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Using first principle calculations, we have studied the structural and electronic properties of two dimensional bilayers of indium and gallium chalcogenides. With density functional theory corrected for van derWaals interactions, the different modes of stacking were investigated in a systematic way, and several of them were found to compete in energy. Then, their band structures were obtained with the GW approximation and found to correspond to indirect bandgap semiconductors with a small dependency on the mode of stacking. Finally, by analysing the electron density, it appeared that GaSe-InS is a promising system for electron-hole separation. [ABSTRACT FROM AUTHOR]

Lead halide perovskite (APbX 3 ) has recently emerged as a promising active layer in light-emitting diodes (LEDs) as well as an absorber for photovoltaic devices. For better LED properties, it is important to understand the fundamental mechanism of the optoelectronic behaviors, e.g., how the nanostructure of the APbX 3 thin film correlates with its emitting properties. We investigated the effect of APbBr 3 (A = CH 3 NH 3 , Cs) crystallite size on the photophysical properties regarding its crystallographic changes and spin-orbit coupling. Photoluminescence lifetime measurements, X-ray and electron diffraction analyses, and density functional theory calculations were performed. We demonstrate that the emitting properties of mesoscale APbBr 3 crystallites are improved due to the formation of a pure cubic phase that leads to the spin- and momentum-allowed carrier recombination. Our findings provide fundamental insights into the emitting behavior of APbBr 3 , which suggests a control of its optoelectronic properties by means of modulating the crystal morphology and resultant electronic band structures.

Using density functional theory, we study the electronic properties of several halide monolayers. We show that their electronic bandgaps, as obtained with the HSE hybrid functional, range between 3.0 and 7.5 eV and that their phonon spectra are dynamically stable. Additionally, we show that under an external electric field some of these systems exhibit a semiconductor-to-metal transition.

New light is shed on the previously known perovskite material, Cs 2 Au 2 I 6 , as a potential active material for high-efficiency thin-film Pb-free photovoltaic cells. First-principles calculations demonstrate that Cs 2 Au 2 I 6 has an optimal band gap that is close to the Shockley-Queisser value. The band gap size is governed by intermediate band formation. Charge disproportionation on Au makes Cs 2 Au 2 I 6 a double-perovskite material, although it is stoichiometrically a single perovskite. In contrast to most previously discussed double perovskites, Cs 2 Au 2 I 6 has a direct-band-gap feature, and optical simulation predicts that a very thin layer of active material is sufficient to achieve a high photoconversion efficiency using a polycrystalline film layer. The already confirmed synthesizability of this material, coupled with the state-of-the-art multiscale simulations connecting from the material to the device, strongly suggests that Cs 2 Au 2 I 6 will serve as the active material in highly efficient, nontoxic, and thin-film perovskite solar cells in the very near future., (© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

A universal methodology to efficiently improve the photocatalyst performance of semiconductors was developed by employing exfoliated RuO2 two-dimensional nanosheets as a conducting hybridization matrix. The hybridization with a RuO2 nanosheet is easily achieved by crystal growth or electrostatically derived anchoring of semiconductor nanocrystals on the RuO2 nanosheet. An enhanced chemical interaction of inorganic semiconductor with hydrophilic RuO2 nanosheet is fairly effective in optimizing their photocatalytic activity and photostability by the enhancement of charge separation and charge mobility. The RuO2 -containing nanohybrids show much better photocatalyst functionalities than do the graphene-containing ones. The present study clearly demonstrates that hydrophilic RuO2 nanosheets are superior hybridization matrices, over the widely used hydrophobic graphene nanosheets, for exploring new efficient hybrid-type photocatalysts., (© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

We report the observation of coherent lattice vibrations in mono- and few-layer WSe2 in the time domain, which were obtained by performing time-resolved transmission measurements. Upon the excitation of ultrashort pulses with the energy resonant to that of A excitons, coherent oscillations of the A1g optical phonon and longitudinal acoustic phonon at the M point of the Brillouin zone (LA(M)) were impulsively generated in monolayer WSe2. In multilayer WSe2 flakes, the interlayer breathing mode (B1) is found to be sensitive to the number of layers, demonstrating its usefulness in characterizing layered transition metal dichalcogenide materials. On the basis of temperature-dependent measurements, we find that the A1g optical phonon mode decays into two acoustic phonons through the anharmonic decay process.

Two-dimensional crystals of beta-copper sulfide are synthesized in an in-situ electron microscopy experiment. Copper crystals are deposited on an amorphous carbon film containing sulfur. The carbon film graphitizes upon heating and electron irradiation and allows the reaction of Cu and S towards two-dimensional Cu(2) S crystals. These are energetically favourable and bonded via van der Waals interactions to the graphitic substrate., (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)