光致发光谱研究自组织InAs双模量子点态填充

采用固态源分子束外延技术在GaAs(100)衬底上,制备了InAs量子点,对样品进行原子力显微镜测试,统计结果表明量子点尺寸呈双模分布。光致发光谱研究表明,在室温和77 K下,小量子点的发光峰均占主导地位,原因可能是：(1)大量子点的态密度小于小量子点;(2)捕获载流子速率,大量子点小于小量子点;(3)大量子点与盖层存在较大的应变势垒和可能出现的位错和缺陷,导致温度变化引起载流子从小尺寸量子点转移到大尺寸的量子点中概率很小。     

生长方向对量子点应变与应变弛豫的影响

由于材料弹性的各向异性与表面能的各向异性, 不同的生长方向或生长面, 量子点有不同的力学性能与行为. 本文基于各向异性弹性理论的有限元方法, 以金字塔型自组织InAs/GaAs半导体量子点为研究对象, 分别在7个常见的生长方向或生长面上, 对其应变能和应变弛豫能、自由能等进行了分析计算, 得到了这些能量随生长方向的变化规律. 结果表明(211)量子点应变弛豫能最大, 而(100)量子点应变弛豫能最小. 这些结果可为可控制备量子点提供理论参考. 关键词： 量子点 生长方向 平衡形态 应变弛豫     

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。     

Controlled positioning of self-assembled InAs quantum dots on (1 1 0) GaAs

We report on a new approach for positioning of self-assembled InAs quantum dots on (1 1 0) GaAs with nanometer precision. By combining self-assembly of quantum dots with molecular beam epitaxy on in situ cleaved surfaces (cleaved-edge overgrowth) we have successfully fabricated arrays of long-range ordered InAs quantum dots. Both atomic force microscopy and micro-photoluminescence measurements demonstrate the ability to control position and ordering of the quantum dots with epitaxial precision as well as size and size homogeneity. Furthermore, photoluminescence investigations on dot ensembles and on single dots confirm the high homogeneity and the excellent optical quality of the quantum dots fabricated.     

红光InAlAs量子点的结构和光学性质

利用ＭＢＥ方法在（００１）衬底上成功地生长密度大、尺寸小、发红光的ＩｎＡｌＡｓ／ＡｌＧａＡｓ量子点结构。通过原子力显微镜观察表明,ＩｎＡｌＡｓ量子的密度和大小都随覆盖厚度的增加而增大;发现Ａｌ原子的表面迁移率决定ＩｎＡｌＡｓ量子点的形貌,光荧光谱证实了量子点的发光峰值在红光范围,并结合形貌的统计得到了量子点的发光峰展宽主要昌受量子点的横向尺寸影响。     

CdSe量子点膨润土复合材料的制备与表征

首先以CdCl₂·2.5H₂O、SeO₂和NaBH₄为反应物,制备巯基丁二酸稳定的CdSe量子点。然后将有机膨润土与CdSe量子点溶液混合并充分搅拌,制备负载CdSe量子点的膨润土发光材料,用荧光光谱、扫描电镜和X射线粉末衍射等分析测试手段对所得材料的光谱性能与微观结构进行表征。光谱分析表明,量子点膨润土复合材料的发光颜色与量子点溶液非常一致;X射线光电子能谱分析表明,复合后的材料中含有Cd 和 Se两种元素;此外,在量子点膨润土复合材料X射线粉末衍射谱中可见CdSe量子点（111）、（220）及（331）3个晶面的衍射峰,2θ=4.3°处出现膨润土（001）衍射峰。数据表明,在制备的CdSe量子点膨润土复合材料中,量子点和膨润土的结构都没有改变。     

基于明胶制备碳量子点及其光学性能的研究

通过水热法采用热解明胶制备出有蓝色荧光的碳量子点,并通过单因素优化实验对制备碳量子点的温度、时间进行优化以选择出制备碳量子点的最佳条件,结果表明在水热反应温度为200 ℃,反应时间为6 h时制备的碳量子点的荧光性能最强。同时,利用透射电子显微镜(TEM)、傅里叶变换红外光谱(FTIR)、X射线光电子能谱(XPS)、Ｘ射线衍射(XRD)、紫外-可见吸收光谱(UV)及荧光光谱(PL)等手段对最佳条件下制备的碳量子点进行测试与表征,结果表明,该方法制备的碳量子点量子产率为39.4%,与不掺杂的碳量子点相比其量子产率相对较高,这可能是因为有N元素的存在使得量子产率有所提高;所制备的碳量子点不仅具有丰富的含氧官能团而且抗光漂白性能良好,形态主要是均匀分散的球形,没有明显的晶格条纹,这与相关文献报道的碳量子点的形态相一致,其在250～300 nm有较弱的吸收,但无明显的特征吸收峰,这可能是由于C=O基团的n-π*跃迁引起的;此外,还讨论了氙灯照射时间、pH、碳量子点浓度、不同类型溶剂及离子强度等因素对碳量子点荧光性能的影响,研究结果表明,氙灯照射时间及离子强度对碳量子点荧光性能几乎无影响,在过酸或过碱的条件下其荧光强度相对较弱,原因可能是在过酸或过碱的条件下发生质子化或非质子化的作用导致其荧光强度减弱;且碳量子点溶液随着其浓度的增加,荧光强度先增加后减小;而对于溶剂类型而言,其在极性溶剂中的荧光强度大于其在非极性溶剂中的荧光强度,说明该方法制备的碳量子点具有良好的水溶性。     

基于CuInS2/ZnS核壳结构量子点的高光效暖白光LED

采用逐步热注射法合成了用于白光LED的CuInS₂/ZnS(CIS/ZnS)核壳结构量子点.通过调整Cu/In的比率, 在CuInS₂(CIS)量子点的基础上, 合成了发射波长在570~650 nm之间可调的CIS/ZnS量子点.与CIS量子点的低量子产率相比, 具有核壳结构的CIS/ZnS量子点的量子产率达到了78%.通过在黄光荧光粉YAG :Ce³⁺表面旋涂CIS/ZnS量子点的方式制备了暖白光LED器件.在工作电流为10 mA时, 暖白光LED的发光效率达到了244.58 lm/W.由于CIS/ZnS量子点的加入, 所制备的白光LED器件的显色指数达到86.7且发光颜色向暖色调发生了转移, 相应的色坐标为(0.340 6, 0.369 0).     

多层GaSb(QDs)/GaAs生长中量子点的聚集及发光特性

通过对多层GaSb量子点的生长研究,发现随着生长层数的增加,量子点尺寸逐渐变大,密度没有明显变化,并且量子点出现了聚集现象;当层数增加到一定数量、量子点聚集到一定大小时,聚集的量子点处会出现空洞。这些现象表明,各层量子点在生长过程中存在关联效应,并且GaAs层不能很好地覆盖在聚集的量子点之上,在继续生长其它量子点层时,聚集的量子点处在高温下出现GaSb的蒸发,从而出现空洞。PL谱出现了很宽的量子点发光峰,这很可能是由于多层量子点在生长时大小分布较宽而导致的结果。     

Transport in graphene nanostructures

Graphene nanostructures are promising candidates for future nanoelectronics and solid-state quantum information technology. In this review we provide an overview of a number of electron transport experiments on etched graphene nanostructures. We briefly revisit the electronic properties and the transport characteristics of bulk, i.e., two-dimensional graphene. The fabrication techniques for making graphene nanostructures such as nanoribbons, single electron transistors and quantum dots, mainly based on a dry etching ??paper-cutting?? technique are discussed in detail. The limitations of the current fabrication technology are discussed when we outline the quantum transport properties of the nanostructured devices. In particular we focus here on transport through graphene nanoribbons and constrictions, single electron transistors as well as on graphene quantum dots including double quantum dots. These quasi-one-dimensional (nanoribbons) and quasi-zero-dimensional (quantum dots) graphene nanostructures show a clear route of how to overcome the gapless nature of graphene allowing the confinement of individual carriers and their control by lateral graphene gates and charge detectors. In particular, we emphasize that graphene quantum dots and double quantum dots are very promising systems for spin-based solid state quantum computation, since they are believed to have exceptionally long spin coherence times due to weak spin-orbit coupling and weak hyperfine interaction in graphene.     

Spatial distribution of elastic deformations in Ge/Si structures with quantum dots

A method is developed for calculating the elastic deformation in coherently strained heterostructures on the basis of the valence force field (VFF) model using the Green’s function of the “atomistic” elastic problem. The spatial distribution of the elastic deformations in a Ge/Si system with pyramidal Ge quantum dots buried in a Si matrix is investigated theoretically. The deformation distribution in and around the pyramids is determined. Near quantum dots, the region near the tip of the pyramid is most strongly intensely. Inside quantum dots the region of the vertex is most relaxed, and the most strained section lies on the contour of the pyramid base. Compression occurs in the plane of the pyramid base inside quantum dots, and stretching occurs along the vertical direction. The picture is reversed near quantum dots: stretching occurs in the lateral direction and compression in the vertical direction. It is shown that the local deformations and their spatial distribution are essentially independent (to within the scaling) of the size of the quantum dots for 10–15 nm pyramid bases.     

用B样条技术研究半导体微晶中激子的量子受限效应

在有效质量近似（EMA）下，采用B样条技术和变分方法，分别研究较大CdTe球量子点(25—35nm)和较小CdS球量子点(025—35nm)中激子的量子受限效应，计算出CdTe和CdS球量子点中受限激子的基态能和束缚能随参数的变化规律，比较两种计算结果得到：（1）较大量子点中受限激子的基态能和束缚能对量子点边界和量子点外部介质的介电常数不敏感，但较小量子点中受限激子的基态能和束缚能对量子点边界和量子点外部介质的介电常数比较敏感.（2）在较强受限区域，大量子点与小量子点的激子基态能和束缚能的变化规律完全不同.（3）B样条技术对于研究这种具有边界的束缚态系统是很精确的方法，这种方法特别适合用于多层结构量子点系统的精确计算. 关键词： B样条技术 量子受限效应 有效质量近似     

Electron–phonon interaction in CdS/ZnS quantum dot

Electron–phonon effects on the two first electronic states in both CdS and GaAs quantum dots are investigated. Both confined longitudinal optical (LO) and surface optical (SO) phonons are considered. We use the intermediate-type variational approach. We find that, shifts caused by phonon contribution on electronic energies are more significant for CdS quantum dot. We find, also, that, contrary to GaAs based quantum dots, we shouldn’t neglect the SO phonon contribution for CdS based ones, especially for small dots.     

Spin relaxation and decoherence of holes in quantum dots

We investigate heavy-hole spin relaxation and decoherence in quantum dots in perpendicular magnetic fields. We show that at low temperatures the spin decoherence time is 2 times longer than the spin relaxation time. We find that the spin relaxation time for heavy holes can be comparable to or even longer than that for electrons in strongly two-dimensional quantum dots. We discuss the difference in the magnetic-field dependence of the spin relaxation rate due to Rashba or Dresselhaus spin-orbit coupling for systems with positive (i.e., GaAs quantum dots) or negative (i.e., InAs quantum dots) g factor.     

Excitons in Ge/Si double quantum dots

The spatial structure of excitons and the oscillator strength characterizing the intensity of interband optical transitions in vertically coupled Ge/Si quantum dots have been theoretically studied. It has been found that the probability of the exciton transition under certain conditions (the sizes of the quantum dots, the separation of the dots) can be much larger (up to a factor of 5) than the value for the case of single quantum dots. It is expected that the results will make it possible to approach the creation of efficient light-emitting and photoreceiving devices based on Si and Ge indirect-band semiconductors.     

大气环境下合成白光发射的Tb3+/Eu3+共掺杂全无机卤化物钙钛矿量子点

为克服因混合不同卤化物钙钛矿量子点发生阴离子交换反应、不稳定的红光发射卤化物钙钛矿量子点等而导致在获取白光发射方面存在的不足,提出了一种可以在大气环境下合成Tb³⁺,Eu³⁺稀土离子共掺杂全无机卤化物钙钛矿量子点的方法。调节Tb³⁺,Eu³⁺稀土离子的掺杂比例,调控从钙钛矿量子点主晶格到Tb³⁺和Eu³⁺离子的能量转移,获得了单一组分、白光发射的钙钛矿量子点(Tb,Eu):CsPbCl3和(Tb,Eu):CsPb(Cl/Br)3,并对量子点的形貌、结构、发光性能及能量传递机理和稳定性进行了详细研究。研究结果表明:在365nm激光激发下,不同含量Tb³⁺/Eu³⁺离子共掺杂的钙钛矿量子点(Tb,Eu):CsPbCl3发射光谱对应的色坐标位于1931色度图中的白光区域。在进料比PbCl2∶TbCl3∶EuCl3为1∶1.5∶1时,量子产率为3.59%,比纯的CsPbCl3量子点的量子产率(0.57%)提高了6倍。进一步研究发现,该(Tb,Eu):CsPbCl3量子点在空气中储存2个月之后,量子产率几乎保持不变(3.63%),保持了良好的稳定性。此外,研究了采用不同溶剂(正辛烷、十八烯)合成Tb³⁺/Eu³⁺共掺杂钙钛矿量子点的发光特性。Tb³⁺/Eu³⁺离子共掺杂的钙钛矿量子点(Tb,Eu):CsPbCl3可实现单一组分的白光发射,有良好的稳定性,具备一定的应用前景。     

Theoretical study of quantum confined Stark shift in InAs／GaAs quantum dots

The quantum confined Stark effect (QCSE) of the self-assembled InAs/GaAs quantum dots has been investigated theoretically. The ground-state transition energies for quantum dots in the shape of a cube, pyramid or “truncated pyramid” are calculated and analysed. We use a method based on the Green function technique for calculating thestrain in quantum dots and an efficient plane-wave envelope-function technique to determine the ground-state electronic structure of them with different shapes. The symmetry of quantum dots is broken by the effect of strain. So the properties of carriers show different behaviours from the traditional quantum device. Based on these results, we also calculate permanent built-in dipole moments and compare them with recent experimental data. Our results demonstrate that the measured Stark effect in self-assembled InAs/GaAs quantum dot structures can be explained by including linear grading.     

Speedy photoelectric exchange of CdSe quantum dots/mesoporous titania composite system

Two classes of important nano-materials, mesoporous titania (MT) and CdSe quantum dots, are prepared via chemical methods. Rapid immobilization of CdSe quantum dots into mesoporous titania has been achieved and the composite system is characterized by high-resolution transmission electron microscopy and photoluminescence. The band-to-band photoluminescence emission of CdSe quantum dots is blue-shifted for 20 nm after the dots are immobilized into mesoporous titania. Loading CdSe quantum dots into mesoporous titania leads to speedy photoelectric exchange process of the system and a significant rise in photoconductivity (for a factor of 8), which would attract great deal of attention in quantum dots solar cell.     

Linearly polarized photoluminescence from an ensemble of wurtzite GaN/AlN quantum dots

Microphotoluminescence from GaN/AlN quantum dots grown by molecular beam epitaxy on sapphire substrates along the (0001) axis has been studied. To produce quantum dots of different average sizes and densities, the nominal amount of deposited GaN has been varied from 1 to 4 ML. The density of the quantum dots was about 10¹¹ cm⁻², which corresponded to about 10³ quantum dots excited in the experiments. The photo-luminescence from the quantum dots was linearly polarized and the maximum polarization degree (15%) has been observed for the sample with the lowest amount of deposited GaN. The photoluminescence intensity from this sample under continuous laser excitation decreased by more than two orders of magnitude for about 30 min and then stabilized. The photoluminescence intensity from other samples under continuous excitation remained constant. We suggest that a rather high polarization degree is caused by anisotropy in the strain and shape of the quantum dots formed near the dislocations, which also act as the centers of nonradiative recombination.     

Effects of crystal shape on the energy levels of zero-dimensional PbS quantum dots

Nanometer-size PbS quantum dots have been made by electrodeposition on a Au(111) substrate. The deposited nanocrystals have a flattened cubic shape. We probed the single-electron energy-level spectrum of individual quantum dots by scanning tunneling spectroscopy and found that it deviates strongly from that of spherical PbS quantum dots. The measured energy-level spectrum is successfully explained by considering strong confinement in a flattened cubic box.