Search

First-principles calculations are carried out to investigate the effect of pressure (up to 40 GPa) on electronic, optical and elastic properties of the scintillator material calcium iodide, CaI2, by using Perdew–Burke–Ernzerh of generalized gradient approximation (PBE-GGA). The electronic band structure reveals an indirect band gap of 3.89 eV and a direct band gap of 4.35 eV for zero pressure. We have also calculated the pressure dependence of optical properties. The elastic constants and elastic moduli increase nonlinearly with pressure. Decreasing Pugh's ratio and increasing hardness with pressure indicate a cross-over from ductile to brittle nature at 5 GPa. Analyses of elastic properties show that CaI2 is mechanically stable up to the applied pressure of 40 GPa. The calculated light yield increases linearly with pressure yielding a pressure coefficient of 1327 ph/MeV per GPa due to the decreasing band gap. Light yield increases as much as 62% at 40 GPa. [ABSTRACT FROM AUTHOR]

Nb2O5 quantum dots (QDs) were grown using a simple technique of vacuum thermal evaporation. QDs were found to be crystalline in nature by selected area electron diffraction (SAED) in TEM. Samples with thickness up to 20 nm did not show any significant residual strain. Residual stress effect on band gap of crystalline Nb2O5 was studied for films thicker than 20 nm. Residual strain was determined using SAED of the films with reference to powder X-ray diffraction (XRD). Films thicker than 45 nm become amorphous as analyzed by both SAED and XRD. The optical absorption of films in the range 25-60 nm indicates significantly varying optical band gap of films. The varying band gap with film thickness scales linearly very well with the variation of residual stress with film thickness. The residual stress dependence of band gap of crystalline films yields stress free band gap as 3.37 eV with pressure coefficient of band gap (∂Eg/∂P)T=-29.3 meV/GPa. From this study, the crystalline to amorphous transformation in tetragonal form of M-Nb2O5 has been determined to be at about 14 GPa. Both pressure coefficient of band gap and crystalline to amorphous transition for tetragonal M-Nb2O5 have been determined for the first time in the literature. [ABSTRACT FROM AUTHOR]

The thermally evaporated layer structured BaI2 grows in various completely preferred (hkl) film orientations with different growth parameters like film thickness, deposition rate, substrate temperature, etc. which were characterized by structural, morphological, and optical absorption measurements. Structural analysis reveals the strain in the films and the optical absorption shows a direct type band gap. The varying band gaps of these films were found to scale linearly with their strain. The elastic moduli and other constants were also calculated using Density Functional Theory (DFT) formalism implemented in WIEN2K code for converting the strain into residual stress. Films of different six (hkl) orientations show stress free anisotropic band gaps (2.48-3.43 eV) and both positive and negative pressure coefficients. The negative and positive pressure coefficients of band gap are attributed to the strain in I-I (or Ba-Ba or both) and Ba-I distances along [hkl], respectively. The calculated band gaps are also compared with those experimentally determined. The average pressure coefficient of band gap of all six orientations (-0.071 eV/GPa) found to be significantly higher than that calculated (-0.047 eV/GPa) by volumetric pressure dependence. Various these issues have been discussed with consistent arguments. The electron effective mass me*=0.66m0 and the hole effective mass mh*=0.53m0 have been determined from the calculated band structure. [ABSTRACT FROM AUTHOR]

Optical absorption measurements were carried out at room temperature on stoichiometric and well characterized vacuum evaporated ZnI2 films as function of film thickness and heat-treatment temperatures. The films were transparent and tetragonally structured. X-ray diffraction analysis reveals residual strain on the crystal planes parallel to film thickness. Optical absorption shows three distinct peaklike structures at 3, 3.5 (attributed to be excitonic), and 4.1 eV. The residual strains as well as all the absorption structures vary with film thickness and treatment temperatures. Combining all the data from the two experiments, we find that the two excitonic peaks blueshift with residual strain of (104). The strain-free values for all the above quantities have been determined by the extrapolation of their linear dependence on residual strain. The excitonic absorption observed at room temperature shows a large excitonic binding energy of 150 meV. [ABSTRACT FROM AUTHOR]

Optical absorption measurements is conducted at room temperature on stoichiometric and well characterized vacuum evaporated ZnI2 films as a function of film thickness and heat-treatment temperatures. Optical absorption reveal three distinct peaklike structures at 3, 3, 5 (attributed to be excitonic), and 4.1 eV.

It is demonstrated that the measured diffuse reflectance spectra of γ-In2Se3 can be used to map the conduction band edge density of states through Kubelka-Munk analysis. The Kubelka-Munk function derived from the measured spectra almost mimics the calculated density of states in the vicinity of conduction band edge. The calculation of density of states was carried out using first-principles approach yielding the structural, electronic, and optical properties. The calculations were carried out implementing various functionals and only modified Tran and Blaha (TB-MBJ) results tally closest with the experimental result of band gap. The electronic and optical properties were calculated using FP-LAPW + lo approach based on the Density Functional Theory formalism implementing only TB-mBJ functional. The electron and hole effective masses have been calculated as m e * = 0.25 m 0 and m h * = 1.11 m 0 , respectively. The optical properties clearly indicate the anisotropic nature of γ-In2Se3. [ABSTRACT FROM AUTHOR]

The quantum dots of antimony trisulphide, a potential semiconductor for various applications, are grown in glass matrix for the first time and are characterized by various techniques. The dependence of the average dot size on growth parameters like growth temperature and time is systematically studied for the dot size range of 5-80 nm. The linear blue shift of band gap of dots with inverse square of dot size clearly indicates the typical behavior of quantum dots in a strong quantum confinement regime. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

Abstract: The crystalline quantum dots of Nb2O5 in the range 1–20nm have been grown by vacuum thermal evaporation using the bottom-up approach of early nucleation and growth stage. The dot sizes were controlled by growth time for an optimized constant molecular flux rate of growth. The dots were characterized by TEM and UV/Vis absorption spectroscopy. The variation of optical band gap of dots scaling with inverse square of their size demonstrates the quantum confinement effect in the dots. The reduced mass of electron–hole pair (exciton) was determined to be 0.532me from this behavior. [Copyright &y& Elsevier]