Polarization of photo-luminescence of ZnS: Cu monocrystals

Different models of luminescence centres are discussed on the basis of measurements of the composition of ZnS monocrystal photo-luminescence in different polarizations and temperature dependence of the degree of polarization. Those of the models submitted by Birman, which assume the polarization to be due to the different force of the oscillators for transitions withEc andEc, or models assuming luminescence polarization to be due to the orientation of the luminescence centres, agree with the results of experiments, i.e. the temperature independence of the degree of polarization and the conformable spectral composition of both polarizations. It is also shown that measurements made up to now of the degree of polarization must be taken as orientational as a consequence of the depolarizing influence of the diffused rays of luminescence on its value.

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ZnS: Cu

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Anisotropic surface effects in Au and Cu monocrystals

Electroreflectance spectra at normal incidence of (100) and (110) faces of gold and copper monocrystals are given, in the spectral range from 0.22–0.7 μm. The fractional change in reflectance is different with (110) faces when light is polarized parallel to the [001] direction and parallel to the [$\text{1}\text{10}$] direction while no anisotropy is seen on (100) faces. This shows that electroreflectance is a powerful tool to investigate metal surfaces where the optical electrons are sensitive to the distribution of the surface atoms.     

Photoluminescence of Cu:ZnS, Ag:ZnS, and Au:ZnS nanoparticles applied in Bio-LED

In this work, transition elements, including Cu²⁺, Ag⁺, and Au³⁺, were used to dope in zinc sulfide (ZnS) by chemical solution synthesis to prepare Cu:ZnS, Ag:ZnS, and Au:ZnS nanoparticles, respectively. Transition elements doping ZnS nanoparticles form the electronic energy level between the conduction band and valance band, which will result in the green light emission. There is a zinc sulfide emission shift from blue (~3.01 eV) to green light (~2.15 eV). We also found that Au:ZnS nanoparticles will emit a green light (~2.3 eV) and a blue light (~2.92 eV) at the same time because the mechanism of blue light emission was not broken after Au element had been doped. Furthermore, we used sodium chlorophyllin copper salt to simulate chlorophyll in biological light emission devices (Bio-LED). We combined copper chlorophyll with Cu:ZnS, Ag:ZnS, and Au:ZnS nanoparticles by a self-assembly method. Then, we measured its photoluminescence spectroscopy and X-ray photoelectron spectroscopy to study its emission spectrum and bonding mode. We found that Au:ZnS nanoparticles are able to emit green and blue light to excite the red light emission of copper chlorophyll, which is a potential application of Bio-LED.     

Luminescence of nanocrystalline ZnS:Cu

Temperature dependent luminescence and luminescence lifetime measurements are reported for nanocrystalline ZnS:Cu²⁺ particles. Based on the variation of the emission wavelength as a function of particle size (between 3.1 and 7.4 nm) and the low quenching temperature (T_q=135 K), the green emission band is assigned to recombination of an electron in a shallow trap and Cu²⁺. The reduction in lifetime of the green emission (from 20 μs at 4 K to 0.5 μs at 300 K) follows the temperature quenching of the emission. In addition to the green luminescence, a red emission band, previously only reported for bulk ZnS:Cu²⁺, is observed. The red emission is assigned to recombination of a deeply trapped electron and Cu²⁺. The lifetime of the red emission is longer (about 40 μs at 4 K) and the quenching temperature is higher.     

核-壳结构的ZnS:Cu/ZnS纳米粒子的制备及发光性质研究

制备了核-壳结构的ZnS:Cu/ZnS纳米粒子以及普通的没有壳的Cu2 掺杂的ZnS纳米粒子,研究了ZnS无机壳层对ZnS:Cu纳米粒子发光性质的影响.透射电子显微镜、激发光谱和发射光谱的研究表明,后加入的Zn2 离子在已经形成的ZnS核表面生长,形成ZnS壳层;而适当厚度的ZnS壳层可以钝化粒子表面,减少无辐射复合中心的数目,抑制表面态对发光的不利影响,提高ZnS:Cu纳米粒子中Cu2 离子在450 nm左右的发光强度.     

Photodielectric Processes in ZnS : Cu Polycrystalline Layers

The frequency dependences of dielectric parameters of zinc sulfide electroluminescent polycrystalline structures doped with copper are studied in the dark and under light excitation in the visible wavelength range. A positive photodielectric effect most pronounced in the low-frequency range was revealed. The experimental results are explained within framework of formation of a space charge in the bulk of a semiconductor. The analysis of data indicates they can be correlated with luminance characteristics of an electroluminescent layer.     

Quenching of photoluminescence in ZnS:Cu single crystals

Experiments on the quenching of photoluminescence in ZnS:Cu single crystals by secondary radiation are reported. Quenching of emission at photon energies of 1.4, 1.7, 2.4 and 2.75 eV by photons at 0.93, 1.6, 2.0, 2.4 eV is found, with a possible fifth peak at 2.7 eV. The effect of each secondary band on each emission band is found to be equivalent. This is explained in terms of a model in which all quenching transitions effectively fill a common ground state for the green/blue emission.     

Luminescence properties of co-doped ZnS:Ni,Mn and ZnS:Cu,Cd nanoparticles

Transition metal doped ZnS:Ni and ZnS:Cu and co-doped ZnS:Ni, Mn and ZnS:Cu, Cd nanoparticles were synthesized through the chemical precipitation method in an air atmosphere. The XRD analysis of co-doped samples shows the formation of cubic phase. The average size of nanoparticles ranges from 3.6 to 5.5 nm. The formation of co-doped nanoparticles was confirmed by XRD and PL analysis. The PL spectra show that the obtained nanoparticles have good crystal quality. An optimum concentration of transition metals was selected in co-doped ZnS nanoparticles.     

Shallow hole traps in ZnS: Cu,Al phosphors

Localized shallow trapping levels for minority carries, i.e., holes, in ZnS: Cu, Al phosphors are studied by the Dishman method, i.e. by measuring the temperature dependence of the ratio of the quantum efficiency for the green luminescence by the host excitation to that by the direct copper acceptor excitation over the temperature range from 4.2 K to room temperature. The ratio shows increases in three temperature ranges of 20–53 K, 58–100 K and 180–260 K. These increases are attributed to the release of holes to the valence band from three kinds of traps. Analyzing results, the depths of these traps are determined as 35, 68 and 306 mev.     

The relation between lattice-parameter and particle size in ZnS:Cu Phosphor

The effect of particle size on life time in electroluminescent phosphor was investigated using fractionated samples. The life time of large-particle size phosphors was longer than that of small-particle size phosphor. It was found that particle size has a close relation with the lattice-parameter magnitude; the lattice parameter was larger in a small-particle size phosphor when the activator and coactivator content were the same. It was concluded that the lattice parameter was an important brightness life-time factor.     

Zum Lumineszenz-Abklingverhalten von ZnS/Cu

The decay characteristics of ZnS/(10^?4) Cu have been investigated by ion pulse excitation. The excitation density could be varied by using ions of different masses. The decay is exponential. The decay times depend on the ion pulse duration and on the penetration depth of the used ions. The green emission shows two different decay times. The intensity of the slower component of the green emission could be altered by excitation with UV-light before decay measurement. In the case of the slower component of the green emission we suppose an interaction between the excited states of the luminescence centres and energetically deep electron traps.     

Barriers participating in the excitation of electroluminescence of ZnS:Cu

It is shown that heterojunctions of the type Cu_xS-ZnS:Cu cannot determine the brightness and quantum yield of electroluminescence of particles of powdered luminophors. The main contribution comes from the excitation of electroluminescence in surface barriers at places where dislocations reach the surface of crystals. State University for Means of Communication, Moscow. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2. pp. 89–92, February, 1998.     

Thermoluminescence characteristics of inorganically and organically capped ZnS:Cu nanophosphors

ZnS:Cu nanophosphors were prepared by wet chemical methods and characterized by X-ray diffraction (XRD). The typical morphologies of the nanophosphors were investigated by scanning electron microscopy (SEM). The thermoluminescence (TL) properties of inorganically and organically passivated ZnS:Cu nanophosphors were investigated after γ-irradiation using a ⁶⁰Co source at room temperature. The TL glow curve of capped ZnS:Cu showed variation in TL peak and intensity as the capping agent was changed. Amongst the synthesized samples the TL glow curve of SiO₂ capped ZnS:Cu showed the highest TL intensity. It has been found that TL response of SiO₂ capped ZnS:Cu is linear in the range 10-550 Gy. A discussion of the obtained results is also presented.     

Anomalous thermal quenching of luminescence in ZnS:Cu phosphor crystals

We studied the thermal quenching of luminescence (TQL) in the case of photoexcitation (PE, _e=313 and 365 nm) and cathodoexcitation (CE) with a high excitation density (10¹⁸ cm^–3· sec^–1) in ZnS:Cu single crystals with various degrees of copper supersaturation, EL-455 and K-83 copper-activated powders, and ZnSCu epitaxial thin films. Our study of TQL in the blue and green bands showed that two types of quenching anomalies are observed in the specimens used. An anomaly of the first type manifests itself in the fact that, with band-to-band excitation (_e = 313 nm and CE). the green band is quenched earlier than the blue band and small steps are observed on the quenching front. This anomaly is explained with the aid of a dislocation model of the blue center. An anomaly of the second type is observed in specimens not supersaturated with copper and consists of the quenching of luminescence beginning earlier at a high excitation (CE) density than at a low excitation (PE) density and proceeds in the same way in the blue and green bands. The thermal depths of the acceptors level are estimated from TQL for the three fundamental luminescence bands 440, 468, and 525 nm which are equal to 0.38 ±0.05, 0.7–0.8, and 0.9–1 eV, respectively.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 38–43, July, 1979.     

Influence factors and mechanism of emission of ZnS:Cu nanocrystals

Copper-doped Zn S(Zn S:Cu) nanocrystals are synthesized by the sol–gel method.The average size of the Zn S:Cu nanocrystals is 3.1 nm.The x-ray diffraction indicates that increasing the Cu-dopant concentration results in a large shift in the diffraction angle.The effects of the dopant concentration,the reactant ratio,and aging temperature on the optical properties of the Zn S:Cu nanocrystals are also investigated.The fluorescence emission mechanism is analyzed by peak deconvolution using Gaussian functions.We find that the emission of the Zn S:Cu nanocrystal is composed of different luminescence centers at 430,470,490,526,and 560 nm.The origins of these emissions are discussed and demonstrated by controlled experiments.     

Epitaxial growth defects and interfacial structures of Cu deposited on TiO2

Epitaxial growth defects and the interfacial structure between vapor deposited Cu and TiO₂(110) have been studied by combined high-resolution electron microscopy (HREM) and image simulations. The Cu film was found to grow epitaxially with an orientation given by Cu(111)//TiO₂(110) and Cu110//TiO₂ [001]. With this relationship, there exist two equivalent domain orientations which are rotated with respect to each other by 180°. Localized misfit dislocations have not been detected, but {111} stacking faults and microtwins were observed which may occur as a result of 3-D island coalescence. HREM observations and image simulations have been used to study the interface atomic structure. The dominant interfacial structure has a stoichiometric (110) TiO₂ surface with bridging rows of O atoms and occasionally, an interfacial structure having a reduced (110) TiO₂ surface terminated by both Ti and O atoms has been observed locally.     

Photophysical and photoluminescence properties of co-activated ZnS:Cu,Mn phosphors

ZnS:Cu,Mn phosphors were prepared by conventional solid state reaction with the aid of NaCl-MgCl₂ flux at 900 °C. The samples were characterized by X-ray powder diffraction, UV-vis absorbance spectra and photoluminescence spectra. All samples possess cubic structure. Cu has a much stronger effect on the absorption property of ZnS than Mn. Incorporation of Mn into ZnS host only slightly enhances the light absorption, while addition of Cu remarkably increases the ability of absorption due to ground state Cu⁺ absorption. The emission spectra of the ZnS:Cu,Mn phosphors consist of three bands centered at about 452, 520 and 580 nm, respectively. Introduction of Mn significantly quenches the green luminescence of ZnS:Cu. The excitation energy absorbed by Cu is efficiently transferred to Mn activators non-radiatively and the Mn luminescence can be sensitized by Cu behaving as a sensitizer (energy donor).     

Laser phenomena in DCEL of ZnS:Cu:Nd:Cl thin films

The possibility of the occurrence of laser phenomena in the DCEL of thin films of ZnS:Cu:Nd:Cl, designed for display purposes, has been investigated. Evidence of stimulated emission at ～ 1080 nm has been found in both DCEL and cathodoluminescence emission of the films and has been attributed to a tendency to population inversion involving the ${}^4\text{F}{}_{\mathrm{<mtext>2</mtext><mtext>3</mtext>}}$ energy level and a sub-level of the ${}^4\text{I}{}_{\mathrm{<mtext>11</mtext><mtext>2</mtext>}}$ energy level. In addition, the expected directional nature of the stimulated emission has been shown to be present at the higher applied voltages.     

On the mechanism of the double-comet-shaped electroluminescence in ZnS:Cu crystals

Brightness waves of electroluminescent comets in ZnS:Cu single crystals have been investigated to decide whether the recombination occurs spontaneously or delayed by one half-period. Brightness waves during rising and decay, in addition to modifications of brightness waves due to disturbances of the stationary excitation by voltage pulses, exhibit delayed emission and a reservoir of excited centres exists in the stationary case. These results together with the time dependent blue-to-green ratio of the emission may be explained best on the basis of a bipolar field-emission model as proposed by Fischer.     

Optical Behavior of ZnS:Cu Microcrystals Embedded in Porous Silica Gels

Emission and excitation spectra of ZnS:Cu microcrystals trapped in porous silica xerogels are presented. This system is characterized by intense, long-lived green emission at room temperature. It was observed that this emission was greatly reduced after compressing the xerogel samples.