N. K. Verma

Lifetime shortening in doped ZnS nanophosphors

ZnS : Mn nanophosphor has been doped with quencher impurity Ni using the colloidal precipitation method with capping agent polyvinylpyrrolidone. The formation of ~2.5 nm sized nanoparticles has been confirmed by x-ray diffraction and transmission electron microscope studies. Energy and time resolved photoluminescence spectra of the synthesized nanophosphors have been studied at room temperature. Photoluminescent spectra for ZnS : Mn, Ni show three peaks at 412 nm, 433 nm and 590 nm corresponding to Ni impurity, sulphide vacancies (Vs) and Mn impurity, respectively. The ZnS : Mn nanophosphor show typical nanosecond lifetimes for defect-related emission and millisecond lifetimes for Mn impurity-related emission. However, the ZnS : Mn, Ni samples showed lifetime shortening, with variation of dopant concentration for defect-related (420 nm) as well as impurity-related emission (590 nm), which is attributed to exchange interaction between Mn2+ and nearest neighbour Ni2+ impurities.

Enhanced photoluminescence decay processes of doped CaS phosphors at low temperature

CaS phosphor samples singly doped with Mn impurity and doubly doped with Mn and X (X= Fe, Co and Ni) have been synthesized using a flux method. Crystal structure and phase identification of doped CaS phosphors have been carried out by X-ray diffraction (XRD) studies. A pulse excitation method has been employed to study the optical parameters of the doped phosphors. Excited state lifetime measurements at liquid nitrogen temperature were carried out with a pulsed UV laser (nitrogen laser) as the excitation source has a short pulse width (10 ns) and high peak power (200 kW per pulse). The results have been compared with room temperature lifetime studies. Enhancement in photoluminescence intensity and lifetime shortening in these synthesized doped phosphors is reported at low temperature. The lifetime values have been found to be in the microsecond time domain for CaS:Mn phosphors while on addition of a quencher impurity Ni to CaS:Mn phosphors, the lifetime values reduce to the nanosecond time domain at 77 K temperature. A thermally activated carrier transfer model has been proposed to explain the observed abnormal temperature behaviour of emission from CaS:Mn, X-doped phosphors.

Laser-induced down-conversion parameters of singly and doubly doped ZnS phosphors

Singly and doubly doped ZnS phosphors have been synthesized using flux method. Laser-induced photoluminescence has been observed in ZnS-doped phosphors when these were excited by the pulsed UV N2 laser radiation. Due to down-conversion phenomenon, fast phosphorescence emission in the visible region is recorded in milliseconds time domain for ZnS:Mn while in the case of ZnS:Mn:killer (Fe, Co and Ni) the lifetime reduces to microseconds time domain. Experimentally observed luminescent emission parameters of excited states such as, lifetimes, trap-depth values and decay constants have been reported here at room temperature. The high efficiency and fast recombination times observed in doped ZnS phosphors make these materials very attractive for optoelectronic applications.

Two-point resolution of Gaussian aperture operating in partially coherent light using various resolution criteria

We have investigated the dependence of two-point resolution by an aberration-free Gaussian aperture on the real part of the complex degree of coherence. The Sparrow and the modified Rayleigh criteria have been used for the partially coherent optical systems, and a comparison between them has been made. To reveal important information concerning the characteristics of the image, the irradiance distribution from two mutually partially coherent point sources has also been computed.

Structural, optical and magnetic properties of cobalt-doped CdSe nanoparticles

Pure and Co-doped CdSe nanoparticles have been synthesized by hydrothermal technique. The synthesized nanoparticles have been characterized using X-ray diffraction (XRD), ultraviolet-visible spectroscopy (UV–Visible), photoluminescence spectroscopy (PL), energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM) and superconducting quantum interference device (SQUID), at room temperature. From XRD analysis, pure and cobalt-doped CdSe nanoparticles have been found to be polycrystalline in nature and possess zinc blende phase having cubic structure. In addition to this, some peaks related to secondary phase or impurities such as cobalt diselenide (CoSe2) have also been observed. The calculated average crystallite size of the nanoparticles lies in the range, 3–21 nm, which is consistent with the results obtained from TEM analysis. The decrease in average crystallite size and blue shift in the band gap has been observed with Co-doping into the host CdSe nanoparticles. The magnetic analysis shows the ferromagnetic behaviour up to 10% of Co-doping concentration. The increase of Co content beyond 10% doping concentration leads to antiferromagnetic interactions between the Co ions, which suppress the ferromagnetism.

Fast photoluminescence decay processes of doped CaS Phosphors

Singly and doubly doped CaS phosphors have been synthesized using the flux method. Excited-state lifetime measurements have been carried out for CaS doped with Mn and quencher impurities (Fe or Co or Ni) having doping concentrations 0.05–0.70% by weight, using pulsed-UV laser (nitrogen laser) as the excitation source having pulse width (10 ns) and peak power (200 kW per pulse). Laser-induced photoluminescence has been observed in CaS-doped phosphors when these phosphors were excited by the pulsed-UV N2 laser radiation. Owing to the downconversion phenomenon, fast photoluminescence emission in the visible region is recorded in microseconds time domain for CaS:Mn, whereas in case of CaS:Mn:Ni, shortest component of lifetime values reduces to nanoseconds time domain.

Diffraction by truncated-Gaussian annular apertures

A theoretical study of the diffraction of a plane Gaussian wave truncated by annular aperture. Kirchhoffs integral has been evaluated to obtain the irradiance distribution in the diffracted field. Results of the calculations conducted for optimum size of the annular aperture for various values of the Gaussian spot size have been presented graphically.

Effect of Killer Impurities on Laser Excited Doped ZnS Phosphors

Zinc sulphide phosphors, doped with Ba and Mg as well as killer impurities of Fe, Co and Ni having variable concentrations have been synthesized and then using ultraviolet laser (Nitrogen Laser) as the excitation source, their decay-curve analysis has been done. Various strong emissions have been detected and the corresponding excited state lifetimes are measured at room temperature. Studies have also been carried out to see the effect of different dopant concentrations on the phosphorescence excited-state life times.

Highly reflective titania nanoparticle‐based coating

Purpose – The purpose of this paper is to investigate the reflective properties of titania (TiO2) nanoparticle‐based coating.Design/methodology/approach – TiO2 nanoparticles, synthesised by sol‐gel method, were characterised by X‐ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and ultraviolet‐visible absorption spectroscopy (UV‐vis). The coating material has been prepared by dispersing titania nanoparticles in an acrylic binder with different pigment to binder weight ratio. The reflectors were prepared by applying this coating material to different coating thicknesses to aluminium sheets.Findings – In the study reported here, the coating material could produce reflectors with diffuse reflectance, ∼99 per cent, using coating material, having binder by weight ratio between 14 and 20 per cent, and thickness, 0.15 mm. On exposing the developed reflectors to different levels of illumination (upto 20,000 lux), they were still found to have diffuse reflectance o...

Room temperature ferromagnetism in Mg-doped ZnO nanoparticles

Zn1-xMgxO (x = 0, 0,10) nanoparticles were successfully synthesized using sol-gel method. X-ray diffraction (XRD) confirms that the synthesized nanoparticles possess wurtzite phase having hexagonal structure. Morphological analysis was carried out using transmission electron microscopy (TEM) which depicts the spherical morphology of ZnO nanoparticles. Energy dispersive spectroscopy (EDS) showed the presence of Mg in ZnO nanoparticles. Electron spin resonance (ESR) signal was found to be decreasing with increasing of Mg-doping concentration. The room temperature ferromagnetism was observed in undoped and Mg-doped ZnO nanoparticles. The increase of Mg-doping concentration resulted in decrease of saturation magnetization value which could be attributed to decrease of oxygen vacancies present in host nanoparticles.