Time-resolved photoluminescence study of the red emission in nanoporous SiGe alloys

Porous Si_1−xGe_x (PSiGe) layers with efficient room temperature visible photoluminescence (PL) were elaborated by anodical etching from p-type doped epitaxial layers with Ge contents from 5 to 30%. The luminescence is characterised by a broad PL band centred at 1.8 eV. Time resolved photoluminescence decay is studied in porous silicon germanium as a function of germanium content, temperature, emission energies and surface passivation. The PL decay line shape is well described by a stretched exponential in all cases. The effective lifetime at low temperature in as prepared porous Si_1−xGe_x is 400 μs, i.e. an order of magnitude less than in porous silicon. After the formation of a 20 Å thick oxide surface layer we observe a decrease of the effective lifetime to 20 μs at T=4 K.     

Time-resolved stimulated emission spectroscopy in the ultrashort domain through pump-probe experiments

The photoemission properties of fluorescent chromophores have a widespread application in many fields ranging from chemical-physics and biology to organic light emitting devices. These systems usually display high fluorescence conversion efficiency, which makes them suitable for transient/gain experiments also in liquid solutions, thin films and eventually in protein environments.Pump and probe methods have been widely employed for wavelength-resolved spectroscopy in the subpicosecond time scale. In our group, we have recently assembled a new experimental setup for pump and probe spectroscopy: preliminary tests on the Rhodamine B dye in ethanol have been performed in order to optimize the setup. The dynamic response of photoinduced changes of the chromophore dispersed into a suitable solvent has been studied with a subpicosecond time resolution.The optically prepared initial state of the Rhodamine B in ethanol solution appears to evolve on a timescale of few picoseconds into a successive state, which could be attributed to an intramolecular charge transfer state.     

Field emission and photoluminescence of ZnO nanocombs

Three kinds of new comb-shape nanostructures of ZnO have been grown on single silicon substrates without catalyst-assisted thermal evaporation of Zn and active carbon powders. The morphology and structure of the prepared nanorods are determined on the basis of field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and x-ray diffraction (XRD). The growth mechanism of the ZnO nanocombs can be explained on the basis of the vapor–solid (VS) processes. In nanocombs 1 and nanocombs 2, the comb teeth grow along [0001] and the comb stem grows along [ $01\overline{1}0$ ], while in nanocombs 3, nanoteeth grow along [ $01\overline{1}0$ ] and stem grows along [0001]. The photoluminescence and field-emission properties of ZnO nanocombs 1–3 have been investigated. The turn-on electric field of ZnO nanocombs 1–3, which is defined as the field required to producing a current density of 10 μA/cm², is 9, 7.7 and 7.1 V/μm, respectively. The field-emission performance relies not only on the tip’s radius of curvature and field enhancement factor, but also on the factor evaluating the degree of the screening effect.     

Structural and photoluminescence properties of ZnO nanoparticles on silicon oxide

Isolated, self assembled ZnO nanoparticles are grown in two steps: by the electron beam evaporation of Zn on oxidised silicon wafers, during which isolated Zn nanodots are grown, and a subsequent annealing in oxygen that results in the desired ZnO nanodots. Low temperature PL measurements of the ZnO nanodots show that the near band edge part of the spectra is dominated by a zero phonon line near 3.36 eV which is an overlap of two emitting lines near 3.363 eV and 3.367 eV. Characterization by TEM and EELS shows that the nanoparticles are zinc oxide single crystals grown with their c-axis perpendicular to the substrate; their distribution, size and crystallinity depend on the deposition parameters of zinc and the growth substrate. We discuss the effect of these parameters on the morphology of the resulting material. Our approach demonstrates a simple method for the growth of high purity isolated ZnO nanodots of similar sizes, distributed uniformly on a large surface. PACS 61.46.Df; 81.05.Dz; 81.07.-b     

The I.R. photoluminescence emission band in ZnO

Measurements of emission spectra, excitation spectra, intensity dependence of the luminescence, decay of the luminescence, and temperature dependence of the luminescence in ZnO are reported. The results for the emission at 1·70 eV, with the exception of the decay of the luminescence, were found to be similar to those of the yellow (2·02 eV) emission band in ZnO. Both bands could be excited at the band edge and directly, the intensity of both bands was found to be linear with excitation strength and the asymptotic regions of the temperature dependence of both bands could be approximated by exponential functions. It is proposed that the luminescent transition is an electron transition from the edge of the conduction band to a hole trapped in the bulk at 1·60 eV above the edge of the valence band, and that the luminescence center is an unassociated acceptor-like center.     

Stable aqueous ZnO nanoparticles with green photoluminescence and biocompatibility

Semiconductor quantum dots (QDs) as a kind of biological labeling material have many unique fluorescence properties relative to conventional organic dyes, which can be used for long-term fluorescence tracking. In this work, a facile method was developed for synthesizing water-stable ZnO nanoparticles with green emission. The as-synthesized ZnO nanoparticles were shown to be highly stable and soluble in water. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) as well as UV–vis and fluorescence spectrophotometry (PL) were employed to investigate the structures and properties of ZnO nanoparticles. Furthermore, hemolysis assay was performed to evaluate the biocompatibility of these ZnO nanoparticles in vitro. The results indicate that the as-prepared ZnO nanoparticles have biocompatibility, which make them a promising cell label.     

Zno纳米晶的室温紫外受激发射特征

ZnO的激子特性对制备氧化锌基的光电子器件至关霞要,因此对ZnO量子点中激子的发光性质及其跃迁过程进行研究显得十分必要.采用溶胶-凝胶法制备了ZnO纳米晶,X射线衍射(XRD)结果表明样品具有六角纤锌矿多晶结构.研究了在不同泵浦功率激发下ZnO纳米晶的紫外发射的时间积分光谱和时间分辨光谱,观察到自南激子发光,激子-激子碰撞和电子-空穴等离子体引起的受激发射,研究了在不同泵浦功率激发下自由激子及激子-激子碰撞随泵浦功率依赖的动力学过程.研究结果对理解激子带边发射有一定帮助,对ZnO材料在短波长半导体光电器件方面有潜在的应用价值.     

Optical processes in the formation of stimulated emission from ZnO nanowires

This paper studies power dependent photoluminescence spectra, the stimulated emission occurring at ultraviolet (UV) band instead of the green emission band of ZnO nanowires, which are prepared with a chemical reduction method. The dynamics of the UV emission and green emission is given to demonstrate the reason of stimulated emission occurring at UV band but not the green emission band under high excitation, which indicates that the slow decay rate of trap state makes it easy to be fully filled and saturated, while the fast decay rate of near-band-edge exciton state makes the UV emission dominate the radiative recombination under high excitation. The UV emission, as well as the corresponding stimulated emission, occurs in competition with the green deep-trap emission. In addition, when pump fluence further increases, the multiple lasing modes appear. The dependence of these lasing modes on the pump fluence is first discussed. This diagram should be helpful to understand and design the optical nanodevices of ZnO nanowires.     

Structure and photoluminescence of VAC-functionalized ZnO nanoparticles by plasma polymerization

A plasma polymerization method was used to modify the surfaces of ZnO nanoparticles, and the effects of plasma surface modification on photoluminescence (PL) property of ZnO nanoparticles were studied. High-resolution transmission electron microscopy images revealed that a thin film of vinyl acetate (VAC) polymer layer (∼4 nm) was uniformly deposited on the surfaces of the ZnO nanoparticles. The chemical structure of the polymer layer was identified by Fourier transform infrared (FTIR) experiments. The photoluminescence (PL) intensity of the ZnO nanoparticles was found to be significantly decreased by the deposited plasma films. For the particle of smaller size, the ultrathin film indicated better ultraviolet (UV) shielding ability.     

Yellowish-white photoluminescence from ZnO nanoparticles doped with Al and Li

ZnO nanoparticles codoped with Al and Li were chemically synthesized with a low temperature drying process. They are crystalline and can be made as small as 5 nm. Intense yellowish white photoluminescence was observed from smaller ZnO nanoparticles with a higher concentration of Al and Li. The photoluminescence peak consists of yellow and green emission bands. Both peak intensities increase with increasing the Al and Li concentrations and with decreasing the size of ZnO nanoparticles. The green and yellow emission bands were attributed to donor–acceptor-pair recombination involving Zn vacancies and lithium as the acceptor state, respectively, and the donor responsible for both emissions to oxygen vacancies. Both enhanced emissions by codoping may be explained by an increase in the number of electrons occupying the deep donor level on account of doping with Al. Although the yellowish white emission decays with time, passivation of the crystallite surface with poly(p-phenylene vinylene) suppresses the degradation. The observed high-intensity and stable yellowish white emission makes PPV-passivated ZnO nanoparticles, codoped with Al and Li, more attractive as a candidate for “white” phosphor.     

Multi-photon-pumped stimulated emission from ZnO nanowires: A time-resolved study

In this work, we study temporal evolution of multi-photon-pumped stimulated emission from ZnO nanowires. In addition to second harmonic generation, ultraviolet stimulated emission is observed in ZnO nanowires under femtosecond pulse excitation at 800 nm. Sharp emission peaks appear when excitation flux reaches a threshold of 80 mJ/cm², which can be interpreted as lasing action in self-formed nanowire microcavities. Temporal evolution of the emission captured by Kerr shutter technique shows strong excitation-power dependence. The dynamic trace of stimulated emission exhibits a fast decay with a lifetime about 4.5 ps at intermediate excitation (∼100 mJ/cm²) and a lifetime about 2 ps at high excitation (>160 mJ/cm²). The difference in the lifetime can be attributed to different gain mechanisms related to excitonic interaction and electron-hole plasma, respectively.     

Probing the excitonic emission of ZnO nanoparticles using UV-VUV excitations

This study deals with the influence of the excitation (UV-lamp, UV-laser and VUV synchrotron radiation) on the 3.31 eV band of ZnO microcrystals and of variously treated nanoparticles. The nanoparticles are synthesized in ultra high vacuum condition and their stoichiometry and crystallinity can be controlled. This provides an efficient way to probe the influence of these factors on the excitonic emission. The energy and intensity of the excitation have a strong influence on the excitonic luminescence and particularly on the 3.31 eV emission band. The result of these experiments are used to probe the origins of this band which is found to be not linked to any surface phenomena. Indeed, the only way to fully explain our results is to consider that the 3.31 eV band involve the superposition of two emissions features: the first due to acceptor defects and the other originates form the LO phononic repliqua of the free exciton.     

White-light emission of ZnO nanoparticles prepared by sol-gel method

In:ZnO nanoparticles are prepared by the sol-gel process. The ratios of In/(Zn+In) are 0%, 5%, 8%, 10%, and 15%, respectively. Crystal phase structures and optoelectronic properties of these samples are characterized and the chromaticity coordinates of different samples are also calculated in CIE-XYZ colour system. The results show that preferred growth direction of ZnO changes from (002) plane to (001) plane and interplanar distance becomes shorter. When the doping amount of In is 5%, Zn atoms are completely replaced by In atoms. The resistivities of the samples first decrease, then increase afterwards with the increase of the amount of In. With the increase of In, the ultraviolet emission is redshifted and new peaks occur at 465 nm, 535 nm, and 630 nm. The sample with 10% indium has white-light emission. The band structures of samples with 0% and 12.5% indium are investigated by the first principle method. The mechanism of white emission is discussed from the viewpoint of additional energy levels.     

ZnS中Te等电子中心的时间分辨光谱研究

用时间分辨光谱研究了很大的Te组分范围内的ZnS_1-xTe_x（x=00 05—085）合金的发光动力学特性，结果表明：不同形态的Te等电子中心具有不同的辐射复 合寿命，从几个ns到几十个ns的范围内变化，当x＝015左右时，寿命达到最大值（约 40ns）.其物理机理源于不同的Te等电子中心具有不同的局域化特性.当Te组分较小时，等 电子中心从Te₁逐渐演变到Te₂，Te₃或Te₄ 时，相应发光寿命增加，表现出不断增强 的激子发光局域化特性；而当Te组分较大时，Te原子团变得较大，其局域势与基体原子势的 相互作用增强，等电子中心的局域化特性减弱，而基体价带扩展态特征变得明显起来，相应 发光寿命逐渐减小.还研究了激子束缚能随Te组分的变化以及发光强度随温度的变化关系， 所得结果进一步支持了时间分辨光谱研究所得到的结论. 关键词： ZnS 等电子中心 时间分辨光谱 局域态     

Deep level emission of ZnO nanoparticles deposited inside UV opal

The temperature-dependent photoluminescence spectra of zinc oxide (ZnO) nanocrystals deposited inside the ultraviolet (UV) opal were studied. ZnO was grown in the voids between FCC packed SiO₂ spheres using spray pyrolysis under ultrasonic vibration in the solution containing a zinc nitrate precursor. The ZnO nanoparticles inside opal matrix with UV photonic band-gap exhibit suppression of the excitonic emission and enhancement of the deep level emission. Suppression of the excitonic lines is due to the inhibition of spontaneous emission, while enhancement and broadening of the DL emission in the green spectral region is due to Purcell effect. The infiltration of ZnO nanoparticles inside the photonic crystal may be a useful technique to increase its emission efficiency in the selected spectral region.     

Temperature and concentration dependences of the photoluminescence of MEH-PPV polymer composite films with ZnO nanoparticles

The absorption and photoluminescence (PL) spectra of MEH-PPV: ZnO composite films have been investigated at different concentrations of ZnO nanoparticles and at different temperatures (in the case of PL). It has been shown that, at 297 K, with increasing concentration of ZnO nanoparticles in the composite, the intensity of the PL lines of MEH-PPV decreases, whereas the intensity of the PL lines of ZnO increases. At a relatively low concentration of ZnO nanoparticles, a decrease in the temperature leads to an increase in the intensity of PL lines associated with MEH-PPV and ZnO, whereas at higher concentrations of ZnO nanoparticles, the intensity of these lines decreases. This is accompanied by a slight shift in the maximum of the PL toward the infrared (IR) region and a narrowing of the PL line of MEH-PPV with a decrease in the temperature and with an increase in the ZnO concentration. The mechanism of energy transfer in composite systems consisting of a polymer and inorganic nanoparticles that can be responsible for the observed effects has been discussed.     

Time-resolved 2-photon photoionization on metallic nanoparticles

Received: 16 October 1998 / Revised version: 11 December 1998     

ZnO粉末中无序激射现象时间分辨的研究

采用抽运-探测时间分辨方法实验研究了半导体材料ZnO纳米颗粒粉末中的无序激射现象.在2 67nm激光的抽运下，通过精确控制抽运光的能量和样品表面的抽运面积，获得了宽度小于1n m的单模无序激射光谱和多峰的多模无序激射光谱.时间分辨的抽运-探测结果显示，此时样 品的上能级寿命仅为几个皮秒，证明了ZnO粉末的单模无序光谱是受激辐射的结果. 关键词： 高散射介质 多重散射 抽运-探测 无序激射