Low-temperature absorption edge and photoluminescence in layered structured Tl2Ga2S3Se single crystals

The absorption edge of undoped Tl₂Ga₂S₃Se crystals have been studied through transmission and reflection measurements in the wavelength range 440–1100 nm and in the temperature range 10–300 K. The absorption edge was observed to shift toward lower energy values with increasing temperature. As a result, the rate of the indirect band gap variation with temperature γ=−2.6×10⁻⁴ eV/K and the absolute zero value of the band gap energy E_gi(0)=2.42 eV were obtained.     

Structural and luminescent properties of silicon nanoparticles incorporated into zirconia matrix

An impact of mechanical stresses on structural and photoluminescent properties of Si nanoparticles (NPs) incorporated into the zirconia thin films is reported. The stresses are found to be responsible for important structural modifications of the NPs. The zirconia matrix doped with the NPs exhibits bright red photoluminescence (PL) at room temperature due to efficient quantum confinement of the photogenerated carriers in the nanoscale Si particles. Spectral position of the PL picks depends on: mean dimension of the NPs, their concentration, stress induced deformation and order degree of the near-surface region. In general, zirconia matrix appears as a robust and reliable host media for Si NPs.     

Temperature dependence of the fundamental band gap parameters in cadmium-rich Zn<Subscript>x</Subscript>Cd<Subscript>1-x</Subscript>Se using photoluminescence spectroscopy

Thin films of ternary Zn_xCd_1-xSe were deposited on GaAs (100) substrate using metalorganic-chemical-vapour-deposition (MOCVD) technique. Temperature dependence of the nearband-edge emission from these Cd-rich Zn_xCd_1-xSe (forx = 0025, 0.045) films has been studied using photoluminescence spectroscopy. Relevant parameters that describe temperature variation of the energy and broadening of the fundamental band gap have been evaluated using various models including the two-oscillator model, the Bose-Einstein model and the Varshni model. While all these models suffice to explain spectra at higher temperatures, the two-oscillator model not only explains low temperature spectra adequately but also provides additional information concerning phonon dispersion in these materials.     

Surfactant-assisted synthesis of defective zirconia mesophases and Pd/ZrO2: Crystalline structure and catalytic properties

Mesoporous zirconia nanophases with structural defects were synthesized by using a surfactant-templated method. Physicochemical properties and crystalline structures of the zirconia nanophases were studied by means of thermogravimetric analysis (TGA), N₂ physosorption isotherm and in situ Fourier transform infrared (FT-IR) spectroscopy, transmission electron microscopy (TEM) and X-ray diffraction (XRD) techniques. The resultant materials show typical mesoporous features which vary with calcination temperature. The cationic surfactant in the network of the solids induces structural deformation and defect creation. The zirconia consists of monoclinic and tetragonal nanophases which contains many structural defects, and its crystalline structure shows microstrain. Both, concentration of lattice defects and degree of the crystal microstrain, decrease as the calcination temperature is increased. When CO is adsorbed on the surface of Pd/ZrO₂, linear bonds of CO–Pd⁰, CO–Pd^δ+ and CO–Zr⁴⁺ are formed, accompanying with CO₂ production. Catalytic evaluation shows that the Pd/ZrO₂ catalyst is very active for CO oxidation and NO reduction. In the case of oxygen absence from reaction mixture, high selectivity to N₂ is achieved without any NO₂ formation. In the oxygen rich condition, CO conversion is enhanced but less than 19% NO₂ is produced. N₂O is formed only in the reducing condition and its selectivity is sensitive to reaction temperature. The possible mechanisms of NO + CO and NO + CO + O₂ reactions over Pd/ZrO₂ catalyst related to reactant dissociation on the Pd metals and to defective structure of the nanozirconia support are discussed.     

On the correlation between the composition of Pr doped garnet type materials and their photoluminescence properties

Lu₃(Al,Ga)₅O₁₂ and (Y,Lu)₃(Al,Mg,Si)₅O₁₂ samples doped with 1% Pr³⁺ were prepared by two aqueous sol-gel chemistry approaches, such as citrate and glycolate methods. All samples were characterised by powder X-ray diffraction (XRD), thermal quenching (TQ), fluorescence lifetime measurements and photoluminescence (PL) techniques. It turned out that the Pr³⁺ emission is very sensitive to the composition of the garnet host lattice. Lu₃Al₅O₁₂:Pr³⁺ samples showed mainly [Xe]4f¹5d¹→[Xe]4f² broad band emission in UV-blue region, whereas [Xe]4f²-[Xe]4f² line emission of Pr³⁺ dominated in Lu₃Ga₅O₁₂ samples. Modification of garnet crystals with Mg²⁺-Si⁴⁺ pair has led to generation of strong crystal field strength resulting in energy transfer from 5d to 4f orbitals. Y₃AlMg₂Si₂O₁₂ composition generated the strongest crystal field splitting out of all synthesised samples and showed practically only Pr³⁺ [Xe]4f²-[Xe]4f² line emission in the red spectral region.     

Structural and optical properties of cadmium magnesium zinc oxide (CdMgZnO) nanoparticles synthesized by sol–gel method

Nanoparticles of ${\mathrm{Cd}}_x{\mathrm{Mg}}_{0.12?x}{\mathrm{Zn}}_{0.88}\mathrm{O}$ $(0\le x\le 0.02)$ were synthesized by a simple sol gel route with the combination of chelating agents. Effect of cadmium on the phase, structural, morphological and optical properties of the synthesized nanoparticles has been studied and reported by using X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy dispersive X-ray (EDX), Fourier transform infrared spectroscopy (FTIR) and UV–Vis diffuse reflectance spectroscopy (UV–vis DRS). The crystal size, lattice parameters, unit cell volume, X-ray density, inter-planar distances and bond length were obtained and analyzed from the XRD data. The X-ray analysis reveals the formation of a single phase with a hexagonal wurtzite structure, where an increase of the cell volume was achieved as the Cd content was increased as well. Synthesized nanoparticle were nearly spherical at nano-size regime and are loosely agglomerated as observed from the SEM analysis. EDX spectra of the composition confirmed the appropriate stoichiometric ratio. A fundamental absorption peak centered at 375 nm was observed from the UV–visible absorption spectra which shifted towards a higher wavelength correlating the narrowing of the energy band gap due to increase in Cd content. The structural adjustment from the IR spectra confirmed the stretching vibration of Zn–O in the ${\mathrm{Cd}}_x{\mathrm{Mg}}_{0.12?x}{\mathrm{Zn}}_{0.88}\mathrm{O}$ lattice with Cd content.     

Optical properties of γ-Fe2O3 nanoparticles dispersed on sol–gel silica spheres

Two types of γ-Fe₂O₃ nanoparticles, pure γ-Fe₂O₃ and γ-Fe₂O₃ dispersed on sol–gel silica spheres (γ-Fe₂O₃/SiO₂) in thin film form were prepared by the sol–gel technique. Transmission electron microscopy, X-ray diffraction, optical transmittance and FTIR studies along with photoluminescence measurements were carried out for characterizing the samples. The X-ray diffraction patterns of both γ-Fe₂O₃ nanoparticles and γ-Fe₂O₃/SiO₂ indicated their phase-pure forms which were supported by the FTIR spectra. The average sizes of the nanoparticles obtained from transmission electron microscopy studies were 4 nm for both types of samples. Optical transmittance studies indicated direct allowed transitions with two band gaps at 2.43 and 3.07 eV. Although both types of samples showed excitonic luminescence at 2.38 eV (at room temperature), the luminescence intensity of the γ-Fe₂O₃/SiO₂ was higher than that of pure γ-Fe₂O₃.