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The thermodynamic stability of hydroxylated graphane, that is, fully sp3 graphene derivatives coordinated with -H and -OH groups, has been recently demonstrated by ab initio calculations. Within the density functional theory approach, we investigate the electronic property modifications of graphane by progressive hydroxylation, that is, by progressively substituting -H with -OH groups. When 50% of graphane is hydroxylated, the energy bandgap reaches its largest value of 6.68 eV. The electronic affinity of 0.8 eV for graphane can widely change in the 0.28-1.60 eV range depending on the geometric configuration. Hydroxylated graphane has two interfaces with vacuum, hence its electron affinity can be different on each interface with the formation of an intrinsic dipole perpendicular to the monolayer. We envisage the possibility of using hydroxylated graphane allotropes with tunable electronic affinity to serve as interfacial layers in 2D material-based heterojunctions.

The scattering theory for two crossing metallic single-walled carbon nanotubes of finite length exposed to an electromagnetic field has been developed based on a synthesis of the quantum transport formalism and classical electrodynamics. The model of the point contact has been developed to be incorporated into the Hall\'en and Pocklington equations for crossing carbon nanotubes. The influence of the contact conductance and position as well as the angle between the tube axes on the tube polarizability has been analyzed in the range of 1 GHz to 10 THz. The physical mechanisms responsible for the electromagnetic interaction between crossing tubes with zero and nonzero intertube contact conductance are shown and discussed. The influence of the coupling between the tubes on the localized plasmon resonance in them has been demonstrated.

An innovative method to quantitatively assess the state of degradation of paper in a non-destructive way has been developed by using THz time-domain spectroscopy (THz-TDS). The method relies on the observation that the hydrolytic and oxidative degradation of cellulose in paper is accompanied with rearrangement of the hydrogen bonds (H-bond) network of cellulose polymers. This gives clear fingerprints in the THz spectral profiles. THz-TDS is a suitable approach to study low-energy vibrational properties of biological materials in a non-destructive way. The THz photon energy range (about 1-40 meV) is particularly suitable to probe the H-bonds between molecules. However, the quantitative assessment of the state of preservation of artworks poses significant challenges, in particular when measurements on low refractive index and thin samples, such as ancient documents and drawings, must be carried out [1]. In the present study, we have successfully solved this problem obtaining a precise determination of the absorption by cellulose fibers in the 0.2-3.5 THz (7-117cm-1) range in single freely standing paper sheets. This has been achieved by developing a new procedure to remove from the experimental signals the spurious interference effects generated by the Fabry-Pérot resonances in the sheet. By using this approach, the THz absorption coefficient of cellulose fibers in ancient and modern samples artificially aged has been obtained. The THz absorption spectra are explained in terms of absorption peaks of the cellulose crystalline phase superimposed to a background contribution due to a disordered H-bonds network [2]. The complex evolution of the spectra as a function of natural and artificial aging is explained with a reduction up to 30% of the H-bond density in the cellulose polymer networks and a parallel increasing of the sample crystallinity evident in the peaks at about 2.1 and 3.1 THz. The comparison with density functional theory ab-initio calculations suggests that the observed THz spectral changes could be related not only to the recrystallization of amorphous regions, but also to the progressive degradation of the external surfaces of crystalline domains. These results demonstrate a significant potential in the non-destructive analysis of the structural modifications of cellulose and paper artifacts by aging, resulting, at a macroscopic scale, in a progressive weakening of the mechanical properties of paper [3]. [1] T. Trafela, M. Mizuno, K. Fukunaga, and M. Strlic, Appl. Phys. A, 111, 2013, 83-90. [2] E. Scarpellini, M. Ortolani, A. Nucara, L. Baldassarre, M. Missori, R. Fastampa, and R. Caminiti, J. Phys. Chem. C 120, 2016, 24088-24097. [3] L. Teodonio, M. Missori, D. Pawcenisc, J. ?ojewskac, F. Valle, Micron 91, 2016, 75-81.

We perform a systematic {\it ab initio} study of the work function and its uniform strain dependence for graphene and silicene for both tensile and compressive strains. The Poisson ratios associated with armchair and zigzag strains are also computed. Based on these results, we obtain the deformation potential, crucial for straintronics, as a function of the applied strain. Further, we propose a particular experimental setup with a special strain configuration that generates only the electric field, while the pseudomagnetic field is absent. Then, applying a real magnetic field, one should be able to realize experimentally the spectacular phenomenon of the collapse of Landau levels in graphene or related two-dimensional materials., 9 pages, 7 figures; final version published in PRB

Development of on-chip integrated carbon-based optoelectronic nanocircuits requires fast and non-invasive structural characterization of their building blocks. Recent advances in synthesis of single wall carbon nanotubes and graphene nanoribbons allow for their use as atomically precise building blocks. However, while cataloged experimental data are available for the structural characterization of carbon nanotubes, such an atlas is absent for graphene nanoribbons. Here we theoretically investigate the optical absorption resonances of armchair carbon nanotubes and zigzag graphene nanoribbons continuously spanning the tube (ribbon) transverse sizes from 0.5(0.4) nm to 8.1(12.8) nm. We show that the linear mapping is guaranteed between the tube and ribbon bulk resonance when the number of atoms in the tube unit cell is \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$2N+4$$\end{document}2N+4, where \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$N$$\end{document}N is the number of atoms in the ribbon unit cell. Thus, an atlas of carbon nanotubes optical transitions can be mapped to an atlas of zigzag graphene nanoribbons., The authors combine ab-initio density functional theory with tight-binding calculations to investigate the optical absorption resonances of armchair carbon nanotubes and zigzag graphene nanoribbons, and show that an atlas of carbon nanotubes optical transitions can be mapped to an atlas of optical resonances of zigzag graphene nanoribbons.

Thanks to the ultrahigh flexibility of 2D materials and to their extreme sensitivity to applied strain, there is currently a strong interest in studying and understanding how their electronic properties can be modulated by applying a uniform or nonuniform strain. In this work, using density functional theory (DFT) calculations, we discuss how uniform biaxial strain affects the electronic properties, such as ionization potential, electron affinity, electronic gap, and work function, of different classes of 2D materials from X-enes to nitrides and transition metal dichalcogenides. The analysis of the states in terms of atomic orbitals allows to explain the observed trends and to highlight similarities and differences among the various materials. Moreover, the role of many-body effects on the predicted electronic properties is discussed in one of the studied systems. We show that the trends with strain, calculated at the GW level of approximation, are qualitatively similar to the DFT ones solely when there is no change in the character of the valence and conduction states near the gap.

International audience; Electronic properties and lattice dynamics of bulk ɛ-GaSe and one, two, and three tetralayers GaSe are investigated by means of density functional and density functional perturbation theory. The few-tetralayer systems are semiconductors with an indirect nature of the fundamental band gap and a Mexican-hat shape is observed at the top of the valence band. The phonon branch analysis reveals the dynamical stability for all systems considered together with the LO-TO splitting breakdown in two-dimensional systems. In-plane (E) and out-of-plane (A) zone-center lattice vibrations dominate the Raman and IR spectra.

Abstract By means of ab initio band structure methods and model Hamiltonians we investigate the electronic, spin and topological properties of four monopnictides crystallizing in bct structure. We show that the Weyl bands around a WP W1 or W2 possess a strong anisotropy and tilt of the accompanying Dirac cones. These effects are larger for W2 nodes than for W1 ones. The node tilts and positions in energy space significantly influence the DOS of single-particle Weyl excitations. The node anisotropies destroy the conventional picture of (anti)parallel spin and wave vector of a Weyl fermion. This also holds for the Berry curvature around a node, while the monopole charges are independent as integrated quantities. The pairing of the nodes strongly modifies the spin texture and the Berry curvature for wave vectors in between the two nodes. Spin components may change their orientation. Integrals over planes perpendicular to the connection line yield finite Zak phases and winding numbers for planes between the two nodes, thereby indicating the topological character. Graphical abstract

We present a first principles study of the stability, and of the electronic and optical properties of graphene with nitrogen doped vacancies. Moreover, we use the vacancies as anchoring sites for Mg, Zn, Pd al Pt atoms and vary the concentration of defects. Decoration of the defects with metal atoms produces semi-metallic systems for any studied size of the cell, with linear bands crossing at the Fermi level. The peculiar electronic properties of massless Dirac fermions in graphene are hence kept, although with anisotropic Fermi velocities. New sharp peaks appear in the optical conductivity in the visible range, thus strongly enhancing the optical response of graphene.

We predict the stability of a graphenelike silicene sheet on one monolayer of aluminum oxide. We find that the honeycomb buckled structure of silicene is not broken upon interaction with one monolayer of ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ in the kagome geometry. As a consequence, the electronic band structure shows unperturbed cones with massless Dirac fermions embedded into ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$-derived bands. The heterostructure conserves the topological character of the freestanding silicene. The optical properties of the silicene/${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ bilayers clearly maintain the characteristic infrared universal limit of $\ensuremath{\pi}\ensuremath{\alpha}$. A quantized absorbance $N\ifmmode\cdot\else\textperiodcentered\fi{}\ensuremath{\pi}\ensuremath{\alpha}$ (with $N$ silicene layers) also occurs for stacking of multilayers.

Using density-functional and many-body perturbation theory, we study the electronic, optical, and excitonic properties of indium nitride as single monolayer and bilayer in comparison with the bulk phase. We investigate the stable geometry for the monolayer, the graphenelike unbuckled honeycomb structure, and for the bilayer the AA' stacking geometry. We demonstrate that the quasiparticle and optical gaps, going from bulk to two-dimensional systems, open dramatically due to strong quantum confinement and reduced screening. Large excitonic effects, which survive at room temperature, are predicted. Our results suggest that low-dimensional InN is a promising material for optoelectronic devices in the visible to near-infrared spectral ranges varying with the number of atomic layers and light propagation.

We propose a characterization tool for studies of the band structure of new materials promising for the observation of topological phase transitions. We show that a specific resonant feature in the entropy per electron dependence on the chemical potential may be considered as a fingerprint of the transition between topological and trivial insulator phases. The entropy per electron in a honeycomb two-dimensional crystal of germanene subjected to the external electric field is obtained from the first principle calculation of the density of electronic states and the Maxwell relation. We demonstrate that, in agreement to the recent prediction of the analytical model, strong spikes in the entropy per particle dependence on the chemical potential appear at low temperatures. They are observed at the values of the applied bias both below and above the critical value that corresponds to the transition between the topological insulator and trivial insulator phases, while the giant resonant feature in the vicinity of zero chemical potential is strongly suppressed at the topological transition point, in the low temperature limit. In a wide energy range, the van Hove singularities in the electronic density of states manifest themselves as zeros in the entropy per particle dependence on the chemical potential., Comment: 8 pages, 5 figures; final version published in PRB

We report on Raman scattering of GaSe for thicknesses varying from bulk down to bilayer. The Raman spectra using 532 nm excitation show a strong dependence on layer thickness. The two out-of-plane modes of GaSe shift in opposite directions as the thickness increases. However, from pentalayer the Raman spectra of GaSe do not show any further change thus reaching bulk nature in terms of optical properties. We also perform ab initio density functional theory calculations for the geometry, the electronic band structure, and the Raman frequencies for bulk down to one monolayer. Good agreement with the experimental results is found, and we reproduce the trend, driven by the number of layers, in the Raman shifts. Our results present the first systematic approach to a layer dependent Raman study of GaSe and answer contradictions reported in the literature.

Using a combination of the density functional theory and tight-binding calculations, we study electronic and optical properties of asymmetric chevron-type graphene nanoribbons recently synthesized with atomic precision. We demonstrate that the low-energy optical selection rules in such infinite chiral ribbons are more reminiscent of those for zigzag rather than for armchair ribbons. It is also shown that, starting from about 25 nm long ribbons, the low-energy absorption and therefore the selection rules of infinitely long ribbons are well reproduced in the finite cluster approach. Hence, ribbons longer than 25 nm (about 28 unit cells) can be treated as infinitely long ones.

We review the behavior of the entropy per particle in various two-dimensional electronic systems. The entropy per particle is an important characteristic of any many body system that tells how the entropy of the ensemble of electrons changes if one adds one more electron. Recently, it has been demonstrated how the entropy per particle of a two-dimensional electron gas can be extracted from the recharging current dynamics in a planar capacitor geometry. These experiments pave the way to the systematic studies of entropy in various crystal systems including novel two-dimensional crystals such as gapped graphene, germanene and silicene. Theoretically, the entropy per particle is linked to the temperature derivative of the chemical potential of the electron gas by the Maxwell relation. Using this relation, we calculate the entropy per particle in the vicinity of topological transitions in various two-dimensional electronic systems. We show that the entropy experiences quantized steps at the points of Lifshitz transitions in a two-dimensional electronic gas with a parabolic energy spectrum. In contrast, in doubled-gapped Dirac materials, the entropy per particles demonstrates characteristic spikes once the chemical potential passes through the band edges. The transition from a topological to trivial insulator phase in germanene is manifested by the disappearance of a strong zero-energy resonance in the entropy per particle dependence on the chemical potential. We conclude that studies of the entropy per particle shed light on multiple otherwise hidden peculiarities of the electronic band structure of novel two-dimensional crystals., Comment: 33 pages,15 figures. Accepted for publication in JETP. arXiv admin note: text overlap with arXiv:1803.02083, arXiv:1712.09118, arXiv:1703.08962, arXiv:1512.08450

Slightly buckled, graphene-like honeycomb crystals made by silicon, silicene, or by other group-IV elements such as germanene and stanene represent atomically thin films, i.e., two-dimensional (2D) systems. The theoretical description of their optical properties suffers from three difficulties, (i) a thickness much smaller than the wavelength of light, (ii) their common modeling by superlattice arrangements with sufficiently large layer distances, and (iii) the inclusion of many-body effects. Here, the solutions of all problems are discussed. (i) The optical response of an individual honeycomb crystal is described by a tensor of 2D optical conductivities or dielectric functions, which are related to the optical response of the corresponding superlattice. (ii) The influence of such a sheet crystal on the transmittance, reflectance and absorbance of a layer system is described. (iii) Excitonic and quasiparticle effects are demonstrated to widely cancel each other. Silicene sheets are investigated in detail. As a consequence of the linear bands and Dirac cones the low-frequency absorbance is defined by the Sommerfeld finestructure constant. Van Hove singularities represented by critical points in the interband structure are identified at higher photon energies. Clear chemical trends along the row C \(\rightarrow \) Si \(\rightarrow \) Ge \(\rightarrow \) Sn are derived. The influence of multiple layers is studied for the cases of bilayer silicene and graphene.

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.

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.

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)"

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.

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.

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 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 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.

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 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.

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.

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.

Calculating the complex dielectric function for optical interband transitions we show that the two-dimensional crystals silicene and germanene possess the same low-frequency absorbance as graphene. It is determined by the Sommerfeld finestructure constant. Deviations occur for higher frequencies when the first interband transitions outside K or K' contribute. The low-frequency results are a consequence of the honeycomb geometry but do not depend on the group-IV atom, the sheet buckling, and the orbital hybridization. The two-dimensional crystals may be useful as absorption normals in silicon technology.

We investigate electronic and optical properties of the topological Weyl semimetals TaAs, TaP, NbAs and NbP crystallizing in bct geometry by means of the ab initio density functional theory with spin-orbit interaction within the independent-particle approximation. The small energetical overlap of Ta5d or Nb4d derived conduction and valence bands leads to electron and/or hole pockets near the Fermi energy at the 24 Weyl nodes. The nodes give rise to two-(three-)dimensional Dirac cones for the W1 (W2) Weyl type. The band dispersion and occupation near the Weyl nodes determine the infrared optical properties. They are dominated by interband transitions, which lead to a deviation from the expected constant values of the imaginary part of the dielectric function. The resulting polarization anisotropy is also visible in the real part of the optical conductivity, whose lineshape deviates from the expected linearity. The details of the Weyl nodes are discussed in relation to recent ARPES results for TaAs and NbP, and compared with measured optical spectra for TaAs. The spectral features of the anisotropic and tilted Weyl fermions are restricted to low excitation energies above absorption onsets due to the band occupation.

Absorption and emission of pristine-like semiconducting monolayers of BN, AlN, GaN, and InN are systematically studied by ab-initio methods. We calculate the absorption spectra for in-plane and out-of-plane light polarization including quasiparticle and excitonic effects. Chemical trends with the cation of the absorption edge and the exciton binding are discussed in terms of the band structures. Exciton binding energies and localization radii are explained within the Rytova-Keldysh model for excitons in two dimensions. The strong excitonic effects are due to the interplay of low dimensionality, confinement effects, and reduced screening. We find exciton radiative lifetimes ranging from tenths of picoseconds (BN) to tenths of nanoseconds (InN) at room temperature, thus making 2D nitrides, especially InN, promising materials for light-emitting diodes and high-performance solar cells.

We present the electromagnetic scattering theory for a finite-length nanowire with an embedded mesoscopic object. The theory is based on a synthesis of the integral equation technique of classical electrodynamics and the quantum transport formalism. We formulate Hall\'en-type integral equations, where the canonical integral operators from wire antenna theory are combined with special terms responsible for the mesoscopic structure. The theory is applied to calculate the polarizability of a finite-length single-walled carbon nanotube (CNT) with a short low-conductive section (LCS) in the microwave and subterahertz ranges. The LCS is modeled as a multichannel two-electrode mesoscopic system. The effective resistive sheet impedance boundary conditions for the scattered field are applied on the CNT surface. It is shown that the imaginary part of the polarizability spectrum has three peaks. Two of them are in the terahertz range, while the third is in the gigahertz range. The polarizability spectrum of the CNT with many LCSs has only one gigahertz peak, which shifts to low frequencies as the number of LCSs increases. The physical nature of these peaks is explained, and potential applications of nanoantennas are proposed.

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.

The thermal properties and the electrical conductance are studied for 2D electron gases in doped Xenes -- graphene, silicene, germanene, stanene, and plumbene -- applying a four-band model to describe the low-energy Dirac-fermion-like electronic excitations. Spin-orbit interactions to discriminate the five Xenes and the influence of an electric field in the normal direction, are taken into account. The density of states and the spectral behavior of the current-current correlation function allow the calculation of the Onsager coefficients. They give analytical formulas for the electronic contributions to the heat capacity and thermal conductance. Also, the electrical conductance can be described within the same framework, if the scattering properties of the electron gases are simulated by constant broadening parameters. For all these thermal and transport properties, only a weak variation with the Xene is found, because of their small spin-orbit-induced gaps, with the exception of plumbene. The heat capacity of Xenes does not show a Schottky anomaly. The thermal conductance increases linearly or quadratically with temperature depending on the temperature range. A similar behavior characterizes the electrical conductance. The dominance of Dirac fermions, i.e., linear bands, determines the ratio of electron thermal conductance and electric conductance, which depends on doping level and temperature. It violates the Wiedemann-Franz law known for 3D electron gases with parabolic energy-momentum dispersion. The Lorenz number is generally much larger for 2D electron gases with almost Dirac character of the dispersion relation.