Luminescence induced by elastic deformation of ZnS:Mn nanoparticles

When the thin film of ZnS:Mn nanoparticles deposited on a glass substrate is elastically deformed by applying a load, then initially the mechanoluminescence (ML) intensity increases with time, attains a peak value I_m at a particular time t_m, and later on it decreases with time. The rise and decay characteristics of the ML produced during release of the load are also similar to those produced during the application of load. Similar rise, occurrence of peak and then decrease in ML intensity are also found, when the film is deformed impulsively by dropping a steel ball of small mass from a low height; however, in this case, the time durations for the occurrence of ML and decay time of ML are very short. In the cases of loading and impulsive deformation ,after t_m, initially the ML intensity decreases at a fast rate and then at a slow rate, in which the decay time of fast decrease is equal to the time-constant for rise of pressure and the decay time for slow decrease is equal to the relaxation time of the surface charges. In the case of loading, the peak intensity I_m and the total intensity I_T of ML increase quadratically with the magnitude of applied pressure; however, in the case of impulsive deformation, both the I_m and I_T increase linearly with the height through which the ball is dropped on to the sample. In the case of deformation of the samples at a fixed strain rate, I_m should increase linearly with the applied pressure. The elastico ML in ZnS:Mn nanoparticles can be understood on the basis of the piezoelectrically-induced electron detrapping model, in which the local piezoelectric field near the Mn²⁺ centres reduces the trap-depth, and therefore, the detrapping of filled electron traps takes place, and subsequently the energy released non-radiatively during the electron-hole recombination excites the Mn²⁺ centres and de-excitation gives rise to the ML. The equal number of photons emitted during the application of pressure, release of pressure, and during the successive applications of pressure, indicates that the detrapped electron-traps get filled during the relaxation of the surface charges induced by the application and release of pressure because the charge carriers move to reduce the surface charges. On the basis of the piezoelectrically-induced electron detrapping model, expressions are derived for different characteristics of the ML of ZnS:Mn nanoparticles and a good agreement is found between the theoretical and experimental results. The expressions explored for the dependence of ML intensity on several parameters may be useful in tailoring the suitable nanomaterials capable of exhibiting ML during their elastic deformation. The values of the relaxation time of surface charges, time-constant for the rise of pressure, and the threshold pressure can be determined from the measurement of the time-dependence of ML. It seems that the trapping and detrapping of charge carriers in materials can be studied using ML.     

Synthesis, optical, structural and thermal characterization of Mn doped ZnS nanoparticles using reverse micelle method

Mn²⁺­doped ZnS nanoparticles have been prepared through the reverse micelles method using sodium bis (2-ethylhexyl) sulfosuccinate (AOT) as a surfactant. The prepared particles were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infra-red spectrometer (FT-IR), UV-vis spectrometry, photoluminescence (PL), electron spin resonance (ESR) and thermogravimetry-differential scanning calorimetry (TG-DSC).     

Relationship between crystal morphology and photoluminescence in polynanocrystalline lead sulfide thin films

Thin films of lead sulfide (PbS) nanoparticles were grown on corning glass and Si(1 0 0) substrates by polyethylene glycol-assisted chemical bath deposition (CBD) method. This paper compares the morphology and the luminescence properties (PL) of the deposited thin films in the presence (or absence) of PEG300 and investigates the effect of deposition temperatures. Surface morphology and photoluminescence properties of samples were analyzed. The PL data show a blue-shift from the normal emission at ∼2900 nm in PbS bulk to ∼360 nm in nanoparticles of PbS thin films. Furthermore, the PL emission of the films obtained without the addition of PEG300 (type 1) was slightly shifted from that of the films obtained in presence of PEG300 (type 2) from ∼360 to ∼470 nm. The blue-shifting of the emission wavelengths from 2900 to ∼360 or 470 nm is attributed to quantum confinement of charge carriers in the restricted volume of nanoparticles, while the shift between the two types of PbS nanoparticles thin films is speculated to be due to an increase in the defect concentration. The blue-shift increased with increase of the deposition temperature, which suggests that there has been a relative depletion in particle sizes during the CBD of the films at higher temperatures. The PbS nanocrystalline thin films obtained in the presence of PEG300 at 60 °C exhibit a high blue luminescence.     

Electronic and optical properties of Cd1-xZnxS nanocrystals

We report a numerical simulation of the conduction and valence band edges of Cd_1-xZn_xS nanocrystallites using a one — dimensional potential model. Electron — hole pairs are assumed to be confined in nanospheres of finite barrier heights. Optical absorption measurements are used to fit the bandgap of the Cd_1-xZn_xS nanocrystal material. A theoretical analysis is also made to calculate the energy location of bound excitons and the oscillator strength of interband transitions as a function of zinc composition. The aim of the latter study is to investigate the optical behavior of Cd_1-xZn_xS nanocrystals. An attempt to explain all the results is presented.     

Improved photoluminescence of ZnS:Mn nanocrystals by microwave assisted growth of ZnS shell

The photoluminescence (PL) of ZnS:Mn nanocrystals was improved greatly by microwave assisted growth of ZnS shell. Under optimized conditions, the luminescence quantum yield of ZnS:Mn nanocrystals increased from 2.8% to 12.1% after the growth of the ZnS shell. Time-resolved fluorescence spectroscopic and electron paramagnetic resonance measurements indicate that the improvement of the dispersivity of the doped Mn ions is responsible for the PL enhancement. Growth of the ZnS shell not only facilitated the diffusion of Mn ions during microwave irradiation but also prohibited the segregation of Mn ions on the particle surface. As a result, more isolated Mn²⁺ ions were produced after the growth of the ZnS shell, and thus the orange luminescence of ZnS:Mn nanocrystals was enhanced greatly.     

核-壳结构的ZnS:Mn纳米粒子的荧光增强

采用反胶束方法制备了ZnS :Mn纳米粒子并对其进行了ZnS壳层修饰 ,采用发射光谱和激发光谱对其光学性质进行了研究。与未包覆的ZnS:Mn纳米粒子相比 ,核 壳结构的ZnS :Mn纳米粒子来自于Mn2 离子的 5 80nm的发光增强了数倍 ,归因于ZnS壳增加了Mn2 离子到纳米颗粒表面的距离 ,减弱了Mn2 离子向表面猝灭中心的传递。样品制备后 ,核 壳结构的ZnS :Mn纳米粒子在 5 80nm的发光随时间略有增强 ,激发光谱的位置未发生明显变化 ,而未包覆的ZnS:Mn纳米粒子在 5 80nm的发光显著增强 ,同时自激活发光减弱 ,激发光谱明显发生红移 ,说明未包覆的ZnS :Mn纳米粒子的尺寸随时间增大 ,而核 壳结构的ZnS :Mn纳米粒子尺寸基本不变。     

Y2O2S:Eu nanocrystals, a strong quantum-confined luminescent system

Trivalent europium-doped yttrium oxysulfide nanocrystals synthesized using sol-gel thermolysis show significant blue shifts in the excitation bands corresponding to fundamental absorption, charge-transfer absorption. A significant blue shift observed in the fundamental absorption edge for the nanocrystals having an average crystallite size (φ) in the range 9-15 nm indicates a strong quantum confinement with a Bohr exciton radius of 5-13 nm. Also, the diffuse reflectance spectra and the corresponding Kubelka-Munk plot indicate the possibility of profound decrease in the absorption coefficient of Eu³⁺-ligand charge-transfer species necessitating further studies in this wide-gap semiconductor nanocrystalline system.     

Photoluminescence and magnetic properties of Fe-doped ZnS nano-particles synthesized by chemical co-precipitation

This paper is focusing on the synthesis of Zn_1−xFe_xS nano-particles with x = 0, 0.1 and 0.2 by chemical co-precipitation method, the prepared of which are characterized by XRD, EDS, TEM, PL, magnetization versus field behavior and M-T curve. In the XRD patterns, Zn_1−xFe_xS nano-particles are shown of cubic zinc blende structure, and the broadening diffraction peaks consistent with the small-size characteristic of nano-materials. The diameter of nano-particles is between 3.3 and 5.5 nm according to the HR-TEM images. The EDS data confirm the existence of Fe ions in Fe-doped ZnS nanoparticles. There we found that Fe-doping did not import new energy bands or defect states, but reduced the intensity of PL peaks. The magnetization versus field behaviors were illustrated by the M-H curves at both 5 K and 300 K, respectively, where no remanence or coercive force was observed. This phenomenon indicates that the Zn_1−xFe_xS (x = 0.1) nano-particles are superparamagnetic. The zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves further reveal that the blocking temperature (T_B) of the superparamagnetic behavior might be below 5 K.     

Effect of ratios of Cd:Se in CdSe nanoparticles on optical edge shifts and photoluminescence properties

This paper addresses the issue related to morphology of CdSe nanoparticles capped with organic molecules. Semiconducting CdSe nanoparticles of 5–16 nm are synthesized using CdO precursor, capped with trioctyl phosphine (TOP)/trioctyl phosphine oxide (TOPO) using different starting precursor ratios of Cd:Se. At an optimum ratio of Cd/Se-2:1, highly luminescent and small sized (5 nm) nanoparticles are obtained. At other Cd/Se precursor ratios (0.5:1, 1:1, 3:1) larger particles are formed with varying photoluminescence (PL) intensity and optical absorption (UV–VIS). X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) are used to determine the crystallinity and stoichiometry of the system, respectively. It is shown that the blue shifts of the optical absorption edge concurrent with the CdSe nanocrystal size reduction, for sizes measured by XRD with respect to the bulk semiconductor, agree perfectly with the strong quantum confinement model. The optical edge shifts are significantly higher for CdSe nanocrystallite as measured by transmission electron microscopy (TEM) than the theoretical prediction based on the strong quantum confinement model. This is understood on the basis of agglomeration effects as observed by TEM for CdSe nanocrystallites. The nano-sized CdSe growth island thus formed comprises of several TOP/TOPO passivated nanocrystals.     

Optical and ESR studies on Fe doped ZnS nanocrystals

Zn_1 − xFe_xS (x=0.0, 0.1, 0.2, 0.4 and 0.6) nanocrystals have been obtained by chemical co-precipitation from homogeneous solutions of zinc and iron salt compounds, with S²⁻ as precipitating anion formed by decomposition of thiophenol. The TEM micrographs show a spherical shape for ZnS nanocrystals and their average size is around 7 nm. The optical absorption spectra indicate a blue shift of the absorption edge with increasing Fe-content. The luminescence of nanoparticles excite at about 370 nm with an emission peak at around 490 nm. At room temperature, ESR signal characteristic of Fe³⁺ was observed in samples of all concentrations.     

Experimental investigation of energy transfer between CdSe/ZnS quantum dots and different-sized gold nanoparticles

Hybrid nanostructures of quantum dots(QDs) and metallic nanostructure are attractive for future use in a variety of optoelectronic devices. For photodetection applications, it is important that the photoluminescence (PL) of QDs is quenched by the metallic nanostructures. Here, the quenching efficiency of CdSe/ZnS core-shell quantum dots (QDs) with different sized gold nanoparticles (NPs) films through energy transfer is investigated by measuring the PL intensity of the hybrid nanostructures. In our research, the gold NPs films are formed by the post-annealing of the deposited Au films on the quartz substrate. We find that the energy transfer from the QDs to the Au NPs strongly depends on the sizes of the Au NPs. For CdSe/ZnS QDs direct contact with the Au NPs films, the largest energy transfer efficiency are detected when the resonance absorption peak of the Au NPs is nearest to the emission peak of the CdSe/ZnS QDs. However, when there is a PMMA spacer between the QDs layer and the Au NPs films, firstly, we find that the energy transfer efficiency is weakened, and the largest energy transfer efficiency is obtained when the resonant absorption peak of the Au NPs is farthest to the emission peak wavelength of CdSe/ZnS QDs. These results will be useful for the potential design of the high efficiency QDs optoelectronic devices.     

Photoluminescence quenching of CdTe/CdS core-shell quantum dots in aqueous solution by ZnO nanocrystals

Photoluminescence (PL) properties of 3-mercaptopropionic acid (MPA) coated CdTe/CdS core-shell quantum dots (QDs) in aqueous solution in the presence of ZnO colloidal nanocrystals were studied by steady-state and time-resolved PL spectroscopy. The PL quenching of CdTe/CdS core-shell QDs with addition of purified ZnO nanocrystals resulted in a decrease in PL lifetime and a small red shift of the PL band. It was found that CdTe(1.5 nm)/CdS type II core-shell QDs exhibited higher efficiency of PL quenching than the CdTe(3.0 nm)/CdS type I core-shell QDs, indicating an electron transfer process from CdTe/CdS core-shell QDs to ZnO nanocrystals. The experimental results indicated that the efficient electron transfer process from CdTe/CdS core-shell QDs to ZnO nanocrystals could be controlled by changing the CdTe core size on the basis of the quantum confinement effect.     

多孔硅的分形结构及其荧光特性

用扫描电子显微镜(SEM)对多孔硅的结构进行了分析.结果显示多孔硅具有分形特性,同计算机模拟结果一致;用荧光光谱仪,研究了多孔硅的荧光特性与激发波长的依赖关系.激发光谱测量结果发现,当激发波长从650 nm变到340 nm时,荧光谱峰位从红端780 nm连续蓝移到500 nm.综合分析说明：正是由于多孔硅的分形微结构以及量子限制效应,导致了多孔硅的荧光特性随激发波长改变的物理现象.     

Electronic structure of HgTe nanocrystals: An observation of p-d weakening

Photoemission studies to identify the electronic structure of the HgTe nanocrystals revealed a new phenomenon of p-d weakening, as a consequence of size quantization effect associated with the mean crystalline size, 5.35 ± 0.83 nm. The weakening of the p-d hybridization by a factor of 0.33, to that of the bulk HgTe suggests the valence band maxima and core level shifts toward higher binding energy. The widening of the band gap due to size quantization is confirmed from optical absorption and photoluminescence measurements. The upward and downward shift of the conduction band minima and the valence band maxima with respect to the bulk value of HgTe are found to be 1.6 eV and 0.54 eV respectively.     

Synthesis and time-resolved photoluminescence spectroscopy of capped CdS nanocrystals

Here, we report the synthesis of colloidal CdS nanoparticles by capping with starch, phenol and pyridine. We also study the photophysical properties of CdS nanoparticles by steady state and time-resolved photoluminescence spectroscopy. The experimental results show that the relaxation of the excited state of CdS nanoparticles is composed of two different components. Our analysis suggests that the fast and slow components decay times of these capped CdS nanocrystals are due to trapping of carriers in surface state and e–h radiative recombination processes, respectively.     

Optical and paramagnetic properties of titanium centres in ZnS

ZnS:Ti crystals grown by different methods are investigated at low temperatures (2 K K) by electron paramagnetic resonance (EPR) and optical spectroscopies, mainly with regard to their photoluminescence properties, including excitation and sensitisation spectra. The more familiar lattice-neutral Ti²⁺(d ² ) ion on a cubic site (AN) presents an isotropic EPR signal with g=1.928 observable up to 80 K. Signals of axial PN and AS sites are detected in the same temperature range. This Ti²⁺ ion exhibits the transition in emission, structured by no-phonon lines (NPL) of centres in various environments, with the AN site represented at 3613 cm^-1, and the ³ T ₁ (F) and ³ T ₁ (P) bands in excitation spectra. Substitutional Ti³⁺(d ¹ ) is identified by an anisotropic EPR spectrum at T = 3.5 K, indicating a quasistatic Jahn-Teller effect at the doubly degenerate ² E(D) ground state, while in the first excited vibronic state a quasidynamic JT effect in the strain-split vibronic ² E/² A ₂ manifold is found. A new luminescence band centred at 4500 cm^-1 with NPL structures near 5000 cm^-1 represents AN, PN and AS sites in the transition. Charge-transfer processes are described in a one-particle model, depicting the Ti³⁺/Ti²⁺ donor level at 12 900 cm^-1 below the conduction-band edge. Received: 3 October 1997 / Revised: 17 December 1998 / Accepted: 18 May 1998     

Optical properties of annealed Mn-doped ZnS nanoparticles

Cysteine stabilized ZnS and Mn²⁺-doped ZnS nanoparticles were synthesized by a wet chemical route. Using the ZnS:Mn²⁺ nanoparticles as seeds, silica-coated ZnS (ZnS@Si) and ZnS:Mn²⁺ (ZnS:Mn²⁺@Si) nanocomposites were formed in water by hydrolysis and condensation of tetramethoxyorthosilicate (TMOS). The influence of annealing in air, formier gas, and argon at 200-1000 °C on the chemical stability of ZnS@Si and ZnS:Mn²⁺@Si nanoparticles with and without silica shell was examined. Silica-coated nanoparticles showed an improved thermal stability over uncoated particles, which underwent a thermal combustion at 400 °C. The emission of the ZnS@Si and ZnS:Mn²⁺@Si passed through a minimum in photoluminescence intensity when annealed at 600 °C. Upon annealing at higher temperatures, ZnS@Si conserved the typical emission centered at 450 nm (blue). ZnS:Mn²⁺@Si yielded different high intensity emissions when heated to 800 °C depending on the gas employed. Emissions due to the Mn²⁺ at 530 nm (green; Zn₂SiO₄:Mn²⁺), 580 nm (orange; ZnS:Mn²⁺@Si), and 630 nm (red; ZnS:Mn²⁺@Si) were obtained. Therefore, with a single starting product a set of different colors was produced by adjusting the atmosphere wherein the powder is heated.     

Mechanoluminescence and thermoluminescence of Mn doped ZnS nanocrystals

This paper reports the synthesis of ZnS:Mn nanocrystals by the chemical route in which mercaptoethanol was used as the capping agent. The particle size of such nanocrystals was measured using XRD and TEM patterns and was found to be in between 3and 5 nm. It was found that the peak position of TL glow curve and the TL intensity of ZnS:Mn nanoparticles increases as the particle size is decreased. The isothermal decay technique is used to determine the trap-depth. The stability of the charge carriers in the traps increase with the decrease in size of the nanoparticles. The higher stability may be attributed to the higher surface/volume ratio and also to the increase in the trap-depth with decreasing particle size. When a ZnS:Mn nanocrystal is deformed the peak intensity I_m increases linearly with the increasing height of the load. After I_m, initially the ML intensity decreases at a fast rate, and later on it decreases at a slow rate. The ML in ZnS:Mn nanocrystals can be understood on the basis of the piezoelectrically induced electron detrapping model.     

Photoluminescence studies on porous silicon/polymer heterostructure

Hybrid devices formed by filling porous silicon with MEH-PPV or poly [2-methoxy-5(2-ethylhexyloxy-p-phenylenevinylene)] have been investigated in this work. Analyses of the structures by scanning electron microscopy (SEM) demonstrated that the porous silicon layer was filled by the polymer with no significant change of the structures except that the polymer was infiltrated in the pores. The photoluminescence (PL) of the structures at 300 K showed that the emission intensity was very high as compared with that of the MEH-PPV films on different substrates such as crystalline silicon (c-Si) and indium tin oxide (ITO). The PL peak in the MEH-PPV/porous silicon composite structure is found to be shifted towards higher energy in comparison with porous silicon PL. A number of possibilities are discussed to explain the observations.     

Optical properties of electroluminescent ZnS doped with Cu+, Mn2+ and Gd3+ ions

ZnS:Gd, ZnS: Cu, Gd and ZnS: Mn, Gd phosphors have been prepared by firing the samples in argon atmosphere. Spectral distributions in these phosphors are discussed with appropriate mechanism. ZnS:Cu, Gd and ZnS:Mn, Gd are found to be examples of multiple band phosphors. Enhancement and quenching of the emission band intensities of all these phosphors have been studied inpel emission. It is observed that Gd³⁺ ions play an important role in transferring their excitation energy to other centres. The voltage and frequency variation ofel brightness are in agreement with collision excitation mechanism in Schottky barrier at the metal semiconductor interface. Studies in phosphorescence and thermoluminescence of these phosphors have also been carried out. It is observed that trap-depth changes slowly with temperature and dopant concentration. The values of trapping parameters have been evaluated. The irregular variation of the life-time of electrons in the traps. with temperature shows the existence of retrapping in these phosphors.