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Dipartimento di Scienze Fisiche, Informatiche e Matematiche sede ex-Fisica
Docente a contratto
Dipartimento di Ingegneria "Enzo Ferrari"

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2023 - A HydroDynamic Model for Trap-Assisted Tunneling Conduction in Ovonic Devices [Articolo su rivista]
Buscemi, F; Piccinini, E; Vandelli, L; Nardi, F; Padovani, A; Kaczer, B; Garbin, D; Clima, S; Degraeve, R; Kar, Gs; Tavanti, F; Slassi, A; Calzolari, A; Larcher, L

Electrical conduction in ovonic threshold switching (OTS) devices is described by introducing a new physical model where the multiphonon trap-assisted tun-neling (TAT) is coupled to a hydrodynamic theory. Static and transient electrical responses from Ge(x)Se(1-x )experimental devices are reproduced, outlining the role played by the material properties like mobility gap and defects in tuning the OTS performances. A clear physical interpretation of the mechanisms ruling the different OTS conduction regimes (off, threshold, on) is presented. A nanoscopic picture of the processes featuring the carrier transport is also given. The impact of geometry, temperature, and material mod-ifications on device performance is discussed providing physical insight into the optimization of OTS devices.

2023 - Device‐to‐Materials Pathway for Electron Traps Detection in Amorphous GeSe‐Based Selectors [Articolo su rivista]
Slassi, Amine; Medondjio, Linda‐sheila; Padovani, Andrea; Tavanti, Francesco; He, Xu; Clima, Sergiu; Garbin, Daniele; Kaczer, Ben; Larcher, Luca; Ordejón, Pablo; Calzolari, Arrigo

The choice of the ideal material employed in selector devices is a tough task both from the theoretical and experimental side, especially due to the lack of a synergistic approach between techniques able to correlate specific material properties with device characteristics. Using a material-to-device multiscale technique, we propose a reliable protocol for an efficient characterization of the active traps in amorphous GeSe chalcogenide. The resulting trap maps trace back the specific features of materials responsible for the measured findings, and connect them to an atomistic description of the sample. Our metrological approach can be straightforwardly extended to other materials and devices, which is very beneficial for an efficient material-device co-design and the optimization of novel technologies.

2022 - Multifunctional Switch Based on Spin-Labeled Gold Nanoparticles [Articolo su rivista]
Lloveras, Vega; El('(i))as-Rodr('(i))guez, Pilar; Bursi, Luca; Shirdel, Ehsan; Go(~(n))i, Alejandro R.; Calzolari, Arrigo; Vidal-Gancedo, Jos('(e))

: The fabrication of multifunctional switches is a fundamental step in the development of nanometer-scale molecular spintronic devices. The anchoring of active organic radicals on gold nanoparticles (AuNPs) surface is little studied and the realization of AuNPs-based switches remains extremely challenging. We report the first demonstration of a surface molecular switch based on AuNPs decorated with persistent perchlorotriphenylmethyl (PTM) radicals. The redox properties of PTM are exploited to fabricate electrochemical switches with optical and magnetic responses, showing high stability and reversibility. Electronic interaction between the radicals and the gold surface is investigated by UV-vis, showing a very broad absorption band in the near-infrared (NIR) region, which becomes more intense when PTMs are reduced to anionic phase. By using multiple experimental techniques, we demonstrate that this interaction is likely favored by the preferentially flat orientation of PTM ligands on the metallic NP surface, as confirmed by first-principles simulations.

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.

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).

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 - 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

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.

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

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.

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.

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 - 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

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 - Nitrocatechol/ZnO Interface: The Role of Dipole in a Dye/Metal-Oxide Model System [Articolo su rivista]
Arnaud, Gaelle Francoise; DE RENZI, Valentina; DEL PENNINO, Umberto; Biagi, Roberto; Corradini, Valdis; Calzolari, Arrigo; Ruini, Alice; 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.

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.

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

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.

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 - Polarization properties of (1-100) and (11-20) SiC surfaces from first principles [Articolo su rivista]
Brandino, Gp; Cicero, G; Bonferroni, Benedetta; Ferretti, Andrea; Calzolari, Arrigo; Bertoni, Carlo Maria; Catellani, A.

We report on first-principles density functional calculations of nonpolar low-index surfaces of hexagonal silicon carbide. We provide an accurate analysis of the macroscopic bulk spontaneous polarization as a function of the hexagonality of the compound, and we describe in detail the electronic and structural properties of the relaxed surfaces. We revise the methodology to achieve a detailed description of the surface polarization effects. Our results on low-index surfaces reveal a strong in-plane polar contribution, opposing the spontaneous polarization field present in hexagonal polytypes. This in-plane surface polarization component has not been considered before, although it is of significant impact in adsorption experiments, affecting functionalization and growth processes, as well as the electronic properties of confined, low-dimensional systems.

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).

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

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.

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

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

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.

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.

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.

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

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

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.

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

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.