ZnS Phosphors Research Articles

Photoluminescence and microstructure of Mn-doped ZnS phosphor powder co-fired with ZnS nanocrystallites

In this study, we correlated the photoluminescence (PL) with the microstructure of ZnS:Mn phosphor powders prepared by firing ZnS with MnO (1mol%), NaCl (1mol%) and ZnS nanocrystallites (NCs) in the range of 0–100wt% at 600–1000°C for 2h in the atmosphere of 3%H2/Ar. ZnS NCs of 10–30nm in size were produced by co-precipitation of zinc nitrate and sodium sulfide solutions at room temperature. Thermal analysis (DTA/TG) and X-ray diffraction (XRD) results indicated that the cubic-hexagonal transformation temperature of ZnS NCs was lowered to approximately 600°C, which was much lower than that of bulk ZnS (1020°C). PL measurements revealed that ZnS:Mn fired with 1wt% ZnS NCs showed the optimal luminescence intensity when compared to those without or with higher ZnS NCs (>1wt%). An appropriate amount of ZnS NCs (1wt%) acting as the flux in the firing process was inferred to avoid the inhomogeneous distribution of Mn2+ as well as the migration of excitation energy to quenching sites and therefore to result in the enhanced PL intensity.

Computational analysis of solid-vapor equilibria for ZnS and SrS phosphor synthesis conditions

The synthesis and postgrowth treatment of luminescent materials frequently involve solid-vapor interactions. Because the underlying multispecies, multireaction equilibria are rather complex, quantitative correlations between synthesis conditions, vacancy concentrations, and emissive properties for predictive purposes are not readily available. In order to support the development of predictive guidelines, a quantitative, computational analysis of the vapor phase over ZnS and SrS in the absence and presence of nonconstituent species has been performed. The results confirm the complexity of these systems and show quantitatively the effects of impurity gases on the critical metal to nonmetal ratio in the ambient atmosphere. The data also reveal the temperature ranges in which desirable M∕X ratios are available for given experimental conditions. The combined results of such computations are useful as reference data for the optimization and definition of reproducible synthesis conditions, and for the estimation of the type and magnitude of vacancies.

Fast photoluminescence decay processes of doped ZnS phosphors at low temperature

Singly and doubly doped ZnS phosphors have been synthesized in the laboratory. The crystal structure has been determined by X-ray diffraction (XRD) studies. Laser-induced photoluminescence has been observed in ZnS-doped phosphors when these were excited by the pulsed UV N 2 laser radiation at liquid nitrogen temperature (77 K). The temperature dependence of lifetimes, trap-depths and decay constant values were investigated for quencher impurities doped ZnS:Mn phosphors. The lifetimes of the orange emission from 4T 1– 6A 1 transition of Mn 2+ ions has been found to decrease at liquid nitrogen temperature. Due to downconversion phenomenon fast phosphorescence /fluorescence emission in the visible region is recorded in milliseconds time domain for ZnS:Mn while in case of ZnS:Mn, X (X=Fe, Co and Ni) phosphors lifetime reduces to nanoseconds time domain. A thermally activated carrier-transfer model has been proposed to explain the observed abnormal temperature behaviour of the lifetimes in ZnS:Mn, X (X=Fe, Co and Ni) phosphors. The high efficiency and fast recombination times observed in doped ZnS phosphors make these materials very attractive for optoelectronic applications.

Electrical properties of Al 2O 3 films for TFEL-devices made with sol–gel technology

Thin films of Al 2O 3 have been deposited on ITO-coated glass substrates by a sol–gel dipcoat process. Aluminium isopropoxide (Al(OC 3H 7) 3) was used as the Al source material. X-ray diffraction measurements show that these films are amorphous. Scanning electron microscopy and atomic force microscopy images of the films have revealed a relatively flat surface with no cracks. The dielectric properties of these aluminium oxide thin films have been investigated in the frequency range of 15 Hz to 1 MHz. We have also compared the electrical behaviour of conventional double insulator thin-film electroluminescent devices where the bottom insulator was partly or fully replaced by a sol–gel layer. These devices behave largely normal but show signs of losses in the sol–gel layer, which are likely linked to its porosity. The interface between the sol–gel bottom insulator and the ZnS phosphor layer shows no severe premature charge transfer as is usual for a similar atomic layer deposition insulator.

P-94: Plasma Modification of ZnS:Cu EL Phosphor

We report nitrogen plasma modification of the ZnS: Cu, Br electroluminescence phosphor. Modification resulted in 30% improvement of ZnS: Cu, Al EL brightness at 220V drive. In the same time emission peak shifted of from 486 to 504nm due to increase of relative intensity of green band in EL spectrum. Thus plasma treatment is a perspective method for the control of EL brightness and spectra of ZnS phosphors.

Manufacturing process of self-luminous glass tube utilizing tritium gas: Optimization of phosphor coating conditions

Domestic research utilizing tritium will become more attractive when tritium is produced from the Wolsong Tritium Removal Facility (WTRF) in Korea, which will start to operate in late 2005. As starting domestic tritium technology research, this study is focused on the mass production of commercially available self-luminous glass tubes (SLGTs) and the design of a new product by simulation. With a low power microscope, SEM-EDX, ICP (Inductively Coupled Plasma) spectrometer, some commercially available SLGTs have been investigated. The inner side of the glass tubes was coated with greenish ZnS phosphor particles with sizes varying from 4–5 Μm, and Cu and Al as an activator and a co-dopant, respectively. Besides, the coating thickness is different for each product. and the thickness range of the products to be considered is 10–100 μm. With the phosphor, a binder package was also selected to meet optimal coating conditions. Cathodoluminescence (CL) device (energy: 0–10 keV, electron flux: nA) was used to simulate β-ray emitted from tritium. From the CL measurement the optimal conditions were 580–600 °C and 30 minutes. At these conditions the degradation of the phosphor by a heat is minimized. We determined all the coating conditions including the phosphor, binder package, coating thickness, and calcinating temperature for the production of SLGTs. Now we are testing our pilot-scale coating device for a mass production with selected experimental conditions

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.

Crystal growth of electroluminescent ZnS:Cu,Cl phosphor and its TiO 2 coating by sol–gel method for thick-film EL device

Bigger-sized spherical particle of ZnS:Cu,Cl to be employed for electroluminescent (EL) device has been synthesized using a new eutectic mixture as flux. The best composition of the flux was the mixture of BaCl 2·2H 2O, MgCl 2·6H 2O and NaCl in the mole percentage ratio of 13.8:39.9:46.2 and when the total amount of the mixture was 6% of the total weight of ZnS. The phosphor was synthesized firing at higher temperatures in two steps. First step firing at 1150 °C gave the hexagonal phase of the ZnS phosphor particles. Low intensity ball-milling of the phosphor pastes in solvents converted hexagonal phase partially to the cubic phase of the phosphor, which is an essential step. Mixing with copper sulfate or copper(I) halides and magnesium chloride and then firing (second step) at 750 °C gave a phosphor with better luminescent characteristics and converting to almost 100% cubic phase. The phosphor particles having size >0.020 mm size were sieved and coated with TiO 2 made by sol–gel process using titanium(IV)-isopropoxide as a precursor. The phosphor particles were coated twice with the TiO 2 sol and finally calcined at 400 °C in nitrogen atmosphere. The EL devices were fabricated with the synthesized phosphor using a screen-printing method.

Mechanisms of flux action for growth of ZnS phosphor particles

Mechanisms of flux action for the growth of ZnS phosphor particles have been investigated by using a sealed quartz capsule, instead of an unsealed crucible used in practical production process. No growth of ZnS particles is found in the melted sodium chloride phase. This result is in remarkable contrast to the classical model by Leverentz, in which melted NaCl acts as flux for the ZnS particle growth. It is revealed that the fluxing agent is the vaporized ZnCl2 gas, which is formed as a result of sequential chemical reactions. A growth mechanism of ZnS particles by the chlorine cycle via ZnCl2 and Cl2 gases is proposed, with emphasis on the importance of growing spaces (voids) in the blend-mixture charged in a crucible. On the basis of the present results, it is stressed that further improvement is possible for the production process of ZnS phosphor powder.

High Brightness ZnS and GaN Electroluminescent Devices Using PZT Thick Dielectric Layers

An improved thick dielectric (TD) layer for inorganic electroluminescent (EL) display devices has been achieved through a composite high-/spl kappa/ dielectric sol-gel/powder route. This composite TD film results in a luminance improvement (up to 10/spl times/) in these TDEL devices with Eu-doped GaN and Mn-doped ZnS phosphor layers. The use of a composite TD film, composed primarily of lead-zirconate-titanate (PZT), results in a significantly higher charge (>3 /spl mu/C/cm/sup 2/) coupling to the phosphor layer. Furthermore, the reduction in porosity of the TD has improved the homogeneity of electric field applied to the phosphor layer, resulting in a steeper luminance-voltage slope. The reduction in porosity has also decreased the diffuse reflection of the TD, which when pigmented, exhibits a diffuse reflectivity of <2% resulting in high display contrast. High luminance levels of up to 3500 cd/m/sup 2/ have been achieved from the ZnS:Mn TDEL devices and 450 cd/m/sup 2/ from GaN:Eu devices. A detailed analysis of the electrical steady-state time-varying characteristics has shown that the electrical performance of TDELs is very similar to TFELs in spite of the physical asymmetry in the device structure. These results demonstrate that three critical requirements for practicality of the TDEL approach (formation on standard display glass, low reflectivity, and electric field homogeneity) can be obtained by careful selection and design of the device materials, fabrication process and device structure.

Doped ZnS Phosphors with a Constant Spectral Density in the 500–750 nm Range

A ZnS: (In, Cu, Cl) electrophosphor that emits with an almost constant spectral density at wavelengths λ in the range 550 < λ < 750 nm and a ZnS: In photophosphor exhibiting the same property in the range 500 < λ < 700 nm are prepared.

Tritium Application: Self-Luminous Glass Tube (SLGT)

To manufacture SLGTs (Self-Luminous Glass Tubes), 4 core technologies are needed: coating technology, tritium injection technology, laser sealing/cutting technology and tritium handling technology. The inside of the glass tubes is coated with greenish ZnS phosphor particles with sizes varying from 4~5 [µm], and Cu, and Al as an activator and a co-dopant, respectively. We also found that it would be possible to produce a phosphor coated glass tube for the SLGT using the well established cold cathode fluorescent lamp (CCFL) bulb manufacturing technology. The conceptual design of the main process loop (PL) is almost done. A delicate technique will be needed for the sealing/cutting of the glass tubes. Instead of the existing torch technology, a new technology using a pulse-type laser is under investigation. The design basis of the tritium handling facilities is to minimize the operator's exposure to tritium uptake and the emission of tritium to the environment. To fulfill the requirements, major tritium handling components are located in the secondary containment such as the glove boxes (GBs) and/or the fume hoods. The tritium recovery system (TRS) is connected to a GB and PL to minimize the release of tritium as well as to remove the moisture and oxygen in the GB.

The cathodoluminescence generated in ZnS phosphor powder with non‐luminescent ZnS coatings: a comparison of experimental data with Monte Carlo calculations

AbstractField emission displays (FEDs) are a possible alternative to other display technologies. They work similarly to CRTs, but instead of an electron gun they have an array of tiny metallic tips acting as electron emitters. This array is situated in close proximity to the phosphor screen and produces light by a process of cathodoluminescence (CL). During prolonged electron beam irradiation a non‐luminescent ZnO layer forms on the irradiated surface of the ZnS phosphor powder according to the electron‐stimulated surface chemical reactions (ESSCR) mechanism. The low‐energy electrons used in FEDs have a shallow penetration depth and since the CL is dependent upon the energy loss in the ZnS bulk itself, growth of the ZnO layer significantly degrades the CL intensity. Monte Carlo techniques may be used to simulate the electron trajectories and energy loss in many different types of materials. Energy loss profiles in the ZnS for different non‐luminescent ZnS coatings of known thickness were determined using the Monte Carlo electron trajectory simulations. Since only energy loss in the ZnS powder results in CL an increase in the thickness of the ZnS coating decreases the CL intensity accordingly. Simulations were performed for electron energies varying from 2 to 10 keV and at incident angles accommodating for the morphology of the phosphor powder. These results compared very well with the experimental results measured by other authors. Copyright © 2004 John Wiley &amp; Sons, Ltd.

A review on ZnS phosphor degradation

Standard ZnS cathodoluminescent phosphors normally lose brightness upon bombardment with electron beams. The main reason for the degradation is the formation of a non-luminescent “dead layer” on the surface due to the electron stimulated surface chemical reaction (ESSCR) mechanism. The decrease in luminance was found to be a result of the growth of the “dead layer”. Calculations showed that the thickness of the oxide layer that formed during electron bombardment, cannot completely explain the magnitude of the decline of the CL intensity. Iso-electronic point defects due to the presence of oxygen that diffuse into the ZnS matrix at the interface further contribute to the degradation. When the ZnS phosphor powder was exposed to the electron beam in a water-rich O2 ambient, a chemically-limited ZnO layer was formed on the surface. A layer of ZnSO4 was formed on the surface during the electron beam degradation of the ZnS phosphor powder in a dry O2 ambient. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Synthesis and photoluminescent properties of ZnS nanocrystals doped with copper and halogen

A novel wet-chemical precipitation method is optimized for the synthesis of ZnS nanocrystals doped with Cu + and halogen. The nanoparticles were stabilized by capping with polyvinyl pyrrolidone (PVP). XRD studies show the phase singularity of ZnS particles having zinc-blende (cubic) structure. TEM as well as XRD line broadening indicate that the average crystallite size of undoped samples is ∼2 nm. The effects of change in stoichiometry and doping with Cu + and halogen on the photoluminescence properties of ZnS nanophosphors have been investigated. Sulfur vacancy (V S) related emission with peak maximum at 434 nm has been dominant in undoped ZnS nanoparticles. Unlike in the case of microcrystalline ZnS phosphor, incorporation of halogens in nanoparticles did not result V Zn related self-activated emission. However, emission characteristics of nanophosphors have been changed with Cu + activation due to energy transfer from vacancy centers to dopant centers. The use of halogen as co-activator helps to increase the solubility of Cu + ions in ZnS lattice and also enhances the donor–acceptor type emission efficiency. With increase in Cu + doping, Cu-Blue centers (Cu Zn −Cu i +), which were dominant at low Cu + concentrations, has been transformed into Cu-Green (Cu Zn −) centers and the later is found to be situated near the surface regions of nanoparticles. From these studies we have shown that, by controlling the defect chemistry and suitable doping, photoluminescence emission tunability over a wide wavelength range, i.e., from 434 to 514 nm, can be achieved in ZnS nanophosphors.

Surface properties of ZnS and AC powder electroluminescent phosphors

Abstract— Phosphor surface properties play an important role in luminescent and processing performance. We studied the distribution of surface active absorption centers (DAC) of ZnS powders and ZnS:Cu electroluminescent phosphors with changes in synthesis and processing conditions. Photometric measurements of the colorimetric indicator adsorption from water solutions demonstrated that in the process of ZnS milling, increasing numbers of active centers are accompanied by the transformation of strong‐acidic centers into weaker ones while moderately basic centers are changed into weaker bases. For the case of phosphor powders, the total number of active centers is inversely related to the intensity of photoluminescence and electroluminescence. This may be explained by considering these centers to be sites of non‐emissive recombinations. Samples with different copper content that distinguished surface centers were studied. It was further found that electron‐beam irradiation causes the formation of strong‐acidic and strong‐basic centers on phosphor surfaces. The DAC acid‐base indicator technique is a useful tool to investigate interfacial processes and surfactant efficiency during phosphor‐based device fabrication.

Present Status of White LED Lighting Technologies in Japan

“The light for the 21st century” Japanese national (Akari) project, which is based on the high-efficient white light-emitting diode (LED) lighting technologies using near ultraviolet (UV) LED and phosphor system, has been started at 1998. The near UV white LED system linked with semiconductor technologies on GaN LED and ZnS phosphors for general lighting applications has for the first time been proposed in the project. The outline and purpose of this project are briefly introduced. In particular, we have demonstrated high-efficient nUV LED having external quantum efficiencies more than 43 % around an emission wavelength of 400 nm. Basic illumination properties of the high luminous efficacy (>40 lm/W) and the high general color rendering index (Ra>90) white LED sources are described. The near UV white LED technologies in conjunction with phosphor blends can offer superior color uniformity, high Ra and the excellent light quality for many lighting applications.

Modelling the effect of a thin ZnO layer on the cathodoluminescence generated in ZnS phosphor powders

A possible alternative to current active matrix liquid crystal displays (AMLCD) and conventional cathode ray tube (CRT) displays are field emission displays (FEDs). It works on a similar principle as CRTs, producing light by cathodoluminescence (CL), but has millions of tiny metallic tips situated near the phosphor screen that act as electron emitters. During prolonged electron beam irradiation, a non-luminescent ZnO layer forms on the surface of the ZnS phosphor. The low energy electrons used in FEDs have a short penetration depth and since the CL is dependent upon the energy loss in the phosphor, growth of the ZnO layer significantly degrades the CL intensity. An expression was derived to quantify the CL generated during electron irradiation using electron energy loss profiles simulated by the Monte Carlo technique. These profiles were used to construct a curve relating the CL intensity to the ZnO thickness. For the ZnS:Cu,Al,Au phosphor the calculated oxide thickness compare extremely well with experimental measurements. However, using the same simulation parameters, the oxide thickness measured on the ZnS:Ag,Cl phosphor is much thinner than the calculated value. This difference is attributed to charge trapping over the range of the primary electrons during electron irradiation of the phosphor which influences the electron-hole (e-h) recombination and rate of oxide growth.

Quantifying the cathodoluminescence generated in ZnS-based phosphor powders

Field emission displays (FEDs) are a possible alternative to other display technologies. They work similar to cathode ray tubes, but instead of an electron gun they have an array of tiny metallic tips acting as electron emitters. This array is situated in close proximity to the phosphor screen and produces light by a process of cathodoluminescence (CL). During prolonged electron beam irradiation a non-luminescent ZnO layer forms on the irradiated surface of the ZnS phosphor powder. This is due to electron-beam-stimulated surface chemical reactions between the ZnS phosphor and water vapour present in the ultrahigh vacuum environment. The low-energy electrons used in FEDs have a shallow penetration depth and, because the CL is dependent upon the energy loss in the ZnS bulk itself, growth of the ZnO layer significantly degrades the CL intensity. Energy loss profiles in the ZnS for different ZnO thicknesses were determined using Monte-Carlo electron trajectory simulations. These profiles, along with photon absorption profiles, were used to determine a curve relating the normalized CL intensity to the ZnO thickness. It was compared with experimentally measured values obtained during CL degradation experiments on standard ZnS : Cu,Al,Au phosphor powders and ZnO thickness measurements with sputter depth profiling. Assuming the presence of a 20 nm thick diffusion interface between the ZnO layer and the ZnS bulk, the theoretical and experimental results agree well. Initially the interface represents the initial stages of oxidation, but after the oxide layer starts to grow as a uniform film it represents a physical interface formed due to diffusion processes. Copyright © 2002 John Wiley & Sons, Ltd.

Detecting the dark electric matter objects (daemons)

A month-long observation of two horizontal mutually light-isolated scintillating screens, 1 m 2 in area and located one above the other a certain distance apart, revealed about 15 correlated signals, whose time shift corresponds to an average velocity of only � 10–15 km s � 1 . We assign the origin of these signals to the negative daemons, i.e. electrically charged Planckian particles which supposedly form the DM in the Galactic disc, captured into the nearEarth orbits. As follows from an analysis of the factors accompanying the signals and governing their properties, the key part in the detection of daemons is played apparently by two processes: (i) nucleon decay in the daemoncontaining nucleus, and (ii) emission of energetic electrons and nucleons in the capture of a nucleus of atom by a daemon. The first process results in the release by daemons of the heavy nuclei captured by them in traversing the components of the system (S, Fe, Zn, Sn etc.), and the second, in excitation of the main part of the scintillations observed in the ZnS(Ag) phosphor. The flux of slow daemons through the Earth’s surface estimated from the measurements is � 10 � 9 cm � 2 s � 1 .