Nature of luminescence transition in low-resistivity n-type ZnS single crystals

The cathodoluminescence spectra of low-resistivity n-type ZnS single crystals were recorded. The emission peak energy shifted towards higher energies with a corresponding decrease in the half-width of the emission band as the temperature was decreased. Time-resolved emission spectra and the decay of luminescence were measured at 77 K, in which the energy of the emission band at its maximum shifted to lower energies during decay. The decay of luminescence was found to be non-exponential. These studies gave experimental evidence that the blue self-activated emission of low-resistivity n-type ZnS crystals is of the donoracceptor pair transition nature, and thereby the difference of emission peak energy for emitting ZnS diodes with different Schottky contacts was interpreted.     

Application studies on red-light emitting ZnS-CdS and Y2O2S:Eu3+ phosphors used in cathode-ray tube screens for television

The (Zn_0.27, Cd_0.73)S:Ag, Cl, (Zn_0.77, Cd_0.23)S:Cu, Cl and Y₂O₂S:Eu³⁺ red-light emitting phosphors have been used in the preparation of cathode-ray tube screens for television. The dependence of screen brightness on both electron-beam accelerating voltage and current densities has been studied. The theoretically calculated intrinsic efficiencies are compared with the experimental efficiencies. The chromaticity of the cathode-ray tube screens was also studied as a function of accelerating high tension and current density. Cathodoluminescence emission spectra for these phosphors at room temperature are also measured.     

Effect of activator distribution on photo- and X-ray excited luminescence properties of ZnS:Cu,Cl phosphors

ZnS:Cu,Cl phosphors were prepared by conventional solid state reaction with the aid of NaCl-MgCl₂ flux. The copper activator was introduced into the phosphor precursors by three different methods: co-precipitated with ZnS (CP), wet-coated onto ZnS powders (WC), and simply mixed with ZnS in a mortar (SM). The samples were characterized by X-ray powder diffraction, photoluminescence spectra and X-ray excited luminescence spectra. The results show that both photo- and X-ray excited luminescence intensities of the as-prepared ZnS:Cu,Cl phosphors are in the decreasing order of CP > WC > SM. The different copper activator distribution in the phosphors resulting from the different methods was the main reason responsible for the different luminescence intensity, and uniform distribution is beneficial to the luminescence of the phosphors.     

Persistent luminescence of Zn<Subscript>2</Subscript>GeO<Subscript>4</Subscript>:Mn<Superscript>2+</Superscript>/Pr<Superscript>3+</Superscript> phosphors

Zn₂GeO₄, Zn₂GeO₄:Mn²⁺, Zn₂GeO₄:Pr³⁺ and Zn₂GeO₄:Mn²⁺/Pr³⁺ phosphors were fabricated by a solid state reaction. The phase and luminescent properties of the fabricated phosphors were investigated. The XRD patterns show that all of the fabricated phosphors have an orthorhombic structure. The fabricated Zn₂GeO₄ shows an emission band in the range of 350–550 nm. The fabricated Zn₂GeO₄:Mn²⁺ and Zn₂GeO₄:Pr³⁺ phosphors show emission bands corresponding to Mn²⁺ and Pr³⁺ ions, respectively. The fabricated Zn₂GeO₄:Mn²⁺/Pr³⁺ phosphor shows the emission band results from Mn²⁺ and the codoped Pr³⁺ enhances the emission intensity of Mn²⁺. Moreover, Zn₂GeO₄:Mn²⁺/Pr³⁺ phosphor exhibits longer decay time than that of Zn₂GeO₄:Mn²⁺. The higher intensity and longer lifetime of Mn²⁺ emission are induced by the energy transfer from Pr³⁺ of various vacancies to Mn²⁺ in Zn₂GeO₄:Mn²⁺/Pr³⁺ phosphors.     

Fabrication of silica nanocoatings on ZnS-type phosphors via a sol–gel route using cetyltrimethylammonium chloride dispersant

In order to apply ZnS-type phosphors in field emission displays (FEDs), their poor ageing performance, resulting from their surface oxidation at high current densities, should be improved. In this study, the green emitting ZnS:Ag,Cl phosphors are covered with uniform and continuous SiO₂ coatings via a sol–gel route, which is expected to inhibit their surface oxidation. During the gelation process, cetyltrimethylammonium chloride (CTAC), a cationic surfactant, is added to increase the dispersibility of phosphors in suspension. Furthermore, the addition of CTAC promotes uniform distribution of charges on the whole phosphor surfaces, thus benefit the formation of continuous and uniform coatings.     

Effect of zinc metal addition on microstructural evolution of Zn0.22Cd0.78S$ phosphors studied by transmission electron microscopy

The effect of zinc powder addition on the microstructural evolution of fine Zn_0.22Cd_0.78S:Ag$, Cl phosphor particles was investigated in relation to the luminescent property of phosphors. The phosphor microstructures were analyzed by high-resolution transmission electron microscopy (HRTEM) using the ultramicrotome technique for specimen preparation. The phosphor baking process at 480 °C for 30 min in air results in significant damage to the surface structure of the phosphors due to the decomposition and oxidation of the phosphor itself. The addition of zinc metal powders into the Zn_0.22Cd_0.78S$ phosphors suppresses the oxidization and decomposition of phosphors by oxidizing zinc metal during the baking process in air. The oxidation of the zinc powders during baking provides the baking furnace with less oxidizing atmosphere for phosphor particles. The structure of phosphors, especially at the surface region, is much less degraded compared to the phosphors baked with no zinc metal addition. © 2000 Kluwer Academic Publishers     

Studies on optical absorption and photoluminescence of thioglycerol-stabilized ZnS nanoparticles

ZnS quantum dots of size 3 nm are prepared at 303 K using ZnSO₄ and Na₂S₂O₃ precursors with thioglycerol as stabilizing agent. Cd²⁺ doped ZnS were prepared by varying doping concentration from 1 to 8 wt.%. ZnS quantum dots were mixed with CdS quantum dots of size 4 nm in the 3:1, 2:1, 1:1, 1:2, 1:3 and 1:4 M ratio. The nanoparticles were characterized by UV–vis, photoluminescence (PL), XRD and high-resolution TEM measurements. The XRD pattern, high-resolution TEM image and SAED pattern reveal that the nanoparticles are in well-crystallized cubic phase. The band gap of ZnS has increased from the bulk value 3.7 to 4.11 eV showing quantum size effect. Excitonic transition is observed at 274 nm in UV absorption and PL emission at 411 nm. Doping with Cd²⁺ red-shifts both UV and PL spectral bands and enhances the PL band of ZnS nanoparticles. Mixing CdS and ZnS quantum dots in different molar ratios shows red-shift of the band edge in the CdS/ZnS hybrid system. In the 1:1 hybrid system of CdS/ZnS nanoparticles, PL band is red-shifted and the intensity is almost doubled with respect to that of CdS nanoparticles.     

A thermoluminescence study on the state of Cl in ZnS:Ag by electron beam

The degradation behavior of ZnS:Ag, Cl as a phosphor for CL by EB irradiation at 7 kV was examined by TL measurement. After EB irradiation, TL intensity decreased and the TL peak shifted to the lower temperature side. By comparing TL thermograms of mechanically damaged ZnS:Ag, Cl, ZnS:Cl with varying Cl concentration as well as ZnS:Ag, Al after EB irradiation, we conclude that the decrease in the effective concentration of Cl⁻, serving as active luminescence center, is responsible for the CL degradation of ZnS:Ag, Cl by EB irradiation.     

Effect of deposition time on structural, optical and photoluminescence properties of Cd0.9Zn0.1S thin films by chemical bath deposition method

The present work investigates the effect of deposition times on the structural, optical and photoluminescence properties of Cd_0.9Zn_0.1S thin films deposited on glass substrate by chemical bath deposition method. The deposition time was varied from 30 to 90 min. The deposited films were uniform and adherent to the glass substrates and amorphous in nature. Structural, optical and photoluminescence properties of Cd_0.9Zn_0.1S thin films were studied through X-ray diffraction, energy dispersive X-ray, scanning electron microscopy, UV–Vis absorption, fourier transform infra red spectroscopy and photoluminescence spectroscopy. The average crystal size was increased from ~1.3 to 2.5 nm with increase in deposition times. The absorption of the films was increased and the absorption peak shifted to lower wavelength side when deposition time increases. The increased energy gap from 2.4 to 2.49 eV with deposition time was due to quantum size effect and better crystallization. The presence of functional groups and chemical bonding were confirmed by FTIR. PL spectra showed two well distinct and strong bands; blue band around 407–415 nm and green band around 537–541 nm due to size effect.     

Band alignment of Cd(1 − x)ZnxS produced by spray pyrolysis method

Cd_(1 − x)Zn_xS thin films have been grown on glass substrates by the spray pyrolysis method using CdCl₂ (0.05 M), ZnCl₂ (0.05 M) and H₂NCSNH₂ (0.05 M) solutions and a substrate temperature of 260 °C. The energy band gap, which depends on the mole fraction × in the spray solution used for preparing the Cd_(1 − x)Zn_xS thin films, was determined. The energy band gaps of CdS and ZnS were determined from absorbance measurements in the visible range as 2.445 eV and 3.75 eV, respectively, using Tauc theory. On the other hand, the values calculated using Elliott-Toyozawa theory were 2.486 eV and 3.87 eV, respectively. The exciton binding energies of Cd_0.8Zn_0.2S and ZnS determined using Elliott-Toyozawa theory were 38 meV and 40 meV, respectively. X-ray diffraction results showed that the Cd_(1 − x)Zn_xS thin films formed were polycrystalline with hexagonal grain structure. Atomic force microscopy studies showed that the surface roughness of the Cd_(1 − x)Zn_xS thin films was about 50 nm. Grain sizes of the Cd_(1 − x)Zn_xS thin films varied between 100 and 760 nm.     

Study of electrical and optical properties of Cd1 − x Zn x S thin films

Thin films of Cd_0.9Zn_0.1S and CdS were prepared by thermal evaporation under vacuum of 10^–6 Torr and with deposition rate of 60 nm/min. X ray diffraction studies confirm the hexagonal structure of both CdS and Cd_0.9Zn_0.1S films. The effect of heat treatments with or without CdCl₂ enhances the grain size growth and improves the crystalline of the films. Moreover, the activation energy is decreased by heat treatment with or without CdCl₂ for all thin films. The optical absorption coefficient of Cd_0.9Zn_0.1S thin films were determined from measured transmittance and reflectance in the wavelength range of 300 to 2500 nm. The optical absorption spectra reveal the existence of direct energy gap for these films. It was found that the optical energy gap decreases upon annealing or CdCl₂ treatments.     

Photoluminescence spectra tuning of Eu2+ activated orthosilicate phosphors used for white light emitting diodes

Divalent europium activated alkaline earth orthosilicate M₂SiO₄ (M = Ba, Sr, Ca) phosphors were synthesized through solid-state reaction technique and their luminescent properties were investigated. Photoluminescence emission spectra of Sr₂SiO₄:Eu²⁺ phosphor was tuned by substitution of Sr²⁺ with 10 mol% Ca²⁺ or Mg²⁺. Two emission bands originated from the 4f–5d transition of Eu²⁺ ion doped into different cation sites in the M₂SiO₄ host lattice were observed under ultraviolet excitation. The Sr₂SiO₄:Eu²⁺ phosphor showed a blue and a green broad emission bands peaked around 475 and 555 nm with some variation for different Eu²⁺ doping concentration. When 10 mol% of Sr²⁺ was substituted by Ca²⁺ or Mg²⁺, the blue emission band blue-shifted to 460 nm and the green emission band shifted to even longer wavelength. An energy loss due to energy transfer from one Eu²⁺ to another Eu²⁺ ion, changing of the crystal field strength and covalence in the host lattice together were assigned for the tuning effect. With an overview of the excitation spectra and the emission spectra in blue and green-yellow color, these co-doped phosphors can become a promising phosphor candidate for white light-emitting-diodes (LEDs) pumped by ultraviolet chip.     

Photo and electroluminescence from terbium doped zinc sulfide films deposited by spray pyrolysis

Undoped, silver doped and silver-terbium doped zinc sulfide photoluminescent thin films have been prepared by the spray pyrolysis technique. The crystalline structure of the deposited films is hexagonal wurzite, in all cases. The films have a preferential growth with the (0 0 2) direction perpendicular to the surface of the substrate. In general, the relative chemical compositions show that the films are formed by a zinc rich ZnS material. The photoluminescent excitation measurements are in agreement with the optical transmission results indicating that the excitation mechanism is related to electron–hole pair generation across the ZnS band gap. The photoluminescent spectra show the typical bands associated with the transition from the ⁵D₄ level to the ⁷F₆, ⁷F₅, ⁷F₄ and ⁷F₃ levels of the Tb³⁺ ions. The emission spectra for the undoped and silver doped films show the well known self-activated emission, centered at 460 and 490 nm, respectively. The dependence of the emission intensity on the temperature shows an energy activation of 0.1 eV for competitive non-radiative transitions. Electroluminescent devices using silver-terbium doped zinc sulfide films as active layer show emission spectra similar to those observed in photoluminescence measurements but with a reduced intensity.     

Photoluminescence studies on energy transfer processes in cerium-doped Gd3Al2Ga3O12 crystals

We have measured photoluminescence properties of cerium-doped Gd₃Al₂Ga₃O₁₂ (Ce:GAGG) crystals at low temperatures with use of synchrotron radiation. Excitation spectra for the Ce³⁺ 5d–4f emission exhibit prominent peaks at Gd³⁺ intra-4f absorption bands. The Gd³⁺ intra-4f emission band is observed at 3.91 eV, but is not in resonance with the lowest energy Gd³⁺ intra-4f absorption band at 3.95 eV. The temperature dependence of the Gd³⁺ emission intensity is not correlated with that of the Ce³⁺ emission intensity. Decay curves of the Ce³⁺ emission were also measured at 9 K under excitation at various photon energies. The decay curve is remarkably changed, depending on the excitation photon energies. The present results give us hints to understand the whole of energy transfer processes in Ce:GAGG crystals.     

SrHfO3-based phosphors and scintillators

The heavily Ce-doped SrHfO₃ (SHO) and the undoped non-stoichiometric SHO sample sets were prepared by solid state reaction in powder form and their luminescence spectra and decay kinetics were measured. Concentration quenching effects and thermally induced ionization of the Ce³⁺ excited states were followed and discussed in the former sample set. In the latter one new emission band at about 334 nm was found in Sr-deficient samples and temperature dependences of its emission intensity and decay times were studied in the detail. Room temperature decay time of about 180 ns and efficient and fast energy transfer from the host to the center of the 334 nm emission make this material promising candidate for X-ray phosphors.     

Synthesis and characterization of ternary chalcogenide ZnCdS 1D nanostructures

1D nanostructures of ternary chalcogenide, ZnCdS-Cd nanocables and ZnCdS nanotubes, have been synthesized through a thermochemical process. The as-prepared nanocables consist of a single-crystal Cd core with a mean diameter of 90 nm and a single-crystal ZnCdS sheath with a thickness of 8 nm. The Cd core and the ZnCdS sheath are coaxially arranged. The as-prepared nanotube has the same structure as the sheath of ZnCdS-Cd nanocables with a hexagonal Zn_0.78Cd_0.22S phase. The formation of ZnCdS nanotubes results from the evaporation of the Cd core. The cathodoluminescence spectrum of ZnCdS-Cd nanocables displays an emission peak at 490 nm, and the emission peak is just between those of ZnS and CdS.     

Electrophosphor brightness enhancement via plasma modification of raw materials

A number of ZnS:Cu,Cl phosphors have been prepared from starting mixtures processed in different plasmas, and their properties have been studied in relation to plasma processing conditions. Plasma processing of the starting mixture is shown to increase the photo- and electroluminescence brightness of the phosphors owing to the increase in activator content and the rise in the intensity of the green emission band relative to that of the blue band.     

X-ray studies and luminescence of ZnS-phosphor doped with silver and cobalt

Measurements were carried out on a series of (ZnS:Ag:Co) with different concentrations of cobalt. The emission spectra were studied at liquid nitrogen and room temperatures. Introducing cobalt dopant into the lattice decreased the efficiency of emission for the first band but produced no shift in the maximum intensity. This phenomenon has been noted by other workers. The approximate average internuclear spacing between activator atoms in hexagonal (ZnS:Ag:Co) was calculated. It is apparent that the average distance between the cobalt atoms for lower concentrations is too large compared with the average distance between the silver atoms. A Siemens D-500 X-ray diffractometer was used to clarify the characteristic features of the phases and the crystallite size estimation.     

Cd1−xZnxS thin films evaporated by electron bombardment II: Optical properties of the films

The variation in the band gap E_g of Cd_1?xZn_xS thin films evaporated from a mixture of CdS and ZnS powders was determined as a function of the film compasition from the visible absorption spectra. The crystallite size was also deduced from these spectra.     

Molten salt synthesis and photoluminescence properties of novel red emitting phosphors Ba5(VO4)3Cl:Eu3+,K+

A series of Ba₅(VO₄)₃Cl:Eu³⁺,K⁺ phosphors have been synthesized by the molten salt synthesis method. The crystalline structure, morphology, photoluminescence properties and lifetimes were characterized using X-ray diffraction (XRD), field emission scanning electron microscope (FE-SEM) and photoluminescence spectroscopy, respectively. XRD indicates that the Ba₅(VO₄)₃Cl:Eu³⁺,K⁺ phosphors are synthesized successfully via molten salt method. SEM image demonstrates that the obtained phosphors have hexagonal polyhedron morphology. The photoluminescence spectra reveal that the as-prepared phosphors exhibit a bright red emission under the excitation of blue or near ultraviolet light. The concentration quenching was also investigated, and the dipole–dipole interaction is responsible for the concentration quenching of fluorescence emission of Eu³⁺ ions in Ba₅(VO₄)₃Cl phosphor. The present work suggests that the Ba₅(VO₄)₃Cl:Eu³⁺,K⁺ phosphors would be a potential candidate for light emitting devices.