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The structural and electronic properties of SiC-based two-dimensional (2D) crystals are studied by means of density functional theory and many-body perturbation theory. Such properties cannot simply be interpolated between graphene and silicene. The replacement of half of the C atoms by Si atoms opens a large direct electronic gap and destroys the Dirac cones. Hydrogenation further opens the gap and significantly reduces the electron affinity to 0.1 or 1.8 eV in dependence on the carbon or silicon termination of the 2D crystal surface, thus showing a unique direction dependent ionization potential. This suggests the use of 2D-SiC:H as electron or hole filter.

We show by first-principles calculations that, due to depressed screening and enhanced two-dimensional confinement, excitonic resonances with giant oscillator strength appear in hydrogenated Si and Ge layers, which qualitatively and quantitatively differ from those of graphane. Their large exciton binding energies and oscillator strengths make them promising for observation of novel physical effects and application in optoelectronic devices on the nanoscale.

We present ab initio quantum calculation of the optical properties of formamide in vapor phase and in water solution. We employ time dependent density functional theory for the isolated molecule and many-body perturbation theory methods for the system in solution. An average over several molecular dynamics snapshots is performed to take into account the disorder of the liquid. We find that the excited state properties of the gas-phase formamide are strongly modified by the presence of the water solvent: the geometry of the molecule is distorted and the electronic and optical properties are severely modified. The important interaction among the formamide and the water molecules forces us to use fully quantum methods for the calculation of the excited state properties of this system. The excitonic wave function is localized both on the solute and on part of the solvent.

The Si(111)2$\ifmmode\times\else\texttimes\fi{}$1 is among the most investigated surfaces. Nonetheless, several issues are still not understood. Its reconstruction is well explained in terms of the Pandey model with a slight buckling (tilting) of the topmost atoms; two different isomers of the surface, conventionally named positive and negative buckling, exist. Usually, scanning tunneling microscopy (STM) experiments identify the positive buckling isomer as the stable reconstruction at room temperature. However, at low temperatures and for high $n$ doping of the substrate, recent scanning tunneling spectroscopy (STS) measurements found the coexistence of positive and negative buckling on the Si(111)2$\ifmmode\times\else\texttimes\fi{}$1 surface. In this work, state-of-the-art ab initio methods, based on density functional theory and on many-body perturbation theory, have been used to obtain structural, electronic, and optical properties of Si(111)2$\ifmmode\times\else\texttimes\fi{}$1 positive and negative buckling. The theoretical reflectance anisotropy spectra (RAS), with the inclusion of the excitonic effects, can provide a way to deepen the understanding of the coexistence of the isomers.

The Si(111)2×1 is one of the most studied surfaces. Its reconstruction is described by the Pandey model with a buckling of the topmost atoms. With relation to the sign of the buckling, there are two slightly different geometric structures (isomers), conventionally named positive buckling and negative buckling. STM measurements suggest that the positive buckling isomer is the stable configuration at room temperature, but a recent work, involving STS measurements, has shown the coexistence of both the isomers, at very low temperature, for highly n-doped Si(111)2×1 specimens. There is hence the necessity to deepen the study of the negative buckling isomer, almost completely neglected in literature especially for what concerns its optical properties. In this work we have studied the structural, electronic and optical properties of both the isomers of Si(111)2×1 within the Density Functional Theory in the Local Density Approximation. Our results show that the response to light of the two isomers is sizearly different; Reflectance Anisotropy Spectroscopy would be the right experimental tool to investigate the coexistence of the two isomers.

The yellowing of paper on aging causes major aesthetic damages of cultural heritage. It is due to cellulose oxidation, a complex process with many possible products still to be clarified. By comparing ultraviolet-visible reflectance spectra of ancient and artificially aged modern papers with ab initio time- dependent density functional theory calculations, we identify and estimate the abundance of oxidized functional groups acting as chromophores and responsible of paper yellowing. This knowledge can be used to set up strategies and selective chemical treatments preventing paper yellowing.

In this paper, we give an overview of the state of the art in calculations of the electronic band structure and absorption spectra of water. After an introduction to the main theoretical and computational schemes used, we present results for the electronic and optical excitations of water. We focus mainly on liquid water, but spectroscopic properties of ice and vapor phase are also described. The applicability and the accuracy of first-principles methods are discussed, and results are critically presented.

We apply reflection anisotropy spectroscopy (RAS) to high-quality atomically smooth H/C(100) $2\ifmmode\times\else\texttimes\fi{}1$ surfaces. The optical signal of the H/C(100) ${0}^{\ifmmode^\circ\else\textdegree\fi{}}$ flat surface, and of the H/C(100) ${2}^{\ifmmode^\circ\else\textdegree\fi{}}$ and H/C(100) ${4}^{\ifmmode^\circ\else\textdegree\fi{}}$ vicinal surfaces, is investigated in terms of single and double steps. A comparison of experimental and theoretical results obtained from ab initio calculations shows that, in the energy range considered (1--5 eV), the RAS response can be interpreted in terms of single height ${S}_{B}$ and double height ${D}_{A}$ steps.

A long-standing puzzle regarding the $\mathrm{Si}(111)\ensuremath{-}2\ifmmode\times\else\texttimes\fi{}1$ surface has been solved. The surface energy gap previously determined by photoemission on heavily $n$-doped crystals was not compatible with a strongly bound exciton known from other considerations to exist. New low-temperature angle-resolved photoemission and scanning tunneling microscopy data, together with theory, unambiguously reveal that isomers with opposite bucklings and different energy gaps coexist on such surfaces. The subtle energetics between the isomers, dependent on doping, leads to a reconciliation of all previous results.

We investigate the optical properties of flavin mononucleotide (FMN) as chromophore of a bacterial protein. FMN is studied both in the oxidized and anionic state, by combining hybrid quantum mechanics/molecular mechanics (QM/MM) dynamics with time-dependent density functional theory (TDDFT). Our first-principles calculations show that the inclusion of the protein biological environment is crucial to reproduce the experimental optical spectra.

The effect of the local fields on the absorption spectra of silicon nanoclusters (NCs), freestanding or embedded in SiO(2), is investigated in the DFT-RPA framework for different size and amorphization of the samples. We show that local field effects have a great influence on the optical absorption of the NCs. Their effect can be described by two separate contributions, both arising from polarization effects at the NC interface. First, local fields produce a reduction of the absorption that is stronger in the low energy limit. This contribution is a direct consequence of the screening induced by polarization effects on the incoming field. Secondly, local fields cause a blue shift on the main absorption peak that has been explained in terms of perturbation of the absorption resonance conditions. Both contributions do not depend either on the NC diameter nor on its amorphization degree, while showing a high sensitivity to the environment enclosing the NCs.

We investigate the optical properties of the tZt - penta -3,5- dieniminium cation, a simplified model for the protonated Schiff base of 11- cis retinal in rhodopsin, along the isomerization pathway by ab-initio calculations based on Many-Body Perturbation Theory using the GW method and the Bethe–Salpeter equation. Our calculations are carried out on a few significant CASSCF geometrical configurations of the isomerization minimal energy path taken from the literature. Our excitation energies are qualitatively in agreement with previous Quantum Monte Carlo and post-Hartree–Fock calculations. We also employ TDDFT based methods, and investigate the outcome of using different approximations and several exchange–correlation functionals.

We present an application of time-dependent density-functional theory (TDDFT) to the study of the optical properties of the $\text{Si}(111)2\ifmmode\times\else\texttimes\fi{}1$ surface from the infrared to the ultraviolet. We have carried out ab initio calculations using different methods, from DFT to Bethe-Salpeter equation (BSE) and, within TDDFT, we have tested the ability of different kernels to describe the optical features in a wide range of energies. We find good agreement between TDDFT and BSE results, by using in TDDFT a long-range frequency-dependent exchange-correlation kernel derived from the many-body formalism (the MB kernel). The agreement between theory and experiment is very good in the whole frequency range studied. Excitonic effects, important in the infrared part of the spectrum, are less pronounced in the visible and UV ranges.

International audience; With the aid of ab-initio, parameter free calculations based on densityfunctional and many-body perturbation theory, we investigate the electronic band structure and electron affinity of diamond surfaces. We focus on clean, ideal (001) and (111) surfaces, and on the effect of hydrogen adsorption. Also single sheets of graphane, that is graphene functionalized upon hydrogen, are investigated. At full H-coverage nearly-free electron states appear near the conduction band minimum in all the systems under study. At the same time, the electron affinity is strongly reduced becoming negative for the hydrogenated diamond surfaces, and almost zero in graphane. The effects of quasi-particle corrections on the electron affinity and on the nearly free electron states are discussed. The fascinating physics of carbon surfaces: first-principles study of hydrogen on C(001), C(111), and grap

The case study of a 32-atoms Si nanocrystallite (NC) embedded in a SiO 2 matrix, both crystalline and amorphous, or free-standing with different conditions of passivation and strain is analyzed through ab-initio approaches. The Si 32 /SiO 2 heterojunction shows a type I band offset highlighting a separation between the NC plus the interface and the matrix around. The consequence of this separation is the possibility to correctly reproduce the low energy electronic and optical properties of the composed system simply by studying the suspended NC plus interface oxygens with the appropriate strain. Moreover through the definition of an optical absorption threshold we found that, beside the quantum confinement trend, the amorphization introduces an additional redshift that increases with increasing NC size, i.e. the gap tends faster to the bulk limit. Finally, the important changes in the calculated DFT-RPA optical spectra upon inclusion of local fields point towards the need of a proper treatment of the optical response of the interface region.

We present ellipsometry data of the dielectric function of wurtzite ZnO in a wide energy range (2.5-32 eV). The ordinary and extraordinary components show a strong anisotropy above 10 eV, a feature for which ZnO deviates from the other II-VI wurtzite compounds. With the aid of ab initio calculations, performed within many-body perturbation theory (MBPT) and within time-dependent density-functional theory (TDDFT), we analyze the origin of the measured optical structures. TDDFT, with the use of a static long-range exchange-correlation kernel, proves to be a cheaper computational tool than MBPT to yield a good description of the whole spectrum. Theoretical results for the zinc-blende phase are also presented.

First-principles calculations within density functional theory and many-body perturbation theory have been carried out in order to investigate the structural, electronic and optical properties of undoped and doped silicon nanostructures. We consider Si nanoclusters co-doped with B and P. We find that the electronic band gap is reduced with respect to that of the undoped crystals, suggesting the possibility of impurity based engineering of electronic and optical properties of Si nanocrystals. Finally, motivated by recent suggestions concerning the chance of exploiting Ge dots for photovoltaic nanodevices, we present calculations of the electronic and optical properties of a Ge35H36 nanocrystal, and compare the results with those for the corresponding Si35H36 nanocrystals and the co-doped Si33BPH36.

Actually, most of the electric energy is being produced by fossil fuels and great is the search for viable alternatives. The most appealing and promising technology is photovoltaics. It will become truly mainstream when its cost will be comparable to other energy sources. One way is to significantly enhance device efficiencies, for example by increasing the number of band gaps in multijunction solar cells or by favoring charge separation in the devices. This can be done by using cells based on nanostructured semiconductors. In this paper, we will present ab-initio results of the structural, electronic and optical properties of (1) silicon and germanium nanoparticles embedded in wide band gap materials and (2) mixed silicon-germanium nanowires. We show that theory can help in understanding the microscopic processes important for devices performances. In particular, we calculated for embedded Si and Ge nanoparticles the dependence of the absorption threshold on size and oxidation, the role of crystallinity and, in some cases, the recombination rates, and we demonstrated that in the case of mixed nanowires, those with a clear interface between Si and Ge show not only a reduced quantum confinement effect but display also a natural geometrical separation between electron and hole.

The nature of the alpha -Sn/Ge(111) surface is still a matter of debate. In particular, two possible configurations have been proposed for the 3× 3 ground state of this surface: one with two Sn adatoms in a lower position with respect to the third one (1U2D) and the other with opposite configuration (2U1D). We report ab initio GW calculations for the alpha-Sn/Ge(111) system, simulating STM images as a function of bias voltage and comparing them with STM experimental results at 78 K. The concurrent application of theory and experiments and the very good match between their results provide unambiguous indications that the stable configuration for the alpha-Sn/Ge(111) surface is the 1U2D. The possible inequivalence of the two down Sn adatoms is also discussed.

Density-functional and many body perturbation theory calculations have been carried out in order to study the optical properties both in the ground and excited state configurations, of silicon nanocrystals in different conditions of surface passivation. Starting from hydrogenated clusters, we have considered different Si/O bonding geometries at the interface. We provide strong evidence that not only the quantum confinement effect but also the chemistry at the interface has to be taken into account in order to understand the physical properties of these systems. In particular, we show that only the presence of a surface Si-O-Si bridge bond induces an excitonic peak in the emission-related spectra, redshifted with respect to the absorption onset, able to provide an explanation for both the observed Stokes shift and the near-visible PL experimentally observed in Si-nc. For the silicon nanocrystals embedded in a SiO matrix, the optical properties are discussed in detail. The strong interplay between the nanocrystal and the surrounding host environment and the active role of the interface region between them is pointed out, in very good agreement with the experimental results. For each system considered, optical gain calculations have been carried out giving some insights on the system characteristics necessary to optimize the gain performance of Si-nc. To cite this article: E. Degoli et al., C. R. Physique 10 (2009). © 2008 Académie des sciences.

We compare, through first-principles pseudopotential calculations, the structural, electronic, and optical properties of different size silicon nanoclusters embedded in a ${\text{SiO}}_{2}$ crystalline or amorphous matrix with that of freestanding, hydrogenated, and hydroxided silicon nanoclusters of corresponding size and shape. We find that the largest effect on the optoelectronic behavior is due to the amorphization of the embedded nanocluster. In that, the amorphization reduces the fundamental gap while increasing the absorption strength in the visible range. Increasing the nanocluster size does not change substantially this picture but only leads to the reduction in the absorption threshold, following the quantum confinement rule. Finally, through the calculation of the optical absorption spectra both in an independent-particle and a many-body approach, we show that the effect of local fields is crucial for describing properly the optical behavior of the crystalline case while it is of minor importance for amorphous systems.

Optical properties of aromatic chromophores are used to probe complex biological processes, yet how the environment tunes their optical properties is far from being fully understood. Here we present a method to calculate such properties on large-scale systems, like biologically relevant molecules in aqueous solution. Our approach is based on many-body perturbation theory combined with a quantum mechanics/molecular mechanics (QM/MM) approach. We include quasiparticle and excitonic effects for the calculation of optical absorption spectra in a QM/MM scheme. We apply this scheme, together with the well-established TDDFT approach, to indole in water solution. Our calculations show that the solvent induces a red shift in the main spectral peak of indole, in quantitative agreement with the experiments, and they point to the relevance of both the electrostatic and geometrical origin of the shift.

Despite the simplicity of the system, the description of the excited-state properties of the cleavage surface of diamond requires the use of nonstandard treatment of many-body effects. Using a simple tight-binding model, we show that the typical assumption concerning the identity of quasiparticle and Kohn-Sham wave functions fails in an important part of the Brillouin zone. This simple model allows computation of the qualitatively different quasiparticle wave functions. The optical properties calculated with these wave functions at the Bethe-Salpeter level show important excitonic effects. Moreover the line shape of the reflectance anisotropy spectrum is significantly improved when compared to the measured one.

International audience; The interaction between small unsaturated hydrocarbon molecules of C2H4 with a vicinal silicon 001 surface is studied by means of reflectance anisotropy spectroscopy and analyzed with first principles calculations. Our results confirm that ethylene adsorbs without breaking the silicon dimers. Comparison of theoretical optical spectra with experimental data shows that the C2H4 molecules lay on top of the silicon dimers from low to high coverage. This occurs even though, from a purely energetic point of view, a bridge configuration would be favorable at 1 monolayer coverage.

Total energy calculations within the Density Functional Theory have been carried out in order to investigate the structural, electronic, and optical properties of un-doped and doped silicon nano-structures of different size and different surface terminations. In particular the effects induced by the creation of an electron-hole pair on the properties of hydrogenated silicon nanoclusters as a function of dimension are discussed in detail showing the strong interplay between the structural and optical properties of the system. The distortion induced on the structure by an electronic excitation of the cluster is analyzed and considered in the evaluation of the Stokes shift between absorption and emission energies. Besides we show how many-body effects crucially modify the absorption and emission spectra of the silicon nanocrystals. Starting from the hydrogenated clusters, different Si/O bonding at the cluster surface have been considered. We found that the presence of a Si—O—Si bridge bond originates significative excitonic luminescence features in the near-visible range. Concerning the doping, we consider B and P single- and co-doped Si nanoclusters. The neutral impurities formation energies are calculated and their dependence on the impurity position within the nanocrystal is discussed. In the case of co-doping the formation energy is strongly reduced, favoring this process with respect to the single doping. Moreover the band gap and the optical threshold are clearly red-shifted with respect to that of the pure crystals showing the possibility of an impurity based engineering of the absorption and luminescence properties of Si nanocrystals.

Electronic transitions involving Zn-3d and O-2s bands are detected.Ab-initio band structure calculations performed at the DFT-LDA level help tointerpret the observed transitions. The calculations are also extended to theGW approximation in order to determine the electronic bandgap. Finally, theplasmon frequency is found to be ~!

The absorption and emission spectra of silicon nanocrystals up to 1 nm diameter including geometry optimization and the many-body effects induced by the creation of an electron-hole pair have been calculated within a first-principles framework. Starting from hydrogenated silicon clusters of different size, different Si/O bonding at the cluster surface have been considered. We found that the presence of a Si-O-Si bridge bond originates significative excitonic luminescence features in the visible range that are in fair agreement with the experimental outcomes., Comment: 12 pages 4 figures

The (1 1 1) face is the cleavage surface of diamond. Understanding its properties is very important for the growing technological interest on the chemistry of diamond surfaces. Within DFT the most stable reconstruction is the Pandey chain model, the atoms on the chain being neither buckled nor dimerised. However this geometry gives rise to a semi metallic band structure in contrast with experimental findings that show the presence of a gap ranging from 0.5 to 2 eV. Here we show that the same equilibrium geometry and thus the same metallic band structure is found relaxing the surface using screened exchange (sX) or Hartree-Fock (HF) functionals. We will discuss in detail how breaking the equivalence of the atoms on the chain affects the band structure and we will show that a buckling would yield a semiconducting surface, but is energetically unfavorable. A semiconducting character can be restored, within the equilibrium geometry, if quasiparticle corrections are carefully included within an iterative GW scheme. The result of the theoretical reflectance anisotropy spectra (RAS) at a DFT-RPA level are also presented and discussed. As expected, a strong anisotropy signal is found at low energies due to transitions between surface states inside the fundamental gap. (C) 2007 Elsevier B.V. All rights reserved.

Low dimensional systems, such as nanodots, nanotubes, nanowires, have attracted great interest in the last years, due to their possible application in nanodevices. It is hence very important to describe accurately their electronic and optical properties within highly reliable and efficient ab-initio approaches. Density functional theory (DFT) has become in the last 20 years the standard technique for studying the ground-state properties, but this method often shows significant deviations from the experiment when electronic excited states are involved. The use of many-body Green's functions theory, with DFT calculations taken as the zero order approximation, is today the state-of-the-art technique for obtaining quasi-particle excitation energies and optical spectra. In this paper we will present the current status of this theoretical and computational approach, showing results for different kinds of low dimensional systems. (C) 2007 Elsevier B.V. All rights reserved.

The absorption and the emission spectra of undoped and doped silicon nanocrystals of different size and surface terminations have been calculated within a first-principles framework. The effects induced by the creation of an electron-hole pair on the atomic structure and on the optical spectra of hydrogenated silicon nanoclusters as a function of dimension are discussed showing the strong interplay between the structural and optical properties of the system. Starting from the hydrogenated clusters, (i) different Si/O bonding at the cluster surface and (ii) different doping configurations have been considered. We have found that the presence of a Si-O-Si bridge bond at the nanocrystal surface gives rise to significant excitonic luminescence features in the near-visible range that are in fair agreement with photoluminescence (PL) measurements on oxidized and SiO$_{2}$ embedded nanocrystals. The study of the structural, electronic and optical properties of simultaneously n- and p-type doped hydrogenated silicon nanocrystals with boron and phosphorous impurities have shown that B-P co-doping is energetically favorable with respect to single B- or P-doping and that the two impurities tend to occupy nearest neighbors sites. The co-doped nanocrystals present band edge states localized on the impurities that are responsible of a red-shifted absorption threshold with respect to that of pure un-doped nanocrystals in agreement with the experiment.

We extend the three-layer model for energy loss calculation to include many-body effects in an ab initio framework. The electron energy loss spectrum of the $\mathrm{C}(100)2\ifmmode\times\else\texttimes\fi{}1$ surface is calculated and compared with the existing experimental results. We show how many-body effects, namely self-energy, local-fields, and electron-hole interaction, deeply influence the dielectric response and how their inclusion is essential to have good agreement with the experiment. A strong anisotropic behavior of local-field effects on the dielectric response has been observed, while the inclusion of the electron-hole attractive interaction produces a surface-state exciton with binding energy of about $1\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, confirming a recent theoretical-experimental study of the optical spectra of this surface.

A Density Functional Theory calculation of the geometrical and electronic structure of the alpha-phase of Sn/Ge(111) surface is presented. The 3 x 3 and root 3 x root 3 reconstructions are studied in details, performing also band structure calculations and simulating STM images. Our results confirm that the IU-2D 3 x 3 structure is the more favorable one, from an energetic point of view but also from a comparison with the available photoemission and STM experiments. The electronic band structure of the flat root 3 x root 3 Surface, instead, hardly compares with the available photoemission data, thus supporting the idea of a dynamical flipping at room temperature of the Sit adatoms.

We present ab initio calculations of the excited state properties of liquid water in the framework of many-body Green's function formalism. Snapshots taken from molecular dynamics simulations are used as input geometries to calculate electronic and optical spectra, and the results are averaged over the different configurations. The optical absorption spectra with the inclusion of excitonic effects are calculated by solving the Bethe-Salpeter equation. The insensitivity of screening effects to a particular configuration make these calculations feasible. The resulting spectra, which are strongly modified by many-body effects, are in good agreement with experiments.

We present a first-principles calculation of the geometry and of the electronic structure for the C1112 1 surface. We find that this surface reconstructs with -bonded chains without any significant dimerization or buckling. At the DFT level, it appears semimetallic, in agreement with previous calculations, but in contrast to experimental evidence. Even the introduction of quasi-particle corrections within the usual GW scheme does not lead to the opening of the gap between surface states. Quasiparticle corrections to the surface band structure are hence calculated within a self-consistent GW scheme. A gap of about 1 eV is found between the surface states, thus finally solving the discrepancy between theory and experiment. The 111 face is the cleavage surface of diamond. Understanding its behavior is important from a technological point of view, the 111 plane is one of the growth surfaces for the chemical vapor deposition CVD of diamond, and from a theoretical point of view, in order to elucidate the principles of surface reconstructions in the other group IV semiconductors. Many potential applications of diamond arise from specific properties of its surfaces: while its negative electron affinity and unique surface conductivity make it attractive for electronic and optoelectronic devices, 1 its extreme surface chemical inertness makes it a good candidate for biomedical applications reducing the chances of rejection; moreover, boron-doped diamond electrodes have been used in water purification and in the detection of many substances, espe

The electronic structure of the $\mathrm{InAs}(110)$ surface is investigated using several theoretical tools: semiempirical tightbinding, density functional theory, and perturbative many-body theory within the GW approximation. Comparison is made with available photoemission data. Moreover, we calculate the optical properties of this surface and perform reflectance anisotropy experiments. The tight-binding and especially the GW approximations provide a very good description of the optical spectra and allow for a more detailed analysis of the origin of the spectral features.

We compare the results of ab initio calculations with measured reflection anisotropy spectra and show that strongly bound surface-state excitons occur on the clean diamond (100) surface. These excitons are found to have a binding energy close to 1 eV, the strongest ever observed at a semiconductor surface. Important electron-hole interaction effects on the line shape of the optical transitions above the surface-state gap are also found.

The structural, optical and electronic properties of silicon nanocrystals are investigated as a function of the dimension as well as the surface passivation. Both the ground‐ and an excited‐state configuration are studied using ab‐initio calculations. Atom relaxation under excitation is taken into account and related with the experimentally observed Stokes shift.

Theoretical approaches for ab initio studies of the electronic and optical properties of matter are here reviewed. Examples within Density Functional Theory, Many-Body perturbation Theory and Time Dependent Density Functional Theory are presented and discussed, pointing out advantages and drawbacks of the different schemes.

We compare the results of ab initio calculations with measured reflection anisotropy spectra and show that strongly bound surface-state excitons occur on the clean diamond (100) surface. These excitons are found to have a binding energy close to 1 eV, the strongest ever observed at a semiconductor surface. Important electron-hole interaction effects on the line shape of the optical transitions above the surface-state gap are also found.

The experimental characterization of surfaces is often obtained through spectro scopic techniques like photoemission inverse photoemission electron energy loss spectroscopy and optical techniques as Reflectance Anisotropy and Differential Reflectivity etc It is hence very important to describe accurately the electronic excitations with highly reliable and efficient ab-initio approaches In the last two decades Density Functional Theory has proven to be a very powerful tool for elec tronic ground state properties but shows significant deviations from the expen ments when excited states are involved The use of many body Green’s functions theory, with DFT calculations as zero order approximation is now the state of the art to obtam quasi particle excitation energies and optical spectra both in bulk and at surfaces In this paper we will present the current status of this theoretical and computational approach showing results for several semiconductor surfaces

We present calculated and measured reflectance anisotropy spectra (RAS) in the energy range from 1 to 6 eV of cleaved GaAs(110) surfaces covered with one monolayer of As and Sb in a (1 × 1) pattern. The spectral range and the accuracy of the data were improved and correlated for the first time with ab initio calculations of RAS spectra for the ECLS (epitaxial continued layer structure) and EOTS (epitaxial on top structure) surface models. The theoretical spectra for the two models completely differ and rule out the EOTS for both adsorbates. For Sb/GaAs(110) this finding agrees with the previous experimental and theoretical results reported on the structure. For As on GaAs(110) the ECLS structure was also suggested, but so far no direct proof for this model has been given. In this paper we show how RAS, thanks to its sensitivity to details of the surface structure and ab initio theoretical description, demonstrates its potential to conclusively determine surface structures.

We perform first-principles total-energy and electronic structure calculations in order to determine the atomic structure of the P-rich $(2\ifmmode\times\else\texttimes\fi{}1)/(2\ifmmode\times\else\texttimes\fi{}2)$ reconstruction commonly observed for GaP(001) surfaces grown by gas-phase epitaxy. A monolayer of buckled phosphorus dimers stabilized by hydrogen adsorbed in an alternating sequence is found to be energetically stable and to be consistent with the experimental data.

We combine low-energy electron diffraction and reflectance anisotropy spectroscopy (RAS) with ah initio calculations of the geometry, band structure, and optical anisotropy to investigate the adsorption of Sb on vicinal Si(001) (1×2). We focus, in particular, on the controversy concerning the Si(001)-(I ×1)-Sb surface. On the basis of total-energy and band-structure calculations, we find that the Sb-undimerised model is unstable and metallic, while experimentally the (1×1) Sb shows no evidence of a Fermi edge. In contrast, the dimerised (2 X 1)-Sb and c(2×2)-Sb reconstructions are found to be semiconducting with a minimal difference in total energy. Furthermore, the RAS spectra calculated for both dimerised reconstructions show strong similarities to one another, and agree well with the experimental RAS data for the Sb-induced (1×1)-Sb surface, with a dominant feature centered at 3.7 eV. We report that these findings are compatible with the (1×1)-Sb surface comprising a mixture of the Sb-dimer-terminated (2×1)-Sb and c-(2×2)-Sb structures.

We demonstrate the potential of recently developed total-energy and electronic-structure methods for the calculation of the optical properties of real surfaces. The many-body effects are fully taken into account by a solution of the combined Dyson and Bethe-Salpeter equations. We show that an initial-value formulation of the polarization function allows an efficient numerical calculation of the optical susceptibility for large slabs consisting of many atoms. As examples we investigate GaP(001) and Si(001) surfaces covered by hydrogen. In the case of P-rich GaP(001)2 x 2-H surfaces the low-energy region of the reflectance anisotropy (RA) is dominated by electron-hole pair excitations in surface states. Surface-induced modifications of bulk excitons near the E-1 and E-2 transitions are responsible for the RA of the monohydride Si(001)2 x 1-H surface. (C) 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.