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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

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 - Conflicting effect of chemical doping on the thermoelectric response of ordered PEDOT aggregates [Articolo su rivista]
Cigarini, Luigi; Ruini, Alice; Catellani, Alessandra; Calzolari, Arrigo

Poly(3,4-ethylenedioxythiophene) (PEDOT) semiconductor plays a relevant role in the development of organic thermoelectric (TE) devices for low-power generation. While dopant counterions are usually needed to provide electrical conductivity, their overall effects on the thermoelectric response of the systems are unknown and uncontrolled. Here, we present a first principles study of the electronic and thermal transport of PEDOT crystalline assemblies, specifically analysing the role played by tosylate dopants on the thermoelectric figure of merit of the doped system. Our results demonstrate that, beside the desired charging effect, the presence of dopants impacts the bulk configuration by inflating the packing structure and worsening the intrinsic transport properties of the PEDOT host. This provides a rationale for the necessity of controlling the optimal amount and the structural incorporation of dopant in order to maximize the thermoelectric response of organic materials.

2018 - Hybridized electronic states between CdSe nanoparticles and conjugated organic ligands: A theoretical and ultrafast photo-excited carrier dynamics study [Articolo su rivista]
Virgili, Tersilla; Calzolari, Arrigo; Suárez López, Inma; Ruini, Alice; Catellani, Alessandra; Vercelli, Barbara; Tassone, Francesco

Formation of densely packed thin films of semiconductor nanocrystals is advantageous for the exploitation of their unique optoelectronic properties for real-world applications. Here we investigate the fundamental role of the structure of the bridging ligand on the optoelectronic properties of the resulting hybrid film. In particular, we considered hybrid films formed using the same CdSe nanocrystals and two organic ligands that have the same bidentate dithiocarbamate binding moiety, but differ in their bridging structures, one bridged by ethylene, the other by phenylene that exhibits conjugation. Based on the results of photo-excited carrier dynamics experiments combined with theoretical calculations on the electronic states of bridged CdSe layers, we show that only the phenylene-based ligand presents a strong hybridization of the molecular HOMO state with CdSe layers, that is a marker of formation of an effective bridge. We argue that this hybridization spread favors the hopping of photo-excited carriers between nanocrystals, which may explain the reported larger photo-currents in phenylene-based hybrid films than those observed in ethylene-based ones.

2017 - Hybridized electronic states between CdSe nanoparticles and conjugated organic ligands: A theoretical and ultrafast photo-excited carrier dynamics study [Articolo su rivista]
Virgili, Tersilla; Calzolari, Arrigo; Suárez López, Inma; Ruini, Alice; Catellani, Alessandra; Vercelli, Barbara; Tassone, Francesco

Formation of densely packed thin films of semiconductor nanocrystals is advantageous for the exploitation of their unique optoelectronic properties for real-world applications. Here we investigate the fundamental role of the structure of the bridging ligand on the optoelectronic properties of the resulting hybrid film. In particular, we considered hybrid films formed using the same CdSe nanocrystals and two organic ligands that have the same bidentate dithiocarbamate binding moiety, but differ in their bridging structures, one bridged by ethylene, the other by phenylene that exhibits conjugation. Based on the results of photo-excited carrier dynamics experiments combined with theoretical calculations on the electronic states of bridged CdSe layers, we show that only the phenylene-based ligand presents a strong hybridization of the molecular HOMO state with CdSe layers, that is a marker of formation of an effective bridge. We argue that this hybridization spread favors the hopping of photo-excited carriers between nanocrystals, which may explain the reported larger photo-currents in phenylene-based hybrid films than those observed in ethylene-based ones. [Figure not available: see fulltext.]

2017 - New energy with ZnS: Novel applications for a standard transparent compound [Articolo su rivista]
D'Amico, Pino; Calzolari, Arrigo; Ruini, Alice; Catellani, Alessandra

We revise the electronic and optical properties of ZnS on the basis of first principles simulations, in view of novel routes for optoelectronic and photonic devices, such as transparent conductors and plasmonic applications. In particular, we consider doping effects, as induced by Al and Cu. It is shown that doping ZnS with Al imparts a n-character and allows for a plasmonic activity in the mid-IR that can be exploited for IR metamaterials, while Cu doping induces a spin dependent p-type character to the ZnS host, opening the way to the engineering of transparent p-n junctions, p-type transparent conductive materials and spintronic applications. The possibility of promoting the wurtzite lattice, presenting a different symmetry with respect to the most stable and common zincblende structure, is explored. Homo- and heterojunctions to twin ZnO are discussed as a possible route to transparent metamaterial devices for communications and energy.

2017 - Probing optical excitations in chevron-like armchair graphene nanoribbons [Articolo su rivista]
Denk, Richard; Lodi-Rizzini, Alberto; Wang, Shudong; Hohage, Michael; Zeppenfeld, Peter; Cai, Jinming; Fasel, Roman; Ruffieux, Pascal; Berger, Reinhard Franz Josef; Chen, Zongping; Narita, Akimitsu; Feng, Xinliang; Müllen, Klaus; Biagi, Roberto; De Renzi, Valentina; Prezzi, Deborah; Ruini, Alice; Ferretti, Andrea; Prezzi, Deborah

The bottom-up fabrication of graphene nanoribbons (GNRs) has opened new opportunities to specifically tune their electronic and optical properties by precisely controlling their atomic structure. Here, we address excitation in GNRs with periodic structural wiggles, the so-called chevron GNRs. Based on reflectance difference and high-resolution electron energy loss spectroscopies together with ab initio simulations, we demonstrate that their excited-state properties are of excitonic nature. The spectral fingerprints corresponding to different reaction stages in their bottom-up fabrication are also unequivocally identified, allowing us to follow the exciton build-up from the starting monomer precursor to the final GNR structure

2017 - Spectroscopic identification of the chemical interplay between defects and dopants in Al-doped ZnO [Articolo su rivista]
Benedetti, S.; Valenti, I.; di Bona, A.; Vinai, G.; Castan-Guerrero, C.; Valeri, S.; Catellani, A.; Ruini, A.; Torelli, P.; Calzolari, A.

The conduction and optoelectronic properties of transparent conductive oxides can be largely modified by intentional inclusion of dopants over a very large range of concentrations. However, the simultaneous presence of structural defects results in an unpredictable complexity that prevents a clear identification of chemical and structural properties of the final samples. By exploiting the unique chemical sensitivity of Hard X-ray Photoelectron Spectra and Near Edge X-ray Absorption Fine Structure in combination with Density Functional Theory, we determine the contribution to the spectroscopic response of defects in Al-doped ZnO films. Satellite peaks in O1s and modifications at the O K-edge allow the determination of the presence of H embedded in ZnO and the very low concentration of Zn vacancies and O interstitials in undoped ZnO. Contributions coming from substitutional and (above the solubility limit) interstitial Al atoms have been clearly identified and have been related to changes in the oxide stoichiometry and increased oxygen coordination, together with small lattice distortions. In this way defects and doping in oxide films can be controlled, in order to tune their properties and improve their performances.

2017 - Thermoelectric figure of merit of polymeric systems for low-power generators [Articolo su rivista]
Cigarini, Luigi; Ruini, Alice; Catellani, Alessandra; Calzolari, Arrigo

The request of thermoelectric materials for low-power and flexible applications fosters the investigation of the intrinsic electron and thermal transport of conducting polymeric chains, which are building blocks of the complex variety of organic composites proposed in experimental samples. Using calculations from first principles and the Landauer approach for both electron and phonon carriers, we study the thermoelectric figure of merit zT of three representative and largely used polymer chains, namely poly(3,4-ethylenedioxythiophene), polyaniline and polyfluorene. Our results provide an upper-limit estimate of zT, due to the intrinsic electronic and vibrational properties of the selected compounds, and pave the way to a microscopic understanding of the mechanisms that affect their electronic and transport characteristics in terms of structural distortions and chemical doping.

2016 - Accurate ab initio tight-binding Hamiltonians: Effective tools for electronic transport and optical spectroscopy from first principles [Articolo su rivista]
D'Amico, Pino; Agapito, Luis; Catellani, Alessandra; Ruini, Alice; Curtarolo, Stefano; Fornari, Marco; Nardelli, Marco Buongiorno; Calzolari, Arrigo

The calculations of electronic transport coefficients and optical properties require a very dense interpolation of the electronic band structure in reciprocal space that is computationally expensive and may have issues with band crossing and degeneracies. Capitalizing on a recently developed pseudoatomic orbital projection technique, we exploit the exact tight-binding representation of the first-principles electronic structure for the purposes of (i) providing an efficient strategy to explore the full band structure E-n (k), (ii) computing the momentum operator differentiating directly the Hamiltonian, and (iii) calculating the imaginary part of the dielectric function. This enables us to determine the Boltzmann transport coefficients and the optical properties within the independent particle approximation. In addition, the local nature of the tight-binding representation facilitates the calculation of the ballistic transport within the Landauer theory for systems with hundreds of atoms. In order to validate our approach we study the multivalley band structure of CoSb3 and a large core-shell nanowire using the ACBN0 functional. In CoSb3 we point the many band minima contributing to the electronic transport that enhance the thermoelectric properties; for the core-shell nanowire we identify possible mechanisms for photo-current generation and justify the presence of protected transport channels in the wire.

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

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.

2015 - Optoelectronic properties and color chemistry of native point defects in Al:ZnO transparent conductive oxide [Articolo su rivista]
Catellani, Alessandra; Ruini, Alice; Calzolari, Arrigo

We present a first principles study on the effect of native point defects in Al:ZnO transparent conductive oxide. Our results indicate that oxygen and zinc vacancies play two completely different roles:the former maintain the electrical properties while worsening the transparency of native Al:ZnO. The latter are strong electron acceptors that can destroy the metal-like conductivity of the system. While the percentage of doping amount is not really relevant, the compensation ratio between Zn vacancies and Al dopants is crucial for the final electrical properties of the system. H impurities always act as electron donors and generally improve the characteristics of the transparent conductor. Finally, we show how the chemistry of the defects affects the color of Al:ZnO samples, in agreement with experimental results. Our results pave the way to defect engineering for the growth of high performance transparent conductive oxides.

2015 - Unconventional co-existence of plasmon and thermoelectric activity in In:ZnO nanowires [Articolo su rivista]
Catellani, Alessandra; Ruini, Alice; Nardelli, Marco Buongiorno; Calzolari, Arrigo

Metal-doped ZnO nanowires exhibit the unique property of being simultaneously thermoelectric transparent conductors and low-loss plasmonic materials in the near-IR and visible range. Using calculations from first principles, we identify the mechanisms that regulate this behavior at the nanoscale and we describe how nanostructuring affects the optoelectronic, vibrational and transport properties of In: ZnO nanowires. Our results reveal that In doping imparts a good electrical conductivity and provides an injected free charge sufficient to sustain a surface-plasmon-polariton excitation. At the same time, surface scattering effects efficiently quench the thermal conductivity along the wire, improving the thermoelectric figure of merit of the system with respect to the bulk material. The coexistence of plasmonic and thermoelectric characteristics fosters the design of a novel class of coupled nanostructured devices for photothermal-electrical energy conversion.

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

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

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 - Effect of ultrathin gold on the Ohmic-to-Schottky transition in Al/ZnO contacts: A first-principles investigation [Articolo su rivista]
Catellani, Alessandra; Calzolari, Arrigo; Ruini, Alice

By using a first principles approach based on Density Functional Theory, we present a study of the manipulation of the Al/ZnO contact, which may be turned from Ohmic to Schottky by simply including an ultrathin Au interlayer at the interface. To understand the rationale behind this contact design, we first characterize the original Al/ZnO interface, which results to be Ohmic, irrespective of the substrate termination and of eventual n-doping. Then, we identify the mechanisms that regulate the contact behavior switch upon gold insertion by highlighting the different charge transfer processes that take place at the interface.

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

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 - Nitrocatechol/ZnO Interface: The Role of Dipole in a Dye/Metal-Oxide Model System [Articolo su rivista]
G. F. Arnaud; Valentina De Renzi; Umberto del Pennino; R. Biagi; V. Corradini; A. Calzolari; A. Ruini; A. Catellani

The electronic properties of a prototype system suitable for dye-sensitized solar cell applications are investigated both experimentally and theoretically by means of electron spectroscopies (high-resolution electron energy loss spectroscopy, HREELS, and ultraviolet and X-ray photoemission spectroscopies, UPS and XPS) and first-principles density functional theory (DFT)-based calculations. The comparison of HREELS and UPS data with the DFT results allows the microscopic description of electronic structure modifications upon interface formation, and provides a quantitative evaluation of the ionization energy and electron affinity changes induced by functionalization: these variations can be associated to the electric dipole of the functional species and, thus, to the formation of an interface dipole layer.

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

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 - Transparent Conductive Oxides as Near-IR Plasmonic Materials: The Case of Al-Doped ZnO Derivatives [Articolo su rivista]
Calzolari, A; Ruini, Alice; Catellani, A.

Using first-principles calculations, we investigate the origin of near infrared plasmonic activity in Al:ZnO transparent conducting oxides. Our results predict realistic values for the plasma frequency and the free electron density as a function of the Al doping and in agreement with recent experimental results. We also provide a microscopic insight on the formation of surface-plasmon polaritons at the Al:ZnO/ZnO interfaces in terms of characteristic lengths that can be measured by experiments. The direct comparison with standard plasmonic metals underlines the promising capabilities of transparent conducting oxides as compact and low-loss plasmonics materials for optoelectronic applications and telecommunications.

2013 - Charge Separation in the Hybrid CdSe Nanocrystal–Organic Interface: Role of the Ligands Studied by Ultrafast Spectroscopy and Density Functional Theory [Articolo su rivista]
Tersilla Virgili;Arrigo Calzolari;Inmaculada Suárez López;Barbara Vercelli;Gianni Zotti;Alessandra Catellani;Alice Ruini;Francesco Tassone

We present a joint experimental and theoretical study of the early stage dynamics of photoexcited charges in a prototypical organic/inorganic interface. By using femtosecond pump probe experiments we compared the photophysic of a layer-by-layer hybrid structure obtained by alternating CdSe nanocrystals and poly(p-styrenesulphonic acid) and the same CdSe nanocrystals capped with hexadec-ylamine and stearic acid diluted in solutions. While in the LBL structure it is dear the appearance of a long-lived charged state, no evidence of this is instead found in the diluted solutions. Density functional calculations indicate that these states are localized dose to the nanoparticle surface, and that electrons and holes are separated across the hybrid interface, pointing out the effects of surfactant capping molecules on the optoelectronic properties of the interface. Our combined approach, allowing for unique access to the photoexcited electronic structure, opens the possibility to the fine tailoring of hybrid organic/semiconducting layers for photovoltaic applications.

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

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 - First principles description of the electronic properties of doped ZnO [Articolo su rivista]
Alessandra, Catellani; Ruini, Alice; Giancarlo, Cicero; Arrigo, Calzolari

We here review how the most common intrinsic defects and dopants modify the electronic properties of ZnO, providing novel results obtained by means of accurate first principles density functional calculations. In particular, we show that interstitial H is responsible for the residual and hardly eliminable n-type character of real ZnO samples. We also show the effects of controlled doping with metal ions to obtain both transparent conductive oxides (Al, Ga, In) and p-type materials (Ag). We demonstrate that Ag inclusions can be linked to the presence of deep acceptors in the host band gap that may cancel the p-type character in Ag-doped ZnO.

2013 - Fluorine-induced enhancement of the oxidation stability and deep-blue optical activity in conductive polyfluorene derivatives [Articolo su rivista]
Calzolari, Arrigo; Vercelli, Barbara; Ruini, Alice; Virgili, Tersilla; Pasini, Mariacecilia

We present a joint experimental/theoretical study on the effects of fluorination on structural, electronic, and optical properties of poly[(9,9-di-n-octylfluoren-2,7-diyl)-alt-tetrafluoro-p-phenylene] (PFTFP), a polyfluorene (PFO)-derived conjugated polymer. The combination of optical (UV-vis) and electrochemical (cyclic voltammetry) techniques with atomistic simulations demonstrates an improved oxidative stability of the fluorinated compound with respect to standard PFO. The resulting excellent luminescence efficiency along with the preservation of the good charge mobility, characteristic of the pristine PFO, make PFTFP a superior material for optoelectronic applications in the deep-blue. The comparison with auxiliary model systems provides a microscopic identification of the peculiar effects of fluorine on the structural and electronic properties of the polymer. © 2013 American Chemical Society.

2013 - Oxygen deficient centers in silica: optical properties within many-body perturbation theory [Articolo su rivista]
N Richard;L Martin-Samos;S Girard;A Ruini;A Boukenter;Y Ouerdane;J-P Meunier

The electronic and optical properties of neutral oxygen vacancies, also called oxygen deficient centers (ODC(I)s), have been investigated in pure and germanium doped silica (both amorphous and alpha-quartz) through first-principles calculations. By means of density functional theory and many-body perturbation theory (GW approximation and the solution of the Bethe-Salpeter equation), we obtain the atomic and electronic structures as well as the optical absorption spectra of pure and Ge-doped silica in the presence of ODCs (SiODC(I) s and GeODC(I) s); our study allows us to interpret and explain the very nature of the optical features in experimental absorption spectra. The theoretical optical absorption signatures of these defects show excellent agreement with experiments for the SiODC(I) s, i.e. two absorption bands arise around 7.6 eV due to transitions between the defect levels. Our theoretical results also explain the experimental difficulty in measuring the GeODC(I) absorption band in Ge-doped silica, which was in fact tentatively assigned to a broad and very weak absorption signature, located between 7.5 and 8.5 eV. The influence of Ge-doping induced disorder on the nature of the defect-related optical transitions is discussed. We find that even if the atomic and electronic structures of SiODC(I) and GeODC(I) defects are relatively similar, the slight network distortion induced by the presence of the Ge atom, together with the increase in the Ge-Si bond asymmetry, completely changes the nature of the optical absorption edge.

2012 - Ab initio complex band structure of conjugated polymers: Effects of hydrid density functional theory and GW schemes [Articolo su rivista]
A. Ferretti; G. Mallia; L. Martin-Samos; G. Bussi; A. Ruini; B. Montanari;N. M. Harrison

The nonresonant tunneling regime for charge transfer across nanojunctions is critically dependent on the so-called beta parameter, governing the exponential decay of the current as the length of the junction increases. For periodic materials, this parameter can be theoretically evaluated by computing the complex band structure (CBS)-or evanescent states-of the material forming the tunneling junction. In this work we present the calculation of the CBS for organic polymers using a variety of computational schemes, including standard local, semilocal, and hybrid-exchange density functionals, and many-body perturbation theory within the GW approximation. We compare the description of localization and beta parameters among the adopted methods and with experimental data. We show that local and semilocal density functionals systematically underestimate the beta parameter, while hybrid-exchange schemes partially correct for this discrepancy, resulting in a much better agreement with GW calculations and experiments. Self-consistency effects and self-energy representation issues of the GW corrections are discussed together with the use of Wannier functions to interpolate the electronic band structure.

2012 - Electronics and Optics of Graphene Nanoflakes: Edge Functionalization and Structural Distortions [Articolo su rivista]
C. Cocchi; D. Prezzi; A. Ruini; M. J. Caldas; E. Molinari

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 - 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: 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 - Surface Effects on Catechol/Semiconductor Interfaces [Articolo su rivista]
A. Calzolari; A. Ruini; A. Catellani

We present a density functional investigation of the surface effects on the catechol sensitization of selected hexagonal semiconductors (SiC, GaN, InN, CdS, CdSe). The atomic relaxation, the ionicity, and the reactivity, which characterize the selected substrates, are found to crucially influence both the bonding geometry and the electronic level alignment at the interface. Our results indicate that surface effects must be considered in order to obtain a correct picture of the optoelectronic response of the system. Our findings pave the way to the fundamental understanding and future design of hybrid catecholate materials for optoelectronic and biomedical applications.

2011 - Anchor Group versus Conjugation: Toward the Gap-State Engineering of Functionalized ZnO(10(1)over-bar0) Surface for Optoelectronic Applications [Articolo su rivista]
A., Calzolari; Ruini, Alice; A., Catellani

Molecular sensitization of the single-crystal ZnO (10 (1) over bar0) surface through absorption of the catechol chromophore is investigated by means of density functional approaches. The resulting type II staggered interface is recovered in agreement with experiments, and its origin is traced back to the presence of molecular-related states in the gap of metal oxide electronic structure. A systematic analysis carried out for further catecholate adsorbates allows us to identify the basic mechanisms that dictate the energy position of the gap states. The peculiar level alignment is demonstrated to be originated from the simultaneous interplay among the specific anchoring group, the backbone conjugation, and the lateral functional groups. The picture derived from our results provides efficient strategies for tuning the lineup between molecular and oxide states in hybrid interfaces with potential impact for ZnO-based optoelectronic applications.

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

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

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

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; A. Ruini; E. Molinari; M. J. Caldas

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.

2010 - Hydration of cyanin dyes [Articolo su rivista]
A., Calzolari; S., Monti; Ruini, Alice; A., Catellani

We investigated the hydration properties of the cyanin dye molecule in the ionic flavylium configuration, through massive classical (force field) and ab initio (Car-Parrinello) molecular dynamics simulations at room temperature. Classical and quantum mechanical results coherently describe the structure of the first solvation shell. We discuss the hydrophobicity/hydrophilicity of the molecule in terms of attractive lateral hydroxyl-water and repulsive carbon pi-water interactions. The analysis of the electronic structure shows a net polarization and a molecular orbital redistribution induced by the polar solvent on the intrinsic (gas phase) properties of the dye. Changing the properties of the molecule, the hydration effects should be carefully taken into account in the further interactions of cyanin with the external environment.

2010 - Substitutional Impurities in PPV Crystals: An Intrinsic Donor-Acceptor System for High V-oc Photovoltaic Devices [Articolo su rivista]
A., Calzolari; Ruini, Alice; C., Cavazzoni; M. J., Caldas

Defects are usually present in organic polymer films and are commonly invoked to explain the low efficiency obtained in organic-based optoelectronic devices. We propose that controlled insertion of substitutional impurities may, on the contrary, tune the optoelectronic properties of the underivatized organic material and, in the case studied here, maximize the efficiency of a solar cell. We investigate a specific oxygen-impurity substitution, the keto-defect -(CH2-C=O)- in underivatized crystalline poly(p-phenylenevinylene) (PPV), and its impact on the electronic structure of the bulk film, through a combined classical (force-field) and quantum mechanical (DFT) approach. We find defect states which suggest a spontaneous electron hole separation typical of a donor acceptor interface, optimal for photovoltaic devices. Furthermore, the inclusion of oxygen impurities does not introduce defect states in the gap and thus, contrary to standard donor-acceptor systems, should preserve the intrinsic high open circuit voltage (V-oc) that may be extracted from PPV-based devices.

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

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

2009 - Optoelectronic Properties of Natural Cyanin Dyes [Articolo su rivista]
A., Calzolari; D., Varsano; Ruini, Alice; A., Catellani; R., Tel Vered; H. B., Yildiz; O., Ovits; I., Willner

An integrated theoretical/experimental study of the natural cyanin dye is presented in terms of its structural and optoelectronic properties for different gas-phase and prototypical device configurations. Our microscopic analysis reveals the impact of hydration and hydroxylation reactions, as well as of the attached sugar, on ground and optically excited states, and it illustrates the visible-light harvesting capability of the dye. Our optical experiments at different and controlled pH concentrations allow for a direct comparison with theoretical results. We analyze the many different contributions to photocurrent of the various portions of a prototypical device and, as a proof of principle, we propose the addition of specific ligands to control the increase of the photocurrent yield in the cyanin-based electrochemical device.

2008 - Competitive chemisorption of bifunctional carboxylic acids on H : Si(100): A first-principles study [Articolo su rivista]

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 - Defect-induced effects on carrier migration through one-dimensional poly(para-phenylenevinylene) chains [Articolo su rivista]
Zoppi, L; Calzolari, A; Ruini, Alice; Ferretti, A; Caldas, M. J.

Defects in one-dimensional (1D) systems can be intrinsically distinct from its three-dimensional counterparts, and polymer films are good candidates for showing both extremes that are difficult to individuate in the experimental data. We study theoretically the impact of simple hydrogen and oxygen defects on the electron transport properties of one-dimensional poly(para-phenylenevinylene) chains through a multiscale technique, starting from classical structural simulations for crystalline films to extensive ab initio calculations within density functional theory for the defects in single crystalline-constrained chains. The most disruptive effect on carrier transport comes from conjugation breaking imposed by the overcoordination of a carbon atom in the vinyl group independently from the chemical nature of the defect. The particular case of the [C=O] (keto-defect) shows in addition unexpected electron-hole separation, suggesting that the experimentally detected photoluminescence bleaching and photoconductivity enhancement could be due to exciton dissociation caused by the 1D characteristics of the defect.

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

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

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.

2007 - Mixing of electronic states in pentacene adsorption on copper [Articolo su rivista]

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 properties of one-dimensional graphene polymers: the case of polyphenanthrene [Articolo su rivista]
D. Prezzi; D. Varsano; A. Ruini; A. Marini; E. Molinari

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 - 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.

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).

2006 - Ab initio exploration of rearrangement reactions: Intramolecular hydrogen scrambling processes in acetone [Articolo su rivista]

The recently developed metadynamics method is applied to the intramolecular hydrogen migration reactions of acetone in the gas phase. Comparison of different sets of collective coordinates allows efficient description of the underlying free energy surface. The simulations yielded numerous reactions: the enol-oxo tautomerism, the decomposition of acetone to various products, and rearrangement reactions. On the basis of the calculated activation barriers it is concluded that the enol-oxo tautomerism is the most frequent intramolecular proton-exchange process the acetone undergoes in the gas phase.

2006 - Ab initio molecular dynamics study of the keto-enol tautomerism of acetone in solution [Articolo su rivista]
C. S., Cucinotta; Ruini, Alice; A., Catellani; A., Stirling

We have studied the keto-enol interconversion of acetone to understand the mechanism of tautomerism relevant to numerous organic and biochemical processes. Applying the ab initio metadynamics method, we simulated the keto-enol isomerism both in the gas phase and in the presence of water. For the gas-phase intramolecular mechanism we show that no other hydrogen-tronsfer reactions can compete with the simple keto-enol tautomerism. We obtain an intermolecular mechanism and remarkable participation of water when acetone is solvated by neutral water. The simulations reveal that C deprotonation is the kinetic bottleneck of the keto-enol transformation, in agreement with experimental observations. The most interesting finding is the formation of short H-bonded chains of water molecules that provide the route for proton transfer from the carbon to the oxygen atom of acetone. The mechanistic picture that emerged from the present study involves proton migration and emphasizes the importance of active solvent participation in tautomeric interconversion.

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.

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 investigation of functionalization-defects on silicon surfaces [Articolo su rivista]
C.S. Cucinotta; B. Bonferroni; A. Ferretti; A. Ruini; M.J. Caldas; E. Molinari

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 - Surface nano-patterning through styrene adsorption on Si(100) [Articolo su rivista]

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

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.

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

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

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 - 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

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-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

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 - 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

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

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 - Tailoring the electronic properties of silicon with cysteine: A first principle study [Articolo su rivista]
Cucinotta, C. S.; Ruini, Alice; Catellani, A.; Caldas, M. J.

We discuss the electronic structure modifications induced on the dihydride-terminated Si(001) surface upon cysteine adsorption by means of ab initio calculations: several stable functionalization schemes are presented, providing different routes for biological recognition, surface nanostructuring, and biomolecular electronics applications. The resulting hybrid systems are discussed and compared in terms of stability, structural, and electronic properties. Based on our results, we propose STM and photoemission experiments to determine unambiguously the adsorption mechanism involved and the attached functional group.

2004 - Ab initio optical absorption in conjugated polymers: the role of dimensionality, [Relazione in Atti di Convegno]

The optical behavior of poly(para-phenylene vinylene) is explored through an ab initio scheme based on the Bethe-Salpeter equation, where the electron-hole interaction is included on top of a density functional theory calculation. Results for different solid-state packings are reviewed, demonstrating that the details of crystalline arrangement dramatically alter the optical properties and lead to a rich excitonic structure, where also charge-transfer states appear (electron and hole on different chains). Moreover, for a typical herringbone packing the excitonic state of the isolated molecule splits in two direct components (with electron and hole on the same chain), one for each non-translationally-invariant chain in the unit cell, and the optical inactivity of the lowest component can crucially quench the luminescence efficiency. Besides the far-field absorption spectra and the description of the excitonic states, a formalism to simulate the near-field spectra is presented that allows one to detect also excitonic states that are dipole-forbidden in the far-field spectra.

2004 - Ab initio study of transport parameters in polymer crystals [Articolo su rivista]
Ferretti A; Ruini A; Bussi G; Molinari E; Caldas MJ

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

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

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

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.

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.

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 Inter-chain Interactions [Articolo su rivista]
Ferretti, Andrea; Ruini, Alice; Molinari, Elisa; M. J., Caldas

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

The sensitivity of interchain interactions to the specific three-dimensional (3D) structure was demonstrated. It was confirmed that the interactions can be used to tailor the transport properties of conjugated-polymer films.

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

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 - Interchain interaction and Davydov splitting in polythiophene crystals: An ab initio approach [Articolo su rivista]

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

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 - Solid State Effects on Exciton States and Optical Properties of PPV [Articolo su rivista]

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

2001 - Ab-initio study of Coulomb-correlated optical properties in conjugated polymers [Articolo su rivista]

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.

1998 - Dynamical-charge neutrality at a crystal surface [Articolo su rivista]
Ruini, Alice; Resta, R.; AND BARONI, S.

For both molecules and periodic solids, ionic dynamical charge tensors-measuring the coupling of electric fields to ionic displacements-are known to obey a dynamical neutrality condition. This condition forces their sum to vanish over the whole finite system, or over the crystal cell, respectively. We extend this sum rule to the nontrivial case of the surface of a semi-infinite solid and show that, in the case of a polar surface of an insulator, the surface ions cannot have the same dynamical charges as in the bulk. The sum rule is demonstrated through calculations for a couple of SiC surfaces.

1997 - Effects of interface morphology on Schottky-barrier heights: A case study on Al/GaAs(001) [Articolo su rivista]
Ruini, Alice; Resta, R.; AND BARONI, S.

The problem of Fermi-level pinning at semiconductor-metal contacts is readdressed starting from first-principles calculations for Al/GaAs. We give quantitative evidence that the Schottky barrier height is very little affected by any structural distortions on the metal side-including elongations of the metal-semiconductor bond (i.e., interface strain)-whereas it strongly depends on the interface structure on the semiconductor side. A rationale for these findings is given in terms of the interface dipole generated by the ionic effective charges.

1995 - Dynamics of electrons in a 2D region coming from a point-contact [Articolo su rivista]

Ballistic and quasi-ballistic transport in mesoscopic systems is, nowadays, a fundamental tool for the investigation of electronic processes in semiconductors. In this work we present some results concerning a numerical simulation of electrons entering a 2D mesoscopic region from a point contact; a magnetic field is applied perpendicular to the structure and influences the electron dynamics. The simulation is performed through a numerical solution of the Schroedinger equation in a finite-difference scheme. It includes a magnetic field and an arbitrary potential V(r). In this way, the quantum effects of impurities on the conductance of the system have been analysed. The resul shows that each impurity configuration characterizes, in a particular way, the transport properties.