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We present a comprehensive study of buckled honeycomb germanene functionalized with alternately bonded side groups hydroxyl (–H), methyl (–CH3) and trifluoro methyl (–CF3). By means of most modern theoretical and computational methods we determine the atomic geometries versus the functionalizing groups. The quasiparticle excitation effects on the electronic structure are taken into account by means of exchange-correlation treatment within the GW framework. The Bethe–Salpeter equation is solved ab initio to derive optical spectra including excitonic and quasiparticle effects. Band edge excitons are investigated in detail. The binding properties are compared with those resulting from model studies. The functionalization leads to significantly modified band structures compared with pristine germanene. The Dirac bands near the K point are destroyed and direct gaps appear at the point. Together with the many-body effects, quasiparticle gaps of 2.3, 1.8 or 1.0 eV result for –H, –CH3 and –CF3 functionalization. Totally different absorption spectra are found for in-plane and out-of-plane light polarization. Strongly bound excitons are visible below the quasiparticle band edge with binding energies of about 0.5, 0.4 or 0.3 eV. The nature of these band-gap excitons is investigated via their wave function, the contribution of various interband combinations and the dipole selection rules. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

By means of ab initio calculations we demonstrate that perturbed hexagonal germanium is a superior material for active optoelectronic devices in the infrared spectral region. Perfect lonsdaleite germanium is a pseudodirect semiconductor, with a direct fundamental band gap but almost vanishing optical transitions. Perturbing the system by replacing a germanium atom with a silicon atom in the primitive cell increases the oscillator strength at the onset gap by orders of magnitude, with a concurrent blue shift of the transition energies. This effect is mainly due to the increased s character of the lowest conduction band because of the perturbation‐induced wave function mixing. A structural distortion of pure lonsdaleite Ge can enhance as well optical oscillator strengths, but their magnitude significantly depends on the specific details of the resulting atomic geometry. In particular, moderate tensile uniaxial strain can induce an order inversion of the two lowest conduction bands, leading to an extremely efficient enhancement of the dipole matrix elements at the minimum gap. In general, chemical and/or structural perturbations are shown to make lonsdaleite germanium a suitable material for light emitting devices. [ABSTRACT FROM AUTHOR]

Exciton binding energies EB measured for α- and β-Ga2O3 crystals seem to be not explainable in the Wannier-Mott picture. However, we demonstrate theoretically that including screening dynamics and effective mass anisotropy, reasonable values EB = 184 meV (α) or 230 meV (β) are derived. Since the binding energies are larger than the phonon frequencies, the exciton formation is so fast that only about 5% of the lattice polarizability contributes to the screening of the electron-hole attraction. Effective dielectric constants possess values between the complete static ones and the electronic high-frequency dielectric constants. They indeed depend on the polymorph. [ABSTRACT FROM AUTHOR]

We report on DFT-TDDFT studies of the structural, electronic and vibrational properties of B 24 N 24 nanocapsules and the effect of encapsulation of homonuclear diatomic halogens ( C l 2 , B r 2 and I 2 ) and chalcogens ( S 2 and S e 2 ) on the interaction of the B 24 N 24 nanocapsules with the divalent magnesium cation. In particular, to foretell whether these BN nanostructures could be proper negative electrodes for magnesium-ion batteries, the structural, vibrational and electronic properties, as well as the interaction energy and the cell voltage, which is important for applications, have been computed for each system, highlighting their differences and similarities. The encapsulation of halogen and chalcogen diatomic molecules increases the cell voltage, with an effect enhanced down groups 16 and 17 of the periodic table, leading to better performing anodes and fulfilling a remarkable cell voltage of 3.61 V for the iodine-encapsulated system. [ABSTRACT FROM AUTHOR]

We present a comprehensive study of the structural and electronic properties of a graphene/phosphorene (G/P) heterostructure in the framework of density functional theory, including van der Waals interaction in the exchange–correlation functional. While the G(4 × 1)/P(3 × 1) superlattice usually used in the literature is subject to a strain as high as about 7%, the in-plane strain could be drastically reduced to under 1% in the G(4 × 13)/P(3 × 12) heterostructure investigated here. Adapting the lattice constants of the rectangular lattices, the equilibrium configuration in the xy plane of phosphorene relative to the graphene layer is optimized. This results in an equilibrium interlayer distance of 3.5 Å and a binding energy per carbon atom of 37 meV, confirming the presence of weak van der Waals interaction between the graphene and the phosphorene layers. The electronic properties of the heterostructure are evaluated under different values of interlayer distance, strain and applied vertical electric field. We demonstrate that G/P heterostructures form an n-type Schottky contact, which can be transformed into p-type under external perturbations. These findings, together with the possibility to control the gaps and barrier heights, suggest that G/P heterostructures are promising for novel applications in electronics and may open a new avenue for the realization of innovative optoelectronic devices. [ABSTRACT FROM AUTHOR]

Both a-Sn and HgTe are expected to have similar topological properties because of their inverted band structure and zero-gap character. We investigate how the different crystal symmetries and the bonding to barrier materials act to the quantum phase transition versus the thickness of the corresponding quantum well (QW) structures. They are simulated by (SnSn)w(CdTe)M and (HgTe)Ar(CdTe)M(l 10) superlattices. Their electronic structures and eigenstates are studied by means of first-principles calculations using the modified Becke-Johnson exchangecorrelation functional and spin-orbit interaction. Significant differences are observed for the two QW materials. A topological transition between trivial insulator and quantum spin Hall phase together with the formation of topologically protected edge states are observed in the case of HgTe QWs, while these features are considerably modified for α-Sn. The different behaviors are discussed in the light of the different symmetry, spin-orbit interaction, and interface bonding. For a better understanding of the influence of the interface electrostatics also results for (HgTe)w(InSb)M(l 10) systems are discussed. [ABSTRACT FROM AUTHOR]

We investigate the edge states of real group IV honeycomb crystals by means of ab initio methods. We fully take spin-orbit coupling into account. Thereby germanene, the germanium-based counterpart to graphene, is used as the model system. We focus on the topological properties depending on the non-, antiferro-, and ferromagnetic edges of the ribbon. We show that, in contrast to model studies, the dispersion of the edge states is strongly nonlinear, destroying gapless Dirac bands. Applying an electric field we further study the topological phase transition to a trivial insulator above a critical field strength. The results are critically discussed and compared with tight-binding predictions. [ABSTRACT FROM AUTHOR]

(HgTe)N(CdTe)M(l 10) and (001) superlattices are studied by means of ah initio calculations versus the thickness of the HgTe quantum wells (QWs). The used approximate quasiparticle theory including spin-orbit coupling (SOC) gives the correct band ordering, band gap, and SOC splitting for bulk HgTe and CdTe. The resulting band discontinuities indicate confinement also for occupied states. In agreement with earlier k . p calculations and experiments we find a topological transition from the topological nontrivial quantum spin Hall state into a trivial insulator with decreasing QW thickness. The spatial localization near the interfaces and the spin polarization are demonstrated for the edge states for QWs with thicknesses near the critical one. They do not depend on the QW orientation and are therefore topologically protected. Below the critical QW thickness, the trivial insulator exhibits drastic confinement effects with a significant gap opening. We show that the inclusion of inversion symmetry, the nonaxial rotation symmetry of the QWs, and the real QW barriers lead to some agreement but also significant deviations from the predictions within toy models. The deviations concern the critical thickness, the number and localization of edge states, and the possibility to find QW subbands between edge states. [ABSTRACT FROM AUTHOR]

We present ab initio studies of NiAs, zinc-blende, wurtzite, and rocksalt MnTe with possible magnetic arrangements of Mn atoms using spin-polarized density functional theory and generalized gradient approximation for exchange and correlation. The impact of an empirical on-site Coulomb interaction U on the lattice constants and ground-state energies is especially studied for antiferromagnetic NiAs and zinc-blende phases, for which contradictory results are found. While U=0 eV gives the correct energetic ordering of the crystal structures, the experimental lattice constants can only be reproduced for finite U=2,...,4 eV. The Hubbard U also strongly influences the magnitude of the magnetic interaction parameters and hence the transition temperatures. In order to obtain Néel temperatures in agreement with experiment a value of about U=4 eV is needed. We demonstrate how U affects the magnetic coupling via the localization of the Mn3 d orbitals. [ABSTRACT FROM AUTHOR]

In many-body perturbation theory (MBPT) the self-energy Σ = iGWT plays a key role since it contains all the many-body effects ofthe system. The exact self-energy is not known; as a first approximation one can set the vertex function T to unity which leads to the GW approximation. The latter properly describes the high-density regime, where screening is important; in the low-density regime, instead, other approximations are proposed, such as the T matrix, which describes multiple scattering between two particles. Here we combine the two approaches. Starting from the fundamental equations of MBPT, we show how one can derive the T-matrix approximation to the self-energy in a common framework with G W. This allows us to elucidate several aspects of this formulation, including the origin of, and link between, the electron-hole and the particle-particle T matrix, the derivation of a screened T matrix, and the conversion of the T matrix into a vertex correction. The exactly solvable Hubbard molecule is used for illustration. [ABSTRACT FROM AUTHOR]

We present an ab initio description of charge transport in organic semiconductors based on a recently developed theory that goes beyond small-polaron and/or narrow-band models. The mobility expression is evaluated with parameters from density functional theory, and application to naphthalene crystals demonstrates substantial progress in the simulated temperature dependence and relative magnitudes for all transport directions. The scattering by phonons is described in a manner that goes beyond the Holstein model for small polarons and, consequently, significantly improves the temperature dependence and anisotropy of carrier mobility with respect to the previous narrow-band theory. The contributions of coherent and incoherent scattering processes are analyzed. Special emphasis is placed on the origin and understanding of the transition from band transport at low temperatures to hopping at high temperatures, both of which are fully included in the theory. Additionally, in contrast to earlier narrow-band theories, the unphysical divergence at zero temperatures is removed. [ABSTRACT FROM AUTHOR]

Wegen der Bedeutung des Halbleiters Silizium (Si) für die moderne Kommunikations‐ und Computertechnologie werden die letzten Jahrzehnte häufig als Siliziumzeitalter bezeichnet. Mit der zunehmenden Miniaturisierung der Schaltkreise steigt aber das Risiko für Störungen der elektrischen On‐Chip‐Kommunikation. Eine mögliche Lösung dieses Problems könnte eine optische Kommunikation mit Lichtemittern und ‐empfängern auf demselben Chip sein. Die kürzlich von unserem europäischen Forscherteam nachgewiesene Lichtemission von hexagonalen Ge‐ und SiGe‐Halbleitern könnte solche Bauelemente ermöglichen. [ABSTRACT FROM AUTHOR]

We conducted a systematic investigation using state-of-the-art techniques on the electronic and optical properties of two crystals of alkaline earth metal fluorides, namely rutile MgF 2 and cubic SrF 2 . For these two crystals of different symmetry, we present density functional theory (DFT), many-body perturbation theory (MBPT), and Bethe–Salpeter equation (BSE) calculations. We calculated a variety of properties, namely ground-state energies, band-energy gaps, and optical absorption spectra with the inclusion of excitonic effects. The quantities were obtained with a high degree of convergence regarding all bulk electronic and optical properties. Bulk rutile MgF 2 has distinguished ground-state and excited-state properties with respect to the other cubic fluoride SrF 2 and the other members of the alkaline earth metal fluoride family. The nature of the fundamental gaps and estimates of the self-energy and excitonic effects for the two compounds are presented and discussed in detail. Our results are in good accordance with the measurements and other theoretical–computational data. A comparison is made between the excitation and optical properties of bulk rutile MgF 2 , cubic SrF 2 , and the corresponding clusters, for which calculations have recently been published, confirming strong excitonic effects in finite-sized systems. [ABSTRACT FROM AUTHOR]

The fascinating electronic and optoelectronic properties of freestanding graphene and the possible inclusion of novel two-dimensional (2D) systems in silicon-based electronics have driven the search for atomic layers consisting of other group-IV elements Si, Ge, Sn, and Pb, which form similar hexagonal lattices and are isoelectronic to graphene. The resulting 2D crystals silicene, germanene, stanene and plumbene, referred as Xenes, but also their functionalized counterparts, e.g. the hydrogenated sheet crystals, named as Xanes, silicane, germanane, and stanane, are in the focus of this review article. In addition, halogenated Xenes are investigated. The consequences of the larger atomic radii on the atomic geometry, the energetic stability, and possible epitaxial preparations are discussed. In the case of honeycomb atomic arrangements, the low-energy electronic excitations are ruled by almost linear bands. Spin–orbit coupling opens small gaps leading to Dirac fermions with finite effective masses. The linear bands give rise to an absorbance of the Xenes determined by the finestructure constant in the long-wavelength regime. While for vanishing photon energies the excitonic influence is still an open question, saddle-point excitons and excitons at M 0 van Hove singularities appear at higher frequencies. After opening substantial fundamental gaps by hydrogenation, the absorption edges of the Xanes, silicane, germanane, and stanane, are dominated by bound excitons with extremely large binding energies. Other chemical functionalizations, but also vertical electric fields, yield electronic structures ranging from topological to trivial insulators. Even a quantum spin Hall phase is predicted at room temperature. The topological character and the possible quantization of the spin Hall conductivity are studied versus gap inversion, chemical functionalization, and Rashba spin–orbit interaction. The drastic changes of the electronic properties of Xenes with chemical functionalization, interaction with the substrate, and external perturbations, open future opportunities for tailoring fundamental properties and, therefore, interesting applications in novel electronic and optoelectronic nanodevices. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

By means of an approximate quasiparticle description we study the electronic structure of wurtzite‐GaP in more detail for the two lowest Γ8c and Γ7c conduction bands and the three highest Γ9v, Γ7+v, and Γ7−v valence bands. We conclude that the corresponding three gaps between the valence bands and the Γ8c conduction band are quasi‐direct, while the ones involving the s‐like Γ7c conduction band are direct. The optical oscillator strengths are also calculated outside the Γ point. Their influence on the observability of excitons in absorption and emission spectra is investigated. The almost dipole‐forbidden transitions into the lowest p‐like Γ8c conduction band become substantial for finite wave vectors perpendicular to the c‐axis. Therefore, the finite extent of the exciton envelope functions gives rise to non‐vanishing transition strengths, which explain the intense photoluminescence observed experimentally by forbidden p‐type excitons. Wurtzite‐GaP is a quasidirect semiconductor with a lowest conduction band of Γ8 symmetry. The corresponding optical transitions from the highest Γ9 and Γ7 valence bands are almost forbidden at Γ. However, the optical matrix elements nearly increase linearly with the in‐plane wave vector. Consequently, the accompanying Wannier–Mott excitons with p character possess finite oscillator strengths, which explain the intense photoluminescence observed experimentally. [ABSTRACT FROM AUTHOR]

We present atomic and electronic structure studies using first-principles calculations of two-dimensional topological insulators, stanene and fluorostanene, deposited on 4 H -SiC(0001) substrates. We demonstrate the stability of H- or F-passivated honeycomb crystals due to a van der Waals interaction between the adsorbate and substrate. Despite destroyed inversion symmetry and biaxial strain the calculations of the band structures and Z2 topological invariants predict that the quantum spin Hall (QSH) phase of stanene on H-passivated SiC as well as fluorostanene on H- and F-passivated SiC survives the interaction with the substrate. Our findings should serve as guidance for the epitaxial growth of tin-based QSH systems on wide-band-gap semiconductors. [ABSTRACT FROM AUTHOR]

The electronic and topological properties of heterovalent and heterocrystalline a-Sn/CdTe(001) quantum wells (QWs) are studied in dependence on the thickness of α-Sn by means of ab initio calculations. We calculate the topological Z2 invariants of the respective bulk crystals, which identify α-Sn as strong three-dimensional (3D) topological insulators (TIs), whereas CdTe is a trivial insulator. We predict the existence of two-dimensional (2D) topological interface states between both materials and show that a topological phase transition from a trivial insulating phase into the quantum spin Hall (QSH) phase in the QW structures occurs at much higher thicknesses than in the HgTe case. The QSH effect is characterized by the localization, dispersion, and spin polarization of the topological interface states. We address the distinction of the 3D and 2D TI characters of the studied QW structures, which is inevitable for an understanding of the underlying quantum state of matter. The 3D TI nature is characterized by two-dimensional topological interface states, while the 2D phase exhibits one-dimensional edge states. The two different state characteristics are often intermixed in the discussion of the topology of 2D QW structures, especially, the comparison of ab initio calculations and experimental transport studies. [ABSTRACT FROM AUTHOR]

We report results of ab initio density functional theory including van der Waals interaction for the formation of a silicene bilayer on top of the Ag(111) surface with significant differences to the monolayer case. We find (2√3 × 2√3)R30° bilayer silicene on (√19 × √19)R23.4° silver substrate to be the most stable. The calculated STM images, however, exhibit a (√3 × √3)R30° symmetry. This translational symmetry, the resulting lattice spacing, and the height of the topmost monolayer agree with recent experimental findings. The band structure of the complete adsorbate system shows conical linear bands near the Fermi level due to the hybridization of adsorbate and substrate states. [ABSTRACT FROM AUTHOR]

We present an accurate ab initio description of the magnetic exchange force microscopy (MExFM). As a prototypical system, the antiferromagnetic NiO(001) surface probed with a Fe tip is investigated. The tip–surface interaction is described on two levels. Short-range chemical and exchange forces between the tip apex and the surface atoms are described in the framework of spin-polarized density functional theory while long-range van der Waals forces are considered within a mesoscopic tip model. For the Ni atoms in the NiO surface as well as the Fe atoms of the tip apex, an on-site repulsion U in the transition-metal 3d shells is included. In order to understand the tip–surface interaction, we investigate the changes in the electronic structure of tip and surface versus distance. The resulting frequency shifts and MExFM images are in good qualitative agreement with experimental data. [ABSTRACT FROM AUTHOR]

By means of first-principles calculations we predict the stability of silicene as buckled honeycomb lattice on passivated substrates of group-IV(111)1 × 1 surfaces. The weak van-der-Waals interaction between silicene and substrates does not destroy its linear bands forming Dirac cones at the Brillouin zone corners. Only very small fundamental gaps are opened around the Fermi level. [ABSTRACT FROM AUTHOR]

We calculate the binding energies of the In4d and Ga3d electrons in wurtzite InxGa1-xN and InxAl1-xN alloys within the quasiparticle approach based on hybrid-functional starting points and the GW approximation. The alloys are described using a cluster approach with 256 configurations arranged in 22 cluster classes. Averaged core level positions are determined by means of a configurational average including statistical weights of the clusters. We demonstrate that even the semicore levels are influenced by the local arrangements of the cations and, hence, the average alloy composition x. Composition-dependent core level shifts are derived for the semicore levels of random alloys. [ABSTRACT FROM AUTHOR]

We investigate structural and electronic properties of graphene-based one-dimensional (1D) superlattices by means of first-principles calculations. The 1D superlattices are modeled by three different chemical and structural modifications of the carbon lattice. We study partial hydrogenation of graphene, substitution of carbon-atom pairs by boron and nitrogen atoms, and incorporation of ribbons of the recently discovered silicene. Our investigations based on density functional theory provide accurate predictions of structural and electronic properties of all systems under consideration. In detail, the formation of 1D interfaces between two-dimensional honeycomb crystals is studied, including the construction of a coincidence lattice. The importance of the interface for the energy alignment and the Fermi-level position is proven. Finally, we discuss the results in light of the possible realization of a quasi-1D metal. [ABSTRACT FROM AUTHOR]

Results for structural and elastic properties of wurtzite and zinc-blende group-III nitrides are calculated using the recently developed AM05 exchange-correlation (XC) functional. They are compared to calculations based on the local-density approximation or the generalized-gradient approximation. We find that AM05 provides a better agreement with experimental results. The atomic geometries are used to compute the quasiparticle band structures within Hedin's GW approximation, based on an initial electronic structure calculated using the HSE hybrid XC functional. Important band parameters such as gap energies, crystal-field splittings, spin-orbit coupling constants, and momentum matrix elements are derived. The less precisely known hole masses of InN and the anisotropic spin-orbit constants for wurtzite are predicted. The wave-vector-induced spin-orbit splittings of the valence and conduction bands are discussed. [ABSTRACT FROM AUTHOR]

A systematic study of ground state properties of cubic (3C) and hexagonal (6H, 4H, 2H) polytypes of normal (nonnitride) III-V compounds is reported using well-converged density-functional calculations within local density approximation, ab initio pseudopotentials, and projector-augmented wave method. Equilibrium results are obtained for lattice parameters, cohesive energies, and bulk moduli. Internal degrees of freedom, i.e., atomic relaxations, are taken into account. Trends with the hexagonality are discussed. Within the axial next-nearest-neighbor Ising (ANNNI) model driving forces of the polytypism as well as the stacking fault formation are derived. The results are compared with available experimental data. [ABSTRACT FROM AUTHOR]

Germanium nanostructures have been predicted to exhibit very strong HOMO-LUMO transitions, contrary to the respective silicon structures. However, few experiments have found luminescence that can clearly be attributed to quantum-confined electron-hole pairs. We resolve this apparent contradiction by identifying four effects that strongly reduce the luminescence intensity in clean, hydrogenated Ge nanocrystals: compression, surface reconstruction, optical excitation, and spin-orbit splitting. The pressure dependence of the HOMO-LUMO gaps and of the radiative lifetimes is explained in terms of the mixing of bulk optical transitions. The localization of electronic states due to a surface reconstruction, and the effect of the optical excitation reduce the luminescence intensity by creating weak transitions below the onset of the absorption spectrum. Spin-orbit coupling reduces the luminescence intensity in a Ge41H60 nanocrystal by a factor of about 5 compared to calculations where it is neglected. Throughout the paper, the results are compared with those of equivalent silicon nanocrystals. [ABSTRACT FROM AUTHOR]

We study the charge transport through crystalline durene which exhibits large hole mobilities. By means of ab initio calculations, we determine the temperature-dependent polaron bandwidth and the mobility tensor of the charge carriers. The origin of the bandlike mobility curves is discussed, and the strong anisotropy of the mobility is analyzed in detail. We put a special focus on the relationship between crystal packing geometry, molecular wave function overlap, and charge transport properties. The results include a visualization of the transport channels in durene which can be regarded as a prototypical herringbone-stacked crystal. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

In order to investigate the applicability of new photovoltaic absorber materials, we show how to use first-principles calculations combined with device simulations to determine the efficiency limits of solar cells made from SiO2/Si superlattices and from coaxial ZnO/ZnS nanowires. Efficiency limits are calculated for ideal systems according to the Shockley¿Queisser theory but also for more realistic devices with finite mobilities, nonradiative lifetimes, and absorption coefficients. Thereby, we identify the critical values for mobility and lifetime that are required for efficient single junction as well as tandem solar cells. [ABSTRACT FROM AUTHOR]

Stepped gold‐stabilized Si(111) surfaces offer much potential for self‐organized assembly of one‐ and two‐dimensional nanostructured arrays of organic molecules due to the presence of regular atomic chains of Au and Si on the surface, parallel to the step edges. In this theoretical study, the adsorption of various simple organic molecules with different functional groups on the Si(553)–Au surface has been investigated. It has been found that adsorption at the step edge site is favored by all species. If the step edge is first passivated by hydrogen, adsorption occurs on Si atoms on the terraces. Instead, adsorption on the gold chains is completely unfavored. The influence of organic molecule adsorption on the electronic band structure and surface metallicity is investigated. [ABSTRACT FROM AUTHOR]

The adsorption of small organic molecules onto vicinal Au-stabilized Si(111) surfaces is shown to be a versatile route towards controlled growth of ordered organic-metal hybrid one-dimensional nanostructures. Density functional theory is used to investigate the site-specific adsorption of toluene-3,4-dithiol (TDT) molecules onto the clean Si(553)-Au surface and onto a co-doped surface whose steps are passivated by hydrogen. We find that the most reactive sites involve bonding to silicon at the step edge or on the terraces, while gold sites are relatively unfavored. H passivation and TDT adsorption both induce a controlled charge redistribution within the surface layer, causing the surface metallicity, electronic structure, and chemical reactivity of individual adsorption sites to be substantially altered. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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 ≪ λ (λ-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 → 0 should be taken in the final expressions. Example spectra are given for the group-IV honeycomb 2D crystals graphene and silicene. [ABSTRACT FROM AUTHOR]

The deposition of silicene on several metals is investigated. For fcc crystals the (111) surfaces while for hexagonal ones the (0001) surfaces are used. The Ca(111)1 × 1 substrate is found to be the most promising candidate. The silicene adsorption on Ca-functionalized Si(111)1 × 1 and 2 × 1 surfaces is also studied. The 1 × 1 substrates lead to overlayer silicene with hexagonal symmetry and Dirac cones. However, the Dirac points are below the Fermi level, and small energy gaps are opened. In the case of 2 × 1 surfaces, strong lattice relaxation occurs. Only rudiments of conical linear bands remain visible. [ABSTRACT FROM AUTHOR]

Several researchers claim to have produced silicene, the graphene‐like allotrope of silicon, that could result in the ultimate miniaturization of the Si‐device technology and in novel electronic and optical properties due to the appearance of massless Dirac electrons. It is sometimes described as a new “wonder material”. Favored substrates are Ag(111) and Ir(111) metal surfaces. However, the growth of silicene layers is a huge challenge. Recent theoretical and experimental studies show a destruction of the Dirac cones due to a lowering of the honeycomb symmetry and the strong interaction with the substrate. In the Letter by Kokott, Matthes, and Bechstedt (pp. 538–541) the possibility of the silicene formation on hydrogen‐passivated Si(111) (as displayed on the front cover) and Ge(111) surfaces is predicted. The stability of a silicene sheet and its exceptional electronic properties are demonstrated by first‐principles studies. The van‐der‐Waals interaction bonds the silicene layer to the inert substrates. Because of the reduced chemical interaction the linear conical bands near the corner points of the hexagonal Brillouin zone survive with an extremely small energy gap. [ABSTRACT FROM AUTHOR]

The uniaxial stress dependence of the band structure and the exciton-polariton transitions in wurtzite ZnO is thoroughly studied using modern first-principles calculations based on the HSE + G0W0 approach, kp modeling using the deformation potential framework, and polarized photoluminescence measurements. The ordering of the valence bands [A(Γ7), B(Γ9), C(Γ7)] is found to be robust even for high uniaxial and biaxial strains. Theoretical results for the uniaxial pressure coefficients and splitting rates of the A, B, and C valence bands and their optical transitions are obtained including the effects of the spin-orbit interaction. The excitonic deformation potentials are derived and the stress rates for hydrostatic pressure are determined based on the results for uniaxial and biaxial stress. In addition, the theory for the stress dependence of the exchange interaction and longitudinal-transversal splitting of the exciton polaritons is developed using the basic exciton functions of the quasicubic approximation and taking the interaction between all exciton states into account. It is shown that the consideration of these effects is crucial for an accurate description of the stress dependence of the optical spectra in ZnO. The theoretical results are compared to polarized photoluminescence measurements of different ZnO substrates as function of uniaxial pressure and experimental values reported in the literature demonstrating an excellent agreement with the computed pressure coefficients. [ABSTRACT FROM AUTHOR]

We investigate the optical properties of hydrogenated α-Sn nanocrystals up to diameters of 3.6 nm in the framework of an ab initio pseudopotential method including spin–orbit interaction (SOI) and the repeated supercell approximation. The fundamental absorption and emission edges are determined including quasiparticle effects and electron–hole interaction. The atomic geometries in the ground and excited electronic states follow from total energy optimization. We discuss the oscillator strengths of the optical absorption near the fundamental gaps for the most important transitions. We demonstrate that the spectra can only be correctly described including SOI. The strongly size-dependent Stokes shifts between optical absorption and emission are shown to be mainly a consequence of the different atomic geometries. [ABSTRACT FROM AUTHOR]

We present structural characterization of the wurtzite crystal structure of GaP nanowires, which were recently shown to have a direct electronic band gap. The structural parameters of the wurtzite phase do consist of two lattice parameters and one internal degree of freedom, determining the Ga-P bond length along the c direction. Using density functional theory calculations, we study the influence of the internal degree of freedom on the band structure. By synchrotron x-ray diffraction studies near the Ga-k edge we determine the lattice parameters a = 3.8419 Å and c = 6.3353 Å as well as the internal degree of freedom u = 0.37385 with high accuracy. We find that different Ga-P bond lengths are not equal, in contrast to the case in the zinc blende bulk phase. As a result, a spontaneous polarization is predicted for wurtzite GaP. [ABSTRACT FROM AUTHOR]

xThe α phase of tin is a zero-gap semiconductor with an inverted band structure with respect to other group-IV elements like Ge. The Γ6c states lie energetically below the Γ8v levels. How these unique electronic properties transform in nanostructures with spatial confinement has not been studied. We apply density-functional theory within the local density approximation to investigate the energetic, structural, and electronic properties of bulk α-Sn and its nanocrystals (NCs) up to a size of 363 Sn atoms. For NCs with larger diameters up to 14 nm the tight-binding method is applied for the electronic states. Spin-orbit coupling is taken into account. The clusters are modeled in such a way that the Td symmetry of the bulk system is conserved. Their surfaces are passivated with hydrogen. We show that the spatial confinement causes not only a decrease of the fundamental gap for increased NC size but also a topological transition where the ordering of s- and p-like highest-occupied molecular orbital and lowest-unoccupied molecular orbital states is interchanged. The influence of quasiparticle and excitonic effects on the lowest pair excitation energies is investigated within approximations based on the hybrid exchange-correlation functional by J. Heyd, G. E. Scuseria, and M. Ernzerhof [ J. Chem. Phys. 118 8207 (2003)] (HSE) and the ΔSCF method. [ABSTRACT FROM AUTHOR]

By combining modern many-body approaches with a cluster expansion scheme, frequency-dependent dielectric functions including excitonic and local-field effects are computed for wurtzitic group-III nitride alloys with varying composition x. The quasiparticle electronic structure required to construct the quasielectron-quasihole pair Hamiltonian for each cluster is approximated using a LDA + U + Δ approach. Two different cluster statistics are employed to perform configurational averages for the frequency-dependent complex dielectric functions. Comparing the resulting composition dependence of peak positions and intensities to experimental data allows conclusions regarding the distribution of the group-III cations in the alloys. [ABSTRACT FROM AUTHOR]

Electronic excitations and optical spectra of CdF2 are calculated up to ultraviolet employing state-of-the-art techniques based on density functional theory and many-body perturbation theory. The GW scheme proposed by Hedin has been used for the electronic self-energy to calculate single-particle excitation properties as energy bands and densities of states. For optical properties many-body effects, treated within the Bethe-Salpeter equation framework, turn out to be crucial. A bound exciton located about 1 eV below the quasiparticle gap is predicted. Within the present scheme the optical absorption spectra and other optical functions show an excellent agreement with experimental data. Moreover, we tested different schemes to obtain the best agreement with experimental data. Among the several schemes, we suggest a self-consistent quasiparticle energy scheme. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We present ab initio calculations of the electronic and transport properties of one-dimensional π-conjugated stacks of guanine- and melanin-based molecules. The electron and hole bandwidths are found to be very large for these organic materials (up to 2 eV), and the effective masses of the charge carriers are rather small. Since these material parameters basically determine the intermolecular charge-transport properties, we predict highly ordered self-assembled one-dimensional stacks of such biological molecules to be promising candidates for applications in organic electronics. [ABSTRACT FROM AUTHOR]

Hydrogen doped ZnO thin films were deposited by radio frequency magnetron sputtering from a ceramic target on c-plane sapphire and fused silica using H2 and O2 as reactive gases. Structural analysis revealed that all films are polycrystalline with the c axis oriented perpendicularly to the substrate surface. The lateral grain size was strongly affected by the oxygen content of the sputtering gas and decreased dramatically above a critical content of 4.5 %. We were able to adjust the carrier density of the films by the deposition parameters to any value between 1014 and 2 × 1020 cm-3. Using temperature-dependent Hall-effect measurements we identified thermionic emission over Coulomb-barriers created by surface trap states at the grain boundaries and tunneling effects to dominate the carrier transport. Preparing and thoroughly characterizing the films is a prerequisite for our investigation of the dependence of the optical band gap energy on the carrier density. We use results from experiment as well as first-principles calculations (including Burstein-Moss shift, band gap renormalization, and excitonic effects) in order to understand the mechanisms that determine how free electrons influence the energy position of the optical absorption onset. [ABSTRACT FROM AUTHOR]

The study of the oxygen vacancy (F center) in MgO has been aggravated by the fact that the positively charged and the neutral vacancy (F+ and F0, respectively) absorb at practically identical energies. Here we apply many-body perturbation theory in the G0 W0 approximation and the Bethe-Salpeter approach to calculate the optical absorption and emission spectrum of the oxygen vacancy in all three charge states. We observe unprecedented agreement between the calculated and the experimental optical absorption spectra for the F0 and F+ center. Our calculations reveal that not only the absorption but also the emission spectra of different charge states peak at nearly the same energy, which leads to a reinterpretation of the F center's optical properties. [ABSTRACT FROM AUTHOR]

The ternary, isostructural, wurtzite-derived group-Ill mononitride alloys InxGa1-xN and InxAl1-xN are reexamined within a cluster expansion approach. Using density functional theory together with the AM05 exchange-correlation functional, the total energies and the optimized atomic geometries of all 22 clusters classes of the cluster expansion for each material system are calculated. The computationally demanding calculation of the corresponding quasiparticle electronic structures is achieved for all cluster classes by means of a recently developed scheme to approximately solve the quasiparticle equation based on the HSE06 hybrid functional and the G0 W0 approach. Using two different alloy statistics, the configurational averages for the lattice parameters, the mixing enthalpies, and the bulk moduli are calculated. The composition-dependent electronic structures of the alloys are discussed based on configurationally averaged electronic states, band gaps, and densities of states. Ordered cluster arrangements are found to be energetically rather unfavorable, however, they possess the smallest energy gaps and, hence, contribute to light emission. The influence of the alloy statistics on the composition dependencies and the corresponding bowing parameters of the band gaps is found to be significant and should, hence, lead to different signatures in the optical-absorption or -emission spectra. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]