Electronic structure and chemical bonding of phosphorus-contained sulfides InPS4, TI3PS4, and Sn2P2S6

The electronic structure of phosphorus-contained sulfides InPS₄, Tl₃PS₄, and Sn₂P₂S₆ was investigated experimentally with X-ray spectroscopy and theoretically by quantum mechanical calculations. The partial densities of electron states calculated with the ab initio multiple scattering FEFF8 code correspond well to their experimental analogues—the X-ray K- and L_2,3-spectra of sulfur and phosphorus. The good agreement between theory and experiment was also achieved for K-absorption spectra of S and P in the investigated sulfides. In spite of the difference in the crystallographic structure of InPS₄, TI₃PS₄, and Sn₂P₂S₆ that influence the form of K-absorption spectra, the electronic structure of their valence bands are rather similar. This is due to the strong interaction of the P and S atoms, which are the nearest neighbors in the compounds studied. The electron densities of p- and s-states of phosphorus are shifted by about 3 eV to lower energies in comparison to the analogous electron states of sulfur. This is connected with the greater electro-negativity of sulfur, and is confirmed by the calculated electron charge transfer from P to S.     

Lattice instability in the non-superconducting compound EuMo6S8

We have performed electrical resistivity and specific heat measurements on bulk Eu_1.1Mo₆S₈ together with low-temperature X-ray powder diffractometry. These investigations revealed a structural phase transition (occuring at 109 K) from the room-temperature rhombohedral structure to a low-temperature triclinic distorted structure in which the compound exhibits a non-metallic behavior.     

Low temperature normal state resistance of ternary molybdenum sulfides

The resistance varies as T² over the range of T from T_c to nearly 40 K, in sintered, sputtered and evaporated thin film Cu_xMo₆S₈, and in sputtered PbMo₆S₈ films. In sintered PbMo₆S₈ the resistance varies as T. These results can neither be explained by existing theory for PbMo₆S₈, nor by a Mo₂S₃ phase argument.     

Electronic properties of Chevrel phases by positron angular correlation

A comparative study of the Chevrel phases Mo₆S₈, Mo₆Se₈, PbMo₆S₈ and Cu_1.8Mo₆S₈ is made by the positron angular correlation technique at room temperature. Electronic properties like the number of conduction electrons per cluster, the density of states at the Fermi level, the electron effective masses and the Fermi velocities are obtained within the framework of the free electron model. The results are discussed in terms of available theoretical and other experimental data.     

Oxidizing agent concentration effect on metal-assisted electroless etching mechanism in HF-oxidizing agent-H2O solutions

The mechanism of metal-assisted electroless etching of silicon in HF-oxidizing agent-H₂O etching system as a function of oxidizing agent concentration was studied. Three types of oxidizing agent were experimented namely Na₂S₂O₈, K₂Cr₂O₇ and KMnO₄. Their concentrations were varied from 0.05 M to 0.3 M. The layers formed on silicon were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy-dispersive X-ray (EDX). It is shown that an insoluble solid-phase film (K₂SiF₆) form on silicon surface when concentration of K₂Cr₂O₇ or KMnO₄ increases in chemical solutions. On other hand, when Na₂S₂O₈ concentration increases, the surface roughness decreases without any chemical complex formation.     

Modification in cell structure for better performance of spray pyrolysed CuInS2/In2S3 thin film solar cell

We report a new structure for CuInS₂/In₂S₃ solar cell, in which both absorber and buffer layers were deposited using chemical spray pyrolysis (CSP) technique. The usual superstrate structure, having buffer layer just above ITO, was not functioning mainly due to diffusion of Cu into In₂S₃ layer as seen from X-ray photoelectron spectroscopy (XPS) results. However, when the configuration of the cell was ITO/CuInS₂/In₂S₃/Ag, cell parameters obtained were V_oc=0.45 V, J_sc=44.03 mA/cm², fill factor (FF) = 29.5% and η=5.87%. Good results could be obtained by using indium sulfide thin films having maximum photosensitivity. The cell was characterized using X-ray diffraction, optical absorption, current–voltage and spectral response measurements. PACS 81.15.Rs; 82.45.Mp; 84.60.Jt     

Metal atom incorporation studies on AxMo6S8 Chevrel phases

Group IIIa metals and Nb, Hg and Pb containing A_xMo₆S₈ Chevrel phases have been synthesized by low temperature reaction (420–430°C) frorn A metal and Mo₆S₈. Some of these phases are hitherto unknown. Electrolysis method of preparation of A_xMo₆S₈ reported by Schöllhorn does not yield the desired phases when A = Pb or Ag. Stoichiometry, thermal stability, electrical and superconducting properties of the compounds have been examined. In the Pb_xMo₆S₈ system, a series of phases with varying x have been prepared and studied. Group IIIa metal and Nb-compounds are found to be semiconductors with low energies of activation exhibiting p-type behavior in the range 77–300 K. Hg-phase behaves as a metal or a degenerate semiconductor and does not become superconducting above 4.2 K. X-Ray, resistivity and T_c studies show that for x ? 0.8 in Pb_xMo₆S₈, the compounds behave more like Mo₆S₈ and only for x ? 1.0 or (PbBi)_1.0, the inherent behavior of the ternary Chevrel phase is exhibited. Seebeck coefficients are small and positive for Mo₆S₈, Cu and Pb-containing phases in the range 77–300 K. The results are discussed in the light of available band structure of these materials.     

X-ray diffraction of the smectic phases of N-(p-n-heptyloxybenzylidene)-p′-n-pentylaniline

Using X-ray diffraction the various smectic phases of N-($\text{p}$-$\text{n}$-heptyloxybenzylidene)-$\text{p}$′-$\text{n}$-pentylaniline can be classified (with increasing temperature) as S_H, S_B, S_C and S_A, respectively. The long molecular axes are probably tilted with respect to the normal to the layers in the S_A, S_C and S_H phases, and perpendicular to the layers in the S_B phase.     

Charge transfer and valence electron concentration in Chevrel phases

Single crystal X-ray diffraction measurements on M_xMo₃S₄ compounds (M = Cu, Ag, Sn, In) yield evidence for the existence of a charge transfer from the interstitial element M to the Mo₆S₈ building blocks. The importance of this transfer was estimated from the contraction of the Mo₆ octahedron as a function of the valency and the concentration of the cations. It is suggested that this effect stabilises the structure and has a strong influence on the superconducting transition temperature. Bonding is discussed using a simple valence bond model which is based on the electron requirement of the Mo₆ octahedron to form 12 covalent bonds.     

Synthesis and structure of the monolayer hydrate K0.3CoO2·0.4H2O

The monolayer hydrate (MLH) K_0.3CoO₂·0.4H₂O was synthesized from K_0.6CoO₂ by extracting K⁺ cations using K₂S₂O₈ as an oxidant and the subsequent intercalation of water between the layers of edge-sharing CoO₆ octahedra. A hexagonal structure (space group P6₃/mmc) with lattice parameters a=2.8262(1) Å, c=13.8269(6) Å similar to the MLH Na_0.36CoO₂·0.7H₂O was established using high-resolution synchrotron X-ray powder diffraction data. The K/H₂O layer in the K-MLH is disordered, which is in contrast to the Na-MLH. At low temperatures metallic and paramagnetic behavior was found.     

Coulometric titration of copper in CuxMo6S8−y(0⩽y⩽0.4) with solid Cu-electrolyte Rb4Cu16I7Cl13

In order to determine the copper content x of copper Chevrel compound Cu_xMo₆S_8?y(0?y?0.4) as a function of copper activity a_Cu and sulfur deficit y, a solid state electrochemical cell, Cu/Rb₄Cu₁₆I₇Cl₁₃/Cu_xMo₆S_8?y/Pt, was constructed and coulometric titration studies were made at 400 K. For the evaluation of the coulometric titration data, measurements were made on the electronic conductivity of copper-ion conductor Rb₄Cu₁₆I₇Cl₁₃. Also, a brief investigation was made on the condition of formation of single phase Chevrel Cu_xMo₆S_8?y with varying x and y at 1000°C. The structural change of Cu_xMo₆S_8?y with a change in x and y was studied by the X-ray diffraction method. It was found that x for constant a_Cu decreases with increasing y. The maximum value x_max of x in Cu_xMo₆S_8?y in equilibrium with metallic copper was found to be expressed by $\text{x}{}_{\mathrm{<mtext>max</mtext>}}\text{=-(}\text{10}\text{3}\text{)y+5}$. In the region of y<0.3, x_max exceeded 4, contrary to a presupposition that x_max is less than 4. X-ray analysis revealed that most of the copper Chevrel compounds denoted by Cu_xMo₆S₈ so far were sulfur deficient ones with y?0.4. The importance of sulfur deficit on the properties of the copper Chevrel compound was emphasized.     

A comparative study of the physical properties of Sb2S3 thin films treated with N2 AC plasma and thermal annealing in N2

As-deposited antimony sulfide thin films prepared by chemical bath deposition were treated with nitrogen AC plasma and thermal annealing in nitrogen atmosphere. The as-deposited, plasma treated, and thermally annealed antimony sulfide thin films have been characterized by X-ray diffraction (XRD), energy dispersive X-ray spectroscopy, scanning electron microscopy, atomic force microscopy, UV-vis spectroscopy, and electrical measurements. The results have shown that post-deposition treatments modify the crystalline structure, the morphology, and the optoelectronic properties of Sb₂S₃ thin films. X-ray diffraction studies showed that the crystallinity of the films was improved in both cases. Atomic force microscopy studies showed that the change in the film morphology depends on the post-deposition treatment used. Optical emission spectroscopy (OES) analysis revealed the plasma etching on the surface of the film, this fact was corroborated by the energy dispersive X-ray spectroscopy analysis. The optical band gap of the films (E_g) decreased after post-deposition treatments (from 2.36 to 1.75 eV) due to the improvement in the grain sizes. The electrical resistivity of the Sb₂S₃ thin films decreased from 10⁸ to 10⁶ Ω-cm after plasma treatments.     

Optical properties of small silver clusters supported at MgO

We present structural and optical properties of silver clusters Ag_n (n=2, 4, 6, 8) at two model support sites of MgO, stoichiometric MgO(100) and F_S-center defect, based on density functional theory and embedded cluster model. Our results provide the mechanism responsible for the absorption and emission patterns due to the specific interaction between the excitations within the cluster and the support site which is strongly cluster size and structure dependent. We propose Ag₄ at stoichiometric site as well as Ag₂, Ag₄ and Ag₆ at F_S-center defects as good candidates for the emissive centers in the visible regime.     

Superconducting tunneling on Chevrel phase thin films

Electron tunneling spectroscopy has been applied for the investigation of Cu_1.8Mo₆S₈, PbMo_6.35S₈ and LaMo₆S₈ d.c. getter-sputtered thin films. Cryogenically deposited Al₂O₃ as the artificial barrier with Al counter-electrode formed the tunnel junctions. The energy gap Δ₀ and the ratio 2Δ₀/k_BT_c were determined from the differential conductance dJ/dV vs. voltage V. Preliminary measurements showed phonon structure in LaMo₆S₈ films.     

Preparation and crystal structure of C60S16

A new fullerene-containing van der Waals-compound, C₆₀S₁₆, has been synthesized and its crystal structure determined from single-crystal X-ray diffraction data collected at room temperature. The compound is monoclinic, space group C 2/c with a=20.867(4) Å, b=21.062(4) Å, c=10.508(2) Å, =111.25(7)° and four formula units per cell. The C₆₀ molecules form a three-dimensional framework with one-dimensional channels along c which contain crown-shaped S₈ rings. The structure has been determined by direct methods and has been refined to atomic resolution on the C₆₀ molecule. The two independent C-C bond distances, averaged under the idealized point symmetry of the free C₆₀ molecule, amount to 1.340(8) Å and 1.448(8) Å, corresponding to a bond alternation as large as 0.108(8) Å. The bond lengths are compared with the results of theoretical calculations of the molecular structure of C₆₀ as well as with bond lengths from various experimental sources.     

Structural dynamics of 4‐cyanobenzaldehyde in S2(ππ*) state

The excited state structural dynamics of 4‐cyanobenzaldehyde (p‐CNB) were studied by using the resonance Raman spectroscopy and the quantum mechanical calculations. The experimental A‐ and B‐band absorptions were, respectively, assigned to the major n_O → π₃* and π₂ → π₃* transitions according to the B3LYP‐TD/6‐31G(d) and CIS/6‐31G(d) computations, and the resonance Raman spectra. It was determined that the actual S₂(π₂π₃) state was in energy lower than S₃(π₁π₃), which was just opposite to the B3LYP‐TD/6‐31G(d) calculated order of the S₂(π₁π₃) and S₃(π₂π₃). The vibrational assignments were carried out for the A‐ and B‐band resonance Raman spectra. The B‐band resonance Raman intensities of p‐CNB were dominated by the C2–C3/C5–C6 symmetric stretch mode ν₈, the overtones nν₈ and their combination bands with the ring C–H bend mode ν₁₇, the C9–N10 stretch mode ν₆, the C7–O8 stretch mode ν₇ and the remaining modes. The conical intersection between S₁(n_Oπ₃) and S₂(π₂π₃) states of p‐CNB was determined at complete active space self‐consistent field (CASSCF)(8,7)/6‐311G(d,p) level of theory. The B‐band short‐time structural dynamics and the corresponding decay dynamics of p‐CNB were obtained by analysis of the resonance Raman intensity pattern and CASSCF computations. The resonance Raman spectra indicated that CI[S₁(n_Oπ₃)/S₂(π₁π₂π₃π₄)] located nearby the Franck–Condon region. The excited state decay dynamics evolving from the S_{2, FC}(π₂π₃) to the S₁(n_Oπ₃) state was proposed. Copyright © 2013 John Wiley & Sons, Ltd.     

Synthesis and characterization of superhydrophobic functionalized Cu(OH)2 nanotube arrays on copper foil

Superhydrophobic functionalized cupric hydroxide (Cu(OH)₂) nanotube arrays were prepared on copper foils via a facile alkali assistant surface oxidation technique. Thus nanotube arrays of Cu(OH)₂ were directly fabricated on the surface of copper foil by immersing in an aqueous solution of NaOH and (NH₄)₂S₂O₈. The wettability of the surface was changed from surperhydrophilicity to superhydrophobicity by chemical modification with 1H,1H,2H,2H-perfluorodecyltriethoxysilane (FAS). The morphologies, microstructures, crystal structure, chemical compositions and states, and hydrophobicity of the films on the copper foil substrates were analyzed by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and water contact angle measurement. It was found that the rough structure of the surface helped to magnify the wettability. The static contact angle (CA) for water is larger than 160° and the contact angle hysteresis (CAH) is lower than 5° on the modified surface. The high roughness of the nanotube arrays along with the generated C-F chains by chemical modification contributed to the improved superhydrophobicity. The present research is expected to be significant in providing a new strategy for the preparation of novel multifunctional materials with potential industrial applications on copper substrates.     

Vibrational spectra of In2S3, CuIn5S8 and AgIn5S8 compounds with a spinel structure

The IR reflection spectra of In₂S₃, CuIn₅S₈, AgIn₅S₈ compounds with a spinel structure are investigated. Using the DA-K-K method, we determined the frequencies of longitudinal and transverse phonons, attenuation coefficients of phonons, and IR intensities, as well as the dielectric constants ε₀ and ε_∞. Belarusian State University of Information Science and Radioelectronics, 6, P. Brovka St., Minsk 220600, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 2, pp. 263–265, March–April, 1997.     

Effect of sintering temperature on the chemical state of ions in the Ba1 − x Sr x TiO3 (x = 0.2) system,according to X-ray photoelectron spectroscopy data

The chemical state of ions in different samples of ceramics of the Ba_{1 ? x} Sr _x TiO₃ (x = 0.2) system produced by varying only a single technological parameter (sintering temperature T _S) is studied by means of X-ray photoelectron spectroscopy. The X-ray electron spectra of the Ba4d, Sr3d, O1s, and Ti2p levels reveal well-marked fine structures. As expected, the energy position and intensity of the features of this fine structure vary with T _S.     

High-pressure polymorphism of As2S3 and new AsS2 modification with layered structure

At normal pressure, the As₂S₃ compound is the most stable equilibrium modification with unique layered structure. The possibility of high-pressure polymorphism of this substance remains questionable. Our research showed that the As₂S₃ substance was metastable under pressures P > 6 GPa decomposing into two high-pressure phases: As₂S₃ → AsS₂ + AsS. New AsS₂ phase can be conserved in the single crystalline form in metastable state at room pressure up to its melting temperature (470 K). This modification has the layered structure with P12₁1 monoclinic symmetry group; the unit-cell values are a = 7.916(2) Å, b = 9.937(2) Å, c = 7.118(1) Å, β = 106.41° (Z = 8, density 3.44 g/cm³). Along with the recently studied AsS high-pressure modification, the new AsS₂ phase suggests that high pressure polymorphism is a very powerful tool to create new layered-structure phases with “wrong” stoichiometry.