Photoluminescence characteristics of ZnS nanocrystallites doped with Ti3+and Ti4+

Direct synthesis of ZnS nanocrystallites doped with Ti³⁺ or Ti⁴⁺ by precipitation has led to novel photoluminescence properties. Detailed X-ray diffraction (XRD), fluorescence spectrophotometry, UV–vis spectrophotometry and X-ray photoelectron spectroscopy (XPS) analysis reveal the crystal lattice structure, average size, emission spectra, absorption spectra and composition. The average crystallite size doped with different mole ratios, estimated from the Debye–Scherrer formula, is about 2.6±0.2 nm. The nanoparticles can be doped with Ti³⁺ and Ti⁴⁺ during the synthesis without the X-ray diffraction pattern being altered. The strong and stable visible-light emission has been observed from ZnS nanocrystallites doped with Ti³⁺ (its maximum fluorescence intensity is about twice that of undoped ZnS nanoparticles). However, the fluorescence intensity of the ZnS nanocrystallites doped with Ti⁴⁺ is almost the same as that of the undoped ZnS nanoparticles. The emission peak of the undoped sample is at 440–450 nm. The emission spectrum of the doped sample consists of two emission peaks, one at 420–430 nm and the other at 510 nm. Received: 27 April 2001 / Accepted: 16 August 2001 / Published online: 17 October 2001     

Fabrication and properties of ZnO:Cu and ZnO:Ag thin films

Thin films of ZnS and ZnO:Cu were grown by an original metal–organic chemical vapour deposition (MOCVD) method under atmospheric pressure onto glass substrates. Pulse photo-assisted rapid thermal annealing of ZnO:Cu films in ambient air and at the temperature of 700–800 C was used instead of the common long-duration annealing in a vacuum furnace. ZnO:Ag thin films were prepared by oxidation and Ag doping of ZnS films. At first a closed space sublimation technique was used for Ag doping of ZnO films. The oxidation and Ag doping were carried out by a new non-vacuum method at a temperature >500 C. Crystal quality and optical properties were investigated using X-ray diffraction (XRD), atomic force microscopy (AFM), and photoluminescence (PL). It was found that the doped films have a higher degree of crystallinity than undoped films. The spectra of as-deposited ZnO:Cu films contained the bands typical for copper, i.e. the green band and the yellow band. After pulse annealing at high temperature the 410 and 435 nm photoluminescent peaks were observed. This allows changing of the emission colour from blue to white. Flat-top ZnO:Ag films were obtained with the surface roughness of 7 nm. These samples show a strong ultraviolet (UV) emission at room temperature. The 385 nm photoluminescent peak obtained is assigned to the exciton–exciton emission.     

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.     

Emission characteristics of SPAN-80 activated ZnS nanocolloids

Quantum surface effects (new emission bands, blueshifts, intensity enhancement) were observed in SPAN-80 activated ZnS nanocolloids and explained in terms of time-dependent density functional theory. The experimental evidences were demonstrated for both undoped and Cu, Mn-doped colloidal phases. The photoluminescence spectra of these materials showed a new green band at 520 nm (ZnS:Cu) and a yellow-orange band at 576 nm (ZnS:Mn) besides a blue band at 465 nm. All bands lie in the visible region and are blueshifted, show sharp emissions with narrow widths and have approximately 20-times stronger intensities in comparison with those of the bulk samples. The time-resolved luminescence spectra showed that the life-times of free electrons were 0.12 μs and 1.9 ms in ZnS:Cu and ZnS:Mn correspondingly.     

The optical properties and energy transition process in nanocomposite of polyvinyl-pyrrolidone polymer and Mn-doped ZnS

This study has been carried out on the optical properties of polyvinyl-pyrrolidone (PVP), the energy transition process in nanocomposite of PVP capped ZnS:Mn nanocrystalline and the influence of the PVP concentration on the optical properties of the PVP capped ZnS:Mn nanocrystalline thin films synthesized by the wet chemical method. The microstructures of the samples were investigated by X-ray diffraction, the atomic absorption spectroscopy, and transmission electron microscopy. The results showed that the prepared samples belonged to the sphalerite structure with the average particle size of about 2–3 nm. The optical properties of samples are studied by measuring absorption, photoluminescence (PL) spectra and time-resolved PL spectra in the wavelength range from 200 to 700 nm at 300 K. From data of the absorption spectra, the absorption edge of PVP polymer was found about of 230 nm. The absorption edge of PVP capped ZnS:Mn nanoparticles shifted from 322 to 305 nm when the PVP concentration increases. The luminescence spectra of PVP showed a blue emission with peak maximum at 394 nm. The luminescence spectra of ZnS:Mn–PVP exhibits a blue emission with peak maximum at 437 nm and an orange–yellow emission of ion Mn²⁺ with peak maximum at 600 nm. While the PVP coating did not affect the microstructure of ZnS:Mn nanomaterial, the PL spectra of the PVP capped ZnS:Mn samples were found to be affected strongly by the PVP concentration.     

Two and four photon absorption and nonlinear refraction in undoped,chromium doped and copper doped ZnS quantum dots

The ZnS quantum dots (QDs) with Cr and Cu doping were synthesized by chemical co-precipitation method. The nanostructures of the prepared undoped and doped ZnS QDs were characterized by UV–vis spectroscopy, Transmission electron microscopy (TEM) and X-ray diffraction (XRD). The sizes of QDs were found to be within 3–5 nm range. The nonlinear parameters viz. Two photon absorption coefficient (β₂), nonlinear refractive index (n₂), third order nonlinear susceptibility (χ³) at wavelength 532 nm and Four photon absorption coefficient (β₄) at wavelength 1064 nm have been calculated by Z-scan technique using nanosecond Nd:YAG laser in undoped, Cr doped and Cu doped ZnS QDs. Higher values of nonlinear parameters for doped ZnS infer that they are potential material for the development of photonics devices and sensor protection applications.     

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).     

Synthesis and photoluminescent properties of ZnS:Cd nanoparticles and their phase-transferred nanocomposite with polyvinylpyrrolidone

ZnS:Cd nanoparticles were synthesized in a reverse micelle system by controlling reaction factors with mercaptoacetic acid (MPA) as a surfactant and N,N-dimethylformamide as an oil phase. X-ray diffraction pattern shows that the ZnS:Cd nanoparticles exhibit a cubic structure and its mean size is calculated around 4 nm. With different molar ratios of Zn²⁺/S^2?, the relative intensity of the emission peaks at 400 and 556 nm changes dramatically due to the more sulfur vacancies which resulted from the imbalance of Zn²⁺ and S²⁺ ions. Furthermore, hydrophobic phase-transferred ZnS:Cd nanoparticles were obtained using octylamine, and a highly luminescent phase-transferred ZnS:Cd/polyvinylpyrrolidone (PVP) nanocomposite was prepared by blending the phase-transferred ZnS:Cd with PVP. Infrared absorption suggests that octylamine has been successfully connected with the MPA-coated ZnS:Cd nanoparticles. Unlike the MPA-coated ZnS:Cd which has a very strong emission at 556 nm, the phase-transferred ZnS:Cd has a strong emission at 435 nm, which is ascribed to surface passivation and electron redistribution. In addition, luminescent intensity enhancement was observed for the phase-transferred ZnS:Cd/PVP nanocomposites with various Cd²⁺ doping concentrations.     

二价铕激活的ZnS磷光体的发光

本文详细描述了ZnS:Eu2+磷光体的合成及光致发光性能。首次报导了这种发光材料的特殊长余辉特性。作者测量了热释发光光谱、不同温度下的发射特性的变化及荧光的激发、发射衰减时间,提出两类缔合Eu中心的模型。用不同的缔合Eu中心较好地解释了它的光谱特性及长余辉现象,认为光谱的两个发射带来自不同的缔合Eu中心,即550nm发射带对与ZnS导带电子陷阱相缔合的Eu中心有关,650nm带来自与电子陷阱和空穴陷阱缔合的Eu中心。发射的余辉主要与导带中某种电源电子陷阱存在有关。此外,本文还对与应用有关的阴极射线发光性能进行了报导。     

First evidence for the two-proton decay of 45Fe

Decays of five ⁴⁵Fe atoms have been observed at the fragment separator of GSI. Whereas one event is consistent with the β-decay of ⁴⁵Fe accompanied by the emission of a 10 MeV proton, four decays are consistent with the emission of particle(s) of total energy 1.1±0.1 MeV. This observation represents the first evidence for two-proton ground-state radioactivity, a decay mode predicted for ⁴⁵Fe. The time distribution of the observed decay events corresponds to a half-life of 3.2^+2.6 _-1.0 ms. Received: 17 May 2002 / Accepted: 28 May 2002     

Spectroscopic studies of the βp and β2p decay of 23Si

²³Si isotopes have been produced as projectile fragments of a ³⁶Ar primary beam at 95 MeV/nucleon at the LISE3 spectrometer of GANIL. After implantation in a detector telescope, β-delayed one-proton and β-delayed two-proton emission has been observed. The main one-proton peaks are at (1.32±0.04)MeV, (2.40±0.04)MeV, and (2.83±0.06)MeV. The total decay energy for the β2p decay is (6.18±0.10)MeV for the decay to the ground state and (5.86±0.10)MeV for the decay to the first excited state in the daughter nucleus. However, energetically possible decays via βpα and β3p emission have not been identified. The spectra allowed us to determine the excitation energy of the isobaric analogue state in ²³Al. This enabled us to calculate the coefficients of the T = 5/2 isobaric multiplet mass equation for A = 23. The mass excess of the ²³Si ground state was deduced. This value is compared to different theoretical predictions. Additionally, we determined the branching ratios for the different decay branches. A half-life measurement yielded T_1/2 = (40.7±0.4)ms.     

高场电致发光过程的动力学分析

本文通过对交流(ZnS:Mn,Cu)和直流(ZnS:Mo,Cu(Cu))两种不同结构的粉末发光材料的发光光谱,时间分辨发光光谱,电流波形,发光亮度波形,发光衰减以及发光强度与掺杂浓度的关系等方面的研究,对Mn和Cu这两种不同类型的发光中心的高场电致发光过程进行了分析,并用一个统一的模型对两种不同类塑的发光中心建立了发光动力学方程,由此解得Mn中心和Cu中心的发光强度随时间的变化规律表达式,它们分别包括了Mn中心和Cu中心发光亮度波形的所有情形。我们从动力学分析出发,找到了影响Cu中心和Mn中心发光强度的主要因素,并从理论上预言了提高发光强度的可能途径。     

Luminescence and decay times of CsI:In+

Luminescence spectra of single crystals of CsI:In⁺ excited in the A(304 nm), B(288 nm), C(268 nm) and D(257 nm) absorption bands have been studied in the temperature range 4.2–300 K. Excitation in the A band at 4.2 K gives rise to the principal emission at 2.22 eV accompanied by a partly-overlapping weak band at 2.49 eV. An additional emission band at about 2.96 eV is observed on excitation in the B, C or D bands. Yet another emission band located at 2.67 eV is excited only in the D band. The relative intensities of the bands are very sensitive to excitation wavelength as well as to temperature. The origin of all these bands is assigned in terms of a model for the relaxed excited states (RES). All the luminescence spectra were resolved into an appropriate number of skew-Gaussian components. Moments analysis leads to a value of (1.35 ± 0.02) × 10¹³ rad s^-1 for the effective frequency (ω_eff) of lattice vibrations coupled to the RES. At the lowest temperature, the radiative decay times of each of the intracenter emission bands (2.22, 2.49 and 2.96 eV) show a slow decay ( ～ 10–100 μs) and a fast decay ( ～ 10–100 ns). The 2.96 eV band, which is assigned to an emission process which is the inverse of the D-band absorption, exhibits a single decay mode ( ～ 10 μs). The intrinsic radiative decay rates (k₁, k₂), the one-phonon transition rate (K) and the second-order spin-orbit splitting (D) for the RES responsible for the principal emission are: k₁ = (6.0±-0.3)×10³ s^-1, k₂ = (1.33±-0.06)×10⁵ s^-1, K = (2.4±-0.4)×10⁷ s^-1 and D = (13.8±-0.5) cm^-1.     

Polarized emission of doped ZnS crystals

Polarization of light emitted in various spectral ranges was studied in two single crystals ZnS:Cu, Cl and ZnS:Ag, Cu, Al. The G-Cu, B-Cu and a small amount of S-A centers have been identified in the ZnS:Cu, Cl crystal by the spectral and polarization methods. The B-Ag band was found in the spectrum of the ZnS:Ag, Cu, Al crystal and its polarization properties investigated. This emission appears to be always polarized perpendicular to the [111]_c axis of the stacking faults independently of the polarization of the exciting light. The symmetry of the B-Ag center is not lower than that of the host lattice. Analogy with G-Cu centers suggests a model for the B-Ag center in which the polarization comes from the symmetry properties of the Ag²⁺ orbitals in the trigonal field of stacking faults.     

Preparation, characterization and study of optical properties of ZnS nanophosphor

In this work the preparation, characterization and photoluminescence studies of pure and copper-doped ZnS nanophosphors are reported, which are prepared by using solid-state reaction technique at a temperature of 100 °C. The as-obtained samples were characterized by X-ray diffraction (XRD) and UV-VIS Reflectance spectroscopy. The XRD analysis confirms the formation of cubic phase of undoped as well as Cu²⁺-doped ZnS nanoparticles. Furthermore it shows that the average size of pure as well as copper-doped samples ranges from 15 to 50 nm. The room-temperature PL spectra of the undoped ZnS sample showed two main peaks centered at around 421 and 450 nm, which are the characteristic emissions of interstitial zinc and sulfur vacancies, respectively. The PL of the doped sample showed a broad-band emission spectrum centered at 465 nm accompanied with shoulders at around 425, 450 and 510 nm, which are the characteristic emission peaks of interstitial zinc, sulfur vacancies and Cu²⁺ ions, respectively. Our experimental results indicate that the PL spectrum confirms the presence of Cu²⁺ ions in the ZnS nanoparticles as expected.     

Preserving long-term emission stability of carbon nanotube field emitter using aluminum layer

Carbon nanotube (CNT) field emitter was fabricated, and then its emission stability was evaluated with three different anode structures; indium tin oxide (ITO)/glass, ZnS:Cu,Al(green phosphor)/ITO/glass, and Al/ZnS:Cu,Al/ITO/glass. It was found that the electron emission from CNTs to the phosphor layer degrades much faster than the emission to ITO layer does. The current decay time from 100 μA/cm² to 50 μA/cm² for ITO/glass and ZnS:Cu,Al/ITO/glass were 250 h and 20 h, respectively. Such rapid decay in emission current with the phosphor-coated anode was found to be attributed to the formation of Zn particles on CNTs during the field emission. However, the deposition of aluminum layer on the phosphor, in other words, using the anode structure of Al/ZnS:Cu,Al/ITO/glass recovered the stability that is comparable to that with an ITO/glass. The aluminum layer was found to efficiently prevent phosphor elements from being degassed, preserving the long-term emission stability of carbon nanotubes.     

Yellow-orange light emission from Mn2+-doped ZnS nanoparticles

ZnS nanoparticles with Mn²⁺ doping (1–2.5%) have been prepared through a simple soft chemical route, namely the chemical precipitation method. The nanostructures of the prepared undoped ZnS and Mn²⁺-doped ZnS:Mn nanoparticles have been analyzed using X-ray diffraction (XRD), Scanning electron microscope (SEM), transmission electron microscope (TEM) and UV–vis spectrophotometer. The size of the particles is found to be in 2–3 nm range. Room-temperature photoluminescence (PL) spectrum of the undoped sample only exhibits a blue-light emission peaked at ∼365 nm under UV excitation. However, from the Mn²⁺-doped samples, a yellow-orange emission from the Mn^{2+ 4}T₁–⁶A₁ transition is observed along with the blue emission. The prepared 2.5% Mn²⁺-doped sample shows efficient emission of yellow-orange light with the peak emission at ∼580 nm with the blue emission suppressed.     

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.     

Effect of chlorine concentration on the spectral characteristics of electroluminescence in ZnS: Cu: Cl phosphor

ZnS: Cu: Cl phosphor prepared under a vacuum firing process is found to give blue electroluminescence with emission peak at 460 nm which remains unaltered with the frequency of the excitation voltage. Addition of excess chlorine in the phosphor gives blue, green and red emission at 460, 520 and 640 nm. The intensity of the blue band decreases and it finally disappears as chlorine concentration is increased. A scheme involving three energy levels attributed to Cu²⁺, Cu⁺ and Cl^- centres in ZnS explains the experimental results completely.     

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.