掺杂纳米半导体超微粒ZnS:Mn2+光学特性研究

九十年代对纳米尺寸(nanoscale)材料的光物理性质的深入研究,导致了介于微观与宏观物理间的新的学科一介观物理(mesoscopic physics)的产生.其科学义在于建立和发展介于原子分子和固体之间所谓介观系统(mesoscopic system)的量子理论,揭示介观物质特性及其相互作用本质,并利用介观特性探索新型结构和功能材料.以往对纳米半导体超微粒材料的研究主要集中于与本征特性相关的量子尺寸效应,或缺陷对超微粒本征特性的影响[1,2].超微粒中过渡金属离子中心发光性质研究首次报道于1993年[3,4]并指出这可能成为崭新的一类发光材料.     

Dy3+单掺和Er3+,Yb3+共掺氧氟微晶玻璃的发光性能

采用高温熔融法和热处理工艺制作了含有GdF3纳米晶的氧氟微晶玻璃。在386 nm激发下,Dy3+掺杂氧氟微晶玻璃的发光强度明显增强,且蓝光对黄光的发光强度比逐渐增大,表明Dy3+已进入到GdF3纳米晶中。在980 nm激光器泵浦下,Er3+,Yb3+共掺氧氟微晶玻璃的上转换发光随着热处理温度的升高明显增强,Er3+的上转换发光出现明显的Stark分裂现象,这亦说明Er3+已进入到GdF3纳米晶相中。通过研究上转换发光强度与泵浦功率的关系,确定绿光上转换发光为双光子过程。     

ZnS:Mn2+,Sm3+粉末材料中Mn2+和Sm3+之间的能量传递

研究了ZnS粉末材料中Mn2+中心和Sm3+中心之间的相互作用.通过测量单独由Mn2+或Sm3+掺杂及Mn2+,Sm3+同时掺杂的ZnS粉末材料的发射光谱、激发光谱、发光衰减以及选择激发发光光谱,证实了Mn2+和Sm3+之间存在偶极子-偶极子相互作用的无辐射能量传递.同时还计算了能量传递几率和传递效率.     

水热法合成ZnS∶Au,Cu超细X射线发光粉

报道了水热法合成的高强度ZnS∶Au,Cu超细X射线发光粉及其光致发光(PL)和X射线激发发光(XEL)的光谱特性。200℃水热处理12h直接合成样品的纳米晶粒约15nm,尺寸分布窄,分散性好,具有纯立方相的类球形结构。氩气保护下1000℃焙烧1h后的样品存在一定的团聚,但团聚后尺寸为1～2μm,为超细X射线发光粉,此时样品为纯六角相的类球形为主的结构。所有样品的PL和XEL光谱均为宽带谱,水热法直接合成样品的XEL强度最强时,样品的Cu/Zn,Au/Cu比值分别为3×10-5和2。在此比值条件下,1000℃焙烧1h样品的XEL发光最强,此时其2个峰值分别位于445和513nm,且与未焙烧前相比强度增强了10倍左右。另外通过比较PL光谱与XEL光谱特性,讨论了PL和XEL光谱的发光机理和其不同的激发机制。     

Energy transfer from organic surface adsorbate-polyvinyl pyrrolidone molecules to luminescent centers in ZnS nanocrystals

Multi-colour emitting doped ZnS nanocrystals surface capped with pyridine (P-ZnS) or polyvinyl pyrrolidone (PVP-ZnS) have been synthesized by wet chemical methods. The photoluminescence studies show that the dopant related emission from P-ZnS nanocrystals are caused by the energy transfer from band-to-band excitation of the host lattice. However, in the case of PVP capped ZnS, considerable enhancement in the emission intensity was observed and the corresponding excitation spectra appeared dramatically broadened due to the presence of multiple excitation bands with peak maxima at 235, 253, 260, 275, and 310 nm. The bands from 235 to 275 nm are assigned to the electronic transitions in the chemisorbed PVP molecules whereas the excitation maximum around 310 nm corresponds to the band-to-band transition within the nanocrystalline ZnS host. The presence of PVP related energy bands in the excitation spectrum indicates the process of energy transfer from the surface adsorbed PVP molecules to dopant centers in ZnS nanocrystals. This study brings out a heterogeneous sensitizer-activator relation between organic surface adsorbate and doped semiconducting nanocrystals.     

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.     

ZnS薄膜参数对有机/无机复合发光器件特性的影响

半导体量子点(QDs)具有发光效率高和发光波长可调等特点。采用胶体CdSe QDs作电致发光器件的有源材料,TPD(N,N′-biphenyl-N,N′-bis-(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine)作空穴传输层,ZnS作电子传输层,研究了有机/无机复合发光器件ITO/TPD/CdSe QDs/ZnS/Ag的电致发光特性。TPD和CdSe QDs薄膜采用旋涂方法、ZnS薄膜采用磁控溅射方法沉积,器件表面平整。CdSe QDs的光致发光和电致发光谱峰位波长均位于～580 nm,属于量子点的带边激子发光。我们与以前的ITO/ZnS/CdSe QDs/ZnS/Ag发光器件结构进行了对比,发现新的器件结构的电致发光谱没有观察到QDs表面态的发光,而且新器件的发光强度是ITO/ZnS/CdSe QDs/ZnS/Ag结构的～10倍。发光效率的提高归因于碰撞激发与载流子注入两种发光机制并存的结果：一方面电子经过ZnS 层加速后,碰撞激发CdSe QDs发光;另一方面,空穴从TPD层注入CdSe QDs 与QDs中激发的电子复合发光。我们进一步研究了ZnS电子加速层厚度对发光特性的影响,选择ZnS薄膜的厚度分别是80,120 和160 nm,发现随着ZnS层厚度增大,器件启亮电压升高,EL强度增大,但是击穿电压降低。EL峰位随着ZnS厚度的减小发生明显蓝移,对上述实验现象进行了机理解释。     

ZnS:Mn2+,Sm3+中Mn2+、Sm3+中心之间能量传递的分时光谱研究

前一阶段我们比较系统地研究了ZnS:Mn2+,Sm3+材料中Mn2+中心和Sm3+中心之间的能量传递。通过测量ZnS:Mn2+、ZnS:Sm3+和ZnS:Mn2+,Sm3+三种材料的发射光谱、激发光谱、选择激发发光光谱,证实了Mn2+中心和Sm3+中心之间存在偶极—偶极相互作用的无辐射能量传递。为了进一步研究Mn2+中心和Sm3+中心之间的相互作用及其物理特点,我们又仔细测量了上述三种不同类型材料的分时光谱,这不仅可以更清楚地了解激发停止后Mn2+中心和Sm3+中心之间的相互作用,而且有效地解决了Mn2+中心发射光谱和Sm3+中心某些特征光谱线交叠引起的测量发光衰减的困难。     

水热法制备ZnS:Cu,Tm超细X射线发光粉

研究了水热法合成的ZnS: Cu,Tm超细X射线发光粉及其光致发光(PL)和X射线激发发光(X-ray excited luminescence,XEL)光谱特性.200 ℃水热处理12 h直接合成样品的纳米晶粒径约15 nm,尺寸分布窄,分散性好,具有纯立方相的类球形结构.氩气保护下900 ℃退火1 h后的样品存在一定的团聚,但团聚后尺寸为200—600 nm,为超细X射线发光粉,此时样品为纯六方相的类球形为主的结构.所有样品的PL和XEL光谱均为宽带谱.水热法直接合成样品的XEL强度最强时,样品的Cu/Zn,Tm/Cu比值分别为3×10^-4和2.在此比值条件下,900 ℃退火1 h样品的XEL发光最强,此时其两个峰值分别位于453,525 nm.发光强度增强的同时粒径很小,对提高成像系统分辨率非常有意义.通过比较PL光谱与XEL光谱特性,讨论了PL和XEL光谱的发光机理和其不同的激发机理. 关键词： ZnS:Cu Tm 水热法 X射线激发发光     

Absorption and luminescence anisotropy of optical centers in thulium glasses for optical amplifiers

A partial polarization of luminescence (1.47 μm) of thulium-activated glasses excited by linearly polarized laser light (0.79 μm) is detected. The degree of polarization depends both on the spectral range of excitation and on the spectral range of detection, reaching a maximum value of ～0.1. An increase in the thulium concentration leads to a decrease in the degree of polarization due to an increasing rate of energy migration between thulium ions.     

Hydrothermal synthesis and optical properties of full-color-emission ZnS:Cu, Al nanocrystals

ZnS:Cu, Al nanocrystals were synthesized by a hydrothermal method at 200 degrees C and their optical properties were studied. The analysis of XRD and TEM show that the spherical-like nanocrystals had a grain size of approximately 15 nm and were well dispersed, with a zinc blende structure. The energy dispersive X-ray spectroscopy (EDX) and atomic absorption spectrometry were applied to the analysis of S, Zn and Cu content in the sample. The results proved that a large number of zinc vacancies exist and Cu is incorporated into the sample lattice. The photoluminescence (PL) spectra were investigated. The PL mechanism is discussed. The excitation spectrum is broad. Under 337 nm excitation the sample emits bright green light. Under 370-410 nm excitation the sample emits white light. The broad emission spectra are almost coincident with any excitation wavelength of between 370 and 410 nm making them attractive as conversion phosphors for LED applications and full-color fluorescence display devices. The emitted white light under 375 nm excitation was found to be the result of blue, green, and orange emission bands. For Cu/Zn, Cu/Al and S/Zn molar ratios of 3 x 10(-4), 2 and 3, respectively, the near blue white light can be observed with the naked eye in daylight.     

Synthesis and photoluminescence characteristics of doped ZnS nanoparticles

Free-standing powders of doped ZnS nanoparticles have been synthesized by using a chemical co-precipitation of Zn²⁺, Mn²⁺, Cu²⁺ and Cd²⁺ with sulfur ions in aqueous solution. X-ray diffraction analysis shows that the diameter of the particles is ∼2–3 nm. The unique luminescence properties, such as the strength (its intensity is about 12 times that of ZnS nanoparticles) and stability of the visible-light emission, were observed from ZnS nanoparticles co-doped with Cu²⁺ and Mn²⁺. The nanoparticles could be doped with copper and manganese during the synthesis without altering the X-ray diffraction pattern. However, doping shifts the luminescence to 520–540 nm in the case of co-doping with Cu²⁺ and Mn²⁺. Doping also results in a blue shift on the excitation wavelength. In Cd²⁺-doped ZnS nanometer-scale particles, the fluorescence spectra show a red shift in the emission wavelength (ranging from 450 nm to 620 nm). Also a relatively broad emission (ranging from blue to yellow) has been observed. The results strongly suggest that doped ZnS nanocrystals, especially two kinds of transition metal-activated ZnS nanoparticles, form a new class of luminescent materials. Received: 16 October 2000 / Accepted: 17 October 2000 / Published online: 23 May 2001     

Structure and photoluminescence properties of ZnS films grown on porous Si substrates

ZnS films were deposited on porous silicon (PS) substrates with different porosities. With the increase of PS substrate porosity, the XRD diffraction peak intensity decreases and the surface morphology of the ZnS films becomes rougher. Voids appear in the films, due to the increased roughness of PS structure. The photoluminescence (PL) spectra of the samples before and after deposition of ZnS were measured to study the effect of substrate porosity on the luminescence properties of ZnS/PS composites. As-prepared PS substrates emit strong red light. The red PL peak of PS after deposition of ZnS shows an obvious blueshift. As PS substrate porosity increases, the trend of blueshift increases. A green emission at about 550 nm was also observed when the porosity of PS increased, which is ascribed to the defect-center luminescence of ZnS. The effect of annealing time on the structural and luminescence properties of ZnS/PS composites were also studied. With the increase of annealing time, the XRD diffraction peak intensity and the self-activated luminescence intensity of ZnS increase, and, the surface morphology of the ZnS films becomes smooth and compact. However, the red emission intensity of PS decreases, which was associated with a redshift. White light emission was obtained by combining the luminescence of ZnS with the luminescence of PS.     

Investigation on trap distribution and photoelectronic effect due to UV,IR and visible light excitation in self-activated zns crystals

Photoelectronic effects in IR sensitive ZnS crystals are found for IR, UV and visible light excitations. The transient rise characteristic of UV excited photoluminescence (UPL) saturates faster than UV produced photoconductivity (UPC). The UPC shows a typical S-shaped rise curve for any 365 run excitation irradiance and temperature. Simultaneously measured transient behaviors of IR induced photoconductivity (INP) and IR stimulated luminescence (STL) have a strong IR excitation intensity dependence for λ_IR = 2.5?4.6 microm. A unique phenomenon, quick rise followed by quick decay during the initial 20 msec, is found in INP before reaching final maximum but is not observed in STL. During UV steady irradiation, additional photoconductivity and luminescence are quickly induced by an abrupt IR excitation of 2.5 microm. Then, the photoconductivity reaches a new steady state level below UPC steady state. However, the luminescence sets the same UPL steady state level. This means that photoconductivity exhibits an IR optical quenching but not stimulation, while neither optical quenching nor stimulation is found in luminescence. It is also possible to quench the UPC response with visible light excitation at 570 nm. These observations support the previously reported discussion that IR absorbing impurity centers and shallow traps, including recombination (or luminescence) centers must be involved in IR stimulable ZnS crystals. They also indicate the presence of deep trap centers for E_T ≈ 2.18 eV, which have a strong role in slow UPC rise.     

Luminescence enhancement of Mn doped ZnS nanocrystals passivated with zinc hydroxide

Mn-doped ZnS nanocrystals prepared by solvothermal method have been successfully coated with different thicknesses of Zn(OH)₂ shells through precipitation reaction. The impact of Zn(OH)₂ shells on luminescent properties of the ZnS:Mn nanocrystals was investigated. X-ray diffraction (XRD) measurements showed that the ZnS:Mn nanocrystals have cubic zinc blende structure. The morphology of nanocrystals is spherical shape measured by transmission electron microscopy (TEM). ZnS:Mn/Zn(OH)₂ core/shell nanocrystals exhibited much improved luminescent properties than those of unpassivated ZnS:Mn nanocrystals. The luminescence enhancement was observed with the Zn(OH)₂ shell thickening by photoluminescence (PL) spectra at room temperature and the luminescence lifetime of transition from ⁴T₁ to ⁶A₁ of Mn²⁺ ions was also prolonged. This result was led by the effective, robust passivation of ZnS surface states by the Zn(OH)₂ shells, which consequently suppressed nonradiative recombination transitions.     

平面偏振光激励的多孔硅的光致发光

研究了以掠入射的平面偏振光激励的多孔硅的光致发光。实验结果显示，光的入射角对多孔硅的发光行为影响不大，然而，以ｚ方向偏振光激励的发光强度明显高于以ｘ方向偏振光激励的发光强度。激励光电场相对于样品表面的不同取向引起光致发光的差异，这反映多孔硅的光学性质是各向异性的，也排除了纯粹的硅量子点的集合作为多孔结构的可能性。     

Er离子注入的富硅SiO2MOS-LED的可见和红外电致发光特性

通过Er离子和Si离子注入并结合高温退火制备了Er掺杂的富硅SiO2薄膜以及ITO/SiON/富硅SiO2:Er/Si MOS结构电致发光器件.研究了富Si浓度的变化对Er3+离子掺杂的电致发光器件的发光性能和传导特性的影响.发现不同Si含量对Er3+离子的不同能级的电致发光会产生不同作用.在富Si量小于5％的条件下,...     

Dependence of photoluminescence of CdSe/ZnS on excitation wavelength

We have found that the photoluminescence (PL) intensity of CdSe/ZnS nanocrystals placed on a thin film of insulator (GaAsOx/GaAs) depends on excitation wavelength through the interference effects of the excitation light. By employing the multi-reflection/interference calculation, the insulator thickness of the underlying non-uniform patterns can be evaluated by the simple observation of CdSe/ZnS PL with a couple of excitation wavelengths. Moreover, the differences observed for the temporal evolution of CdSe/ZnS PL (blue shifts and degradation) among the excitation wavelengths suggest that the photo-induced changes of chemical composition and surface ligands are responsible for blue shifts and degradation, respectively.     

Controllable and switchable chiral near-fields in symmetric graphene metasurfaces

A strong chiral near-field plays significant roles in the detection, separation and sensing of chiral molecules. In this paper, a simple and symmetric metasurface is proposed to generate strong chiral near-fields with both circularly polarized light and linearly polarized light illuminations in the mid-infrared region. Owing to the near-field interaction between plasmonic resonant modes of two nanosheets excited by circularly polarized light, there is a strong single-handed chiral near-field in the gap between the two graphene nanosheets and the maximum enhancement of the optical chirality could reach two orders of magnitude. As expected, the intensity and the response wavelength of the chiral near-fields could be controlled by the Fermi level and geometrical parameters of the graphene nanosheets, as well as the permittivity of the substrate. Meanwhile, based on the interaction between the incident field and scattered field, the one-handed chiral near-field in the gap also could be generated by the linearly polarized light excitation. For the two cases, the handedness of the chiral near-field could be switched by the polarized direction of the incident light. These results have potential opportunities for applications in molecular detection and sensing.     

Photoluminescence polarization of semiconductor nanocrystals

We review the polarization properties of photoluminescence (PL) in nanocrystals (NCs) from both theoretical and experimental points of view. We show that, under linearly polarized excitation, NCs emit partly polarized light owing to their uniaxial structure or their anisotropic shape. In elongated NCs, the anisotropy may have two origins, the electronic confinement or the effect of depolarizing field created by the light-induced charges on the interfaces. Results of polarization studies in porous silicon are presented. They are explained by the shape of the Si NCs. Experiments in CdSe NCs reveal the fine structure of the excitonic levels and show evidence of the enhancement of the electron-hole exchange energy with decreasing NC size. Spin orientation in wurtzite-type NCs is achieved by optical pumping with circularly polarized light. The effect of a magnetic field on the degree of circular polarization and the mechanisms of spin relaxation are discussed. Results in large-size NCs are presented.