Optical Spectra and Band Structure of Ag x Ga x Ge 1-x Se 2 (x = 0.333, 0.250, 0.200, 0.167) Single Crystals: Experiment and Theory (original) (raw)
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Science of Advanced Materials
Ag 2 In 2 GeS 6 and Ag 2 InSiS 6 are two interesting quaternary-sulfide single crystals. Starting from our previous investigation on Ag 2 In 2 GeS 6 single crystals, the Ag 2 In 2 SiS 6 is investigated here. We demonstrate the effect of replacing Ge by Si on the electronic structure and the bonding properties. We have used X-ray diffraction (XRD) data for Ag 2 In 2 Ge(Si)S 6 single crystals as input to our theoretical calculations using the all-electron full potential linearized augmented plane wave method to solve the Kohn Sham Density Functional Theory (DFT) equations. As remarkable finding, our calculations show that, on replacing Ge by Si atom, the environment of the S atoms is changed significantly. The energy gap depends on the exchange correlation function. For the local density approximation (LDA) the energy gap is 0.76 eV, while, based on the modified Becke-Johnson approximation (mBJ), the energy gap increases to 1.98 eV. We should emphasize that this energy gap in Ag 2 In 2 SiS 6 is almost the same as that obtained for Ag 2 In 2 GeS 6 (1.96 eV). Another significant finding is that when we replace Ge by Si the conductions bands move away from the Fermi energy while the valence bands are almost unchanged. In order to support the theoretical calculation the calculated total density of states below E F (TDOS-VB) of Ag 2 In 2 SiS 6 single crystals is compared with our experimentally measured valence-band X-ray photoelectron spectroscopy (XPS-VB). The theoretical spectrum reproduces the general features structure of the measured XPS-VB faithfully.
Ag 2 In 2 GeS 6 and Ag 2 InSiS 6 are two interesting quaternary-sulfide single crystals. Starting from our previous investigation on Ag 2 In 2 GeS 6 single crystals, the Ag 2 In 2 SiS 6 is investigated here. We demonstrate the effect of replacing Ge by Si on the electronic structure and the bonding properties. We have used X-ray diffraction (XRD) data for Ag 2 In 2 Ge(Si)S 6 single crystals as input to our theoretical calculations using the all-electron full potential linearized augmented plane wave method to solve the Kohn Sham Density Functional Theory (DFT) equations. As remarkable finding, our calculations show that, on replacing Ge by Si atom, the environment of the S atoms is changed significantly. The energy gap depends on the exchange correlation function. For the local density approximation (LDA) the energy gap is 0.76 eV, while, based on the modified Becke-Johnson approximation (mBJ), the energy gap increases to 1.98 eV. We should emphasize that this energy gap in Ag 2 In 2 SiS 6 is almost the same as that obtained for Ag 2 In 2 GeS 6 (1.96 eV). Another significant finding is that when we replace Ge by Si the conductions bands move away from the Fermi energy while the valence bands are almost unchanged. In order to support the theoretical calculation the calculated total density of states below E F (TDOS-VB) of Ag 2 In 2 SiS 6 single crystals is compared with our experimentally measured valence-band X-ray photoelectron spectroscopy (XPS-VB). The theoretical spectrum reproduces the general features structure of the measured XPS-VB faithfully.
Revista Mexicana de Física
The primary goal of this study is to investigate the effect of different exchange-correlation functionals on the optoelectronic and elastic properties of the Ag2O chalcogenide compound. For the electronic structures and optical spectra, the Tran-Blaha modified Becke-Johnson approach combined with GGA and with GGA+U (mBJ-GGA-PBEsol and mBJ-GGA-PBEsol+U, respectively) was used. The available theoretical and experimental data for the bandgap energy were reported to determine whether there is a correlation with our results. The electronic structure revealed that our compound is a direct semiconductor at the R-symmetry point with a bandgap of 1.22 eV, which this value agrees well with the experimental values for the first time. The elastic constants were also evaluated using the IRelast package, which revealed that the compound was mechanically stable. Finally, the optical response was systematically studied, and it was found that Ag2O exhibited excellent optical efficiency.
Structural Studies and Valence Band Splitting Parameters in Ordered Vacancy Compound AgGa7Se12
Journal of Inorganic and Organometallic Polymers and Materials, 2012
The threefold absorption in the fundamental absorption region of the ternary chalcopyrite AgGa 7 Se 12 , an ordered defect compound of AgGaSe 2 , is analyzed to elucidate the three closely spaced band gaps in its valence band due to the lifting of degeneracy of the C 15 level. Hopfield's quasi cubic model is employed to extract the crystal-field and spin-orbit splitting parameters and the linear hybridisation of orbitals model for evaluating the percentage contribution of Ag d-orbital and Ga and Se p-orbitals to the p-d hybridization of orbitals. The observed optical properties are correlated with the structural parameters like deformation parameter, anion displacement and anion-cation bond lengths that are deduced from X-ray diffraction data. The compound films for the studies are prepared by a modified form of Gunther's three temperature technique and the compositional analysis was done by energy dispersive analysis of X-rays. X-ray photoelectron spectroscopy confirms the compound formation while atomic force microscopic technique was used for surface morphological analysis. The electrical resistivity of these n-type semi-conducting films is assessed to be *5 Xm and the films are found to be photosensitive.
Journal of Physics: Condensed Matter, 2003
The structural, electronic and optical properties of tetragonal nonlinear optical (NLO) crystals, AgGa(S x Se 1−x) 2 (x = 0.0, 0.25, 0.5, 0.75, and 1.0), were investigated theoretically and experimentally. The results obtained indicated that the electronic bandgaps, optical properties and bulk moduli of these compounds were linearly dependent on the substitution concentration of cations. From partial density of state analysis, it was found that the electronic states near the band edges of AgGa(S x Se 1−x) 2 were a simple proportional mixture of the atomic orbitals of sulfur and selenium. A cell-volume effect was proposed as the major cause of the linear dependence of material properties on the substitution concentration. It was calculated that the second-order NLO susceptibilities were scaled with the cubic power of bandgap, although a minor deviation existed. This deviation arose from the optical transition moment products.
Journal of Materials …
We report measurements of the X-ray diffraction and X-ray photoelectron spectrum on single crystals of Ag2In2GeS6. We also present first principles calculations of the band structure and density of states using the state-of-the-art full potential augmented plane wave method with different possible approximation for the exchange correlation potential. In this paper, we make a detailed comparison of the density of states deduced from the X-ray photoelectron spectra with our calculations. The theoretical results of the density of states are in reasonable agreement with the X-ray photoelectron spectroscopy (VB-XPS) measurements with respect to peak positions. The calculated density of states shows there is a strong hybridization between the states in the valence and conduction bands states. We have calculated the electron charge density distribution in the (100) and (110) planes. In the plane (100), there exists Ag, In, and S atoms, while the plane (110) Ag, S, and Ge atoms are present. The bonding properties are obtained from the charge density distributions. The calculation show that there is partial ionic and strong covalent bonding between Ag–S, In–S, and Ge–S atoms depending on Pauling electro-negativity difference of S (2.58), Ge (2.01), Ag (1.93), and In (31.78) atoms.
Crystallography and optical energy gap values for AgGa(Se1−zTez)2alloys
Journal of Applied Physics, 1985
Polycrystalline samples of Cd~x(AgIn),nln~,Te, (z + y + z = 1) alloys are prepared by a melt and anneal technique. Debye-Scherrer X-ray powder photographs are used to determine equilibrium conditions and lattice parameter values. It is found that in addition to the zincblende and chalcopyrite structures, a partially ordered structure is obtained, plus a two-phase field at higher zvalues. Room temperature measurements of optical absorption are made t o give values of the optical energy gap E , for all single phase samples. It is found that the variation of a is practically linear with composition. However, while E, varied linearly with composition inside a phase field, the resulting lines have different aiming points a t z = I, the values being 2.83 eV for zinc blmde, 1.90 eV for the ordered phase, and 1.36 eV for the chalcopyrite phase. Thus the values of E, give a very good indication of the phase boundaries. Des kchantillons polycristallins d'alliages de Cdi,(AgInj,Mnn,Te, (x + y + z = 1) ont 6th prbparks par une technique de fonte e t recuit. Les conditions d'kquilibre et les valeurs des parametres de maille ont ktb determinbes B partir des films de diffraction des rayons-X par la mkthode Debye-Scherrer. On trouve qu'en plus des structures blende et chalcopyrite, une structure partiellement ordonnee est obtenue, ainis qu'un champ oh deux phases coexistent B plus hautes valeurs de z. Des mesures d'absorption optique ont btk faites 8. l'ambiante pour obtenir les valeurs de bandes interdites optiques E , sur tous les kchantillons presentant une seule phase. On trouve que la variation de a est pratiquement linbaire avec la composition. Toutefois, alors qu'Eg varie lineairement avec la composition B l'intbrieur d'un champ de phase donn8, les droites ainsi obtenues s'extrapolent sur des valeurs diffbrentes pour z = 1, ces valeurs 6tant de 2,83 eV pour la phase blende, 1,90 eV pour la phase or donnee et 1,36 eV pour la phase chalcopyrite. Done les valeurs d'E, fournissent de tres bonnes indications des limites des phases.
Structure-Property Relationships of 2D Ga/In Chalcogenides
Nanomaterials, 2020
Two-dimensional MX (M = Ga, In; X = S, Se, Te) homo- and heterostructures are of interest in electronics and optoelectronics. Structural, electronic and optical properties of bulk and layered MX and GaX/InX heterostructures have been investigated comprehensively using density functional theory (DFT) calculations. Based on the quantum theory of atoms in molecules, topological analyses of bond degree (BD), bond length (BL) and bond angle (BA) have been detailed for interpreting interatomic interactions, hence the structure–property relationship. The X–X BD correlates linearly with the ratio of local potential and kinetic energy, and decreases as X goes from S to Te. For van der Waals (vdW) homo- and heterostructures of GaX and InX, a cubic relationship between microscopic interatomic interaction and macroscopic electromagnetic behavior has been established firstly relating to weighted absolute BD summation and static dielectric constant. A decisive role of vdW interaction in layer-dep...
Metal chalcogenide semiconductors have a significant role in the development of materials for energy and nanotechnology applications. First principle calculations were applied on CsAgGa 2 Se 4 to investigate its optoelectronic structure and bonding characteristics, using the full-potential linear augmented plane wave method within the framework of generalized gradient approximations (GGA) and Engel-Vosko GGA functionals (EV-GGA). The band structure from EV-GGA shows that the valence band maximum and conduction band minimum are situated at C with a band gap value of 2.15 eV. A mixture of orbitals from Ag 4p 6 /4d 10 , Se 3d 10 , Ga 4p 1 , Se 4p 4 , and Ga 4s 2 states have a primary role to lead to a semiconducting character of the present chalcogenide. The charge density iso-surface shows a strong covalent bonding between Ag-Se and Ga-Se atoms. The imaginary part of dielectric constant reveals that the threshold (first optical critical point) energy of dielectric function occurs 2.15 eV. It is obvious that with a direct large band gap and large absorption coefficient, CsAgGa 2 Se 4 might be considered a potential material for photovoltaic applications.
Ab initio band structure calculations of the low-temperature phases of Ag 2Se, Ag 2Te and Ag 3AuSe 2
Journal of Physics and Chemistry of Solids, 2002
Ab initio band structure calculations were performed for the low-temperature modi®cations of the silver chalcogenides b-Ag 2 Se, b-Ag 2 Te and the ternary compound b-Ag 3 AuSe 2 by the local spherical wave (LSW) method. Coordinates of the atoms of b-Ag 2 Se and b-Ag 3 AuSe 2 were obtained from re®nements using X-ray powder data. The structures are characterized by three, four and ®ve coordinations of silver by the chalcogen, a linear coordination of gold by Se, and by metal±metal distances only slightly larger than in the metals. The band structure calculations show that b-Ag 3 AuSe 2 is a semiconductor, while b-Ag 2 Se and b-Ag 2 Te are semimetals with an overlap of about 0.1±0.2 eV. The Ag 4d and Au 5d states are strongly hybridized with the chalcogen p states all over the valence bands. b-Ag 2 Se and b-Ag 2 Te have a very low DOS in the energy range from about 20.1 to 10.5 eV. The calculated effective mass b-Ag 2 Se is about 0.1±0.3 m e for electrons and 0.75 m e for holes, respectively. q