巯基丙酸分子对CdSe核壳量子点和 ZnO纳米粒子薄膜之间电荷转移的影响

通过稳态光谱和时间分辨荧光光谱研究了巯基丙酸(MPA)分子对由量子点到ZnO纳米粒子薄膜的电荷转移过程的影响。研究发现,相对于CdSe纳米粒子薄膜样品,没有MPA分子参与作用的CdSe/ZnO薄膜样品和有MPA分子连接的CdSe/MPA/ZnO薄膜样品中都存在从CdSe量子点到ZnO纳米粒子薄膜的有效电荷分离过程,但是相对于CdSe/ZnO样品, CdSe/MPA/ZnO样品中电荷转移速率明显变小。这表明MPA分子本身它并不能促进CdSe到ZnO电荷分离过程,因此可以认为用金属氧化物薄膜直接吸附量子点吸收材料,将能获得高功率转换效率的量子点敏化太阳能电池。     

Optical properties of two-electron quantum dots in low lying para- and ortho-states

Two low lying energy levels of 3D two-electron quantum dot with rigid confinement (the wave functions vanish at the surface of the quantum dot) are obtained by the variational and perturbation methods. There are two kind of quantum dots: para- and ortho-dots with antiparallel and parallel electron spins, respectively. An ensemble of the two-electron quantum dots contains para-dots in the ground state and ortho-dots in the lowest metastable state at low enough temperatures. The optical parameters of GaAs two-electron quantum dot are calculated with the help of obtained energy levels and compared with the optical parameters known for the one electron GaAs quantum dot. The Coulomb interaction between electrons is responsible for the blue shift of maxima of the absorption coefficient and refractive index of two-electron quantum dots.     

Growth and time-resolved photoluminescence study of self-organized CdSe quantum dots in ZnSe

Employing two different growth methods: standard molecular beam epitaxy (MBE) and low-temperature atomic layer epitaxy (ALE) with subsequent annealing, we have obtained high-quality quantum dot structures consisting of CdSe embedded in ZnSe. Single dot emission lines are observed in micro-luminescence. The samples have been investigated by further optical methods including time-resolved photoluminescence under resonant excitation at 4.2 K. Distinct properties of systems with three-dimensional confinement are observed such as the suppression of the interaction between isolated quantum dots (QDs). In standard quantum wells tunneling/hopping processes generally lead to a pronounced red shift of the luminescence over time due to a lateral localization of excitons in potential fluctuations. A much less pronounced red shift is observed for the QDs reflecting only the different lifetimes of single dots and higher excited states. The red shift completely vanishes under resonant excitation that selectively excites only a few QDs of the ensemble in the layer. Typical behaviour is also observed from the halfwidth of the quantum dot emission.     

Temperature dependent time-resolved exciton luminescence in GaAs/AlGaAs quantum wires and dots

Transient photoluminescence of GaAs/AlGaAs quantum wires and quantum dots formed by strain confinement is studied as a function of temperature. At low temperature, luminescent decay times of the wires and dots correspond to the radiative decay times of localized excitons. The radiative decay time can be either longer or shorter than that of the host quantum well, depending on the size of the wires and dots. For small wires and dots (∼ 100 nm stressor), the exciton radiative recombination rate increases due to lateral confinement. Exciton localization due to the fluctuation of quantum well thickness plays an important role in the temperature dependence of luminescent decay time and exciton transfer in quantum wire and dot structures up to at least ∼ 80 K. Lateral exciton transfer in quantum wire and dot structures formed by laterally patterning quantum wells strongly affects the dynamics of wire and dot luminescence. The relaxation time of hot excitons increases with the depth of strain confinement, but we find no convincing evidence that it is significantly slower in quasi 1-D or 0-D systems than in quantum wells.     

Photoluminescence study of ZnCdSe/ZnSe quantum dot systems

The optical properties of the populated ZnCdSe/ZnSe quantum dots have been studied by photoluminescence spectra measured with different laser excitation apertures at temperatures from 22 to 300 K. The differences of spectral features between small and large excitation spot suggest the existence of quantum dot size fluctuation in the system. The temperature evolution of photoluminescence spectral features revealed that two types of quantum dots with different densities and sizes coexist in ZnCdSe/ZnSe system. The energy spacings of the two kinds of quantum dot emissions are about 50 meV at various temperatures. The thermally activated lateral transfer processes of carriers populated in the two sorts of quantum dots are investigated by temperature dependences of spectral intensities.     

The strain relaxation of InAs/GaAs self-organized quantum dot

This paper presents a detailed analysis of the dependence of degree of strain relaxation of the self-organized InAs/GaAs quantum dot on the geometrical parameters. Differently shaped quantum dots arranged with different transverse periods are simulated in this analysis. It investigates the total residual strain energy that stored in the quantum dot and the substrate for all kinds of quantum dots with the same volume, as well as the dependence on both the aspect ratio and transverse period. The calculated results show that when the transverse period is larger than two times the base of the quantum dots, the influence of transverse periods can be ignored. The larger aspect ratio will lead more efficient strain relaxation. The larger angle between the faces and the substrate will lead more efficient strain relaxation. The obtained results can help to understand the shape transition mechanism during the epitaxial growth from the viewpoint of energy, because the strain relaxation is the main driving force of the quantum dot's self-organization.     

分子束外延生长ZnSe自组织量子点光、电行为研究

分别应用光致发光、电容电压和深能级瞬态傅里叶谱技术详细研究ZnSe自组织量子点样品的光学和电学行为.光致发光温度关系表明ZnSe量子点的光致发光热猝火过程机理.两步猝火过程的理论较好模拟和解释了相关的实验数据.电容电压测量表明样品表观载流子积累峰出现的深度(样品表面下约100nm处)大约是ZnSe量子点层的位置.深能级瞬态傅里叶谱获得的ZnSe量子点电子基态能级位置为ZnSe导带下的011eV,这与ZnSe量子点光致发光热猝火模型得到的结果一致.     

Optical study of single InAs on In0.12Ga0.88As self-assembled quantum dots: biexciton binding energy dependence on the dots size

Single self-assembled InAs quantum dots embedded in a In_0.12Ga_0.88As quantum well and emitting in the near infrared have been optically investigated. The dependence on the excitation power of the single quantum dot photoluminescence has been used to identify the emission of the biexciton complex. The biexciton binding energy, which has been measured for a dozen dots, increases with increasing exciton transition energy for the dot sizes investigated in the present work, as a consequence of stronger confinement in a smaller quantum dot. The obtained data is compared with experimental results available in the literature for InAs quantum dots. PACS 78.67.Hc; 73.21.La; 78.55.Cr     

PHOTOLUMINESCENCE STUDY OF InAs/GaAs SELF-ORGANIZED QUANTUM DOTS WITH BIMODAL SIZE DISTRIBUTION

We have investigated the temperature dependence of the photoluminescence (PL) spectrum of self-organized InAs/GaAs quantum dots. A distinctive double-peak feature of the PL spectra from quantum dots has been observed, and a bimodal distribution of dot sizes has also been confirmed by scanning tunneling microscopy image for uncapped sample. The power-dependent PL study demonstrates that the distinctive PL emission peaks are associated with the ground-state emission of islands in different size branches. The temperature-dependent PL study shows that the PL quenching temperature for different dot families is different. Due to lacking of the couple between quantum dots, an unusual temperature dependence of the linewidth and peak energy of the dot ensemble photoluminescence has not been observed. In addition, we have tuned the emission wavelength of InAs QDs to 1.3 μm at room temperature.     

Electron energy state spin-splitting in 3D cylindrical semiconductor quantum dots

In this article we study the impact of the spin-orbit interaction on the electron quantum confinement for narrow gap semiconductor quantum dots. The model formulation includes: (1) the effective one-band Hamiltonian approximation; (2) the position- and energy-dependent quasi-particle effective mass approximation; (3) the finite hard wall confinement potential; and (4) the spin-dependent Ben Daniel-Duke boundary conditions. The Hartree-Fock approximation is also utilized for evaluating the characteristics of a two-electron quantum dot system. In our calculation, we describe the spin-orbit interaction which comes from both the spin-dependent boundary conditions and the Rashba term (for two-electron quantum dot system). It can significantly modify the electron energy spectrum for InAs semiconductor quantum dots built in the GaAs matrix. The energy state spin-splitting is strongly dependent on the dot size and reaches an experimentally measurable magnitude for relatively small dots. In addition, we have found the Coulomb interaction and the spin-splitting are suppressed in quantum dots with small height. Received 15 May 2001 / Received in final form 14 May 2002 Published online 13 August 2002     

Geometrical effects on the optical properties of quantum dots doped with a single magnetic atom

The emission spectra of individual self-assembled quantum dots containing a single magnetic Mn atom differ strongly from dot to dot. The differences are explained by the influence of the system geometry, specifically the in-plane asymmetry of the quantum dot and the position of the Mn atom. Depending on both these parameters, one has different characteristic emission features which either reveal or hide the spin state of the magnetic atom. The observed behavior in both zero field and under magnetic field can be explained quantitatively by the interplay between the exciton-manganese exchange interaction (dependent on the Mn position) and the anisotropic part of the electron-hole exchange interaction (related to the asymmetry of the quantum dot).     

Purcell effect in triangular plasmonic nanopatch antennas with three-layer colloidal quantum dots

A model describing a plasmonic nanopatch antenna based on triangular silver nanoprisms and multilayer cadmium chalcogenide quantum dots is introduced. Electromagnetic-field distributions in nanopatch antennas with different orientations of the quantum-dot dipoles are calculated for the first time with the finite element method for numerical electrodynamics simulations. The energy flux through the surface of an emitting quantum dot is calculated for the configurations with the dot in free space, on an aluminum substrate, and in a nanopatch antenna. It is shown that the radiative part of the Purcell factor is as large as 1.7 × 10₂ The calculated photoluminescence lifetimes of a CdSe/CdS/ZnS colloidal quantum dot in a nanopatch antenna based on a silver nanoprism agree well with the experimental results.     

Collective modes of quantum dot ensembles in microcavities

Emission spectra of quantum dot arrays in zero-dimensional microcavities are studied theoretically. It is shown that their form is determined by the competition between collective superradiant mode formation and inhomogeneous broadening. A random sources method is used to calculate the photoluminescence spectra from an nonresonant pumped microcavity, and a standard diagram technique is used to provide a microscopic justification for the random sources method. The emission spectra of a microcavity are analyzed taking into account the spread of exciton energy due to inhomogeneous distribution of quantum dots and tunneling between them. It is demonstrated that the luminescence spectra of strongly tunnel-coupled quantum dots are sensitive to the dot positions, and the collective mode can (under certain conditions) be stabilized by random tunneling links.     

Carrier tunneling in asymmetric coupled quantum dots

Exciton spin relaxation at low temperatures in InAlAs–InGaAs asymmetric double quantum dots embedded in AlGaAs layers has been investigated as a function of the barrier thickness by the time-resolved photoluminescence measurements. With decreasing the thickness of the AlGaAs layer between the dots, the spin relaxation time change from 3 ns to less than 500 ps. The reduction in the spin relaxation time was considered to originate from the spin-flip tunneling between the ground state in InAlAs dot and the excited states in InGaAs dot, and the resultant tunneling leads to the spin depolarization of the ground state in InGaAs dot.     

量子点液晶显示背光技术

量子点材料兼具极高的色纯度、发光颜色可调以及的荧光量子产率高等特点,已成为显示领域中的明星材料,在提升显示器件的色域方面具有巨大潜力。基于量子点材料的液晶显示背光技术是目前量子点材料在显示器件中的主流应用方向,引起了学术界和工业界的广泛关注。本文将综述量子点液晶显示背光技术的研究进展,主要包括量子点材料的选择、背光结构的应用以及材料复合与封装技术的发展现状,重点介绍了目前产业界广泛关注的量子点光学膜技术,特别是国内自主知识产权的低成本钙钛矿量子点光学膜技术,由于其具备广色域(124%NTSC)、易加工、低成本等特点,已成为具有成长潜力的技术路线。     

不同盖层对InAs/GaAs量子点结构和光学性质的影响

利用金属有机化合物气相沉积设备生长了不同盖层结构的InAs/GaAs量子点,采用原子力显微镜和光致发光光谱仪对量子点的结构和光学性质进行了研究.量子点层之间的盖层由一个低温层和一个高温层组成.对不同材料结构的低温盖层的对比研究表明,In组分渐变的InGaAs低温盖层有利于改善量子点均匀性、减少结合岛数目、提高光致发光强度;当组分渐变InGaAs低温盖层厚度由6.8 nm增加到12 nm,发光波长从1256.0 nm红移到1314.4 nm.另外,还对不同材料结构的高温盖层进行了对比分析,发现高温盖层采用In组分渐变的InGaAs材料有利于光致发光谱强度的提高. 关键词： 半导体量子点 盖层 组分渐变     

Generation of dislocation loops in strained quantum dots embedded in a heterolayer

The generation of prismatic dislocation loops in strained quantum dots is investigated. The quantum dots are embedded in a film-substrate heterostructure with mechanical stresses caused by the difference between the lattice parameters of the film (heterolayer) and the substrate. The intrinsic plastic strain ?_m of a quantum dot arises from the misfit between the lattice parameters of the materials of the quantum dot and the surrounding matrix. The interface between the heterolayer and the substrate is characterized by a misfit parameter f. The critical radius of a quantum dot R _c at which the generation of a dislocation loop in the quantum dot becomes energetically favorable is analyzed as a function of the intrinsic plastic strain ?_m and the misfit parameter f.     

氧化硅稳定的氧化锡量子点薄膜的制备及其光学性能

 用溶胶-凝胶-水热过程制备了氧化硅稳定的氧化锡量子点,然后将其分散到氧化硅溶液中,用旋转涂膜的方法制备光学性能良好的氧化硅稳定的氧化锡量子点薄膜。X射线衍射和高分辨透射电镜表征显示氧化锡量子点具有良好的四方金红石晶型,平均粒径约4.0 nm。室温光致发光显示这种氧化硅稳定的氧化锡量子点薄膜在356 nm和388 nm处分别有很强的激子发光和缺陷态发光。根据透射谱拟合得到了氧化锡量子点薄膜的光学禁带宽度,其值约为3.96 eＶ。     

Quantum interference in exciton-Mn spin interactions in a CdTe semiconductor quantum dot

We show theoretically and experimentally the existence of a new quantum-interference effect between the electron-hole interactions and the scattering by a single Mn impurity. The theoretical model, including electron-valence-hole correlations, the short- and long-range exchange interaction of a Mn ion with the heavy hole and with electron and anisotropy of the quantum dot, is compared with photoluminescence spectroscopy of CdTe dots with single magnetic ions. We show how the design of the electronic levels of a quantum dot enables the design of an exciton, control of the quantum interference, and hence engineering of light-Mn interaction.     

Splitting and storing excitons in strained coupled self-assembled quantum dots

We present a novel self-assembled quantum dot structure designed to spatially separate and store photo-generated electrons and holes in pairs of strain coupled quantum dots. The spatial separation of electron–hole pairs into quantum dots and strain-induced quantum dots has been investigated and verified by photoluminescence experiments. Results from time-resolved PL demonstrates that at low temperatures (3 K) the electron–hole pair can be stored for several seconds.