1,721,026 research outputs found
Optoelectronic properties and color chemistry of native point defects in Al:ZnO transparent conductive oxide
Full text linkWe 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
Electronic properties of guanine-based nanowires
No full textWe 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
Optical properties of anthocyanins in the gas phase
No full textThe gas-phase optical properties of the six most common anthocyanins are studied using time-dependent density-functional theory. Different anthocyanins are classified into three groups, according to the number of low-frequency peaks displayed in the UV-vis spectrum. This behavior is analyzed in terms of one-electron transitions and interaction effects, the latter being rationalized using a suitable double-pole model. Moving from PBE to hybrid exchange-correlation functionals results in a hypsochromic shift of the optical gap. While the colors thus predicted do not quite match those observed in solution, thus highlighting the importance of solvation effects, adoption of hybrid functionals remarkably determines a greater chromatic uniformity of different molecules, in qualitative agreement with experimental evidence in acidic solutions
Ab-Initio Study of Model Guanosine Assemblies: the Role of pi-pi Coupling and Band Transport
No full textSeveral 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
Unconventional co-existence of plasmon and thermoelectric activity in In:ZnO nanowires
No full textMetal-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
Electron channels in biomolecular nanowires
No full textWe 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
Multimodel Approach to the Optical Properties of Molecular Dyes in Solution
No full textWe introduce a multimodel approach to the simulation of the optical properties of molecular dyes in solution, whereby the effects of thermal fluctuations and of dielectric screening on the absorption spectra are accounted for by explicit and implicit solvation models, respectively. Thermal effects are treated by averaging the spectra of molecular configurations generated, by an ab initio molecular-dynamics simulation where solvent molecules are treated explicitly. Dielectric effects are then dealt with implicitly by computing the spectra upon removal of the solvent molecules and their replacement with an effective medium, in the spirit of a continuum solvation model. Our multimodel approach is validated by comparing its, predictions with those of a fully explicit-solvation simulation for cyanidin-3-glucoside (cyanin) chromophore in water. While multimodel and fully explicit-solvent spectra may differ considerably for individual configurations along the trajectory, their time averages are remarkably similar, thus providing a solid benchmark of the former and allowing us to save considerably on the computer resources needed to predict accurate absorption spectra. The power of the proposed methodology is' finally demonstrated by the excellent agreement between its predictions and the absorption spectra of cyanin measured at strong and intermediate acidity conditions.ST
Ab initio study of model guanine assemblies: The role of pi-pi coupling and band transport
No full textSeveral 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.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 (formula presented) orbitals in adjacent guanine molecules. (formula presented) 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. © 2001 The American Physical Society
Fluorine-induced enhancement of the oxidation stability and deep-blue optical activity in conductive polyfluorene derivatives
No full textWe 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
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