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The exotic electrodynamics properties of graphene come from the linearly dispersive electronic bands that host massless Dirac electrons. A similar behavior was predicted to manifest in freestanding silicene, the silicon counterpart of graphene, thereby envisaging a new route for silicon photonics. However, the access to silicene exploitation in photonics was hindered so far by the use of optically inappropriate substrates in experimentally realized silicene. Here we report on the optical conductivity of silicon nanosheets epitaxially grown on optically transparent Al2O3(0001) from a thickness of a few tens of nanometers down to the extreme two-dimensional (2D) limit. When a 2D regime is approached, a Dirac-like electrodynamics can be deduced from the observation of a low-energy optical conductivity feature owing to a silicene-based interfacing to the substrate. © 2018 American Chemical Society.

The accurate calculation of electronic excited states of large and electronically correlated biological chromophores in their complex protein environment still represents a challenge for quantum chemistry. Two ab initio techniques are recently emerging as candidates to correctly tackle this issue: Quantum Monte Carlo (QMC) calculations for the ground state geometry optimization, and Many Body Green's Function Theory (MBGFT) for exited state energies. In the present work we use the Variational Monte Carlo (VMC) to carry out structural optimizations and we present an extension of MBGFT to complex environments, using a Quantum Mechanics/Molecular Mechanics framework. This technique is applied to evaluate the optical properties of the retinal protonated Schiff base (RPSB) chromophore in rhodopsin, the protein responsible for dim light vision in the vertebrates. Vertical energies for the bright excitation of RPSB calculated solving the Bethe-Salpeter equation in gas phase and in the protein environment correspond to 2.19 eV and 2.58 eV, respectively. These data are in fair agreement with the available experimental findings. The comparison between these excitation energies and that obtained for a gas phase calculation on a retinal geometry obtained in the protein environment (2.03 eV) reveals the essential role of the protein field in the spectral tuning of the molecule. The proposed VMC/MBGFT methodologies can therefore be applied to obtain a reliable ab initio evaluation of optical properties of biological chromophores. © 2014 Elsevier B.V. All rights reserved.

In this paper, we present results concerning ab initio calculations of the optical properties of group IV two-dimensional sheets and compare them with the prediction of an analytical model for excitons in two dimensions, which we explicitly derive. This comparison helps to understand the physics at the basis of the strong many-body effects present in the optical spectra of this class of materials. Although it demonstrates the need of full ab initio calculations for an accurate description of the two-dimensional excitons, it validates, at the same time, this simple approach to estimate, through a density functional theory calculation of the two-dimensional electronic and optical properties, the binding energy and the radius of the excitons in this class of materials.

Colloidal nanocrystals electronic energy levels are determined by strong size-dependent quantum confinement. Understanding the configuration of the energy levels of nanocrystal superlattices is vital in order to use them in heterostructures with other materials. A powerful method is reported to determine the energy levels of PbS nanocrystal assemblies by combining the utilization of electric-double-layer-gated transistors and advanced ab-initio theory.

Paper is an important material for many applications. During the centuries, it has been the most widely used writing support and therefore paper degradation is a major issue for cultural heritage. The main component of paper is cellulose, one of the most abundant biomaterials on Earth. Cellulose oxidation is mainly responsible for the yellowing of the ancient samples, through the formation of chromophores. In order to investigate this issue and the chromophores’ role, we exploit the optical properties combining non-destructive experiments and theoretical calculations based on ab-initio techniques. In this paper, we illustrate the method and show its application to three ancient paper samples. The procedure we describe is a precious tool for cultural heritage preservation: indeed, the approach we present is based on non-invasive and non-destructive measurements that allows a microscopic understanding of cellulose-based artifacts degradation.

The relative energy stability between Si(111)2 × 1 positive and negative buckling is still a puzzle. Both isomers possess comparable energies in Density Functional Theory (DFT) calculations and, hence, should occur with the same probability. This is in contrast to experimental findings, which show that the positive buckling is the observed structure at low and room temperature. Here we investigate the role of the exchange-correlation (xc) approximation in DFT functionals. We find that the particular choice of the xc functional does not change the relative stability between the Si(111)2 × 1 isomers. Moreover, we investigate the effect of donor atoms on ground-state and optical properties of both isomers, highlighting the differences with respect to the undoped case for several possible positions of the phosphorus (P) donor atoms. Considering the most stable position of P, relative stability between the isomers is not strongly affected by doping whereas geometric parameters are different with respect to the no doping case. The optical response bears some resemblance with the undoped case but the intensity of reflectance anisotropy peak is decreased for both isomers. © 2013 Elsevier B.V.

We study the electronic properties of two-dimensional (2D) group-III nitrides BN, AlN, GaN, InN, and TlN by first-principles approaches. With increasing group-III atomic number, a decrease of the electronic gap from 6.7 eV to 0 eV takes place. 2D GaN and 2D InN in honeycomb geometry present a direct gap at Γ, while the honeycomb structures of BN and AlN tend to be indirect semiconductors with the valence band maximum at K. Alloying of the nitrides allows tuning the gap with cation composition. Interestingly, Inx Ga1-xN and Inx Tl1-xN alloys enable, with varying x, to construct type I or type II heterostructures. We demonstrate that it is possible to tailor the electronic and optical response from UV to IR. We suggest that 2D InGaN and InTlN heterostructures may efficiently harvest light and serve as building blocks for a future generation of III-V solar cells. Finally, 2D InTlN with a low In content is eligible as the emitter and detector for THz applications.

The prediction of silicene [1], a 2D honeycomb crystal of silicon analogous to graphene, lead to intensive theoretical study [2]. Experimental realization has, on the one hand, been successful: although synthesis of free-standing silicene has remained elusive, various reports of silicene growth on Ag(110), Ag(111), ZrB2, and Ir(111) have appeared, culminating in the recent demonstration of a field-effect silicene-based transistor [3]. On the other hand, many of these reports have since been questioned as the strongly hybridized nature of the Si-substrate bonding apparently destroys the key electronic property (Dirac cone) [4]. In this presentation we report first principles calculations of the structure and optical properties of silicene-like nanoribbons reported to form on Ag(110) and as 2D overlayers on Ag(111). In the case of Si/Ag(110), several structural models have been proposed that suggest a Si-stabilized missing row reconstruction based on zigzag [5] or pentamer [6] chains. Refined experimental studies using grazing incidence x-ray diffraction (GIXD), scanning tunneling microscopy (STM), and surface optical spectra (reflectance anisotropy spectroscopy RAS and surface differential reflectance spectroscopy SDRS), are interpreted using density-functional theory (DFT) and confirm the correct structure as being a pentamer chain reconstruction [7], definitively ruling out silicene nanoribbon structures [8]. The case of Si/Ag(111) is complicated by the abundance of single and multilayer phases [9], and we attempt to cast light on the atomic geometry by interpreting recent experimental SDRS measurements [9,10] using ab initio DFT simulations of the full Si/Ag overlayer phase. The importance of including many-body effects in the optical spectra calculations is examined [11]. [1] G.G Guzman-Verri and L.C. Lew Van Yoon, Phys. Rev. B. 76, 075131 (2007) [2] See e.g. C. Grazianetti, E. Cinquanta and A. Molle, 2D Mater. 3, 012001 (2016) [3] L. Tao et al, Nature Nanotechnology 10 227 (2015) [4] R. Quhe et al, Sci. Rep. 4, 5476 (2014) [5] C. Hogan et al Phys. Rev. B 92, 115439 (2015). [6] J. I. Cerdá, et al, Nat. Commun. 7, 13076 (2016). [7] G. Prévot, C. Hogan, et al, Phys. Rev. Lett. 117, 276102 (2016); [8] P. De Padova, et al, APL 96, 261905 (2010); B. Aufray et al, APL 96, 183102 (2010); C. Lian and J. Ni, Physica B: Condensed Matter 407, 4695 (2012). [9] Y. Borensztein et al, Phys. Rev. B. 92, 155407 (2015) [10] Y. Borensztein, G. Prevot, L. Masson, Physical Review B 89, 245410 (2014); [11] C. Hogan, O. Pulci, D. Martin, A. Curcella, Y. Borensztein, in preparation.

Using ab initio density functional theory the band structure and the dielectric function of a bct Cd3As2 crystal are calculated. We find a Dirac semimetal with two Dirac nodes k± near the Γ point on the tetragonal axis. The bands near the Fermi level exhibit a linear behavior. The resulting Dirac cones are anisotropic and the electron-hole symmetry is destroyed along the tetragonal axis. Along this axis the symmetry-protected band linearity only exists in a small energy interval. The Dirac cones seemingly found by ARPES in a wider energy range are interpreted in terms of pseudo-linear bands. The behavior as 3D graphene-like material is traced back to As p orbital pointing to Cd vacancies, in directions which vary throughout the unit cell. Because of the Dirac nodes the dielectric functions (imaginary part) show a plateau for vanishing frequencies whose finite value is proportional to the Sommerfeld fine structure constant but varies with the light polarization. The consequences of the anisotropy of the Dirac cones are highlighted for the polarization dependence of the infrared optical conductivity.

Paper is the most widely used writing support due to the remarkable properties of its principal component - cellulose - one of the most abundant biomaterials present on Earth. However, due to the complexity of the material, an exhaustive picture of its degradation pathways is still missing. In this paper, we will present recent results and progresses obtained in the comprehension of the role of cellulose oxidation in the yellowing of ancient paper. Visible and ultraviolet spectra of cellulose in ancient paper samples and reference modern samples artificially aged have been interpreted with the aid of ab-initio Time-Dependent Density Functional Theory calculations. Through the comparison of measured and calculated absorption spectra, several oxidized forms of cellulose polymers, acting as chromophores, and responsible for ancient paper yellowing were identified. The relative concentration of ketones and aldehydic groups depends on the environmental conditions in which samples were stored along their life.

Transition metal monopnictides represent a new class of topological semimetals with low-energy excitations, namely, Weyl fermions. We report optical properties across a wide spectral energy range for TaAs, TaP, NbAs and NbP, calculated within density functional theory. Spectra are found to be somewhat independent of the anion and the light polarization. Their features are explained in terms of the upper $s$, $p$, $d$, and $f$ electrons. Characteristic absorption features are related to the frequency dependence of the Fresnel reflectivity. While the lower part of the energy loss spectra is dominated by plasmonic features, the high-energy structures are explained by interband transitions., Comment: Added reference 21 to "S.-Y. Xu, et al, Science 349, 613 (2015)"

Spectral properties of chromophores are used to probe complex biological processes in vitro and in vivo, 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 quantum-mechanics/molecular-mechanics (QM/MM) approach. We show here how to include quasi-particle 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 redshift in the main spectral peak of indole, in quantitative agreement with the experiments and point to the importance of performing averages over molecular dynamics configurations for calculating optical properties.

The microscopic study of complex systems has reached a high level of accuracy that allows for a deep understanding of their structure, electronic properties, and optical spectra. The theoretical investigation of surfaces is nowadays routinely done within density functional theory, for ground state properties, and, with a larger computational load, within many-body perturbation theory, for excited states properties. In this paper we present and discuss examples of calculations for group IV two-dimensional systems such as a clean silicon surface, a tin–germanium interface, graphene, and graphane, pointing out the importance of a pertinent treatment of many-body effects.

We present an ab initio investigation of the optical properties of 3-tert-butylcyclohexene in both its conformers. The optical spectra, here the photoabsorption cross section, have been obtained within density-functional theory at the independent-particle level, and within time-dependent density-functional theory. The optical spectra of the two conformers show small but visible differences, hence suggesting that optical absorption experiments can discriminate among the two molecular geometries. To cite this article: K. Gaal-Nagy et al., C. R. Physique 10 (2009).

In this work, we present the results of ab-initio theoretical calculations concerning the electronic and optical properties of Si(001) surface covered by small hydrocarbon molecules, namely acetylene and ethylene. As a result of the adsorption process, both molecules are covalently bonded to the surface, and we point out analogies and differences of the two systems under study. The most stable configurations have been taken into account at different coverages. We will show how band structure and reflectance anisotropy spectra calculations supply additional indirect information concerning the adsorption geometry and the saturation coverage, that have been longly debated.

We review the theoretical framework of ab-initio excited state properties calculations showing the application of these methods to systems of different dimensionality. Many body perturbation theory within the GW approximation of the self energy for the calculation of electronic band structures is presented and applied to liquid water. The Bethe Salpeter equation for the computation of optical properties is illustrated and used for the case of surfaces and liquid water. An alternativemethod for the calculation of optical properties, the Time Dependent Density Functional Theory, is also briefly illustrated. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The ab initio calculation of optical spectra of sheet crystals usually arranges them in a three-dimensional superlattice with a sufficiently large interlayer distance. We show how the resulting frequency-dependent dielectric tensor is related to the anisotropic optical conductivity of an individual sheet or to the dielectric tensor of a corresponding film with thickness $d$. Their out-of-plane component is taken into account, in contrast to usual treatments. We demonstrate that the generalized transfer-matrix method to model the optical properties of a layer system containing a sheet crystal accounts for all tensor components. As long as $d\ensuremath{\ll}\ensuremath{\lambda}$ ($\ensuremath{\lambda}$-wavelength of light) this generalized formulation of the optical properties for anisotropic two-dimensional (2D) systems of arbitrary thickness reproduces the limits found in literature that are based either on electromagnetic boundary conditions for a conducting surface or on an isotropic dielectric tensor. For $s$-polarized light, the results are independent of the sheet description. For oblique incidence of $p$-polarized light, the tensor nature of the optical conductivity (or the dielectric function) of the sheet crystal strongly impacts on reflectance, transmittance, and absorbance due to the out-of-plane optical conductivity. The limit $d\ensuremath{\rightarrow}0$ should be taken in the final expressions. Example spectra are given for the group-IV honeycomb 2D crystals graphene and silicene.

The low-temperature phase of Sn/Ge(1 1 1) 3 × 3 is studied by means of first-principle calculations at the DFT level and in the GW approximation. Experimental STM images taken at 80 K are presented and compared with theoretical predictions. This analysis allows to give support to a geometry in which one Sn adatom is higher than the other two in the surface unit cell and excludes the complementary geometry in which two Sn adatoms are higher than the third. Concerning the very low temperature phase (T < 30 K), we show that from a purely energetic point of view a sqrt3xsqrt3 geometry cannot be excluded, being almost degenerate with the 3 × 3 phase.

The absorption and, for the first time, the emission spectra of doped silicon nanocrystals have been calculated within a first-principles framework including geometry optimization. Starting from hydrogenated silicon nanocrystals, simultaneous n- and p-type doping with boron and phosphorous impurities have been considered. We found that the B-P co-doping results to be easier than simple 13- or P-doping and that the two impurities tend to occupy nearest neighbours sites inside the nanocrystal itself. The co-doped nanocrystals bandstructure presents band edge states that are localized on the impurities and are responsible of the red-shifted absorption threshold with respect to that of pure un-doped nanocrystals in fair agreement with the experimental outcorne. The emission spectra show a Stokes shift with respect to the absorption due to the structural relaxation after the creation of the electron-hole pair. Moreover, the absorption and emission spectra have been calculated for a small co-doped nanocrystal beyond the single particle approach by introducing the self-energy correction and solving the Bethe-Salpeter equation scheme. Our procedure shows the important role played by the many-body effects. (C) 2006 Elsevier B.V. All rights reserved.

The proton disorder in ice has a key role in several properties such as the growth mode, thermodynamical properties, and ferroelectricity. While structural phase transitions from proton disordered to proton ordered ices have been extensively studied, much less is known about their electronic and optical properties. Here, we present ab initio many body perturbation theory-based calculations of the electronic and optical properties of cubic ice at different levels of proton disorder. We compare our results with those from liquid water, that acts as an example of a fully (proton- and oxygen-)disordered system. We find that by increasing the proton disorder, a shrinking of the electronic gap occurs in ice, and it is smallest in the liquid water. Simultaneously, the excitonic binding energy decreases, so that the final optical gaps result to be almost independent on the degree of proton disorder. We explain these findings as an interplay between the local dipolar disorder and the electronic correlation.

We present an ab initio study of two semiconductor surfaces, the alpha phase of Sn on Ge(111) and the cleavage surface of diamond. The theoretical tools used (density functional theory (DFT) and many-body perturbation theory) are discussed in detail, and the advantages and disadvantages of the two approaches are pointed out. We show that in the case of diamond it is essential to go beyond the DFT single-particle approach, and to introduce quasiparticle effects through the GW approximation.

We show an approximate method to calculate the polarizability of a many-electron system within Green's function theory in a similar way as within time-dependent density functional theory (TDDFT). The basic idea is to join the computational simplicity of the latter with the accuracy of the former. We apply this approach to a prototype system, LiF. For comparison, we also show results obtained within other approximated TDDFT-based methods. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The electronic and optical properties of silicon nanocrystals passivated with hydrogen and oxygen have been investigated both in the ground- and in an excited-state configuration, through different ab-initio techniques. The presence of an electron–hole pair leads to a strong interplay between the structural and optical properties of the system. The structural distortion of the nanocrystals induced by an electronic excitation is analysed together with the role of the symmetry constraint during the relaxation. The structural distortion can account for the experimentally observed Stokes Shift. Size-related aspects are also analysed and discussed.

In this paper we review the first-principles theoretical framework of the Bethe–Salpeter equation which is nowadays the state-of-the-art approach for including self-energy, local fields and excitonic effects in the surface optical response. Two different approaches for calculating the dielectric screening will be described. In both cases a parallel and efficient iterative algorithm to find numerically the solution of the BSE equation has been implemented. In fact, if the surface states are not energetically separated from the bulk states and if one is interested in describing a large energy window, the traditional approach involving a full diagonalization of a very large excitonic Hamiltonian is prohibitive. The reflectance anisotropy spectrum of the monohydride Si(100) is considered and compared with experiments. A comparison of the results obtained within different treatments of the electron–hole screening is made. Convergence and numerical problems are discussed.

We describe an application of the many-body GW method to calculate the electronic band structure and optical properties of the clean Si(0 0 1) 1 × 2 surface and its changes upon antimony adsorption. We find that the so called (1 × 1) reconstruction of Si(0 0 1):Sb is actually a mixture of (2 × 1) and c(2 × 2)-Sb domains.

We present the results of a first principles study of the adsorption of acetylene on Si(001). Several adsorption sites are investigated, and the electronic structures and reflectance anisotropy spectra (RAS) are calculated for the energetically most favorable geometries. We find that the end-bridge adsorption sites, both at 0.5 ML and at 1ML C2H2 coverage do not give metallic surfaces, at odd with other calculations. First principles study of optical properties for different configurations of acetylene adsorbed on Si(001) show a strong change of the spectra, thus suggesting that RAS experiments may give a definitive answer on the adsorption sites and saturation coverage. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Preview Synonyms Optical reflectance spectroscopy of ancient paper; Optical spectroscopy for ancient paper diagnostic Definition Optical spectroscopy of ancient paper is the study of the interaction between paper and ultraviolet, visible and near infrared radiation. This approach is aimed to nondestructive evaluation of the degradation of ancient artifacts. Cellulose and Ancient Paper Cellulose has had a unique role in civilization because it has been widely used for the acquisition, storage, and dissemination of human culture. In fact, one of the main cellulose derivatives is paper, which was invented in China in the second century A.D. and, independently, in Mesoamerica in the seventh century A.D. The technological knowledge of making paper spread, in the Middle Ages, from the Far East to Arab World and Europe. As a result, for centuries, a growing number of books, documents, and artistic drawings have been accumulating in archives, libraries, and museums, all over the world [1]. The preservation ...

The Institute for Complex Systems, in collaboration with the University of Rome "Sapienza" and Tor Vergata and the University of Sussex (UK), has recently developed a THz spectroscopy laboratory aimed to academic and industrial research. The access to the wide THz electromagnetic band (which lies between optical waves and microwaves in the electromagnetic spectrum) has been limited for a long time by the lack of cost-effective and practical intense sources. The recent development of technologies for broadband THz generation is changing this scenario allowing a rapidly evolving field in research, industry, and security applications. Fingerprinting of spectroscopic lines in the THz region helps identifying chemical or biochemical molecules. Imaging with THz waves can be used for pharmaceutical, security, or identification of substances such as drugs, explosives or weapons. THz spectroscopy and imaging is used to quality control and optimization of industrial manufacturing processes. As example, one can look for tiny variations or defects in polymer and plastic materials, characterize the quality and morphology of paper products, textiles and wood products. Other examples span from measuring the thickness of coatings, non-destructive diagnostics for historical art conservation, and last but not the least, quality control of chocolate products with THz imaging.

In this paper, we present a new noninvasive and nondestructive approach to recover scattering and absorption coefficients from reflectance measurements of highly absorbing and optically inhomogeneous media. Our approach is based on the Yang and Miklavcic theoretical model of light propagation through turbid media, which is a generalization of the Kubelka-Munk theory, extended to accommodate optically thick samples. We show its applications to paper, a material primarily composed of a web of fibers of cellulose, whose optical properties are strongly governed by light scattering effects. Samples studied were ancient and industrial paper sheets, aged in different conditions and highly absorbing in the ultraviolet region. The recovered experimental absorptions of cellulose fibers have been compared to theoretical ab initio quantum-mechanical computational simulations carried out within time-dependent density functional theory. In this way, for each sample, we evaluate the absolute concentration of different kinds of oxidized groups formed upon aging and acting as chromophores causing paper discoloration. We found that the relative concentration of different chromophores in cellulose fibers depends on the aging temperature endured by samples. This clearly indicates that the oxidation of cellulose follows temperature-dependent reaction pathways. Our approach has a wide range of applications for cellulose-based materials, like paper, textiles, and other manufactured products of great industrial and cultural interest, and can potentially be extended to other strongly absorbing inhomogeneous materials. © 2014 American Physical Society.

The dependence of the GaP and InP(0 0 1) surface reconstructions on the chemical potentials of their constituents is explored. Based on first-principles pseudopotential plane-wave calculations the surface phase diagram is obtained. For both GaP(0 0 1) and Inp(0 0 1) surfaces, P-top dimer reconstructions are predicted for P-rich conditions, while for group III-element rich preparations a mixed dimer (III-P) on a complete III-layer is favoured. For GaP, in addition, a Ga-top dimer reconstruction seems to be a possibility for very Gallium rich growth conditions. STM images are calculated in order to contribute to a solution of the structural puzzle.

The microscopic structure of the GaP(001) surface is not well understood, in marked contrast to other III–V compounds, in particular GaAs(001) (see Ref. [1] for a recent review). It has been suggested that ion bombardment and annealing of GaP(001) results in a (4 2) reconstructed Ga-rich surface [2, 3] in analogy to GaAs. On the other hand, based on ion scattering spectroscopy and low-energy electron diffraction (LEED) experiments, a (2 4) translational symmetry has been proposed [4, 5]. In solid-source molecular beam epitaxy a (2 4) diffraction pattern shows up under both Ga and P supply [6]. In addition, (4 4) spots have been observed. A slightly different behaviour is found for samples grown by metal-organic vapour phase epitaxy or chemical beam epitaxy as well as for GaP(001) surfaces prepared by thermal desorption of a protective arsenic/phosphorous double layer cap under ultrahigh vacuum conditions [7–9]. After annealing to 690 K the de-capped GaP(001) surfaces show a (2 1)=(2 2)-like LEED pattern. Annealing to higher temperatures (785 K) leads to a (2 4) translational symmetry. The many-step annealing procedure is accompanied by a transition from a more P-rich to a more Ga-rich GaP(001) surface. Previous total-energy (TE) calculations [8, 9] show the stability of several (2 4) reconstructions. For Ga-rich surfaces they are characterized by single Ga–Ga or mixed Ga–P dimers on top of a complete Ga layer. For lower Ga contents one Ga pair is missing in this layer and one (d structure) or two P dimers (b2 structure) terminate the surface. (4 2) reconstructions with Ga or P dimers in the uppermost atomic layer are unstable. The comparison of measured and calculated reflectance anisotropy spectra support the mixed-dimer reconstruction of the Ga-rich GaP(001) surface [8, 9]. This has recently been confirmed by a comparison of calculated and measured scanning-tunneling microscopy (STM) images [10]. The (2 4) mixed-dimer structure agrees with recent measurements of the surface core-level shifts [5]. The structure of the (2 1)=(2 2) reconstruction of the GaP(001) surface remains unknown. Auger electron spectroscopy indicates a P-rich surface. The similarities with

We have studied the quasiparticle (QP) wave functions and energies of the GaAs(1 1 0) surface and of the cluster Na4 within Hedin's GW approximation. We found that the commonly accepted assumption that the Density Functional Theory (DFT)–Local Density Approximation (LDA) wave functions are a very good approximation for the true QP ones can be wrong. We analyze the effect of the resulting changes in the wave functions on the optical spectra.

We infer the structure of the GaP(001)(2x4) surface from a study of scanning tunneling microscopy (STM) images obtained under UHV conditions on metal-organic vapor phase epitaxy grown samples. STM images are compared with results of first-principles calculations for models energetically most favorable under Ga-rich growth conditions. The comparison shows that the GaP(001)(2x4) surface unit cell consists of a mixed Ga-P dimer on top of a complete gallium layer, hence ruling out the Cia-Ga dimer model.

The dependence of the InP(001) surface reconstruction on the chemical potentials of its constituents is explored. Based on first-principles pseudopotential plane-wave calculations the surface phase diagram is constructed. 17 structural models are studied for the phosphorus-rich InP(001) surfaces, with (2×1), p(2×2) and c(2×2) translational symmetry. P-top dimer reconstructions are favoured. Under less P-rich preparation conditions a tendency for disorder is predicted. STM images are also calculated in order to contribute to a solution of the structural puzzle.

We present ab initio calculations for both the wave functions and the energies of single quasiparticles. The conventional quasiparticle approach evaluates energy-level corrections to first order in the difference between the self-energy and the Kohn-Sham exchange-correlation potential. Here, we also recalculate the quasiparticle states. At the example of the GaAs(110) surface we show that this nonperturbative treatment is important for surfaces with electronic states close in energy but different with respect to their localization. As a sensitive observable the reflectance anisotropy is studied.

We present a calculation of the electronic and optical properties of the GaAs(110) : H surface performed within the first-principles density functional theory (DFT) in the local density approximation (LDA). The geometry and electronic structure are analyzed and compared with those of the clean surface. The reflectance anisotropy spectrum and the differential reflectivity are then computed. Taking into account the usual underestimation of the LDA gaps with respect to the experimental values, the comparison between our theoretical spectra with available experimental reflectance data shows a satisfactory agreement, allowing for the interpretation of the main structures in terms of electronic transitions.

We discuss the progress in realistic calculations of the optical response of semiconductor surfaces. It concerns numerical developments as well as the better inclusion of many-body effects. We show that the starting point of such calculations, the atomic structure and the surface equilibrium phase, can be now derived with high accuracy. The different steps are described in detail for the computation of the reflectance anisotropy and the exploration of the In-rich InP(001)2 4 surfaces.

We present a first-principles calculation of self-energy effects on the optical properties of the GaAs(110) surface. Three main results are obtained. (a) The self-energy shifts for the valence bands are larger for surface-localized states, at odds with the commonly assumed ``scissor operator'' hypothesis. (b) The computed shifts display an almost linear dependence on the surface localization of the wave function; this allows us to realize a well-converged calculation of optical properties based on the $\mathrm{GW}$-corrected spectrum. (c) The agreement with experimental data is improved with respect to local-density-approximation calculations.

We apply the method of random matrices to a calculation of the optical properties of porous silicon and silicon nanopowders, analyzing the effects of disorder on the visible luminescence properties. The maximum of the luminescence peal; displays a blueshift, which adds up to the effects induced by other mechanisms, such as quantum confinement or surface passivation, and agrees with the experimental findings, The long-wavelength luminescence edge, instead, is redshifted by the presence of disorder, thus partially compensating for the gap-opening effect induced by the other mechanisms.

Silicene and related 2D crystals with honeycomb symmetry are of increasing interest because of their compatibility with the Si-device technology. By means of density functional and many-body perturbation theory we model their principal properties and growth on substrates. In the case of hydrogenated crystals also excitons in two dimensions are studied.

Graphene, a 2D layer of sp2 bonded carbon atoms in honeycomb arrangement, is a recently synthesized material, with exceptional mechanical, electronic, optical and thermal properties. Although theoretically studied since a long time, as it is the building block of graphite (a stacking of graphene layers), it was isolated as single or few layers only in 2004. Today graphene can be produced by several methods that depend on the quality and dimension of the material to be obtained. Applications of graphene are extensive in electronics, photonics, sensing and also in the field of energy generation and storage. Belonging to the same chemical group of carbon, also silicon could be in principle the ingredient of a 2D material similar to graphene, named silicene. It shares with graphene the peculiar optical and electronic properties and could provide a more easy compatibility with the existing silicon-based microelectronics technology. An extremely high thermal conductivity in the range 3000-5000 Wm-1K-1 has been measured for graphene. Only few results exist instead on the thermal properties of silicene. In this work we report ab-initio calculations, based on density functional theory and density functional perturbation theory, that provide the ingredients for the determination of the thermal conductivity and specific heat of graphene, silicene and 2D silicon carbide. Ab-initio calculations solve Schrodinger equation without the need of empirical parameters, only knowing the chemical nature of the system constituents. The use of consolidated approximations employed in density functional theory allows the accurate prediction of several properties of materials.

We review here some of the most reliable and efficient computational theoretical ab initio techniques for the prediction of optical and electronic spectroscopic properties and show some important applications to molecules, surfaces, and solids. We investigate the role of the solvent in the optical absorption spectrum of indole molecule. We study the excited-state properties of a photo-active minimal model molecule for the retinal of rhodopsin, responsible for vision mechanism in animals. We then show a study about spectroscopic properties of Si(1 1 1) surface. Finally we simulate a bulk system: paper, that is mainly made of cellulose, a pseudo-crystalline material representing 40% of annual biomass production in the Earth.

We show that the low-frequency absorbance of undoped graphene, silicene, and germanene has a universal value, only determined by the Sommerfeld fine-structure constant. This result is derived by means of ab initio calculations of the complex dielectric function for optical interband transitions applied to two-dimensional (2D) crystals with honeycomb geometry. The assumption of chiral massless Dirac fermions is not necessary. The low-frequency absorbance does not depend on the group-IV atom, neither on the sheet buckling nor on the orbital hybridization. We explain these findings via an analytical derivation of the relationship between absorbance and fine-structure constant for 2D Bloch electrons. The effect of deviations of the electronic bands from linearity is also discussed. DOI: 10.1103/PhysRevB.87.035438

Nanostructuring of a material leads to enormous effects on its excited state properties. This study, through the application of different state-of-the-art ab initio theoretical tools, investigates the effect of size on the electronic gap of germanium nanocrystals highlighting similarities and differences with respect to equivalent silicon nanostructures. We performed both GW and ?SCF calculations for the determination of their electronic structure. While it is known that ?SCF corrections to the Kohn-Sham gap vanish for extended systems, the two approaches were expected to be equivalent in the limit of small clusters. However, it has been recently found that for hydrogenated Si clusters the ?SCF gaps are systematically smaller than the GW ones, while the opposite is true for Ag clusters. In this work we find that the GW gaps are larger than the ?SCF ones for all the Ge dots, with the exception of the smallest one. Such crossing between the ?SCF and the GW gap values was not expected and has never been observed before. Moreover, also for hydrogenated Si nanocrystals we found a similar behavior. The origin of this crossing might be found in the Rydberg character of the LUMO of the smallest clusters and can also explain the qualitative differences in the comparison between GW and ?SCF found in the previous studies. © 2013 American Chemical Society.

The recently reported synthesis of silicene in the form of nanoribbons on Ag(110) or 2D epitaxial sheets on Ag(111) aroused considerable interest in the scientific community. Both overlayers were reported to display signatures of Dirac fermions with linearly dispersing electronic bands. In this work, we study the electronic structure of these adsorbate systems within density functional theory. We show that the conical features apparent in angle-resolved photoelectron spectroscopy measurements are not due to silicon but to the silver substrate, as an effect of band folding induced by the Si overlayer periodicity.

We present first-principles studies of the optical absorbance of the group IV honeycomb crystals graphene, silicene, germanene, and tinene. We account for many-body effects on the optical properties by using the non-local hybrid functional HSE06. The optical absorption peaks are blueshifted due to quasiparticle corrections, while the influence on the low-frequency absorbance remains unchanged and reduces to a universal value related to the Sommerfeld fine structure constant. At the Dirac points spin-orbit interaction opens fundamental band gaps; parabolic bands with a very small effective mass emerge. Consequently, the low-frequency absorbance is modified with a spin-orbit-induced transparency region and an increase of the absorbance at the fundamental absorption edge.

The relaxation dynamics of unaligned multi-walled carbon nanotubes decorated with metallic nanoparticles have been studied by using transient optical measurements. The fast dynamics due to the short-lived free-charge carriers excited by the pump are not affected by the presence of nanoparticles. Conversely, a second long dynamics, absent in bare carbon nanotubes, appears only in the decorated samples. A combination of experiment and theory allows us to ascribe this long dynamics to relaxation channels involving electronic states localized at the tube-nanoparticle interface.