U. Pal

Effects of crystallization and dopant concentration on the emission behavior of TiO2:Eu nanophosphors

Uniform, spherical-shaped TiO2:Eu nanoparticles with different doping concentrations have been synthesized through controlled hydrolysis of titanium tetrabutoxide under appropriate pH and temperature in the presence of EuCl3·6H2O. Through air annealing at 500°C for 2 h, the amorphous, as-grown nanoparticles could be converted to a pure anatase phase. The morphology, structural, and optical properties of the annealed nanostructures were studied using X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy [EDS], and UV-Visible diffuse reflectance spectroscopy techniques. Optoelectronic behaviors of the nanostructures were studied using micro-Raman and photoluminescence [PL] spectroscopies at room temperature. EDS results confirmed a systematic increase of Eu content in the as-prepared samples with the increase of nominal europium content in the reaction solution. With the increasing dopant concentration, crystallinity and crystallite size of the titania particles decreased gradually. Incorporation of europium in the titania particles induced a structural deformation and a blueshift of their absorption edge. While the room-temperature PL emission of the as-grown samples is dominated by the 5D0 - 7Fj transition of Eu+3 ions, the emission intensity reduced drastically after thermal annealing due to outwards segregation of dopant ions.

Photoluminescence and Raman Scattering in Ag-doped ZnO Nanoparticles

Effects of Ag doping on the crystallinity and optical properties of zinc oxide (ZnO) nanoparticles have been studied by x-ray diffraction, diffuse reflectance spectroscopy, micro-Raman, and photoluminescence spectroscopy. It has been observed that while Ag-doping at low concentration improves the optoelectronic properties of ZnO nanostructures, Ag-doping at high concentrations drastically modify the emission behavior and lattice vibrational characteristics of the nanostructures. High Ag content in ZnO nanostructures causes lattice deformation, induces silent vibrational modes in Raman spectra, and reduces excitonic UV emission due to concentration quenching.

Correlations among size, defects, and photoluminescence in ZnO nanoparticles

We studied the correlations among size, defects, and photoluminescence emissions in ZnO nanoparticles of sizes ranging from 25 to 73 nm. The impurities and defects were characterized by Fourier-transform infrared spectroscopy and Raman spectroscopy. Particles of larger size revealed fewer surface impurities and enhanced E2 mode of hexagonal ZnO crystals, while the oxygen vacancy centers did not vary significantly with particle size. A simultaneous increase of excitonic luminescence and defect luminescence intensities with the increase of particle size is shown, indicating both emissions are subjected to nonradiative quenching by near surface defects. The study on the size-dependent green luminescence in our samples suggests that the emission might be a bulk property instead of having a surface origin in nanostructured ZnO. Two different radiative recombination processes are involved in the excitonic emission of ZnO. While the slow decay component (370 ps) did not depend on particle size, the fast componen...

Scattering of electromagnetic radiation by a multilayered sphere

The computer implementation of the algorithm for the calculation of electromagnetic radiation scattering by a multilayered sphere developed by Yang, is presented. It has been shown that the program is effective, resulting in very accurate values of scattering efficiencies for a wide range of size parameters, which is a considerable improvement over previous implementations of similar algorithms. The program, named scattnlay, would be the first of its kind to be publicly available.

Optical constants of vacuum-evaporated polycrystalline cadmium selenide thin films

The optical constants (n,K) of vacuum‐evaporated polycrystalline CdSe thin films are determined over 900–3100 nm photon wavelengths. Variation of band gap and optical constants with film thickness and substrate temperature is studied. Anomalous variation of refractive index near the band gap is explained by the volume and surface imperfections. Average spin‐orbit splitting of valence band (0.32) is estimated for the films deposited on mica substrates. A theoretical plot of refractive index near the band edge is done. The dispersion of refractive index in films is studied by considering a single‐oscillator model.

Optical characterization of vacuum evaporated cadmium sulfide films

The optical constants (n,k) of cadmium sulfide (CdS) thin films were determined in the spectral range of 0.55 Ixm to 1.80 Ixm from the optical absorption and transmittance measurements. The optical band gap (Eg) was determined for the films deposited at different substrate temperatures. Scanning electron microscopy (SEM), energy dispersive X-ray (EDX) analysis and X-ray diffraction (XRD) techniques were used to determine the morphology, composition, crystalline structure and crystallite size of the films. Evaporated CdS films show a predominant hexagonal phase with small crystallites. The optical band gap values of the films varied from 2.38 to 2.41 eV depending on the substrate temperature. It has been observed that the band gap and refractive index of the films have a close relationship with the size of the crystallites, The lower estimated value of band gap of the films is explained considering the effects of excitons and (or) some impurities.

Some optical properties of evaporated zinc telluride films

The absorption and transmission of ZnTe thin films are measured at wavelengths of 0.32-2.3 mu m. The optical constants (n, K) are measured in this range, although the short wavelength refractive index n measurement is limited by the band gap. A theoretical calculation of n near the band gap is made from experimental parameters. An explanation is given for the abnormal decrease of refractive index near the band gap. The effects of film thickness and doping with impurities like In, PbCl2, BaF2 have also been investigated. Effective crystallite size and strain have been determined by the method of variance analysis of the X-ray diffraction line profile on the same films. Lattice constants have also been calculated using the Nelson-Riley plots. It has been observed that there is an increase in optical band gap with decrease in crystallite size, increase in strain and decrease in lattice constant value.

Effects of morphology, surface area, and defect content on the photocatalytic dye degradation performance of ZnO nanostructures

ZnO nanostructures of different morphologies were fabricated through ultrasound-assisted hydrolysis of zinc acetate at room temperature, by controlling the pH of the reaction mixture. It has been observed that the pH of the reaction solution affects both the morphology and defect content of the nanostructures. To study the effects of morphology and other parameters like specific surface area, defect content, and surface contamination on photocatalytic activity, both the as-grown and air-annealed nanostructures were tested for methylene blue (MB) degradation under UV light. While all the above mentioned parameters have been seen to affect the photocatalytic performance of ZnO nanostructures, specific surface area, defect content, and carbon contamination at the surface have been seen to be the most important parameters, and should be controlled for their application in photocatalysis. Therefore, for photocatalytic applications of ZnO nanostructures, not only their morphology or the specific surface area are important, but care should be taken to control their defect contents and surface contaminants.

Effect of metal-ion doping on the optical properties of nanocrystalline ZnO thin films

Optical properties of metal (Al, Ag, Sb, and Sn)-ion-implanted ZnO films have been studied by ultraviolet-visible spectroscopy and spectroscopic ellipsometric techniques. The effects of metal-ion doping on the optical band gap (Eg), refractive index (n), and extinction coefficient (k) of nanocrystalline ZnO films have been studied for the similar implantation dose of all the metal ions. The ellipsometric spectra of the ion-implanted samples could be well described by considering an air/roughness/ZnO–M (layer 1)/ZnO (layer 2)/glass model. The band gap of ZnO films increases with Al ion doping and decreases with doping of Ag, Sb, and Sn ions. The refractive index of ZnO films in the visible spectral region increases substantially on Sb and Sn ion doping, while it decreases to some extent with Al ion doping.

Linear optical response of metallic nanoshells in different dielectric media

Metal nanoshells, which consist of nanometer-scale dielectric cores surrounded by thin metallic shells, have been designed and studied for their linear optical responses. The plasmon resonance of metal nanoshells displays geometric tunability controlled by the ratio of shell thickness either to the core radius or to the total radius of the particle. Using Mie theory the surface plasmon resonance (SPR) of metallic nanoshells (Au, Ag, Cu) is studied for different geometries and physical environments. Considering a final radius of about 20 nm, the SPR peak position can be tuned from 510 nm (2.43 eV) to 660 nm (1.88 eV) for Au, from 360 nm (3.44 eV) to 560 nm (2.21 eV) for Ag, and from 553 nm (2.24 eV) to 655 nm (1.89 eV) for Cu, just by varying the ratio t/RShell and the environments inside and outside. With the decrease of the t/RShell ratio the SPR peak position gets redshifted exponentially and the shift is higher for a higher refractive index surroundings. The plasmon linewidth strongly depends on the surface scattering process and its FWHM increases with the reduction of shell thickness.