Advanced Materials Research Vol. 665

Abstract: The optical characterization of Molybdenum diselenide (MoSe₂) and polyaniline (PANI) has been carried in the wavelength range 200 nm to 2500 nm. The detailed analysis of the optical properties has been carried out only for a range 200 nm to 800 nm from which the indirect band gap around 1.42 eV for MoSe₂ and 1 eV and 2.5 eV for PANI was evaluated. It was interesting to note that π π* transitions lead to two distinct orders of energy gaps. The hybrid cells were fabricated using a photosensitive interface between MoSe₂ and PANI. Various parameters of these heterostructure hybrid cells have been evaluated and it was found that the photoconversion efficiency was around 1%. Using the solar cell characteristics, the presence of trapping centers at the n-MoSe₂/ p-PANI interface has been confirmed.

Abstract: Group II-VI compounds have been investigated largely in last two decades due to their interesting optoelectronic properties. ZnTe, a member of this family, possesses a bandgap around 2.26eV. This material is now a day investigated in thin film form due to its potential towards various viable applications. In this paper, the authors report their investigations on the preparation of ZnTe thin films using vacuum evaporation technique and their structural and optical characterizations. The structural characterization, carried out using an X-ray diffraction (XRD) technique shows that ZnTe used in present case possesses a cubic structure. Using the same data, the micro strain and dislocation density were evaluated and found to be around 1.465×10^-3 lines-m² and 1.639×10¹⁵ lines/m²respecctively. The optical characterization carried out in UV-VIS-NIR region reveals the fact that band gap of ZnTe is around 2.2eV in present case. In addition to this, it was observed that the value of bandgap decreases as the thickness of films increases. The direct transitions of the carries are involved in ZnTe. Using the data of UV-VIS-NIR spectroscopy, the transmission coefficient and extinction coefficient were also calculated for ZnTe thin films. Besides, the variation of extinction coefficient with wavelength has also been discussed here.

Abstract: Transparent thin film of pure TiO₂ and 3% Sr-doped TiO₂ (Ti_0.97Sr_0.03O₂) were prepared by spin coating technique onto well-cleaned glass substrate. The dielectric behaviour of the films were studied at room temperature for different frequencies. The capacitance of both films were is found to decrease with increasing frequency and at higher frequencies capacitance becomes constant. AC conductivity is found to continuously increase with increase in the frequency. At high frequency it doesnt show any change and remains almost constant.

Abstract: CdSe is a II-VI group semiconducting material with optimum bulk band gap of 1.74eV. It is a promising material due to its wide range of technological applications in optoelectronics devices. CdSe nanoparticles have been synthesized at different temperatures starting from Room temperature to 80°C using appropriate precursor solutions containing Cadmium acetate, Triethanolamine (TEA), Ammonia and Sodium selenosulphate. The pH of Solution was around 10.50 ± 0.10 during synthesis. We confirmed the elemental analysis by Energy Dispersive X-ray Analysis (EDAX) and X-ray Photoelectron Spectroscopy (XPS) technique. X-Ray Diffraction (XRD) studies shows that the synthesized nanoparticles belonged to cubic phase with crystallite size lying between 2nm-4nm. The effect of temperature on particle size, lattice parameter, density of dislocation and strain were investigated. Blue shift of 103nm to125nm has been observed from optical absorption spectra and raman measurements performed at room temperature using He-Ne laser (632nm, 5mW) showed the presence of longitudinal optical phonon modes. Photoluminescence (PL) studies shows a shift of 30nm when compared with the bulk PL emission peak.

Abstract: A unique evidence of crystalline phase, known as Wigner crystal (WC), is explored in Electron-Hole quantum bilayers (EHBL) by tuning electron layer density independently at zero-temperature and zero-magnetic field. The quantum or dynamical version of Singwi, Tosi, Land and Sjölander (qSTLS) approach is used for obtaining the static density susceptibility. The static density susceptibility plays very important role for exploration of WC phase by showing a divergent behavior at finite wavevector, corresponds to the reciprocal lattice wavevector, during the phase transition from liquid state to WC ground-state. A comparison of present results with a recent results of dependently tunable EHBL system [Phys. Rev. B 66, 205316 (2002)] shows that the onset of Wigner crystallization now occurs at sufficiently lower interlayer spacing by tuning the electron layer density and keeping hole layer density fixed. Further, the prediction of WC phase gets support from the structure factor which exhibits a strong peak near the phase transition point.

Abstract: Cholesterol is a main component in mammalian cell membranes and lipoproteins. High level of cholesterol in blood can damage arteries and potentially linked with cardiovascular diseases. Cholesterol is also major constituent of gall stones. Cholesterol crystals were grown by single diffusion gel growth technique using neutral silica gel set by acetic acid. Solution of cholesterol prepared in acetone was poured as a supernatant solution on set gel. Transparent, thin, platelet type crystals were grown. Powder XRD and FT IR studies were carried out for the confirmation of crystal system and chemical bonding. The surface micro topographical study was carried out by using SEM and AFM and interesting features were observed. As cholesterol possesses high dielectric constant, it functions as an insulting cover for the transmission of electric impulses in the nervous tissues; therefore, the dielectric study was carried to understand its dielectric behavior in the frequency range from 50 Hz to 5 MHz. The dielectric constant decreased as the frequency of applied field increased. The a.c. conductivity and a.c. resistivity were also calculated.

Abstract: Tin Selenide thin films have been deposited using thermal evaporation technique on chemically and ultrasonically cleaned glass substrates. The stoichiometry of deposited films has been studied using Energy Dispersive Analysis of X-rays (EDAX).The orthorhombic structure and polycrystalline nature of the films were also revealed by X-ray diffraction (XRD) and Transmission Electron Microscopy (TEM) analysis. The well characterized thin film of SnSe was then used to fabricate Ag/p-SnSe/In Schottky barrier diode. The I-V characteristics of prepared diodes have been investigated over the temperature range of 303 K to 393 K. The forward biased I-V characteristics of prepared structure has been analyzed using TE theory and different device parameters have been evaluated and discussed in present paper. The Richardson constant was also determined from the conventional Richardson plot and it is found close to the reported value.

Abstract: The structural, electronic, optical and elastic properties of zinc-blende compounds (CdX, X = S, Se and Te), were studied using full-potential augmented plane wave plus local orbitals method (FP-LAPW+ lo) within density functional theory, using generalized gradient approximation (GGA). Geometrical optimization of the unit cell (lattice constant, bulk modulus and its pressure derivative) is in good agreement with experimental data. Results for band structures, density of states, and elastic constants (C₁₁, C₁₂ and C₄₄) are presented. We also report our results on optical properties like the complex dielectric functions and the refractive index (n) of these compounds. Our results are in reasonable agreement with the available theoretical and experimental data.

Abstract: Cadmium sulphide (CdS), a member of group II-VI semiconductors is one of the promising materials from its applications point of view. The present investigations are about the preparation, structural and optical characterization of CdS thin films and their use as Schottky diode with Aluminum. Thin films of CdS having thickness around 700nm have been deposited by thermal evaporation. The chemical composition of the deposited CdS thin films has been made using EDAX technique. The structural characterization of this films was carried out using XRD. The structure of CdS after the deposition was found to be Cubic. Also, the lattice parameters were evaluated from the XRD data. From TEM of CdS thin films, the polycrystalline nature was confirmed. Optical characterization has been carried out using UV-VIS-IR spectroscopy. The direct as well as indirect band gaps obtained are 1.64eV and 1.48eV respectively. Schottky junctions were formed by a thermal vapor-deposition of 500nm Al films on pre-coated CdS glass substrates. Diode parameters, such as the zero bias barrier height φb0, the flat band barrier height φbf and the ideality factorη, were calculated using thermionic emission theory at room temperature.

Abstract: Tin Selenide compounds have attracted considerable attention because of their important properties useful for optoelectronics, holographic recording systems, electronic switching and infrared production and detection. Moreover, SnSe is a semiconductor with a band gap of about 1 eV that makes it potential candidate for solar cell material. Further, the optoelectronic properties of SnSe can be modified using doping of lead, copper etc. In this context, we report here results of the preparation and characterization of flash evaporated lead doped SnSe thin films. Thin films of SnSePb_0.1 having thickness of 100nm and 500nm were deposited on to chemically cleaned glass substrates. The chemical composition of the deposited SnSePb_0.1thin films has been evaluated using EDAX technique. The structure of SnSePb_0.1was studied using low angle XRD and it shows that the deposited films are polycrystalline in nature having orthorhombic structure. Various lattice parameters along with micro strain for both 100nm and 500nm thin films have been evaluated. Electrical transport properties have been also investigated using high temperature Hall Effect measurement in the temperature range 300 400K. The decrease in the resistivity of deposited films with increasing temperature confirms the semiconducting behavior of SnSePb_0.1 thin films.