Effect of substrate and annealing on the structural and optical properties of ZnO:Al films

ZnO:Al thin films with c-axis preferred orientation were deposited on glass and Si substrates using RF magnetron sputtering technique. The effect of substrate on the structural and optical properties of ZnO:Al films were investigated. The results showed a strong blue peak from glass-substrate ZnO:Al film whose intensity became weak when deposited on Si substrate. However, the full width at half maxima (FWHM) of the Si-substrate ZnO:Al (0 0 2) peaks decreased evidently and the grain size increased. Finally, we discussed the influence of annealing temperature on the structural and optical properties of Si-substrate ZnO:Al films. After annealing, the crystal quality of Si-substrate ZnO:Al thin films was markedly improved and the intensity of blue peak (∼445 nm) increased noticeably. This observation may indicate that the visible emission properties of the ZnO:Al films are dependent more on the film crystallinity than on the film stoichiometry.     

Influence of holmium doping on the optical properties of quaternary InGaAsP epitaxial layers

Photoluminescence and photoconductivity measurements were used to study the influence of Ho doping on the optical properties of InGaAsP layers grown by liquid phase epitaxy (LPE). The full width at half maximum (FWHM) of the photoluminescence peak was found to decrease as the amount of Ho increases. When the amount of Ho is 0.11 wt%, the FWHM has a minimum value of 7.93 meV, about 46% lower than that of the undoped InGaAsP. The absorption tails observed in the photoconductivity were analyzed with the Urbach tail model and the Urbach energies were obtained from the fits. The Urbach energy decreases as the amount of Ho increases, indicating that Ho doping greatly reduces the amount of residual impurities in LPE-grown layers.     

Effect of annealing temperature on the microstructures and photocatalytic property of colloidal ZnO nanoparticles

Colloidal ZnO nanoparticles were prepared in ethanol solutions and annealed at different temperatures (150-500 °C) subsequently. The size, morphology and surface characteristics of ZnO nanoparticles were examined by TEM, XRD, UV-vis absorption spectrum and FTIR technique. With the increase of annealing temperature, the mean size of ZnO nanoparticles was increased from 10 to 90 nm, while the bonding structure of acetate groups coordinating with zinc ions evolved from unidentate to bidentate type. The UV-induced degradation results of methyl orange verified that the photocatalytic process of colloidal ZnO nanoparticles without annealing and the sample annealed at 150 °C was unstable for the weakly bonding unidentate type of acetate groups. However, the sample annealed above 150 °C demonstrated their photocatalytic stability in the whole catalytic process for the stable bidentate bonding type of acetate groups. In addition, the change of particle size in the annealing process significantly affected the catalytic activity of photocatalysts. ZnO nanoparticles annealed at 300 °C would be a prospective photocatalysts with a high catalytic activity and stability compared with the other samples.     

Magnetic and optical properties of self-organized InMnAs quantum dots

Ten layers of self-assembled InMnAs quantum dots with InGaAs barrier were grown on high resistivity (1 0 0) p-type GaAs substrates by molecular beam epitaxy (MBE). The presence of ferromagnetic structure was confirmed in the InMnAs diluted magnetic quantum dots. The ten layers of self-assembled InMnAs quantum dots were found to be semiconducting, and have ferromagnetic ordering with a Curie temperature, T_C=80 K. It is likely that the ferromagnetic exchange coupling of sample with T_C=80 K is hole mediated resulting in Mn substituting In and is due to the bound magnetic polarons co-existing in the system. PL emission spectra of InMnAs samples grown at temperature of 275, 260 and 240 °C show that the interband transition peak centered at 1.31 eV coming from the InMnAs quantum dot blueshifts because of the strong confinement effects with increasing growth temperature.     

Catalyst efficiency, photostability and reusability study of ZnO nanoparticles in visible light for dye degradation

Polydispersed ZnO nanoparticles (ZnO1000 and ZnO600) with two different windows of particle size distributions (∼120 and 30 nm) were synthesized using citrate gel route and different annealing treatments (1000 and 600 °C, respectively). Photocatalytic efficiency of these samples was compared with TiO₂ in its commercial form-P25, on two dyes, Methylene blue (MB) and Methyl orange (MO). The X-ray diffraction data showed wrutzite ZnO and anatase and rutile phases of P25. UV-visible absorbance spectra of ZnO1000 showed broad absorption range from UV-to-visible (from 382 to 700 nm), as against sharp absorption peaks in UV range for both ZnO600 and P25. The microstructural morphology as seen through scanning electron micrographs showed ZnO1000 with tetrapod-like structures while the ZnO600 showed almost spherical morphologies. Upon subjecting these catalysts to dye solutions in sunlight it was found that both the dyes were completely decolorised within 20 min by ZnO1000, as against partial decolorisation by ZnO600 and P25 ( 53% and 78% for MO and 77% and 88% for MB samples). The effect of catalyst loading (from 125 mg to 1 g) on decolorisation showed that ZnO1000 had good efficiency for all concentrations which was followed by P25 and then by ZnO600. Small perturbations are attributed to the competition between sunlight scattering-induced, reduced irradiation field and the exposed surface area offered by catalyst, which work as active sites for decolorisation. The reusability of the catalysts when studied on fresh dye samples (4 trials), the decolorisation efficiency decreased merely from 99.2% to 99.12% for ZnO1000 as compared to ZnO600 (53.3% to 19.94%) and P25 (78.3% to 31.42%), indicating the efficient reusability of ZnO1000. The effective half life of the catalysts, in terms of number of reuses, were calculated and found to be ∼3 for both ZnO600 and P25 and was >3000 for ZnO1000, which justifies its extremely high reuse. The byproduct analysis (compared with standards prescribed by World Health Organisation (WHO) and Central Pollution Control Board of India (CPCB)) showed cleavage of the chromophore and of other bonds with opening of benzene rings, indicating degradation of the dyes in concurrence with decolorisation, in the stipulated time. Further, cytotoxicity studies performed on SiHa cell lines showed non-toxicity of the byproducts with ZnO1000 as compared to ZnO600 and P25.     

Hydrogen passivation effect on the yellow-green emission band and bound exciton in n - ZnO

Photoluminescence (PL) measurements performed on as-grown, hydrogenated, and annealed n-type ZnO bulk samples investigated the origins of their yellow (2.10 eV) and green (2.43 eV) emission bands. After hydrogenation, the defect-related peak at 2.10 eV was no longer present in the room temperature PL spectrum, the peak intensity at 2.43 eV was unchanged, and the intensity of the emission peak at 3.27 eV increased significantly. These results indicate that yellow band emission is due to oxygen vacancies, as the emission peak at 2.10 eV disappears when hydrogen atoms passivate these vacancies. The emission peak at 2.43 eV originates from complexes between oxygen vacancies and other crystal defects. We discuss the shallow donor impurities arising due to these hydrogen atoms in the ZnO bulk sample.     

Effects of an electric field on the electronic and optical properties of zigzag boron nitride nanotubes

We have investigated the electro-optical properties of zigzag BNNTs, under an external electric field, using the tight binding approximation. It is found that an electric field modifies the band structure and splits the band degeneracy. Also the large electric strength leads to coupling the neighbor subbands which these effects reflect in the DOS and JDOS spectrum. It has been shown that, unlike CNTs, the band gap of BNNTs can be reduced linearly by applying a transverse external electric field. Also we show that the larger diameter tubes are more sensitive than small ones. The semiconducting metallic transition can be achieved through increasing the applied fields. The number and position of peaks in the JDOS spectrum are dependent on electric field strength. It is found that at a high electric field, the two lowest subbands are oscillatory with multiple nodes at the Fermi level.     

Theoretical investigations on the structural, lattice dynamical and thermodynamic properties of XC (X=Si, Ge, Sn)

First-principles calculations, which is based on the plane-wave pseudopotential approach to the density functional perturbation theory within the local density approximation, have been performed to investigate the structural, lattice dynamical, and thermodynamic properties of SiC, GeC, and SnC. The results of ground state parameters, phase transition pressure and phonon dispersion are compared and agree well with the experimental and theoretical data in the previous literature. The obtained phonon frequencies at the zone-center are analyzed. We also used the phonon density of states and quasiharmonic approximation to calculate and predict some thermodynamic properties such as entropy, heat capacity, internal energy, and phonon free energy of SiC, GeC, and SnC in B3 phase.     

Influence of deposition parameters on the structural and optical properties of CdS thin films obtained by micro-controlled SILAR deposition

Polycrystalline CdS films were obtained by a micro-controlled SILAR deposition technique, using aqueous solutions of cadmium acetate and thiourea as precursors. The structural and optical properties of the films were found to be influenced by various deposition parameters such as number of immersion cycles, concentration of the precursors and temperature of the solutions. Contrary to the observations made by some researchers, we found that the thickness of the films increased continuously with number of immersion cycles and also with concentration of the precursor solutions. We also found that the films covered the substrates uniformly, without any voids, unlike the films obtained by others. Effect of deposition parameters on thickness, substrate coverage, grain size, chemical composition, optical band gap and other properties of the films is discussed in detail.     

Annealing effect on the morphologies and photoluminescence properties of ZnO nanocombs

In order to examine the annealing effect on morphologies and photoluminescence properties of the as-synthesized ZnO nanocombs, they were annealed in argon and in air at 800 °C for 10 h, respectively. The SEM, TEM, HRTEM, and PL examination results show that the morphologies and photoluminescence properties of ZnO nanocombs annealed in argon are quite different from those of ZnO nanocombs annealed in air. To anneal ZnO nanocombs in air caused the disappearance of combs, the enhancement of ultraviolet emissions, and the disappearance of emissions in the visible spectral region. On the other hand, the combs still existed, and the green emission centering at 500 nm still existed after annealing ZnO nanocombs in argon. The mechanisms for the morphological and photoluminescence evolution of annealed ZnO nanocombs are also proposed.     

Ab initio calculations of structural,electronic, optical and thermodynamic properties of alkaline earth tellurides BaxSr1−XTe

Structural, electronic and thermodynamic properties of SrTe and BaTe compounds and their ternary mixed crystals Ba_xSr_1−xTe in the rock-salt structure have been studied with density functional theory (DFT), whereas the optical properties have been obtained by using empirical methods such as the modified Moss relation. The exchange-correlation potential was calculated using the generalized gradient approximation (GGA) of Perdew–Burke–Ernzerhof (PBE) and the local density approximation (LDA) of Teter–Pade (TP). In the present work, we used the virtual-crystal approximation (VCA) to study the effect of composition (x). The calculated lattice parameters at equilibrium volume and the bulk modulus for x=0 and x=1 are in good agreement with the literature data. Furthermore, the Ba_xSr_1−xTe alloys are found to be an indirect band gap semiconductor. In addition, we have also predicted the heat capacities (C_V), the entropy(S), the internal energy (U) and the Helmholtz free energy (F) of the parent compounds SrTe and BaTe.     

Effects of S incorporation on Ag substitutional acceptors in ZnO:(Ag, S) thin films

Ag–S codoped ZnO thin films have been fabricated on Si substrates by radio frequency (RF) magnetron sputtering using a thermal oxidation method. XRD and SEM measurements showed that the sample has hexagonal wurtzite structure with a preferential (002) orientation and the surface is composed of compact and uniform grains. Ag_Zn–nS_O defect complexes were observed in the Ag–S codoped ZnO films by XPS analysis. Low temperature PL spectra showed neutral acceptor bound exciton emission related to Ag_Zn–nS_O. The corresponding acceptor ionization energy of 150 meV is much lower than that of monodoped Ag (246 meV), which is favorable for p-type doping of ZnO.     

Effect of annealing temperature on the optical and electrical properties of amorphous As45.2Te46.6In8.2 thin films

The optical absorption of the As-prepared and annealed As_45.2Te_46.6In_8.2 thin films are studied. Films annealed at temperatures higher than 453 K show a decrease in the optical energy gap (E_o). The value of E_o increases from 1.9 to 2.43 eV with increasing thickness of the As-prepared films from 60 to 140 nm. The effect of thickness on high frequency dielectric constant (?_∞) and carrier concentration (N) is also studied. The crystalline structures of the As_45.2Te_46.6In_8.2 thin films resulting from heat treatment of the As-prepared film at different elevated temperatures is studied by X-ray diffraction. An amorphous-crystalline transformation is observed after annealing at temperatures higher than 453 K. The electrical conductivity at low temperatures is found due to the electrons transport by hopping among the localized states near the Fermi level. With annealing the films at temperatures higher than 473 K (the crystallization onset temperature) for 1 h, the electrical conductivity increases and the activation energy decreases, which can be attributed to the amorphous-crystalline transformations.     

First principles study on the structural and optical properties of the high-pressure ZnO phases

A new high-pressure tetragonal phase (B10) of ZnO is investigated with an ab initio calculation based on density functional theory and is compared with the cubic B1 (rocksalt structure) and B2 (CsCl structure) phases at high pressure. It is found that the B10 phase has a more covalent nature than the B2 phase. The B1, B2, and B10 phases are semiconductors and their band gap energies are determined to be 3.73, 3.15, and 3.27 eV, respectively. The B10 phase has a similar optical response to the B2 phase, but not the B1 phase. The similarity of dielectric function between B10 and B2 phases are the result of the similar profiles of electronic density of state.     

Influence of pH on microstructural and optical properties of electrosynthesized CdSe thin films

Cadmium selenide (CdSe) thin films were electrosynthesized onto well cleaned stainless steel and fluorine-doped tin oxide (FTO) coated glass (10-15 Ω/cm²) substrates at different pH of the electrolytic solution. X-ray diffraction study reveals a cubic structure with preferential orientation along the (1 1 1) direction. The structural parameters such as grain size (D), lattice constant (a), strain (ε), dislocation density (δ), average internal stress (S) and degree of preferred orientation (I₁₁₀/I₂₂₀) in the film are calculated and they are found to be dependent on the pH of the depositing bath. EDAX analysis confirms nearly stoichiometric composition of the film deposited at pH 2.70. AFM analysis shows uniform deposition of the film over the entire substrate surface. In optical studies, the transition of the deposited film is found to be a direct allowed transition. The optical energy gaps are found to be in the range from 1.87 to 2.04 eV depending on the pH of the depositing bath. Photoluminescence (PL) spectrum shows blue shift in PL peak position and reduction in luminescence intensity for the film deposited at pH other than 2.70.     

Structural and optical characterization of CdS nanoparticles prepared by chemical precipitation method

In the present study we have synthesized CdS semiconducting quantum dots by the chemical precipitation method using Thioglycerol as the capping agent. X-ray powder diffraction (XRD) and transmission electron microscopy (TEM) are employed to characterize the size, morphology and crystalline structure of the as-prepared material. The synthesized QPs have a mixture of cubical and hexagonal crystal symmetry with 12 nm average diameter. Ultraviolet-visible (UV-vis) absorption spectroscopy is used to calculate the band gap of the material and blue shift in absorption edge. Confinement of the optical phonon modes in the QPs is studied by Raman spectroscopy, while FTIR for identification of chemical bonds in the nanomaterial. Multiple cadmium and sulphur defects were observed by employing the photoluminescence (PL) method.     

Pressure dependent elastic properties of diluted magnetic semiconductors: Hg1−xMnxS (x=0.02 and 0.07)

A theoretical study of the elastic properties in diluted magnetic semiconductors Hg_1−xMn_xS (x=0.02 and 0.07) using an effective interionic interaction potential (EIoIP) in which long-range Coulomb interactions, charge transfer mechanism (three body interaction) and the Hafemeister and Flygare type short-range overlap repulsion extending up to the second neighbor ions and the van der Waals (vdW) interaction is considered. Particular attention is devoted to evaluate Poisson's ratio ν, the ratio R_S/B of S (Voigt averaged shear modulus) over B (bulk modulus), elastic anisotropy parameter, elastic wave velocity, average wave velocity and thermodynamic property as Debye temperature is calculated. By analyzing Poisson's ratio ν and the ratio R_S/B we conclude that Hg_1−xMn_xS is brittle in zinc blende (B3). To our knowledge this is the first quantitative theoretical prediction of the pressure dependence of ductile (brittle) nature of Hg_1−xMn_xS compounds and still awaits experimental confirmations.     

Electronic structure and optical properties of RbPb2Br5

We report on density functional theory (DFT) calculations of the total and partial densities of states of rubidium dilead pentabromide, RbPb₂Br₅, employing the augmented plane wave+local orbitals (APW+lo) method as incorporated in the WIEN2k package. The calculations indicate that the Pb 6s and Br 4p states are the dominant contributors to the valence band: their main contributions are found to occur at the bottom and at the top of the band, respectively. Our calculations reveal that the bottom of the conduction band is formed predominantly from contributions of the unoccupied Pb 6p states. Data of total DOS derived in the present DFT calculations are found to be in agreement with the experimental X-ray photoelectron valence-band spectrum of this compound. The predominant contributions of the Br 4p states at the top of the valence band of rubidium dilead pentabromide are confirmed by comparison on a common energy scale of the X-ray emission band representing the energy distribution of the valence Br p states and the X-ray photoelectron valence-band spectrum of the RbPb₂Br₅ single crystal. Main optical characteristics of RbPb₂Br₅, such as dispersion of the absorption coefficient, real and imaginary parts of dielectric function, electron energy-loss spectrum, refractive index, extinction coefficient and optical reflectivity are explored for RbPb₂Br₅ by the DFT calculations.     

The effect of the Bi source on optical properties of Bi2Te3 nanostructures

nanostructures were synthesized by using different Bi sources via a simple solvothermal process, in which and BiCl₃ were used as the Bi sources. Optical properties of nanostructures prepared with and BiCl₃ as the Bi sources were investigated by micro-Raman spectroscopy. The Raman scattering spectrum of hexagonal nanoplates prepared by using as the Bi source shows that the infrared (IR) active mode A_1u, which must be odd parity and is Raman forbidden for bulk crystal due to its inversion symmetry, is greatly activated and shows up clearly in the Raman scattering spectrum. We attribute the appearance of the infrared active A_1u mode in the Raman spectrum to crystal symmetry breaking of hexagonal nanoplates. However, the Raman scattering spectrum of nanostructures with irregular shape prepared by using as the Bi source only exhibits the two characteristic Raman modes of crystals. Micro-Raman measurements on nanostructures with different morphologies offer us a potential way to tailor optical properties of nanostructures by controlling the morphologies of the nanostructures, which is very important for practical applications of nanostructures in thermoelectric devices.     

Electronic structure and optical properties of BiSI crystal

Electronic structure and optical properties of BiSI crystal were investigated by the full potential linearized augmented plane wave (FL-LAPW) method with density functional theory (DFT). The complex dielectric function and optical constants, such as optical absorption coefficient, refractive index, extinction coefficient, energy-loss spectrum and reflectivity, were calculated. The optical properties of BiSI crystal were studied experimentally by spectroscopic ellipsometry. The experimental results were compared with the theoretical spectra of complex dielectric functions and with the spectra of a pseudo-dielectric function (PDF). This method shows that experimental spectra consist of four Laurence lines sum.