High‐pressure Raman and photoluminescence of highly anisotropic CdS nanowires

Raman and photoluminescence of CdS nanowires of diameter 80 nm and lengths up to several tens of micrometers were studied at pressure up to 60 kbar using a Jobin‐Yvon T64000 micro‐Raman system in conjunction with the diamond‐anvil cell technique. The phase transition pressure of wurtzite to rock salt was observed at 38 kbar, which is higher than that of bulk CdS. In contrast with the transition pressure of different‐sized CdS nanocrystal, this elevated phase transition pressure cannot be explained well by the size effect. Thus the contribution of particle morphology of such a system, which represents the low‐energy surface structure, should be considered. The pressure dependence of photoluminescence is also discussed. Copyright © 2007 John Wiley & Sons, Ltd.     

Photoluminescence Spectra in Polycrystals of Chirally Pure and Racemic Phases of Tryptophan

The photoluminescence spectra of three crystalline phase modifications of aromatic amino acid tryptophan are compared: the left phase of L-tryptophan, the right phase of D-tryptophan, and the racemic phase of DL-tryptophan. The photoluminescence spectra were recorded using a fiber-optic procedure, a small-size FSD-8 spectrometer, and a data processing system that enables comparison of the analyzed spectrum with the spectrum of a reference substance. Photoluminescence was excited by a pulsed-periodic ultraviolet laser at a wavelength of 266 nm.

     

Study of the structural and morphological changes during the phase transition of ZnS to ZnO

Nanocrystalline zinc sulfide (ZnS) (nanorods) was synthesized by chemical precipitation method, and they were annealed in air at different temperatures in the range 200–700 °C for the phase transition of ZnS to ZnO. The characterization of the system was done by different techniques such as X-ray diffraction (XRD), high-resolution transmission electron microscopy, UV–Vis spectroscopy, photoluminescence spectroscopy and differential scanning calorimetry (DSC). From the XRD and TEM analysis, the crystal structure is found to convert from cubic ZnS phase to the hexagonal ZnO phase and its morphology from nanorods to nanoparticles with the increasing annealing temperatures. The UV–Visible absorption and photoluminescence measurements revealed that the relative changes in the phases alter the band gap and introduce new kinds of defects in the system. The percentage of the ZnS and ZnO phase has been found to be proportional to the annealing temperature for a fixed time interval, and the DSC measurement has also found similar results as in the XRD pattern during the phase transitions.     

调制掺杂ZnSe/BeTe Ⅱ型量子阱结构中的内秉电场和新型带电激子

报道了调制掺杂的ZnSe/BeTe/ZnSe Ⅱ型量子阱(type-Ⅱ QW)在低温(2—5 K)条件下的光致发光(PL),光致发光激发(PLE)和磁性光致发光(magneto-PL)光谱的实验结果.　观察到非掺杂样品的PL有两个很强的主发光峰而掺杂样品只有一个的奇异发光.　PL直线偏振度和PLE的测量结果都表明了这些空间间接型跃迁PL是来自两个异质结界面的贡献,非掺杂样品的两个主发光峰的分离则是起因于QW结构中的内秉电场(built-in electric field).在平行于QW生长方向的强磁场中, 关键词： 光致发光 二维电子气 带电激子 Ⅱ型量子阱     

Porous silicon with β-cyclodextrin modified surface for photoluminescence sensing of organic molecules in gas and liquid phase

Porous silicon surface was modified by photochemically activated hydrosilylation reaction with permethyl-6^I-alkenoylamino-6^I-deoxy-β-cyclodextrins terminated with linear alkenoyl spacers of various lengths. As compared to unmodified surface, derivatized surfaces revealed modified photoluminescence response in the presence of controlled amounts of various organic molecules in gas and liquid phase. For the selected set of analytes we observed most significant modification of photoluminescence response for aromatic compounds what corresponds to optimum molecular size for strong host–guest interaction with β-cyclodextrin cavity. Aliphatic compounds quenched photoluminescence from both unmodified and surface modified porous silicon. For low gas phase concentrations of aromatic analytes β-cyclodextrin modified porous silicon revealed photoluminescence enhancement, at higher concentrations common photoluminescence quenching was observed. The size-dependent host–guest interaction between β-cyclodextrin cavity and detected molecule was observed in photoluminescence quenching in the presence of aliphatic molecules in liquid phase. The role of the strength of host–guest interactions between detected analytes and β-cyclodextrin cavity on photoluminescence sensor response is discussed.     

Size evolution and photoluminescence of silicon nanocrystallites in evaporated SiOx thin films upon thermal processing

Silicon suboxide thin films have been fabricated by physical vapor deposition of silicon monoxide in vacuum at controlled oxygen partial pressure. These films undergo a phase separation into silicon- and oxygen-enriched regions upon thermal processing. At temperatures around 900 °C, the onset of Si nanocrystallite formation is observed, regardless of film stoichiometry. With increasing initial oxygen content of the films, the mean size of created nanocrystallites decreases whereas the corresponding photoluminescence emission blueshifts. The photoluminescence intensity increases with increasing annealing temperature up to 1050 °C. Upon resonant excitation at low temperatures, the photoluminescence exhibits phonon replica signature. Therefore, the emission may be attributed to excitonic recombination in the nanocrystallites. Received: 3 July 2001 / Accepted: 6 August 2001 / Published online: 17 October 2001     

Low-temperature MOVPE growth of ZnO thin films by using a buffer layer

ZnO thin films have been grown on a-plane (1,1,−2,0) sapphire substrates by metalorganic vapor phase epitaxy (MOVPE) at low substrate temperature of 350 °C. It is showed that the crystal and electrical quality of the thin films was improved by using a ZnO buffer layer. The photoluminescence (PL) measurements indicate that the ZnO thin films grown at such a low substrate temperature have a strong UV emission.     

Effects of nanostructure on optoelectronic properties of β phase polyoctylfluorene thin film

β phase polyoctylfluorene thin films were obtained by exposure to toluene vapor for various annealing times or dipping into a THF/methanol mixture. The photoluminescence and electroluminescence of PFO thin films decrease with increasing annealing time. Grazing incident X-ray diffraction of the thin films indicates that more and larger β phase crystallites will be generated in thin film exposed for longer time, which will lead to more defects that reduce photoluminescence and electroluminescence. By analyzing the mechanism of formation of β phase, we assume that the defects mainly come from the formation of out-of-plane crystalline structure. The assumption is confirmed by higher photoluminescence of β phase polyoctylfluorene thin film achieved by dipping into a THF/methanol mixture that has less out-of-plane crystalline structure.     

Photoluminescence of a soluble polypyrrole based on N-vinylpyrrole

Photoluminescence of a novel polypyrrole based on N-vinylpyrrole was systematically observed in organic solutions. The polymer, which has a unique molecular structure, exhibited good photoluminescence in organic solutions. The emission peak of the polymer exhibited one strong green emission band at around 510 nm in common organic solutions. The maximum fluorescence quantum yield of the polymer was found to be 0.16 in NMP solution with fluorescein as standard. At the same concentration, the photoluminescence intensity increased in the order of CHCl₃, THF, DMSO, CH₂Cl₂ and NMP. The photoluminescence spectrum had a slight red shift as the polarity of the solvents increased. The photoluminescence intensity also increased with the polarity of the solvent, except DMSO. This is because of its hygroscopicity in air and its viscosity. In THF solutions, the photoluminescence intensity increased until the concentration reached a certain weight percent (3.0×10⁻² wt.%) and then decreased with higher concentrations. This was most likely due to quenching in the aggregate phase. Furthermore, iron ion was a quencher in the DMSO solution. In a mixed solvent system of DMSO and water, water showed a typical quenching effect.     

ZnO/CuO 核壳结构纳米线光致发光性能 与CuO壳层厚度的关系

通过分别生长核层与壳层制备出了ZnO/CuO核壳结构的纳米线。形貌和结构分析表明,ZnO核为单晶纳米线而CuO则以多晶形式覆盖在核层表面上。光致发光(PL)研究表明,ZnO纳米线PL强度随CuO壳层厚度的变化而变化。当壳层比较薄时ZnO的PL强度增大,这主要是由于CuO壳层对ZnO核层的修饰减少了表面态,而当壳层厚度增加到一定程度时,ZnO的PL强度不再变化,这主要是由于在核壳结构中形成了type-I型结构的原因。我们对这一现象做了详细的讨论。     

The temporal evolution of the structure and luminescence properties of CdSe semiconductor quantum dots grown at low temperatures

Mono-dispersed CdSe quantum dots have been prepared by water based route using 2-mercaptoethanol at low temperatures. The structures of the CdSe nanocrystals were investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). The XRD pattern showed that the prepared CdSe has a cubic phase with zinc blende structure. The temporal evolution of the absorption and photoluminescence spectra was used to follow the reaction process and to characterize the optical properties of the prepared CdSe quantum dots. The results exhibited clear exciton peaks in the absorption spectra. The influence of the temperature and/or time of reaction on the properties of the CdSe nanocrystals were investigated. It is found that the size of CdSe nanoparticles increases, as the reaction temperature and/or time are increased. The results showed that the Stokes shift between photoluminescence emission peak and absorption peaks is increased with the increase of the reaction temperature.     

On luminescence enhancement in the narrow-gap phase of PbS-CdS heterogeneous semiconductor

A great enhancement of photoluminescence from CdS-doped PbS [1] can be attributed to the recombination flux from the wide-into narrow-gap phase due to the field of the variband transition region. A semiconductor heterostructure model accounting for the enhancement of luminescence in the PbS-CdS system is presented. The concentration profile of nonequilibrium charge carriers and the integral intensity of luminescence at the interface between the narrow-and wide-gap phase are numerically studied.     

钛酸锶钡薄膜的室温光学性能研究

采用溶胶-凝胶法制备了(Ba0.75Sr0.25)TiO3薄膜,研究了不同退火温度下样品的物相结构、薄膜的光致发光性能和光学透过率。结果表明:室温下非晶钛酸锶钡薄膜在蓝光激发下具有明显的发光现象,发光波长范围是500～650 nm,峰值在525 nm附近。延长非晶态薄膜的退火时间能够显著提高样品的发光强度,且发光强度随薄膜厚度增加而增大。晶态薄膜有微弱的发光现象。透射谱测试结果表明,钛酸锶钡薄膜在可见光范围内具有良好的光学透过率。     

Pressure induced phase transition in n-type InP at low temperature studied by photoluminescence

Abstract

Low temperature (5 K) photoluminescence of n-type InP has been studied in the 0–12 GPa range. The luminescence band energy follows the pressure dependence of the band gap, provided effective mass variations are taken into account. No evidence of a direct to indirect gap crossover has been observed up to the phase transition pressure (10.3 GPa). The phase transition is uncompleted at 12 GPa     

Analysis of optical spectra of CdS and CdSe films deposited on a sapphire substrate by laser ablation technique

CdSe and CdS films, deposited on a sapphire substrate by means of pulsed laser ablation technique, have been investigated by means of reflectivity and photoluminescence measurements in order to study the effect of such a transparent substrate on the optical properties of the deposited epilayers. The reflectivity spectra at low temperature have been studied by means of an analytical model which permits one to obtain the energies of the excitonic resonances. The photoluminescence spectra show that our CdSe and CdS films present excitonic emission at low temperature, differently from the same films deposited on quartz. The temperature dependence of the excitonic energy has been analysed by taking into account the contribution of both the thermal expansion and electron-phonon interaction. The exciton linewidth has been analysed according to well known phenomenological models. Received 21 June 2001 and Received in final form 18 November 2001     

Temperature dependence of the optical properties of ZnSe films deposited on quartz substrate

ZnSe thin films were deposited by pulsed laser ablation on quartz substrate. The films were investigated by different characterization techniques, such as X-ray diffraction, Raman microspectroscopy, absorption, reflectivity, and photoluminescence spectroscopy. The XRD analysis showed the formation of cubic phase polycrystalline films. The Raman spectra confirmed the formation of ZnSe by the presence of TO and LO peaks at 202 cm^-1 and 252 cm^-1, respectively. The analysis of absorption and reflectivity measurements permits evaluation of the band gap and excitonic energy at low temperature and the temperature dependence of the energy gap. The photoluminescence measurements indicated the possibility of obtaining intrinsic band-band radiative emission up to room temperature. PACS 52.38.Mf; 78.55.-m; 78.55.Et     

脉冲激光沉积法在多孔铝衬底上生长的ZnO薄膜的结构与光学性质

ZnO是一种性质优良很有前途的紫外光电子器件材料,多孔铝是一种良好的模板型衬底,试图将二者结合起来以制备出一种全新的光电功能材料。制备了三种不同孔径多孔铝衬底,采用脉冲激光沉积法,在真空背景下,在多孔铝衬底上生长了氧化锌薄膜。利用扫描电子显微镜、X射线衍射和光致荧光对样品进行了测试和分析。研究表明:利用不同孔径的多孔铝衬底生长的氧化锌薄膜的结构和光学性质差异很大。样品A的光致发光主要是394nm的紫外发射和498nm的蓝绿光发射;样品B的光致发光主要是417nm的紫光发射和466nm蓝光发射;样品C的光致发光主要是415nm的紫光发射和495nm的蓝绿光发射。由于薄膜是富锌的,随着在空气中氧化的进行,光谱发生变化。利用固体能带理论对光谱进行了全面的分析。     

BCS wave-function approach to the BEC-BCS crossover of exciton-polariton condensates

The crossover between low and high density regimes of exciton-polariton condensates is examined using a BCS wave-function approach. Our approach is an extension of the BEC-BCS crossover theory for excitons, but includes a cavity photon field. The approach can describe both the low density limit, where the system can be described as a Bose-Einstein condensate (BEC) of exciton-polaritons, and the high density limit, where the system enters a photon-dominated regime. In contrast to the exciton BEC-BCS crossover where the system approaches an electron-hole plasma, the polariton high density limit has strongly correlated electron-hole pairs. At intermediate densities, there is a regime with BCS-like properties, with a peak at nonzero momentum of the singlet pair function. We calculate the expected photoluminescence and give several experimental signatures of the crossover.     

Electron-hole plasma luminescence in GaP

We have studied the photoluminescence spectrum of the electron-hole gas phase in presence of electron-hole liquid in GaP. At low temperature, free excitons are clearly present. With increasing temperatures, i.e. density of the vapour phase, the exciton features evolve into a broad band shifted in energy, which we attribute to an electron-hole plasma. The results are discussed in view of an insulator-metal Mott transition.     

PbSe量子点近红外光源的CH_4气体检测

研制了一种新型的PbSe量子点近红外光源,其光致发光谱较窄,能有效匹配气体的红外吸收峰。采用配位溶剂法合成出5.1 nm的PbSe量子点,并将其沉积到Ga N芯片上(沉积厚度为165.5μm),经过紫外光照处理和固化后制成了光致发光的近红外光源。该光源第一激子吸收峰位于1 592 nm,光致发光峰位于1 665 nm,其发射光谱包含了1 625~1 840 nm之间的CH_4气体的全部的吸收谱。利用其进行CH_4气体浓度检测实验,结果表明,系统的检测下限可以达到100×10~(-6),检测误差为2%。这种由PbSe量子点近红外光源构成的新型检测系统具有低功耗、低成本和高效能等优点,将其应用在气体检测中时可以略去滤光片,其在红外气体检测领域中有着较广阔的应用前景。