of a standard linear-response calculation of phonon dispersions in the harmonic approximation, resulting in a similarly good agreement with experiments. The microscopic mechanisms responsible for the decay are revealed and shown to be different for different materials and to depend sensitively on the applied pressure.

1995 - Fast exciton and free-carrier kinetics in semiconductors: A Monte Carlo simulation [Articolo su rivista]
Selbmann, Pe; Gulia, M; Molinari, Elisa; Rossi, F; Lugli, P.
abstract

The coupled dynamics of free carriers and excitons after non-resonant excitation with short laser pulses is investigated by means of an Ensemble Monte Carlo method which includes phonon-assisted formation and dissociation of free excitons. Simulations for bulk GaAs reveal that excitonic binding under participation of LO phonons is a very fast and effective process. The temporal rise of exciton luminescence depends on the particular excitation conditions and is determined by all interaction mechanisms in the system.

1995 - MICROSCOPIC CALCULATION OF THE ELECTRON OPTICAL-PHONON INTERACTION IN ULTRATHIN GAAS/ALXGA1-XAS ALLOY QUANTUM-WELL SYSTEMS [Articolo su rivista]
Lee, I; Goodnick, Sm; Gulia, M; Molinari, Elisa; Lugli, P.
abstract

MICROSCOPIC CALCULATION OF THE ELECTRON OPTICAL-PHONON INTERACTION IN ULTRATHIN GAAS/ALXGA1-XAS ALLOY QUANTUM-WELL SYSTEMS

1995 - Magnetic-field effects in the luminescence of V-shaped quantum wires. [Articolo su rivista]
Rinaldi, R; Giugno, Pv; Cingolani, R; Rossi, F; Molinari, Elisa; Ferrara, M; Marti, U; Martin, D; Moriergemoud, F; Ruterana, P; Reinhart, F.
abstract

Quantum wire heterostructures, such as V- and T-shaped wires, are very promising candidates for low-threshold lasing. A crucial issue is the excitonic vs. free-carrier nature of the radiative recombination. Here, we report on magnetophotoluminescence studies of GaAs and InGaAs V-shaped wires that allow to discriminate different regimes of radiative recombination.

1995 - ULTRAFAST CARRIER RELAXATION AND VERTICAL-TRANSPORT PHENOMENA IN SEMICONDUCTOR SUPERLATTICES - A MONTE-CARLO ANALYSIS [Articolo su rivista]
Rossi, F; Meier, T; Thomas, P; Koch, Sw; Selbmann, Pe; Molinari, Elisa
abstract

ULTRAFAST CARRIER RELAXATION AND VERTICAL-TRANSPORT PHENOMENA IN SEMICONDUCTOR SUPERLATTICES - A MONTE-CARLO ANALYSIS

1995 - ULTRAFAST RELAXATION OF PHOTOEXCITED CARRIERS IN SEMICONDUCTOR QUANTUM WIRES - A MONTE-CARLO APPROACH [Articolo su rivista]
Rota, L; Rossi, F; Lugli, P; Molinari, Elisa
abstract

A detailed analysis of the cooling and thermalization process for photogenerated carriers in semiconductor quantum wires is presented. The energy relaxation of the nonequilibrium carrier distribution is investigated for the ''realistic'' case of a rectangular multisubband quantum-wire structure. By means of a direct ensemble Monte Carlo simulation of both the carrier and the phonon dynamics, all the nonlinear phenomena relevant for the relaxation process, such as carrier-carrier interaction, hot-phonon effects, and degeneracy, are investigated. The results of these simulated experiments show a significant reduction of the carrier-relaxation process compared to the bulk case, which is mainly due to the reduced efficiency of carrier-carrier scattering; on the contrary, the role of hot-phonon effects and degeneracy seems to be not so different from that played in bulk semiconductors.

1994 - AN INVESTIGATION OF CARRIER DYNAMICS IN SEMICONDUCTOR QUANTUM WIRES FOLLOWING FEMTOSECOND LASER EXCITATION [Articolo su rivista]
Rota, L; Rossi, F; Lugli, P; Molinari, Elisa
abstract

A Monte Carlo analysis of the carrier relaxation dynamics in a GaAs quantum wire system following laser photoexcitation is presented. Relaxation mechanisms due to electron-electron and electron-polar optical phonon interaction are included within a multisubband picture taking into account both intrasubband and intersubband scattering mechanisms for the case of rectangular quantum wire structures. Degeneracy and hot-phonon effects are also investigated as a function of carrier density and kinetic energy.

1994 - Exciton binding energy in GaAs V-Shaped quantum wires [Articolo su rivista]
R., Rinaldi; M., Lepore; M., Ferrara; Im, Catalano; R., Cingolani; F., Ross; L., Rota; Molinari, Elisa; P., Lugli; U., Marti; D., Martin; F., MORIER GENOUD; P., Ruterana; Fk, Reinhart
abstract

Exciton binding energy in GaAs V-Shaped quantum wires

1994 - HOT PHONONS IN QUANTUM WIRES - A MONTE-CARLO INVESTIGATION [Articolo su rivista]
Rota, L; Ryan, Jf; Rossi, F; Lugli, P; Molinari, Elisa
abstract

We have investigated the effect of non-equilibrium phonons on carrier relaxation dynamics in a quantum wire following ultrafast photoexcitation. We show that phonon build-up produces a considerable reduction of the cooling rate of photoexcited carriers for densities of the order of 10(6) cm(-1). In this respect, the results for quantum wires are found to be similar to the bulk case. An important consequence of the reduced dimensionality of wires is found in the non-equilibrium phonon distribution, which is populated even at very small wave vectors.

1994 - INPLANE RAMAN-SCATTERING OF (001)-SI/GE SUPERLATTICES - THEORY AND EXPERIMENT [Articolo su rivista]
Schorer, R; Abstreiter, G; Degironcoli, S; Molinari, Elisa; Kibbel, H; Presting, H.
abstract

Using a micro-Raman setup, in-plane Raman scattering was performed on short-period (001)-Si/Ge superlattices, and the complete phonon spectrum (longitudinal and transverse, optical and acoustic modes) could be studied. In the relevant wave-vector range, the in-plane dispersion was found to affect the frequencies of folded-acoustic modes, while being negligible for confined optical modes. The comparison with theoretical spectra, calculated by means of first-principles interatomic force constants, shows that the observed deviations of ''unfolded'' confined optical modes from the bulk dispersions can be attributed to interface roughness and is well described by a simple alloy layer model (2-3 intermixed SiGe atomic layers at interfaces).

1994 - INVESTIGATION OF QUANTUM STATES IN V-SHAPED GAAS QUANTUM WIRES [Relazione in Atti di Convegno]
Rinaldi, R; Cingolani, R; Rossi, F; Rota, L; Ferrara, M; Lugli, P; Molinari, Elisa; Marti, U; Martin, D; Moriergenoud, F; Reinhart, Fk
abstract

The quantized states of GaAs V-shaped quantum wires have been investigated by means of photoluminescence and photoreflectance. The experimental results are successfully compared with theoretical calculations of eigenstates based on a realistic model for the two-dimensional confining potential.

1994 - METASTABLE TRANSITION OF EL2 IN GAAS - THE ELECTRON-PHONON-COUPLING CHANNEL [Articolo su rivista]
Caldas, Mj; Molinari, Elisa
abstract

We propose a theoretical approach to the general problem of phonon-coupling between localised electronic states in semiconductors, which takes advantage of results from first-principles calculations. Application of this scheme to the isolated antisite defect As(Ga) in GaAs reveals that electron-phonon coupling can provide the mechanism for the metastable transition characteristic of EL2.

1994 - OPTICAL PHONON PROBES OF THE LATERAL SCALE OF INTERFACE ROUGHNESS - A THEORETICAL INVESTIGATION [Articolo su rivista]
Degironcoli, S; Molinari, Elisa
abstract

We study the effect of the different lateral scale of interface roughness on the optical photon spectrum of thin Si/Ge (001) superlattices. We find that the first Si-like confined optical mode does localize either in the thinnest or in the thicker part of the Si well already for relatively small lateral sizes of the interface terraces, contrary to the corresponding behaviour in GaAs/AlAs structures. We show that this gives rise to distinct changes in the Raman lineshape, which should be useful to discriminate between short-range intermixing and long-range interface corrugation in Si/Ge superlattices. We finallyy discuss the optimal conditions to allow experimental observations of this effect.

1994 - PHONONS AND ELECTRON-PHONON INTERACTION IN GAAS QUANTUM WIRES [Articolo su rivista]
Rossi, F; Bungaro, C; Rota, L; Lugli, P; Molinari, Elisa
abstract

Phonon dispersions and potential profiles for thin rectangular GaAs wires embedded in AlAs are calculated by means of a microscopic approach based on ab-initio microscopic force constants. Besides non-dispersive vibrations, well confined in GaAs, we find phonons with interface character along one or both of the in-plane directions, the latter including modes with maxima at the edges of the wire. While confined phonon potentials are well described within the dielectric continuum (DC) model, interface mode profiles turn out to be more complex. Not only macroscopic models do not give analytic results in this case, but also numerical implementations of the DC model reproduce only partially the microscopic results in the thin-wire regime.

1994 - VIBRATIONAL PROPERTIES OF A CONTINUOUS SELF-SIMILAR STRUCTURE [Articolo su rivista]
Petri, A; Alippi, A; Bettucci, A; Craciun, F; Farrelly, F; Molinari, Elisa
abstract

We present an extensive investigation of the normal modes of vibration of a prototype hierarchical continuous system, consisting of a Cantor-like sequence of piezoelectric and resin elements. From a detailed analysis of the density of states, displacement profiles, and eigenvalue-spacing distributions, evidence is found for the existence of multiple fracton and phonon regimes. The role of resonant modes and the effect of disorder, which may be of primary importance in real systems, are also discussed in detail.

1994 - VIBRATIONAL PROPERTIES OF SI/GE SUPERLATTICES - THEORY AND INPLANE RAMAN-SCATTERING EXPERIMENTS [Articolo su rivista]
Schorer, R; Abstreiter, G; Degironcoli, S; Molinari, Elisa; Kibbel, H; Kasper, E.
abstract

Phonons in short-period (001)-Si(n)Ge(n) Superlattices (SL's) have been studied both theoretically, by a first priniciples approach including strain and interface intermixing, and experimentally by micro-Raman spectroscopy where in-plane scattering geometries allow the observation of both longitudinal (L) and transverse (T) modes. Experimental data are found to deviate considerably from theoretical predictions for SL's with ideally sharp interfaces, both in frequency of higher-order confined Si-like modes and in the polarization dependence of the SiGe-like ''interface'' peak (lineshape and L-T splitting). Supercell calculations representing interface intermixing within a simple model by 2-3 monolayers of SiGe alloy at the interfaces are found to reproduce these major experimental findings. Measurements of Raman resonance profiles of various SL phonon modes strongly confirm their calculated different spatial localization.

1993 - INTERFACE MODE IN SI/GE SUPERLATTICES - THEORY AND EXPERIMENTS [Articolo su rivista]
Degironcoli, S; Molinari, Elisa; Schorer, R; Abstreiter, G.
abstract

The Raman peak associated with Si-Ge vibrations in short-period Si/Ge (001) superlattices is studied both experimentally and theoretically. On the experimental side, we use a microprobe technique which allows us to investigate the longitudinal (L) and transverse (T) spectra, and find an unexpected behavior of the line shape and L-T spitting of this peak. By means of first-principles calculations, taking into account both strain and interface intermixing, we show that such behavior is consistent with the picture of an intermixed alloy layer at the interfaces, and we are able to identify the character and spatial localization of the individual atomic clusters contributing to the vibrations.

1993 - Micro-Raman scattering in ultrathin-layer superlattices: Evidence of zone-center anisotropy of optical phonons [Articolo su rivista]
G., Scamarcio; M., Haines; G., Abstreiter; Molinari, Elisa; S., Baroni; A., Fischer; K., Ploog
abstract

Raman spectra of (001) (GaAs)m(AlAs)n, 1 less-than-or-equal-to m, n less-than-or-equal-to 7 superlattices have been measured in backscattering along x' \\ [110BAR], x \\ [010], and z \\ [001] directions with a microprobe. Confined longitudinal (LO) and transverse (TO) optical phonons with q either parallel or normal to the z axis have been studied in all the independent scattering geometries. A strong anisotropy is observed when the direction of the transferred momentum is changed from the z axis to x' or x. The most evident feature is that the Raman spectra in the z(x'x')zBAR geometry axe dominated by LO1 confined phonons, whereas no signal at this energy is present in the y'(x'x')y'BAR geometry. Modes evolving from TO1 and LO(n) (n odd) phonons give rise to structures at intermediate frequencies in the x'(y'y')x'BAR spectrum. In contrast to LO1, the measured TO1 energy remains constant, showing the isotropy of the lower-energy branch of this doubly degenerate mode. These results axe explained by microscopic lattice-dynamical calculations performed in an ab initio scheme, which properly accounts for angular dispersion and mode mixing. We have also found that in the ultimate limit of confinement, when only one principal mode per branch can be considered, the phonon frequencies can be reproduced by the macroscopic dielectric continuum model, provided that the effect of phonon confinement is considered.

1993 - PHONONS IN THIN GAAS QUANTUM WIRES [Articolo su rivista]
Rossi, F; Rota, L; Bungaro, C; Lugli, P; Molinari, Elisa
abstract

Phonon frequencies and potentials for an array of thin rectangular GaAs wires embedded in AlAs are calculated within a microscopic scheme. The confined and interface character of optical modes are clearly evident from their dispersion and from the spatial profiles. Our results allow us to conclude that macroscopic models based on the dielectric continuum scheme are adequate to describe confined phonon profiles at wave vectors relevant to el-ph scattering, in contrast with approaches based on mechanical boundary conditions, which yield modes with the wrong symmetry sequence. The implications for electron-phonon scattering rates are discussed.

1993 - REDUCED CARRIER COOLING AND THERMALIZATION IN SEMICONDUCTOR QUANTUM WIRES [Articolo su rivista]
Rota, L; Rossi, F; Goodnick, Sm; Lugli, P; Molinari, Elisa; Porod, W.
abstract

By using a Monte Carlo analysis of the carrier relaxation in GaAs quantum wires following laser photoexcitation, we show that carrier cooling due to phonon emission and internal thermalization due to electron-electron interaction are significantly decreased with respect to bulk systems. This decreased thermalization is mainly attributed to the reduced efficiency of intersubband processes and to the reduced effect of electron-electron intrasubband scattering.

1993 - Si-GaAs(001) superlattice structure [Articolo su rivista]
Sorba, Lucia; G., Bratina; A., Franciosi; L., Tapfer; G., Scamarcio; V., Spagnolo; A., Migliori; P., Merli; Molinari, Elisa
abstract

We examined the structural properties of Si layers embedded in GaAs and AlAs-GaAs structures by means of X-ray diffraction and transmission electron microscopy to identify growth parameters for the synthesis of Si-GaAs superlattices. We find that localized pseudomorphic Si layers can be obtained by molecular beam epitaxy at 500-540-degrees-C up to a thickness of 7-8 monolayers. Fifteen period Si-GaAs(001) superlattices involving Si layers 2-3 monolayer thick were then synthesized using the same growth parameters. X-ray diffraction measurements and Raman spectroscopy studies confirm that pseudomorphic growth and relatively abrupt interfaces were achieved.

1992 - Atomic intermixing in short period GaAs/AlAs superlattices [Articolo su rivista]
Bernard, Jusserand; Francis, Mollot; Richard, Planel; Molinari, Elisa; Stefano, Baroni
abstract

We compare the confined optical frequencies measured by Raman scattering and predicted by ab initio lattice dynamics including atomic intermixing. We combine the structural information deduced from both GaAs and AlAs phonons. The residual Ga (Al) concentrations in the AlAs (GaAs) layers is estimated. They strongly decrease with decreasing growth temperature. However, a significant amount of gallium in the AlAs is evidenced even at 400°C.

1992 - Direct experimental observation of fracton mode patterns in one-dimensional Cantor composites [Articolo su rivista]
F., Craciun; A., Bettucci; Molinari, Elisa; A., Petri; A., Alippi
abstract

By measuring fracton mode displacements in artificial piezoelectric composities with hierarchical structure, we find direct evidence of their localized and self-similar character. Clear indications of the existence of multiple fracton and phonon regimes are also presented.

1992 - Effects of disorder on the Raman spectra of GaAs/AlAs superlattices [Articolo su rivista]
Molinari, Elisa; Baroni, S; Giannozzi, P; de Gironcoli, S.
abstract

The vibrational properties of GaAs/AlAs (001) superlattices are studied theoretically by means of an ab initio approach-based on interatomic force constants-that allows one to treat the effects of compositional disorder using very large supercells. We find that the experimental Raman spectra in thin samples cannot be explained without taking into account disorder occurring at the interfaces. Moreover, we show that some of the AlAs-like LO modes are extremely sensitive to disorder, and they are therefore suitable for a rather precise characterization of the samples.

1992 - INTERACTION OF ELECTRONS WITH INTERFACE PHONONS IN GAAS/ALAS AND GAAS/ALGAAS HETEROSTRUCTURES [Articolo su rivista]
P., Lugli; Bordone, Paolo; Molinari, Elisa; H., Rucker; A. M., de Paula; A. C., Maciel; J. F., Ryan; M., Shayegan
abstract

The interaction of electrons with interface phonons is predicted to be of major importance in narrow quantum wells. Time-integrated Raman measurements of non-equilibrium phonons in GaAs/AlAs structures show strong coupling to AlAs interface modes, in good agreement with theoretical predictions based on a microscopic phonon model. Monte Carlo simulations of time-resolved Raman measurements of interface phonons in GaAs/AlGaAs structures provide further confirmation of this result.

1992 - Infrared reflectivity and Raman spectra of (GaAs)m(AlAs)n ultrathin layer superlattices [Articolo su rivista]
G., Scamarcio; Molinari, Elisa; L., Tapfer; M., Lugarà; K., Plooga
abstract

Confined TO and LO photon frequencies of (GaAs)m(AlAs)n superlattices with 1 less-than-or-equal-to m, n less-than-or-equal-to 7 are measured by means of far-infrared reflectivity and Raman scattering, respectively, and compared with those calculated in the framework of an ab-initio scheme for superlattices with abrupt interfaces.

1992 - LOW-THRESHOLD SUBHARMONIC GENERATION IN COMPOSITE STRUCTURES WITH CANTOR-LIKE CODE [Articolo su rivista]
Alippi, A; Shkerdin, G; Bettucci, A; Craciun, F; Molinari, Elisa; Petri, A.
abstract

We show experimental evidence of extremely low thresholds for subharmonic generation of ultrasonic waves in one-dimensional artificial piezoelectric plates with Cantor-like structure, as compared to the corresponding homogeneous and periodical plates. The origin of this apparent anomaly is theoretically investigated by studying anharmonic coupling between normal modes. We demonstrate that the large enhancement of nonlinear interaction results from the more favorable frequency and spatial matching of coupled modes (fractons and phonons) in the Cantor-like structure, with no need to invoke anomalous modifications of the nonlinear elastic constants.

1992 - Microscopic calculation of the electron-phonon interaction in quantum wells [Articolo su rivista]
H., Rücker; Molinari, Elisa; P., Lugli
abstract

The electron–optical-phonon scattering rates in GaAs/AlAs quantum wells are calculated on the basis of a fully microscopic description of the phonon spectra. The results indicate the great importance of confined as well as GaAs-like and AlAs-like interface phonons. By comparing our results with those of several macroscopic models, we resolve a long-standing controversy on their ability to describe the relevant vibrations.

1992 - Phonons in Si/GaAs superlattices [Articolo su rivista]
G., Scamarcio; V., Spagnolo; Molinari, Elisa; L., Tapfer; Sorba, Lucia; G., Bratina; A., Franciosi
abstract

Phonon dispersion in strained heterovalent (Si)n/(GaAs)m (001) superlattices has been calculated using first-principles force constants. Raman scattering experiments have been conducted on fifteen-period (Si)2/(GaAs)28 and (Si)3/(GaAs)50 superlattices synthesized by molecular-beam epitaxy. In addition to folded acoustic modes, confined Si-like and quasiconfined GaAs-like optical modes appear in the superlattice spectra, in spite of the extremely small thickness of the Si layers. Quantitative agreement is found between measured and calculated phonon frequencies, confirming that a description in terms of strain- and confinement-induced shifts of the optical phonons is appropriate for these structures.

1992 - Si‐GaAs(001) superlattices [Articolo su rivista]
Sorba, Lucia; G., Bratina; A., Franciosi; L., Tapfer; G., Scamarcio; V., Spagnolo; Molinari, Elisa
abstract

Si‐GaAs(001) superlattices have been grown by molecular beam epitaxy. X‐ray interference measurements and Raman spectroscopy studies in the acoustic range for (Si)2(GaAs)28 and (Si)3(GaAs)50 superlattice structures demonstrate that pseudomorphic growth conditions were achieved. Raman data in the optical range show large (∼50–70 cm−1) confinement‐ and strain‐induced shifts of the Si‐like optical modes.

1992 - Threshold lowering for subharmonic generation in Cantor-like composite structures [Articolo su rivista]
A., Alippi; G., Shkerdin; A., Bettucci; F., Craciun; Molinari, Elisa; A., Petri
abstract

We show evidence of extremely low thresholds for subharmonic generation in one-dimensional artificial composites with hierarchical structure, as compared to the corresponding homogeneous and periodic structures. The displacement profiles of the anharmonically coupled normal modes are probed experimentally, indicating that the enhanced non-linear interaction is due to the fact that, for a given fundamental mode, a second mode with much more favourable frequency and spatial matching is found in the hierarchical composite than in the non-fractal structures.

1992 - Vibrational properties of Si/GaAs superlattices [Articolo su rivista]
G., Scamarcio; V., Spagnolo; Molinari, Elisa; L., Tapfer; Sorba, Lucia; G., Bratina; A., Franciosi
abstract

Raman scattering experiments and first principles phonon calculations have been performed on (Si)m (GaAs)n superlattices grown by molecular beam epitaxy. In spite of the small thickness of the Si layers, folded acoustic modes, confined Si-like and quasi-confined GaAs-like optical modes are clearly observed in the spectra. The experimental frequencies compare well with the calculated ones, confirming that a description of optical phonons in terms of strain- and confinement-induced shifts is appropriate for these novel heterostructures.

1992 - Vibrational properties of isolated AlAs monolayers embedded in GaAs: a theoretical study of the effects of disorder [Articolo su rivista]
Molinari, Elisa; Baroni, S; Giannozzi, P; de Gironcoli, S.
abstract

We present ab-initio calculations of Raman spectra of a prototype superlattice (SL) formed by the periodic repetition of an AlAs single monolayer embedded between two thicker GaAs layers. By comparing results for the ideal structure and for disordered configurations characterized by cationic intermixing, we demonstrate that AlAs-like longitudinal optical modes are extremely sensitive to disorder and may allow a quantitative characterization of the actual composition profiles obtained in real samples. The general implications of this conclusion for the study of interface intermixing in GaAs/AlAs SL's are discussed.

1991 - Electron-phonon interaction in 2-dimensional systems - a microscopic approach [Articolo su rivista]
P., Lugli; Molinari, Elisa; H., Rucker
abstract

We present a calculation of the electron optical-phonon scattering rates in GaAs/AlAs quantum wells, based on a very accurate microscopic description of the phonon spectra. The results show that — besides the contribution of confined modes — a very large contribution originates from interface phonons of both GaAs-like and AlAs-like character. We then compare our results for phonon displacements and potentials, as well as for scattering rates, with those obtained from several macroscopic phonon models. We are therefore able to provide indications for selecting the model which allows the most appropriate simplified description of vibrations at the wavevectors relevant to the interaction with carriers.

1991 - Infrared reflectivity by transverse-optical phonons in (GaAs)m/(AlAs)n ultrathin-layer superlattices [Articolo su rivista]
G., Scamarcio; L., Tapfer; W., König; A., Fischer; K., Ploog; Molinari, Elisa; S., Baroni; P., Giannozzi; S., de Gironcoli
abstract

We report far-infrared reflectivity measurements on (GaAs)m/(AlAs)n superlattices (SL's) with systematically varied layer thicknesses in the range 1 less-than-or-equal-to m,n less-than-or-equal-to 7. Taking advantage of an appropriate choice of the total SL thickness D less-than-or-equal-to 0.3-mu-m, we measure the frequencies of AlAs-like TO1 confined phonons from the peak of the reststrahlen band. The GaAs-like TO1 frequencies are obtained by fitting reflectivity spectra to the SL dielectric-response-theory model. Microscopic calculations of confined TO frequencies are performed within an ab initio scheme and successfully compared with the experimental data.

1990 - ANISOTROPY IN THE OPTICAL-SPECTRUM OF THE GAAS(110) SURFACE [Articolo su rivista]
Manghi, Franca; Molinari, Elisa; Selloni, A; Delsole, R.
abstract

A Comment on the Letter by X. Zhu et al., Phys. Rev. Lett. 63, 2112 (1989).

1990 - ANISOTROPY OF SURFACE OPTICAL-PROPERTIES FROM 1ST-PRINCIPLES CALCULATIONS [Articolo su rivista]
Manghi, Franca; Delsole, R; Selloni, A; Molinari, Elisa
abstract

The optical properties of GaAs(110) and GaP(110) surfaces are studied by means of self-consistent local-density calculations. A very large contribution to the reflectance anisotropy is found to be related to transitions which do not involve surface states. These transitions give a substantial—yet smaller—contribution also to the differential reflectivity. Comparison is made with relevant experimental data.

1990 - Calculations of phonon spectra in III–V and SiGe superlattices: A tool for structural characterization [Articolo su rivista]
A., Fasolino; Molinari, Elisa
abstract

This paper reviews some of the basic concepts of phonons in semiconductor superlattices with emphasis on the aspects which are of relevance for characterization.

1990 - Chemisorption of H on GaAs(110): a first-principle calculation. [Articolo su rivista]
Bertoni, Carlo Maria; M., BUONGIORNO NARDELLI; F., Bernardini; F., Finocchi; Molinari, Elisa
abstract

We calculate the equilibrium structure of a monolayer of H chemisorbed on GaAs(110), and show that is characterized by the full removal of the substrate relaxation and by a counterrelaxation of about -5 degrees.The corresponding vibration frequencies of the H-subtrate bonds and chemisorption induced changes in the electronic structure are also calculated, and found in good agreementwith the available experimental data.

1990 - Electron-phonon interaction in quasi-two-dimensional systems [Articolo su rivista]
H., Rücker; Molinari, Elisa; P., Lugli
abstract

We present a calculation of the electron-LO-phonon scattering rate in quasi-two-dimensional systems, based on a fully microscopic description of the phonon spectra. The results obtained for a GaAs/AlAs quantum-well structure indicate the great importance of interface phonons and allow us to solve a long-standing controversy on the validity of simplified macroscopic models for describing the relevant vibrations.

1990 - Hydrogen adsorption on compound semiconductor surfaces [Articolo su rivista]
Bertoni, Carlo Maria; M., Buongiorno Nardelli; Molinari, Elisa
abstract

We present the results of the total-energy calculations for different adsorption geometries of atomic hydrogen on GaAs (110) and the corresponding changes in the surface electronic states. In particular we discuss the problem of the quenching of the substrate relaxation.

1990 - Phonon spectra of ultrathin GaAs/AlAs superlattices: An ab initio calculation [Articolo su rivista]
Stefano, Baroni; Paolo, Giannozzi; Molinari, Elisa
abstract

Phonon spectra of ultrathin (GaAs)n(AlAs)n (001) superlattices are studied theoretically using linear-response density-functional techniques. Results are presented for n=1,2,3 superlattices, along with prototype supercell calculations aimed at simulating a completely disordered (alloy) as well as some partially disordered superlattices. Besides interfacial disorder, which modifies the effective confinement length of low-order longitudinal-optic phonons, we find that—in the ultrathin regime—some degree of cationic mixing must also affect inner planes in order to explain experimental findings.

1990 - Planar force-constant method for lattice dynamics of superstructures [Articolo su rivista]
A., Fasolino; Molinari, Elisa; K., Kunc
abstract

We present a simple and accurate method for dealing with phonons in superlattices, and illustrate it with the example of GaAs/AlAs heterostructures. In this approach the vibrations of the superlattice are described in terms of planar force constants, which are determined ab initio for one constituent (e.g., GaAs bulk) and extended to the other constituent (e.g., AlAs) by introduction of the ‘‘mass and charge approximation.’’ Within this approximation, the dynamical problem of the superlattice is represented as that of an infinite bulk crystal (e.g., GaAs), which is modified by a sequence of on-site perturbations. We systematically develop the formalism for the one-dimensional description of phonons propagating along the superlattice growth direction, derive its relation with the general three-dimensional description, and clarify its physical meaning. The representation of the Coulomb interactions in the one-dimensional formalism is described in some detail. Applications of the method to (GaAs)m/(AlAs)n superlattices of several thicknesses are discussed.

1989 - 1ST-PRINCIPLES CALCULATION OF ANISOTROPIC REFLECTANCE AT THE GAAS(110) SURFACE [Articolo su rivista]
Manghi, Franca; R., Delsole; Molinari, Elisa; A., Selloni
abstract

The optical properties of the GaAs(110) surface are studied by means of self-consistent local-density slab calculations. It is found that a large contribution to the calculated anisotropy of the clean surface reflectance is related to transitions which do not involve surface states. Comparison is made with differential reflectivity and reflectance anisotropy experiments.

1989 - A coupled-mode theory for periodic piezoelectric composites [Articolo su rivista]
F., Craciun; L., Sorba; Molinari, Elisa; M., Pappalardo
abstract

A coupled oscillator model to calculate the resonance spectrum of a one-dimensional piezoelectric composite plate, used in ultrasonic transducers, is proposed. Two resonant modes, one produced by the elastic wave reflection on the plate boundaries (thickness resonance) and the other by the reflection on the periodic discontinuities (lateral resonance) are considered. A Kronig-Penney model is used to calculate the lateral resonances. The thickness resonance is obtained with an effective medium model. The coupling of these two modes is described by a biquadratic equation whose solutions are the resonant frequencies of the piezoelectric composite plate. A criterion for a distribution of phases to keep the spurious lateral resonances away from the thickness resonance vicinity is obtained

1989 - Calculated phonon spectra of Si/Ge (001) superlattices: Features for interface characterization [Articolo su rivista]
Molinari, Elisa; Fasolino, Annalisa
abstract

We present a model calculation or phonon spectra of Si/Ge superlattices along the (001) growth direction. The most relevant result for structural characterization is that interface modes involving the SiGe bonds at the interface are predicted only in the transverse and not in the longitudinal polarization. The detection of such modes between the Si‐like and Ge‐like optical modes in backscattering Raman spectra from the (001) surface must then be ascribed to interface disorder or alloying.

1989 - Finite‐size effects in the frequency response of piezoelectric composite plates [Articolo su rivista]
A., Alippi; F., Craciun; Molinari, Elisa
abstract

The resonance spectrum of composite plates made of alternating piezoelectric ceramic and epoxy elements is studied both theoretically and experimentally in the frequency range below the thickness resonance. The transmissivity of the finite structure to plate modes is calculated, taking into account the effective plate velocities of the two constituent materials and the finite number of elements in the structure. The results are in excellent agreement with the electrically excited acoustical resonances of samples with different geometry and number of elements. In particular, the effects of the finite size on the number and frequency of modes, as well as their selection rules in the given experimental setup, are successfully interpreted. New ‘‘surface’’ resonances, attributed to the end elements, are observed in the stopbands when the surface elements have lower impedance

1989 - Origin of surface anisotropies in the optical spectra of III-V compounds [Articolo su rivista]
Manghi, Franca; Molinari, Elisa; R., Del Sole; A., Selloni
abstract

We present results of ab initio calculations of optical spectra of GaP and GaAs (110) surfaces, and show that transitions between bulk single-particle states modified by the crystal truncation account for the largest contribution to above-gap optical anisotropy of these systems.

1989 - Resonant quasiconfined optical phonons in semiconductor superlattices [Articolo su rivista]
Fasolino, Annalisa; Molinari, Elisa; J. C., Maan
abstract

We point out that resonant phonon modes with quasiconfined behavior may arise in semiconductor superlattices in the continuum frequency range, i.e., where both constituents have allowed bulk frequencies with real wave vector. With reference to the cases of Si/Ge and InAs/GaSb (001) superlattices, we show that such modes appear close to the edge of overlapping optical frequencies, with displacement patterns and Raman strengths comparable to those of true confined modes. However their degree of confinement and their actual number and frequency location are found to be more sensitive to the adjacent layer and to the details of the interfaces. Their study can therefore yield additional structural information on the interface region with respect to the study of true confined modes.

1989 - Surface confined phonons in semiconductor superlattices [Articolo su rivista]
Sorba, Lucia; Molinari, Elisa; Fasolino, Annalisa
abstract

We show that the phonon spectra of finite GaAs/AIAs(001) superlattices along their growth direction display a new type of confined surface modes which split off the superlattice confined optical branches. Their frequency location and decay inside the superlattice is investigated by means of a model calculation and compared with the behaviour of surface states of semi-infinite GaAs.

1988 - Phonons in semiconductor superlattices [Articolo su rivista]
Molinari, Elisa; Fasolino, Annalisa
abstract

The phonon spectra of semiconductor superlattices along the growth direction are studied by means of a one-dimensional approach with “long range” interplanar force constants derived from ab initio interplanar forces for the bulk components. Results are shown for frequencies and displacements of , and (001) superlattices, with particular emphasis on the presence of resonant modes and of interface modes of microscopic origin, and for confined optical modes and surface modes in finite (001) superlattices.

1988 - Piezoelectric plate resonances due to 1st lamb symmetrical mode [Articolo su rivista]
A., Alippi; F., Craciun; Molinari, Elisa
abstract

The resonances of the first Lamb s0 mode in parallelepiped piezoelectric plates excited by surface electrodes have been measured. Below the first thickness mode, the resonances are found to reproduce the dispersion of s0 in an infinite plate even for plates of small length. Moreover, a significant difference is observed between the behavior of the fundamental and its harmonics, reflecting the different effect of metallization in the two cases.

1988 - Stopband edges in the dispersion curves of Lamb waves propagating in piezoelectric periodical structures [Articolo su rivista]
A., Alippi; F., Craciun; Molinari, Elisa
abstract

The fractional volume dependence of stopbands in piezoelectric periodical composite plates has been studied experimentally by exciting the band‐edge resonances. The frequency and symmetry of such modes below the thickness resonance of the plate are successfully interpreted in terms of a theoretical model which provides approximate dispersion curves of the lowest Lamb waves propagating in the composite plate.

1987 - Bonding and surface electronic structure of an Sb overlayer on GaP(110) [Articolo su rivista]
Manghi, Franca; CALANDRA BUONAURA, Carlo; Molinari, Elisa
abstract

We calculate the electronicstructure of an Sboverlayer on GaP(110) using a self-consistent pseudopotential approach. A detailed analysis of the energy position and the orbital nature of the overlayer induced states shows that the main contribution to the bonding between the adatoms and the substrate comes from states around 4 eV below the valence band maximum. A model description of the outcomes of the full calculation is presented, which differs from previous interpretations.

1987 - CALCULATED PHONON SPECTRA OF Si/Ge SUPERLATTICES : ANALOGIES WITH OTHER SYSTEMS AND NEW FEATURES [Articolo su rivista]
Fasolino, Annalisa; Molinari, Elisa
abstract

The phonon spectra of Si/Ge superlattices are calculated along the growth direction. Besides folded and confined modes, as in the more studied case of GaAs/AlAs systems, along (001) they show interface modes in the transverse polarization and resonant modes in the longitudinal one at frequencies just below the edge where the Si and Ge bulk modes start overlapping. The thickness dependence, dispersion and displacement patterns of the latter modes show their similarities to proper confined modes.

1987 - Calculated longitudinal superlattice and interface phonons of InAs/GaSb superlattices [Articolo su rivista]
A., Fasolino; Molinari, Elisa; J. C., Maan
abstract

The longitudinal phonon spectrum and the associated Raman strength of (100) InAs/GaSb superlattices are calculated along the growth direction. Apart from confined and extended modes, as found also in GaAs/AlAs superlattices, this system shows new modes localized at the interface. The energy and the Raman strength of these latter modes depend crucially on the nature of the interface (InSb or GaAs) and suggest that they are experimentally observable.

1987 - GaAs/AlAs monolayer superlattices: A new candidate for a highly spin-polarized electron source [Articolo su rivista]
F., Ciccacci; Molinari, Elisa; N. E., Christensen
abstract

Using the results of an ab-initio self-consistent local density Dirac calculation of its electronic structure, we suggest that the GaAs/AlAs monolayer superlattice should be a good candidate for a high quality spin-polarized photoelectron source, giving a theoretical polarization of electrons of 100% together with favourable conditions for their extraction from the solid.

1987 - Microscopic calculation of differential reflectivity of GaP(110) [Articolo su rivista]
Manghi, Franca; Molinari, Elisa; R., del Sole; A., Selloni
abstract

We compute the optical properties of semiconductor surfaces starting from the self-consistent electronic structures determined using local pseudopotentials with a plane-wave basis set. Calculations for a 9 layer (110) slab of GaP are carried out with both clean and H covered surfaces. Differential reflectivity spectra and their anisotropies are interpreted in terms of transitions involving surface states.

1986 - Calculated superlattice and interface phonons of InAs/GaSb superlattices [Articolo su rivista]
A., Fasolino; Molinari, Elisa; J. C., Maan
abstract

The phonon spectrum of InAs/GaSb superlattices is calculated with a linear-chain model with interplanar force constants. Apart from confined and extended modes, as found also in GaAs/GaAlAs superlattices, this system shows new modes localized at the interface. These latter modes depend crucially on the nature (InSb or GaAs) of the interface and some of them do not coincide with any other possible superlattice mode. This fact would make them easily experimentally identifiable

1986 - Confined longitudinal and transverse phonons in GaAs/AlAs superlattices [Articolo su rivista]
Molinari, Elisa; Fasolino, Annalisa; K., Kunc
abstract

The phonon spectrum of GaAs/AlAs superlattices along the (001) growth direction is calculated using a new approach based on a realistic unified treatment of the interactions in both bulk constituents, which naturally lends itself to dealing with superlattice geometries. Results for a (GaAs)3(AlAs)3 superlattice are shown with emphasis on different aspects of confinement.

1986 - Superlattice Effects on Confined Phonons [Articolo su rivista]
Molinari, Elisa; Fasolino, Annalisa; K., Kunc
abstract

In a recent Letter, Sood, Menendez, Cardona, and Ploog (SMCP) reported Raman measurements of confined LO and TO phonons in GaAs/A1As superlattices. We focus on the results shown in Fig. 4 of SMCP, where the observed frequencies of the modes confined in GaAs layers deviate towards higher energy with respect to the folded bulk modes for q & n/a (100).

1985 - GREEN'S FUNCTION MATCHING METHOD FOR REALISTIC CALCULATIONS OF INTERFACES [Articolo su rivista]
Molinari, Elisa; G. B., Bachelet; M., Altarelli
abstract

A new method for the calculation of the electronic structure of interfaces is proposed, which avoids the introduction of artificial periodicities. The basic ingredients of the method are (a) the Green's function matching formalism, put forward by Garcia-Moliner et al. ; and (b) the self-consistent Green's function approach, as successfully applied to defects in semiconductors. Preliminary results, which indicate the feasibility of the procedure, are presented.

1985 - Green function approach to realistic calculations of the electronic structure of semiconductor interfaces [Articolo su rivista]
Molinari, Elisa; G. B., Bachelet; M., Altarelli
abstract

A new method for realistic calculations of the electronic structure of semiconductor interfaces is proposed, which is able to deal with the semi-infinite geometry of the system avoiding the use of repeated slab schemes. The matching formalism due to Garcia-Moliner and Rubio is exploited in order to obtain an explicit expression of the interface Green function on a localized orbital basis set in terms of the bulk band structure of the two component materials. The self-consistency loop scheme based on the Dyson equation — and already successfully applied to the case of defects in semiconductors — is then reformulated in order to treat both the short-range and the long-range (dipole layer) terms in the potential rearrangement. An example is briefly discussed to show that the numerical work required in this procedure is manageable.

1985 - Long range order in Al0.5Ga0.5As: Local density calculation of the electronic structure [Articolo su rivista]
N. E., Christensen; Molinari, Elisa; G. B., Bachelet
abstract

Using self-consistent, relativistic band structure calculations it is demonstrated that optical spectra measured for ordered AlGaAs2 should contain features that allows a distinction from the disordered structures. The results will be relevant for MBE-grown (1, 1) superlattices.

1984 - ELECTRONIC-PROPERTIES OF THE Cs-GaAs(110) INTERFACE AT MONOLAYER COVERAGE [Articolo su rivista]
Manghi, Franca; CALANDRA BUONAURA, Carlo; Bertoni, Carlo Maria; Molinari, Elisa
abstract

The electronic properties of the interface obtained by a monolayer deposition of Cs onto GaAs(110) are studied theoretically by modelling the overlayer with a thin jellium slab of appropriate electron density. The calculations are carried out self-consistently with a pseudopotential approach for two different substrate geometries, corresponding to the ideal and the relaxed configuration of the surface atoms. In both cases a band of Cs-induced states is found in the gap which hybridizes with the dangling and back bond states of the GaAs surface. The change in the work function upon Cs deposition is calculated and found for both the substrate geometries in good agreement with the experimental data. Comparison with electron energy loss and photoemission experiments indicates that the model with the surface in the relaxed configuration is the most appropriate one.

1983 - ELECTRON-STATES OF AN Sb-ORDERED OVERLAYER ON GaAs(110) [Articolo su rivista]
Bertoni, Carlo Maria; CALANDRA BUONAURA, Carlo; Manghi, Franca; Molinari, Elisa
abstract

The electronic properties of an Sb overlayer deposited onto a GaAs(110) surface have been calculated using a self-consistent-pseudopotential approach and assuming the ordered-overlayer geometry proposed in recent low-energy electron diffraction studies. The results show that Sb adatoms are bound by strong covalent bonds to the substrate and that various overlayer or chemisorption-induced states appear throughout the valence band. Comparison with photoemission data allows us to assign a major Sb-induced structure appearing in the energy distribution curves.

1982 - Aspects of self-consistent procedures in surface pseudopotential calculations [Articolo su rivista]
Manghi, Franca; Molinari, Elisa
abstract

The authors present a discussion of the main features of self-consistency for clean and chemisorbed surfaces in the local density functional formalism. The technical details of the procedure are analysed together with the behaviour of the various contributions to the surface potential. They show that the iterative approach to self-consistency can be made efficient and quick when the starting potential is properly chosen and the components of the potential more sensitive to the self-consistent process are singled out.

1982 - SELF-CONSISTENT PSEUDOPOTENTIAL CALCULATION OF THE ELECTRONIC-PROPERTIES OF THE INP (110) SURFACE [Articolo su rivista]
Manghi, Franca; Molinari, Elisa; Bertoni, Carlo Maria; CALANDRA BUONAURA, Carlo
abstract

Self-consistent pseudopotential calculations for InP(110) surface have been carried out using the repeated slab method and assuming relaxed surgace geometry. Various surface states have been found and their character and symmetry properties have been investigated by analyzing the corresponding pseudocharge density maps. Comparison between self-consistent and non-self-consisting calculations shows significant differences.The results are in agreement with the experimental finding obtained by different techniques.

1982 - THEORETICAL INVESTIGATION OF HYDROGEN CHEMISORPTION ON GA-CONTAINING III-V-COMPOUNDS [Articolo su rivista]
Manghi, Franca; Bertoni, Carlo Maria; CALANDRA BUONAURA, Carlo; Molinari, Elisa
abstract

A fully self-consistent pseudopotential calculation of the electronic properties of atomic hydrogen chemisobed on GaAs and GaP (110) surface is reported. Different chemisorption geometries ans substrate coverages are considered. The results are compared with the experimental information to select a structural model.

1981 - Theoretical study of the electronic structure of GaP(110) [Articolo su rivista]
Manghi, Franca; Bertoni, Carlo Maria; CALANDRA BUONAURA, Carlo; Molinari, Elisa
abstract

A self-consistent pseudopotential approach has been used to calculate the electronic structure of GaP(110) surface in both ideal and relaxed configurations. Calculations have been performed using the repeated slab method and a local form of the bare ionic pseudopotential. An efficient self-consistent procedure, which allows us to obtain quick convergence and eliminates some difficulties found in previous applications of the method, has been used. Particular care has been devoted to have complete consistency between bulk and slab calculations. Our results for the ideal surface show various surface states, whose distribution and nature are similar to those found in tight-binding calculations. For the geometry of the relaxed surface we assumed a rotation-relaxation model determined by a recent low-energy electron diffraction study. With this geometry our results show that a nonvanishing density of empty surface states, to a large extent due to backbonds, remains in the gap. The orbital composition of these states, as well as of all the other surface features, is detailed, together with the mirror-plane symmetries relevant in the interpretation of angle-resolved photoemission data. Our results are in agreement with the experimental data provided by various different measurements.

Università degli studi di Modena e Reggio Emilia

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