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MoS2 is a two-dimensional layered transition metal dichalcogenide with unique electronic and optical properties. The fabrication of ultrathin MoS2 is vitally important, since interlayer interactions in its ultrathin varieties will become thickness-dependent, providing thickness-governed tunability and diverse applications of those properties. Unlike with a number of studies that have reported detailed information on direct bandgap emission from MoS2 monolayers, reliable experimental evidence for thickness-induced evolution or transformation of the indirect bandgap remains scarce. Here, the sulfurization of MoO3 thin films with nominal thicknesses of 30 nm, 5 nm and 3 nm was performed. All sulfurized samples were examined at room temperature with spectroscopic ellipsometry and photoluminescence spectroscopy to obtain information about their dielectric function and edge emission spectra. This investigation unveiled an indirect-to-indirect crossover between the transitions, associated with two different Λ and K valleys of the MoS2 conduction band, by thinning its thickness down to a few layers. [ABSTRACT FROM AUTHOR]

Ab initio density functional perturbation theory is used to calculate the elastic constants of CdGa2S4, CdGa2Se4, CdGa2Te4, and ZnGa2Se4 thiogallates. The values of the bulk modulus, shear modulus, Young's modulus, and Poisson's ratio are determined. The force constants of interatomic bonds in CdGa2Te4 and ZnGa2Se4 compounds are calculated. Regularities in the dependence of the frequencies of Raman‐active phonons on the atomic masses are established. [ABSTRACT FROM AUTHOR]

A single crystalline layered semiconductor In1.2Ga0.8S3 phase was grown, and by intercalating p-aminopyridine (NH2-C5H4N or p-AP) molecules into this crystal, a new intercalation compound, In1.2Ga0.8S3·0.5(NH2-C5H4N), was synthesized. Further, by substituting p-AP molecules with p-ethylenediamine (NH2-CH2-CH2-NH2 or p-EDA) in this intercalation compound, another new intercalated compound—In1.2Ga0.8S3·0.5(NH2-CH2-CH2-NH2) was synthesized. It was found that the single crystallinity of the initial In1.2Ga0.8S3 samples was retained after their intercalation despite a strong deterioration in quality. The thermal peculiarities of both the intercalation and deintercalation of the title crystal were determined. Furthermore, the unit cell parameters of the intercalation compounds were determined from X-ray diffraction data (XRD). It was found that increasing the c parameter corresponded to the dimension of the intercalated molecule. In addition to the intercalation phases' experimental characterization, the lattice dynamical properties and the electronic and bonding features of the stoichiometric GaInS3 were calculated using the Density Functional Theory within the Generalized Gradient Approximations (DFT-GGA). Nine Raman-active modes were observed and identified for this compound. The electronic gap was found to be an indirect one and the topological analysis of the electron density revealed that the interlayer bonding is rather weak, thus enabling the intercalation of organic molecules. [ABSTRACT FROM AUTHOR]

ZnGa2S4 luminescent compound is prepared by solid‐state reaction, which is confirmed by X‐ray diffraction to have a tetragonal structure. The photoluminescence (PL) emission and excitation spectra, infrared (IR) reflectivity and Raman scattering of the defective ZnGa2S4 compound are studied experimentally. High stability of the shape and peak positions of the PL spectra is observed in a wide range temperature range (100–290 K). It was determined that PL intensity increases with decreasing temperature. Higher correlated color temperature (CCT) value >6000 K implies the suitability of using this material for light emitting diodes. Five modes of optical phonons are found in both IR reflectance and Raman spectra of ZnGa2S4 and identified from point group symmetry analysis. The calculations of the dynamic properties are carried out within the framework of the density functional theory (DFT) with full optimization of the lattice parameters. Results of calculations show that, the phonon‐dispersion curve is sensitive to S–Se replacement in ZnGa2S4 and gives rise to clearly pronounced energy gaps in the phonon spectrum. [ABSTRACT FROM AUTHOR]

ZnGa2S4 luminescent compound is prepared by solid‐state reaction, which is confirmed by X‐ray diffraction to have a tetragonal structure. The photoluminescence (PL) emission and excitation spectra, infrared (IR) reflectivity and Raman scattering of the defective ZnGa2S4 compound are studied experimentally. High stability of the shape and peak positions of the PL spectra is observed in a wide range temperature range (100–290 K). It was determined that PL intensity increases with decreasing temperature. Higher correlated color temperature (CCT) value >6000 K implies the suitability of using this material for light emitting diodes. Five modes of optical phonons are found in both IR reflectance and Raman spectra of ZnGa2S4 and identified from point group symmetry analysis. The calculations of the dynamic properties are carried out within the framework of the density functional theory (DFT) with full optimization of the lattice parameters. Results of calculations show that, the phonon‐dispersion curve is sensitive to S–Se replacement in ZnGa2S4 and gives rise to clearly pronounced energy gaps in the phonon spectrum. [ABSTRACT FROM AUTHOR]

The phonon structure of the silver sulfide Ag2S was investigated, experimentally using Raman spectroscopy, and theoretically using the density-functional perturbation theory for the first time. Seven Raman-active modes were observed and identified at 23, 39, 42, 44, 62, 65, and 243 cm−1. Symmetry assignments of all the vibrational modes were derived from considerations of point group symmetry. The phonon band structure and the relative Raman intensities were also investigated by ab initio calculations and compared with the experimental data. The temperature, laser power, and illumination time dependencies of frequency, linewidth, and intensity of the Raman-active modes are discussed. In the Raman spectra at higher frequencies 1300-1700 cm−1, additional broad Raman modes observed in all samples at higher laser powers 8-10 mW were ascribed to luminescence from β-Ag2S. The phonon and Raman spectra of the β-Ag2S provide a useful insight into the β-Ag2S → α-Ag2S phase transition. Finally, calculated infrared vibrational mode frequencies were compared with measured infrared mode frequencies. [ABSTRACT FROM AUTHOR]

Electronic band structure, together with formation energy of native point defects has been studied for β-Ag2S superionic material. The density-functional full-potential calculations have been carried out for perfect crystal and defect structure. In latter case a 96-atom supercell has been composed and account of structural relaxation around a vacancy has been taken up to second nearest neighbor. Formation energies of point defects have been obtained and compared. According to comparison, cation vacancies are the dominant intrinsic defects in β-Ag2S and transition into a state with high ionic conductivity is probably related with their abrupt concentration increase with increasing temperature. (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

Two peculiar types of electronic dispersive structures, irreproducible in bulk band calculations, have been observed in quasi-one-dimensional TlSe studied by angle-resolved photoemission spectroscopy. One is of the cross type centered at the Γ point and extending along the Γ-H-T direction parallel to the chains, which are formed along the crystallographic c-axis. The other is of the pyramid type with truncated pyramids regularly arranged along the Γ-N direction perpendicular to the chains. To an extent given by the obtained excitation photon energy dependence, the above electronic structures can be assigned to high-lying surface conduction bands resulting from secondary electron emission. [ABSTRACT FROM AUTHOR]

Bulk electronic structures of covalent-chain antiferromagnets TlFeS2 and TlFeSe2 have been investigated by hard X-ray photoemission spectroscopy (HAXPES) and band calculations based on the full-potential linearized augmented plane wave method. Competition between localized and delocalized characters of Fe 3d electrons has been suggested by the slight discrepancies between valence-band HAXPES spectra and calculated density of states, as well as Fe 2p spectra with a broad charge-transfer satellite. It is assumed that this competition closely related to the crystallographic onedimensionality of these materials. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

Ab initio calculations based on density functional theory (DFT) are performed to study the structural, electronic, and magnetic properties of 2D free‐standing honeycomb CrAs. It is shown that CrAs has a low buckled stable structure. Magnetic CrAs has larger buckling than nonmagnetic CrAs. 2D CrAs is a ferromagnetic semiconductor at lattice constant a ≤ 3.71 Å, and above this lattice constant CrAs is a half‐metal ferromagnet. 2D CrAs is shown to be half‐metal ferromagnetic with a magnetic moment of 3.0 μB per unit cell, at equilibrium structure. The dz2 orbital of the eg band is completely empty in the spin‐down state, whereas it is almost occupied in the spin‐up state, and the magnetic moment in the eg band is mainly dominated by the dz2 orbital of Cr. The dzx/dzy and dxy orbitals of the t2g band are partially occupied in the spin‐up state and behave like a metal, whereas they are insulators in the spin‐down state. Phonon calculations confirm the thermodynamic stability of 2D CrAs. The ferromagnetic (FM) and antiferromagnetic (AFM) interactions between the Cr atoms reveal that the FM state is more stable than the AFM state of 2D CrAs. [ABSTRACT FROM AUTHOR]

Electronic, optical, and lattice vibrational properties of β‐Ag2Te are investigated, experimentally using spectroscopic ellipsometry and Raman spectroscopy, and theoretically using the density‐functional perturbation theory (DFPT) for the first time. First‐principles calculations of the electron spectra and dielectric functions are performed with the modified Becke–Johnson (mBJ) exchange potential, which solves the band‐gap problem found with generalized gradient approximations (GGA). The calculated phonon spectra of β‐Ag2Te reveal that low‐frequency optic modes overlap with acoustic modes and should scatter strongly with acoustic modes during heat transport. The possible Ag disorder and strong scattering of acoustic phonons by optic modes lead to an extremely low lattice thermal conductivity of β‐Ag2Te observed in the experiment. The phonon structure was also investigated experimentally using Raman spectroscopy. Three Raman‐active modes were observed and identified at 92, 120.6, and 140.5 cm−1. Symmetry assignments of all the vibrational modes were derived from considerations of point group symmetry. First‐principles calculations of the electron spectra and dielectric functions performed with the modified Becke–Johnson exchange potential correct the band‐gap value of β‐Ag2Te and improve an agreement with experimental results. Phonon spectra of β‐Ag2Te show that low‐frequency optic modes and acoustic modes overlap and as a result, the lattice thermal conductivity strongly decreases. Together with Ag disorder, this leads to a high thermoelectric effect in this material. [ABSTRACT FROM AUTHOR]

Bulk electronic structures of covalent-chain antiferromagnets TlFeS2 and TlFeSe2 have been investigated by hard X-ray photoemission spectroscopy (HAXPES) and band calculations based on the full-potential linearized augmented plane wave method. Competition between localized and delocalized characters of Fe 3d electrons has been suggested by the slight discrepancies between valence-band HAXPES spectra and calculated density of states, as well as Fe 2p spectra with a broad charge-transfer satellite. It is assumed that this competition closely related to the crystallographic onedimensionality of these materials. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

MoS 2 is a two-dimensional layered transition metal dichalcogenide with unique electronic and optical properties. The fabrication of ultrathin MoS 2 is vitally important, since interlayer interactions in its ultrathin varieties will become thickness-dependent, providing thickness-governed tunability and diverse applications of those properties. Unlike with a number of studies that have reported detailed information on direct bandgap emission from MoS 2 monolayers, reliable experimental evidence for thickness-induced evolution or transformation of the indirect bandgap remains scarce. Here, the sulfurization of MoO 3 thin films with nominal thicknesses of 30 nm, 5 nm and 3 nm was performed. All sulfurized samples were examined at room temperature with spectroscopic ellipsometry and photoluminescence spectroscopy to obtain information about their dielectric function and edge emission spectra. This investigation unveiled an indirect-to-indirect crossover between the transitions, associated with two different Λ and K valleys of the MoS 2 conduction band, by thinning its thickness down to a few layers.