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In this work, we predict and analyze the optical spectra of pyrimidinic uracil-like nucleobases thymine (THY), uracil (URA), and 5-fluorouracil (5-FU) and their reflection anisotropy spectra (RAS) upon adsorption on the silicon (001) surface. First-principles results based on plane-wave density functional theory show chemically sensitive features in gas phase optical absorption spectra that redshift/blueshift according to the orbitals involved in the corresponding transition. In the RAS, a characteristic lineshape is found, typical of the energetically favored "dimer bridge" configuration, and remarkably similar for all the investigated Si(001):X systems (X = THY, URA, 5-FU). We show that molecular tilting and breaking of the glide plane symmetry have a negligible effect on the optical spectra, despite their influence on the surface bandstructure. Contrarily to gas phase spectra, chemically sensitive RAS features only appear above 4.5 eV, and can be recognized as molecular contributions consistent with gas phase optical absorption results whereas substrate effects dominate at lower energies. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

We study the electronic properties of the Si(001):Uracil, Si(001):Thymine, and Si(001):5-Fluorouracil systems, focusing on the Si dimer-bridging configuration with adsorption governed by carbonyl groups. While the overall structural and electronic properties are similar, with small differences due to chemical substitutions, much larger effects on the surface band dispersion and bandgap show up as a function of the molecular orientation with respect to the surface. An off-normal orientation of the molecular planes is favored, showing larger bandgap and lower total energy than the upright position. We also analyze the localization of gap-edge occupied and unoccupied surface states.

Oligoacenes in their crystalline state are increasingly used in the field of electronics and photonics. Following a recent systematic theoretical study of the five smallest oligoacene molecules (naphthalene, anthracene, tetracene, pentacene, and hexacene), we evaluate their one- and two-particle properties (electron affinities, ionisation energies, quasiparticle correction to the highest occupied molecular orbital—lowest unoccupied molecular orbital gap, exciton binding energy, electronic absorption), using different exchange-correlation functionals in the framework of Density Functional Theory (DFT) and Time Dependent DFT. The hybrid functional B3LYP is found to yield the best agreement with the available experimental data.

One- and two-particle effects in the electronic and optical spectra of the fluoride compound BaF2 are determined using density functional theory and a many-body perturbation scheme. A wide energy range has been considered, including the visible and all the ultraviolet region. The GW approximation for the electronic self-energy has been used to tackle the one-particle excitations problem, enabling us to determine the electronic energy bands and densities of states of this fluoride. For the optical properties, the two-particle effects calculated with the Bethe-Salpeter scheme turn out to play a fundamental role. A bound exciton positioned at about 1.5 eV below the one-particle gap is forecasted. The optical absorption and the electron energy loss spectra together with other optical functions are in good agreement with the experimental results up to 15 eV. In fact, for this part of the spectrum a self-consistent one-particle scheme along with the Bethe-Salpeter approach produces notable results. Less satisfactory results for the higher energy region in the spectra have been produced with the proposed method. Possible causes of these discrepancies are fully discussed.

Electronic ground-state properties and quasiparticle energies of the charge-transfer insulating oxides SrO and MgO have been computed. Structural properties calculations have been performed within DFT–LDA, while for the electronic excitations an efficient DFT–GW scheme, founded on the use of a model dielectric function, has been used. The comparison of our results with available theoretical and experimental data turns out to be satisfactory either for the ground state or for the excitation spectrum.

Relaxations at nonpolar surfaces of semiconductor III-V compounds result from a competition between dehybridization and charge transfer. First-principles calculations for the (110) and (10$1\ifmmode\bar\else\textasciimacron\fi{}$0) faces of zinc-blende and wurtzite AlN, GaN, and InN reveal an anomalous behavior as compared with ordinary III-V semiconductors. Additional calculations for GaAs and ZnO suggest close analogies with the latter. We interpret our results in terms of the larger ionicity (charge asymmetry) and bonding strength (cohesive energy) in the nitrides with respect to other III-V compounds, both essentially due to the strong valence potential and absence of p core states in the lighter anion. The same interpretation applies to Zn II-VI compounds.

Electronic excitations and optical spectra of $CdF_{2}$ are calculated up to ultraviolet employing state-of-the-art techniques based on density functional theory and many-body perturbation theory. The GW scheme proposed by Hedin has been used for the electronic self-energy to calculate single-particle excitation properties as energy bands and densities of states. For optical properties many-body effects, treated within the Bethe-Salpeter equation framework, turn out to be crucial. A bound exciton located about 1 eV below the quasiparticle gap is predicted. Within the present scheme the optical absorption spectra and other optical functions show an excellent agreement with experimental data. Moreover, we tested different schemes to obtain the best agreement with experimental data. Among the several schemes, we suggest a self-consistent quasiparticle energy scheme., Accepted on PRB

We compare models of diagonal and off-diagonal screening with LDA-RPA full calculations in Si and cubic GaN. Simplified GW calculations relying on these models are also compared with full GW calculations for the same materials. A recipe for simplified GW calculations is obtained which works well for small, moderate and wide gap semiconductors.

An efBcient method developed for the calculation of quasiparticle corrections to densityfunctional-theory — local-density-approximation (DFT-LDA) band structures of diamond and zincblende materials is generalized for crystals with other cubic, hexagonal, tetragonal, and orthorhombic Bravais lattices. Local-field efFects are considered in the framework of a LDA-like approximation. The dynamical screening is treated by expanding the self-energy linearly in energy. The anisotropy of the inverse dielectric matrix is taken into account. The singularity of the Coulomb potential in the screened-exchange part of the electronic self-energy is treated using auxiliary functions of the appropriate symmetry. An application to the electronic quasiparticle band structure of wurtzite 2HSiC is presented within the approach of norm-conserving, nonlocal, fully separable pseudopotentials and a plane-wave expansion of the wave functions for the underlying DFT-LDA. Electronic-structure calculations for solids using density-functional theory (DFT) within the local-density approximation (LDA) are not capable of yielding reliable excitation energies. For semiconductors this fact is known as the so-called energy band gap problem. The correction to the Kohn-Sham eigenvalues giving the accurate quasiparticle (QP) energies of the crystal electrons may be obtained by first-order many-body perturbational theory, with respect to the difference between the exchange-correlation (XC) self-energy and the corresponding XC potential of the DFT-LDA. Thereby the self-energy is treated within the GTV approximation of Hedin. One of the crucial points in such calculations is the evaluation of the self-energy operator, which may be numerically very expensive, because of the required knowledge of the full inverse dielectric matrix of the system and the careful treatment of the singularity, due to the Coulomb potential entering the screenedexchange (SX) contribution to this operator. In order to decrease this remarkable numerical effort, one may follow the efBcient calculational scheme introduced by Cappellini et al. , 4 for semiconductors with diamond or zinc-blende structure. The local-field contributions to the screened Coulomb potential are described within a LDA-like treatment. The electron density governing the screening properties is replaced by state-averaged values of the local density. The dynamics of the screening is considered by a linear expansion of the self-energy operator around the Kohn-Sham eigenvalues. A substantial reduction of the computational effort arises from the use of a model dielectric function. This gives rise to an analytical representation of the static Coulomb-hole (COB) contribution to the self-energy. The Coulomb singularity in its SX part is treated according to the notion of Gygi

An analytical model for the static screened Coulomb interaction of a semiconductor sphere that accurately reproduces the screening properties of an icosahedral C 60 cluster is presented. Compared with the static screened Coulomb interaction determined after polarizability calculations it yields the same value for nearest-neighbor interaction energy and the same antiscreening effect for distant sites for points located on the nuclear grid of n free molecule. Also the static dipole molecular polarizability turns out to be in good agreement with that calculated after different theoretical methods. When extended to finite frequencies, without introducing other parameters, the present model reproduces the main resonance around 20 eV of the optical absorption spectrum for the gas phase C 60 and the giant plasmon resonance around 28 eV observed in C 60 fullerites via electron energy loss spectroscopy. These results turn out to be largely insensitive to the details of the model

The electronic structure of fluorite crystals are studied by means of density functional theory within the local density approximation for the exchange correlation energy. The ground-state electronic properties, which have been calculated for the cubic structures $CaF_{2}$,$SrF_{2}$, $BaF_{2}$, $CdF_{2}$, $HgF_{2}$, $\beta $-$PbF_{2}$, using a plane waves expansion of the wave functions, show good comparison with existing experimental data and previous theoretical results. The electronic density of states at the gap region for all the compounds and their energy-band structure have been calculated and compared with the existing data in the literature. General trends for the ground-state parameters, the electronic energy-bands and transition energies for all the fluorides considered are given and discussed in details. Moreover, for the first time results for $HgF_{2}$ have been presented.

We present our ab initio calculation of linear optical properties in cubic (c-BN) and layered hexagonal BN (h-BN). Our results show a reasonable agreement with existent theoretical results and with measured data for the imaginary part of the dielectric function in the case of h-BN, while for c-BN discrepancies arise between theory and experiment. We discuss the possible reasons of this mismatch.

We present a method for the efficient calculation of the electronic structure of semiconductors within the GW approach. It approximately includes dynamical-screening and local-field effects, previously disregarded in simplified GW approaches, without increasing the computational effort. Such effects substantially affect the gap corrections. We find quasiparticle shifts in good agreement with the complete GW calculations or experiment for Si, AlAs, GaAs and ZnSe.

We discuss and test a combined method to efficiently perform ground- and excited-state calculations for relaxed structures using both a quantum first-principles approach and a classical molecular-dynamics scheme. We apply this method to calculate the ground state, the optical properties, and the electronic excitations of Ge nanoparticles embedded in a cubic SiC matrix. Classical molecular dynamics is used to relax the large-supercell system. First-principles quantum techniques are then used to calculate the electronic structure and, in turn, the electronic excitation and optical properties. The proposed procedure is tested with data resulting from a full first-principles scheme. The agreement is quantitatively discussed between the results after the two computational paths with respect to the structure, the optical properties, and the electronic excitations. The combined method is shown to be applicable to embedded nanocrystals in large simulation cells for which the first-principle treatment of the ionic relaxation is presently out of reach, whereas the electronic, optical and excitation properties can already be obtained ab initio. The errors incurred from the relaxed structure are found to be non-negligible but controllable.

Quasiparticle corrections to the electronic energies have been calculated for small GaP fullerenes, a class of nanoscaled materials recently predicted to be stable. These clusters have been also characterized by us for their optical absorption spectra using time-dependent density functional theory. The comparison between single-particle and optical absorption spectra supports the evidence of strong excitonic effects with bonding energy up to $3.5\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. The quasiparticle corrected highest-occupied-molecular-orbital--lowest-unoccupied-molecular-orbital energy gaps confirm the high stability predicted for such molecules using ground-state computational schemes. The present results will be useful to identify the successful synthesis of such systems via optical absorption and quasiparticle spectra.

We present state of the art first-principles calculations of optical spectra and the loss function of bulk cubic boron nitride $(c\text{\ensuremath{-}}\mathrm{B}\mathrm{N})$, starting from a density functional Kohn-Sham band structure. We investigate the influence of many-body effects beyond the random phase approximation (RPA) on the optical spectra through the inclusion of self-energy and excitonic effects by a GW calculation and the solution of the Bethe-Salpeter equation. For the loss function we only perform RPA calculations, since Bethe-Salpeter results are already available in the literature. We show to which extent, and in which kind of spectra, the description of many-body effects is important for a meaningful comparison with experiment, and when they can be neglected due to mutual cancellation. We also present results obtained within time-dependent density functional theory, both in the adiabatic local density approximation (TDLDA) and using a recently proposed long-range approximation for the exchange-correlation kernel. Our results show that the latter corrects a big part of the error with respect to RPA or TDLDA; however, the corrections are not sufficient to qualify the method for further quantitative predictions, in particular for the study of the optical gap. In fact, since experiments often quote a relatively low (around $6.4\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$) band gap, whereas the calculated optical absorption spectrum already in the random-phase approximation appears blueshifted by more than $2\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ with respect to the available experimental curve, we study in particular the question of the optical gap in this material. It turns out that, although there is evidence for a weakly bound exciton in $c\text{\ensuremath{-}}\mathrm{B}\mathrm{N}$, the optical gap of pure monocrystalline cubic BN should be around $11\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, hence significantly bigger than has sometimes been quoted from experiments.

We present parameter-free calculations of the quasiparticle band structure of systems described by huge supercells. They are based on a pseudopotential--plane-wave method to calculate the electronic structure in the ground state. All-electron wave functions are constructed using the projector-augmented wave method. The electronic self-energy is calculated within the GW approximation using an efficient approach to the screening. It includes a simplified treatment of dynamical and local-field effects. The approach is carefully tested by computing the quasiparticle band structure of group-IV semiconductors within nonprimitive unit cells containing 216 atoms. The success of the method is demonstrated by the calculation of the electronic structure of Ge and Si nanocrystallites embedded in a SiC matrix.

We present a calculation of the linear optical properties of the nonpolar (110) surface of cubic boron nitride, performed within the first-principle density-functional theory and local-density approximation scheme. The reflectance anisotropy (RA) spectrum is analyzed in relation to the better known spectrum of the GaAs(110) surface and to the existing tight-binding results for GaN(110). Previous results for the RA spectrum of GaN(110) obtained in tight-binding scheme are here confirmed, including a negative structure at the onset. For BN(110) the present first ab initio results show the fundamental role played by surface-states transitions at the onset of the RA spectrum. The origin of the optical structures is studied in comparison with the RA spectra of GaAs(110) and GaN(110), in connection with the differences in ionicity between nitrides and GaAs, and considering the specific relaxation occurring at nitrides (110) surfaces.

Linear optical functions of cubic and hexagonal BN have been studied within first principles density functional theory in the local density approximation. Calculated energy-loss functions show reasonable agreement with experiments and previous theoretical results both for h-BN and for c-BN. Discrepancies arise between theoretical results and experiments in the imaginary part of the dielectric function for c-BN. Possible explanations of this mismatch are proposed and evaluated: lattice constant variations, h-BN contamination in c-BN samples, and self-energy effects.

Since their discovery carbon nanotubes have attracted much interest for their peculiar electronic properties which go from metalic to semiconducting behaviour, depending both on diameter and chirality. The exact vaue of their band gap is obviously a crucial point to be addressed because it enters in the nanotube application as microelectronic devices. By making use of an efficient GW scheme, previousy tested on bulk systems, as well as of a model screening function, we obtained for the first time excitation energies and band-gap vaues for carbon nanotubes. Results for (6,0) and (7,0) will be presented and discussed.

The ab initio pseudopotential method within the local-density approximation and the quasiparticle approach have been used to investigate the electronic excitation properties of hexagonal (6H, 4H, 2H) and zinc-blende (3C) silicon carbide. The quasiparticle shifts added to the density-functional eigenvalues are calculated using a model dielectric function and an approximate treatment of the electron self-energy concerning local-field effects and dynamical screening. The inverse dielectric function and the auxiliary function are generalized to hexagonal crystals. Good agreement with the experimental results is obtained for the minimum indirect energy gaps. The k-space location of the corresponding conduction-band minima is clarified. Other excitation energies are predicted. The infuence of the quasiparticle effects on band discontinuities and the electron effective masses is studied.

We present local density functional theory (DFT-LDA) studies of the structural properties of boron nitride in the layered hexagonal (h-BN), zincblende (c-BN), and wurtzite (w-BN) structures, performed using a fast implementation of the norm-conserving pseud-opotential plane-wave method. Quasiparticle band structures are then calculated for all phases by means of an efficient GW self-energy scheme. To our knowledge, these are the first GW quasiparticle calculations for wurtzite BN including local-field and dynamical screening effects. DFT-LDA band gaps as functions of pressure and uniaxial distorsion for h-BN are also discussed.

We present a model for the dielectric function of semiconductors. It has been tested successfully for Si, Ge, GaAs, and ZnSe. In conjunction with the single plasmon-pole approximation it yields plasmon-energy dispersions in fair agreement with experiments. It allows one, moreover, to deduce an analytical expression for the Coulomb-hole part of the static self-energy operator.

We present first-principles calculated electronic and optical properties of some SiGe alloys. The ground-state, electronic excitations and optical properties have been calculated with Ge and Si atoms arranged in different ways among the sites of a diamond-type lattice. For the ground state a DFT-LDA scheme and for the electronic excitations a DFT-GW approach have been respectively used. For the optical properties the DFT-LDA-RPA scheme has been applied for alloys going in composition from Si(100%) to Ge(100%): obtained results have been compared with existing experimental and theoretical data. For the noticeable Si(50%)Ge(50%) alloy also two-particle effects have been evaluated using the Bethe-Salpeter equation.

The electronic excitation energies of boron nitride in the layered hexagonal (h-BN), zincblende (c-BN), and wurtzite (w-BN) structures under applied pressure and lattice stretching have been studied. Hydrostatic pressure applied on denser phases c-BN and w-BN has been considered, on the other hand for h-BN both hydrostatic pressure and compression along the c-axis have been studied. Pressure- and strain-dependent electronic excitation energies have been calculated using a DFT-GW method including quasiparticle corrections in GW approximation. The model we introduced to reproduce the dielectric screening of the system under study depends both on lattice parameters of the crystal and on its dielectric constant. Excitation spectra for the three phases under positive or negative stresses up to ∼ 0.01 Mbar show the same fundamental gap ordering found at equilibrium. On the other hand for h-BN our DFT-GW results predict, at about 19% c-axis enlarging, a minimal gap crossover that may play an important role when the material is intercalated.