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The dielectric tensor of Ga2O3 in the monoclinic (β) phase was determined by generalized spectroscopic ellipsometry in a wide spectral range from 0.5 eV to 8.5 eV as well as by density functional theory calculations combined with many-body perturbation theory including quasiparticle and excitonic effects. The dielectric tensors obtained by both methods are in excellent agreement with each other and the observed transitions in the dielectric function are assigned to the corresponding valence bands. It is shown that the off-diagonal element of the dielectric tensor reaches values up to |εxz| ≈ 0.30 and cannot be neglected. Even in the transparent spectral range where it is quite small (|εxz| < 0.02) it causes a rotation of the dielectric axes around the symmetry axis of up to 20°.

We study the incorporation of Mg atoms into the InN(0001) surface. Energies and atomic geometries are described within density functional theory, while the electronic structure is investigated by an approximate quasiparticle method that yields a gap value of 0.7 eV for bulk InN. The formation of substitutional Mg is energetically favored in the surface layer. The surface electronic structure is less influenced by Mg-derived states. The Fermi level is pinned by In-derived surface states. With increasing depth of Mg beneath the surface the Fermi-level position moves toward the valence band top, suggesting formation of holes and, hence, p-doping of Mg in bulk-like layers.

We compute the optical conductivity of 2D honeycomb crystals beyond the usual Dirac-cone approximation. The calculations are mainly based on the independent-quasiparticle approximation of the complex dielectric function for optical interband transitions. The full band structures are taken into account. In the case of silicene, the influence of excitonic effects is also studied. Special care is taken to derive converged spectra with respect to the number of k points in the Brillouin zone and the number of bands. In this way both the real and imaginary parts of the optical conductivity are correctly described for small and large frequencies. The results are applied to predict the optical properties reflection, transmission and absorption in a wide range of photon energies. They are discussed in the light of the available experimental data.

The electronic properties of PbTe nanocrystals (NCs) embedded in a CdTe matrix are investigated using an ab initio pseudopotential method and a repeated supercell approximation. The asymmetric distribution of polar dot-matrix interfaces leads to the presence of an electrostatic field within the NC region. The consequences of this field on the electronic properties of the dot-matrix system are discussed in detail. The resulting spatial separation of electrons and holes inside the NC region gives rise to a quantum-confined Stark effect. It influences the photoluminesence of the NCs. [ABSTRACT FROM AUTHOR]

We present a comprehensive study of buckled honeycomb germanene functionalized with alternately bonded side groups hydroxyl (-H), methyl (-CH 3 ) and trifluoro methyl (-CF 3 ). 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 Γ point. Together with the many-body effects, quasiparticle gaps of 2.3, 1.8 or 1.0 eV result for -H, -CH 3 and -CF 3 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., (© 2024. The Author(s).)

Hexagonal Si 1- x Ge x with suitable alloy composition promises to become a new silicon compatible direct bandgap family of semiconductors. Theoretical calculations, however, predict that the binary end point of this family, the bulk hex-Ge crystal, is only weakly dipole active. This is in contrast to hex-Si 1- x Ge x , where translation symmetry is broken by alloy disorder, permitting efficient light emission. Surprisingly, we observe equally strong radiative recombination in hex-Ge as in hex-Si 1- x Ge x nanowires, but scrutinizing experiments on the radiative lifetime and the optical transition matrix element of hex-Ge remain hitherto unexplored. Here, we report an advanced spectral line shape analysis exploiting the Lasher-Stern-Würfel (LSW) model on an excitation density series of hex-Ge nanowire photoluminescence spectra covering 3 orders of magnitude. The analysis was performed at low temperature where radiative recombination is dominant. We analyze the amount of photoinduced bandfilling to obtain direct access to the excited carrier density, which allows to extract a radiative lifetime of (2.1 ± 0.3) ns by equating the carrier generation and recombination rates. In addition, we leveraged the LSW model to independently extract a high oscillator strength of 10.5 ± 0.9, comparable to the oscillator strength of III/V semiconductors like GaAs or GaN, showing that the optical properties of hex-Ge nanostructures are perfectly suited for a wide range of optoelectronic device applications., Competing Interests: The authors declare no competing financial interest., (© 2024 The Authors. Published by American Chemical Society.)

Silicon is indisputably the most advanced material for scalable electronics, but it is a poor choice as a light source for photonic applications, due to its indirect band gap. The recently developed hexagonal Si 1-x Ge x semiconductor features a direct bandgap at least for x > 0.65, and the realization of quantum heterostructures would unlock new opportunities for advanced optoelectronic devices based on the SiGe system. Here, we demonstrate the synthesis and characterization of direct bandgap quantum wells realized in the hexagonal Si 1-x Ge x system. Photoluminescence experiments on hex-Ge/Si 0.2 Ge 0.8 quantum wells demonstrate quantum confinement in the hex-Ge segment with type-I band alignment, showing light emission up to room temperature. Moreover, the tuning range of the quantum well emission energy can be extended using hexagonal Si 1-x Ge x /Si 1-y Ge y quantum wells with additional Si in the well. These experimental findings are supported with ab initio bandstructure calculations. A direct bandgap with type-I band alignment is pivotal for the development of novel low-dimensional light emitting devices based on hexagonal Si 1-x Ge x alloys, which have been out of reach for this material system until now., (© 2024. The Author(s).)

Graphene-covered hexagonal SiC substrates have been frequently discussed to be appropriate starting points for epitaxial overlayers of Xenes, such as plumbene, or even their deposition as intercalates between graphene and SiC. Here, we investigate, within density functional theory, the plumbene deposition for various layer orderings and substrate terminations. By means of total energy studies we demonstrate the favorization of the intercalation versus the epitaxy for both C-terminated and Si-terminated 4H-SiC substrates. These results are explained in terms of chemical bonding and by means of layer-resolved projected band structures. Our results are compared with available experimental findings., (© 2024. The Author(s).)

We report on DFT-TDDFT studies of the structural, electronic and vibrational properties of B24N24 nanocapsules and the effect of encapsulation of homonuclear diatomic halogens (Cl2, Br2 and I2) and chalcogens (S2 and Se2) on the interaction of the B24N24 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.

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.

We present a computationally efficient and accurate methodology to compute Z 2 topological invariants for systems without inversion symmetry including quasiparticle (QP) effects within the density functional theory (DFT)-1/2 method. The Wannier charge center evolution is applied to compute the Z 2 topological invariant and investigate the topological properties of group-IV graphene-like systems, graphene, silicene, germanene and stanene, whose inversion symmetry is broken by simultaneous functionalization with hydrogen and fluorine atoms. Different atomic arrangements are studied. The systems are stable with cohesive energy decreasing along the row from carbon to tin. A similar trend is observed for band gaps. The resulting topological invariants are compared with values obtained within conventional DFT and using a hybrid exchange-correlation functional. The variation of the results with the treatment of exchange and correlation demonstrates the importance of QP corrections for the prediction of the topological or trivial character., (© 2021 IOP Publishing Ltd.)

The allotropic affinity for bulk silicon and unique electronic and optical properties make silicene a promising candidate for future high-performance devices compatible with mature complementary metal-oxide-semiconductor technology. However, silicene's outstanding properties are not preserved on its most prominent growth templates, due to strong substrate interactions and hybridization effects. In this letter, we report the optical properties of silicene epitaxially grown on Au(111). A novel in situ passivation methodology with few-layer hexagonal boron nitride enables detailed ex situ characterization at ambient conditions via μ-Raman spectroscopy and reflectance measurements. The optical properties of silicene on Au(111) appeared to be in accordance with the characteristics predicted theoretically for freestanding silicene, allowing the conclusion that its prominent electronic properties are preserved. The absorption features are, however, modified by many-body effects induced by the Au substrate due to an increased screening of electron-hole interactions.

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 report a systematic investigation on the electronic and optical properties of the smallest stable clusters of alkaline-earth metal fluorides, namely, MgF 2 , CaF 2 , SrF 2 , and BaF 2 . For these clusters, we perform density functional theory (DFT) and time-dependent DFT (TDDFT) calculations with a localized Gaussian basis set. For each molecule ((MF 2 ) n , n = 1-3, M = Mg, Ca, Sr, Ba), we determine a series of molecular properties, namely, ground-state energies, fragmentation energies, electron affinities, ionization energies, fundamental energy gaps, optical absorption spectra, and exciton binding energies. We compare electronic and optical properties between clusters of different sizes with the same metal atom and between clusters of the same size with different metal atoms. From this analysis, it turns out that MgF 2 clusters have distinguished ground-state and excited-state properties with respect to the other fluoride molecules. Sizeable reductions of the optical onset energies and a consistent increase of excitonic effects are observed for all clusters under study with respect to the corresponding bulk systems. Possible consequences of the present results are discussed with respect to applied and fundamental research., Competing Interests: The authors declare no competing financial interest., (Copyright © 2020 American Chemical Society.)

Silicon crystallized in the usual cubic (diamond) lattice structure has dominated the electronics industry for more than half a century. However, cubic silicon (Si), germanium (Ge) and SiGe alloys are all indirect-bandgap semiconductors that cannot emit light efficiently. The goal 1 of achieving efficient light emission from group-IV materials in silicon technology has been elusive for decades 2-6 . Here we demonstrate efficient light emission from direct-bandgap hexagonal Ge and SiGe alloys. We measure a sub-nanosecond, temperature-insensitive radiative recombination lifetime and observe an emission yield similar to that of direct-bandgap group-III-V semiconductors. Moreover, we demonstrate that, by controlling the composition of the hexagonal SiGe alloy, the emission wavelength can be continuously tuned over a broad range, while preserving the direct bandgap. Our experimental findings are in excellent quantitative agreement with ab initio theory. Hexagonal SiGe embodies an ideal material system in which to combine electronic and optoelectronic functionalities on a single chip, opening the way towards integrated device concepts and information-processing technologies.