Brighter glow in ZnS nanocrystals with polarized light

No known reports exist on luminescence enhancement under polarized light excitation. In this study, ZnS nanocrystals have been observed to produce brighter luminescence when excited by polarized light. ZnS:Mn bulk and nanocrystals have shown fivefold to tenfold increase in photoluminescence (PL) intensity when excited with linearly polarized light at 305 nm and 340 nm. Luminescence enhancement to a lesser degree was observed with linearly polarized light excitation for ZnS:Cu, Al and ZnS:Ag, Al nanocrystals. The observations suggest emission intensity dependence on the degree of anisotropy, which could be correlated mainly with the symmetry of the luminescence center and also to a lesser extent with nanoparticle shape asymmetry.     

Photoluminescence of ZnS:Cu crystals at various concentrations of copper

Doping of ZnS crystals with background impurities and ZnS:Al crystals by various concentrations of copper from a bismuth melt has been carried out. The photoluminescence spectra of the starting ZnS crystals annealed in the bismuth melt and doped by copper have been investigated. Interpretation of the experimental results according to the model in which the associates (Cu _Zn ^′ Cu _i ^● ) are responsible for the B-Cu band (≈460 nm) and the donor—receptor pairs [Cu _i ^● -(Cu _Zn ^′ Cu _i ^● )] are responsible for the G-Cu band (≈505 nm) is given. It is assumed that doping by copper from a bismuth melt is accompanied by the separation of a Cu_xS-type phase. It has been shown that heat treatment of ZnS crystals in the Bi melt does not lead to the appearance of luminescence centers based on Bi_Zn, Bi_S, and Bi_i. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 6, pp. 794–798, November–December, 2005.     

Thermo- and photostimulated processes of polarization and depolarization in CdI<Subscript>2</Subscript>: Ag

Currents of thermogradient polarization and depolarization of an electret state that arises in a photochromic crystal CdI₂: Ag during one-side cooling of the sample in the dark from 325 to 90 K in the presence of a temperature gradient directed along the crystallographic axis C ₆ have been found. At 90 K, the crystal polarized in the thermogradient electromotive force field is characterized by the photosensitivity in the near-edge, impurity, and infrared spectral regions. It is revealed that the electret state in the CdI₂: Ag crystal is also formed at room temperature during photolysis under irradiation of the samples by integrated light from a xenon lamp. Models of photosensitive centers formed upon doping of the CdI₂ crystal from the melt by the Ag⁺ impurity and during the occurrence of thermo- and photostimulated chemical reactions are proposed. The mechanism of the photochromic effect, including the change in the charge state of silver impurity ions, is considered.     

Activation of zinc sulfide luminescence by modifying its surface by organic solvents

The influence of the properties of the environment interacting with the surface of crystal phosphor on its radiative recombination has been considered. The influence of treatment with hexane and toluene on the state of the surface of powdery ZnS:Cu:Cl has been investigated. It has been shown that treatment with hexane and toluene leads to an increase in the photoluminescence intensity of the luminophor under investigation. On the basis of the analysis of the luminescence and photoluminescence excitation spectra, a model explaining the change in the concentration of radiationless recombination centers on the surface of powdery ZnS:Cu:Cl is proposed. It has been shown that the interaction between the above organic liquids and the luminophor surface leads to its passivation and improvement of the luminescent properties of the luminophor. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 6, pp. 799–804, November–December, 2005.     

The influence of doping element on structural and luminescent characteristics of ZnS thin films

For the fabrication of green and blue emitting ZnS structures the elements of I, III, and VII groups (Cu, Al, Ga, Cl) are used as dopants. The influence of type of impurity, doping technique, and type of substrate on crystalline structure and surface morphology together with luminescent properties was investigated. The doping of thin films was realized during the growth process and/or post-deposition thermal treatment. ZnS thin films were deposited by physical (EBE) and chemical (MOCVD) methods onto glass or ceramic (BaTiO₃) substrates. Closed spaced evaporation and thermodiffusion methods were used for the post-deposition doping of ZnS films. X-ray diffraction (XRD) techniques, atomic force microscopy (AFM), and measurements of photoluminescent (PL) spectra were used for the investigations. It was shown that the doping by the elements of I (Cu) and III (Al, Ga) groups does not change the crystal structure during the thermal treatment up to 1000 C, whereas simultaneous use of the elements of I (Cu) and VII (Cl) groups leads to decrease of the phase transition temperature to 800 C. The presence of impurities in the growth process leads to a grain size increase. At post-deposition treatment Ga and Cl act as activators of recrystallization process. The transition of ZnS sphalerite lattice to wurtzite one leads to the displacement of the blue emission band position towards the short-wavelength range by 10 nm.     

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.     

Spin-lattice coupling coefficients of Mn2+ ions in stacking faults of ZnS

The spin-lattice coupling coefficients (SLCC) of Mn²⁺ ions in axial sites PN of cubic ZnS crystals containing stacking faults have been measured by an uniaxial stress method. These coefficients were correctly interpreted in an ionic model by taking into account the Blume-Orbach mechanism adapted to a C_3v symmetry and a relativistic mechanism whose importance has been previously demonstrated. Finally, a comparison between the experimental SLCC's for Mn₂₊ in the cubic sites and two axial sites PN and AS due to stacking faults shows that they are roughly identical as predicted theoretically.     

ZnS单晶生长层线区结构的X射线衍射分析

汽相生长的ZnS·Cu单晶交流电致发光线的疏密与生长层线的疏密相对应。用X射线周转晶体法对1050℃汽相生长的ZnS晶体结构进行了分析。结果表明,生长层线疏密不同,结构差别很大,层线区的结构特点是层错的无规分布。ZnS·Cu单晶交流电致发光线的产生与结构中层错的无规分布有关。     

Orientation of luminescence centers in self-activated ZnS crystals

Results concerning polarization of the self-activated (SA) luminescence in ZnS crystals are presented. It is assumed that the orientations of absorption centers — circular oscillators (rotators) — and emission centers — rotators and dipoles — are along the crystal 3-fold symmetry axes. The calculated polarization diagrams fit well to experimental results.     

Photoluminescence properties of impurity-doped ZnS nanocrystals fabricated by sequential ion implantation

We have studied photoluminescence (PL) spectrum and dynamics of Cu- and Al-doped ZnS (ZnS:Cu,Al) nanocrystals fabricated by sequential implantation of Zn⁺, S⁺, Cu⁺, and Al⁺ ions into Al₂O₃ matrices. These samples exhibit intense green PL under UV light excitation. The space- and time-resolved PL measurements show that the broad green PL is due to the donor–acceptor (DA) pair luminescence of single ZnS:Cu,Al nanocrystals.     

The self-activated luminescence of ZnS1−xSex

The self-activated luminescence (SA) of ZnS : I, ZnSe : Cl, ZnSe : A1, and some ZnS_1?xSe_x : Cl single crystals has been investigated at LHeT under excitation with light. The SA-luminescence band has been identified in each case by the temperature dependence and by a direct excitation band resulting in polarized emission. Symmetry C_3v in case of ZnS : I and ZnSe : Cl and symmetry C_s in case of ZnSe : A1 for the SA-centre have been unambiguously revealed from angular dependences of polarization. Thus the nearest neigbour-compensated cation-vacancy has been identified as complex responsible for SA-luminescence.     

Electro- and photoluminescence of compounds based on doped zinc sulfide in the 500–750-nm range

The influence of atmospheric passivation on the electro- and photoluminescent properties of ZnS powders doped with In and/or CuCl is investigated. The processes proceeding in the material during thermal doping with In and/or CuCl as well as participation of oxygen in forming the electro- and photoluminescent radiation centers are discussed. The possibility of creating electro- and photoluminophors based on ZnS that have a continuous spectrum in the visible range with the same spectral density is shown. An electroluminophor based on ZnS:In,Cu,Cl that emits radiation with practically the same spectral density in the 550–750-nm range has been created as well as a photoluminophor based on ZnS:In that emits similarly within the range 500 < < 700 nm.__________Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 1, pp. 90–93, January–February, 2005.     

The identity of the EPR A-center and the self-activated luminescence center in ZnS: Cl

Generation and bleaching with polarized light of the EPR signal associated with the zinc vacancy-chlorine donor pair (A-center) in ZnS : Cl are described. Excitation bands at 3550 and 3300Å and bleaching bands at 9000 and 13,000Å are revealed and their polarization properties determined. The close correlation with the polarized luminescence studies of Koda and Shionoya on the ‘self-activated’ luminescence center confirms that the luminescence arises from the zinc vacancy-chlorine pair.     

AC electroluminescence of ZnS: Cu,Cl, Mn thin films in the structure In2O3(Sn)-ZnS: Cu,Cl, Mn-SiOx-Al

AC electroluminescence of ZnS: Cu, Cl, Mn thin films in the structure In₂O₃(Sn) - ZnS: Cu, Cl, Mn-SiO_x Al was studied. Vacuum-evaporated films 0.5 to 2.0 μm thick, excited with sinusoidal voltage of 80–200 V and up to 2 kHz gave the luminance response fulfilling Alfrey-Taylor's relation. Thus the electroluminescence model, suggested by these authors for a ZnS monocrystal, can be applied also for ZnS thin films.     

Cr+ centres in mixed polytype ZnS single crystal

The glow peaks of mixed polytype ZnS: Cu, Cr, Cl crystals were identified as produced by three Cr⁺ centres observed by electron paramagnetic resonance (EPR): one cubic (C) and two axial (H₁ and H₂). The characteristics of these centres were partly found by the methods of interrupted thermoluminescence, infrared stimulation, “thermo-EPR” (variation of the EPR signal with heating of a previously excited crystal) and also by decay of glow areas and of the three Cr⁺EPR signals after excitation at different temperatures. Moreover, these measurements showed the important part played by the non-thermal emptying of the Cr⁺ traps. This emptying occurs through photoexcitation of the trapped electrons by the reabsorbed luminescence light or through recombination of these electrons with free holes. Both of these processes diminish the efficiency of phosphorescence when the crystal is cooled; so it was necessary to take them into account when analysing the experimental results. The percentage of H₁ and H₂ found by EPR and luminescence is in agreement with the percentage of the hexagonalicity as found from the birefringence measurements. The crystal field calculation gives account of three centres in such a mixed polytype crystal.     

Synthesis and characterization of ZnS:Cu,Al phosphor prepared by a chemical solution method

A novel and simple synthesis route for the production of ZnS:Cu,Al sub-micron phosphor powder is reported. Both the host and activator cations were co-precipitated from an ethanol medium by mixing with a diluted ammonium sulfide solution. The co-precipitated ZnS:Cu,Al was in cubic zinc blende structure after an intermediate-temperature furnace annealing. Strong photoluminescent and cathodoluminescent (CL) emission were observed, which was attributed to the 3d¹⁰-3d⁹4s¹ radiative transition at those copper sites. At an accelerating voltage of 1 kV, the CL intensity of the co-precipitated ZnS:Cu,Al sample was recorded 94% of the commercial reference phosphor with the same composition made by high temperature solid-state-reaction method. The particle size of the co-precipitated phosphor powders was found to be controllable simply through adjusting the reactant concentrations. The particle size of the annealed samples was measured by dynamic light scattering, which showed a mean particle diameter between 200 and 700 nm depending on the co-precipitation conditions.     

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.     

Excitonic excitation spectra in ZnS: Cl crystal under pressure

The 3363 Å peak in the excitation spectrum of the S-A luminescence in the ZnS:Mn Cl cubic crystal at R.T. is ascribed to the A exciton. Its pressure coefficient is found as 6.4±0.2 meV/kbar, close to ρ = 6.3±0.2 meV/kbar reported for the gap in ZnS cubic crystal from the reflectivity measurements. This peak decreases by cooling and has not been observed at 85 K. The uv-excitation of the Mn-luminescence at R.T. is due to the energy transfer from the S-A to the Mn²⁺ centers, whereas the excitation peak at the exciton energy at 85 K comes from the excitation of the Mn²⁺ centers by the recombination energy of the Mn²⁺ bound excitons.     

147Pm激发的ZnS：Cu,Cl发光粉的研究

以高纯ZnS粉末为基质,采用高温转相、扩散,以及表面涂敷工艺,制得了¹⁴⁷Pm激发的ZnS:Cu,Cl发光粉。分析了ZnS:Cu,Cl的晶体结构,测量了ZnS:Cu,Cl的激发光谱、发射光谱、发光亮度。其晶体结构主要是六方纤锌矿型结构,激发光谱峰值波长为341nm,发射光谱峰值波长为513nm,初始发光亮度达到312mcd/m₂。由激发光谱的峰值波长341nm推算得到六方ZnS晶体的禁带宽度为3.64eV。分析了¹⁴⁷Pm激发的ZnS:Cu,Cl发光粉的发光寿命,其发光寿命达到5年以上。还探讨了该放射性发光粉的发光机理。¹⁴⁷Pm激发的ZnS:Cu,Cl的稳定发光,实际上是激发过程与复合过程的准平衡。ZnS:Cu,Cl的绿色发光来源于深施主-深受主对的复合发射。实验结果的分析表明,ZnS:Cu,Cl中深施主-深受主之间的能级间隔约为2.42eV。     

Optical alignment of Cu<Superscript>2+</Superscript> axial centers in KTaO<Subscript>3</Subscript>: Spectral dependence of the effect

The Cu _Ta ²⁺ -V_O axial centers in crystalline KTaO₃ were found to undergo alignment under the action of polarized light. The sign of the effect is shown to change depending on the wavelength of the aligning light. A parallel study of the spectral response of photoconductivity of the same samples led to the conclusion that the alignment of the copper centers is driven not by reorientation but rather by an anisotropic recharging of the centers, which involves both the conduction and valence bands of the crystal. This interpretation was supported by a study of the kinetics of thermal destruction of the copper center alignment.