Exciton Dynamics in Semiconductor Nanocrystals

This review article provides an overview of recent advances in the study and understanding of dynamics of excitons in semiconductor nanocrystals (NCs) or quantum dots (QDs). Emphasis is placed on the relationship between exciton dynamics and optical properties, both linear and nonlinear. We also focus on the unique aspects of exciton dynamics in semiconductor NCs as compared to those in bulk crystals. Various experimental techniques for probing exciton dynamics, particularly time‐resolved laser methods, are reviewed. Relevant models and computational studies are also briefly presented. By comparing different materials systems, a unifying picture is proposed to account for the major dynamic features of excitons in semiconductor QDs. While the specific dynamic processes involved are material‐dependent, key processes can be identified for all the materials that include electronic dephasing, intraband relaxation, trapping, and interband recombination of free and trapped charge carriers (electron and hole). Exciton dynamics play a critical role in the fundamental properties and functionalities of nanomaterials of interest for a variety of applications including optical detectors, solar energy conversion, lasers, and sensors. A better understanding of exciton dynamics in nanomaterials is thus important both fundamentally and technologically.     

Core/Shell Semiconductor Nanocrystals

Colloidal core/shell nanocrystals contain at least two semiconductor materials in an onionlike structure. The possibility to tune the basic optical properties of the core nanocrystals, for example, their fluorescence wavelength, quantum yield, and lifetime, by growing an epitaxial‐type shell of another semiconductor has fueled significant progress on the chemical synthesis of these systems. In such core/shell nanocrystals, the shell provides a physical barrier between the optically active core and the surrounding medium, thus making the nanocrystals less sensitive to environmental changes, surface chemistry, and photo‐oxidation. The shell further provides an efficient passivation of the surface trap states, giving rise to a strongly enhanced fluorescence quantum yield. This effect is a fundamental prerequisite for the use of nanocrystals in applications such as biological labeling and light‐emitting devices, which rely on their emission properties. Focusing on recent advances, this Review discusses the fundamental properties and synthesis methods of core/shell and core/multiple shell structures of II–VI, IV–VI, and III–V semiconductors.

     

Silicon Nanocrystals: Size Matters

This paper reviews new approaches to size‐controlled silicon‐nanocrystal synthesis. These approaches allow narrowing of the size distribution of the nanocrystals compared with those obtained by conventional synthesis processes such as ion implantation into SiO₂ or phase separation of sub‐stoichiometric SiO_x layers. This size control is realized by different approaches to introducing a superlattice‐like structure into the synthesis process, by velocity selection of silicon aerosols, or by the use of electron lithography and subsequent oxidation processes. Nanocrystals between 2 and 20 nm in size with a full width at half maximum of the size distribution of 1 nm can be synthesized and area densities above 10¹² cm^–2 can be achieved. The role of surface passivation is elucidated by comparing Si/SiO₂ layers with superlattices of fully passivated silicon nanocrystals within a SiO₂ matrix. The demands on silicon nanocrystals for various applications such as non‐volatile memories or light‐emitting devices are discussed for different size‐controlled nanocrystal synthesis approaches.     

Chemical Synthesis of ZnO Nanocrystals

A new chemical process to synthesize pure ZnO nanocrystal colloidal for bioimaging applications is reported. Zinc acetate dihydrate was dissolved and refluxed in a methanol solution at a low temperature (68degC) and ambient environment. Biocompatible surface capping agents were introduced to the synthesis process to control the particle nucleation and growth and, therefore, the particle size and its surface chemistry. Five types of capping agents were investigated for their effectiveness in limiting the particle growth. Three capping agents-3-amino- propyl trimethoxysilane (Am), tetraethyl orthosilicate (TEOS), and mercaptosuccinic acid (Ms)-were found effective in capping the ZnO nanoparticles and limiting the growth of the particles, while the other two-3-mercaptopropyl trimethoxysilane (Mp) and polyvinylpyrrolidone (Pv)-caused agglomeration or forming large clusters in the solutions. Particles synthesized were in the size range of 10-30 nm after capping, and grew to 60 nm and 100 nm in 3 weeks and 6 weeks respectively during storage at ambient conditions. Refluxing time was found to affect only the first precipitation time. Washing by ethanol and slow drying were found critical in converting Zn(OH)₂ into ZnO. XRD analyses revealed forming of single phase ZnO Wurzite (P63mc) structure. TEM analysis determined the single crystal size of 6 nm. Photo- luminescence (PL) spectra showed high intensity in UV emission and low intensity in the visible emission, which imply a good surface morphology of the ZnO nanoparticles with few surface defects. Optical absorption spectra indicated absorption at the wavelength of 380 nm from the uncapped ZnO, corresponding to the bandgap of bulk ZnO. The capped ZnO absorbed at a shorter wavelength (350 nm) indicating a much smaller particle size. Capping effectiveness of each agent is discussed through possible capping mechanisms and chemical reactions.     

The Growth of Micro-and Nanocrystals

正Based on the given reaction condition and medium,the growth of micro-and nanocrystals can be divided into four types,growth in solution at normal pressure,hydrothermal growth,solvothermal growth,and molten-salt growth.When the water or organic solvent as the reaction medium,surfactant,such as sodium dodecyl benzene sulfonate,can be added to regulate the     

Facet-Dependent Photocatalytic Properties of AgBr Nanocrystals

Polyhedral AgBr nanocrystals, evolved from cubes through truncated cubes and finally to high-symmetry octahedra, which corresponds to a progressive shrinkage of exposed {100} facets and enlargement of exposed {111} facets, are prepared by facile precipitation reactions. The as- prepared AgBr nanocrystals exhibit facet-dependent catalytic properties, with the {111}-dominated octahedra showing the highest photocatalytic activities.     

Shape Control of Colloidal Metal Nanocrystals

Colloidal metal nanoparticles are emerging as key materials for catalysis, plasmonics, sensing, and spectroscopy. Within these applications, control of nanoparticle shape lends increasing functionality and selectivity. Shape‐controlled nanocrystals possess well‐defined surfaces and morphologies because their nucleation and growth are controlled at the atomic level. An overall picture of shaped metal particles is presented, with a particular focus on solution‐based syntheses for the noble metals. General strategies for synthetic control are discussed, emphasizing key factors that result in anisotropic, nonspherical growth such as crystallographically selective adsorbates and seeding processes.     

Enantiomeric Control of Intrinsically Chiral Nanocrystals

The chiral aspect of inorganic crystals that crystallize in chiral space groups has been largely ignored until recently, partly due to difficulties in characterizing the chiroptical properties of bulk crystals, and also due to the difficulty in separating (sub)micrometer-scale chiral crystal enantiomers. In recent years, the colloidal synthesis of intrinsically chiral nanocrystals (NCs) of several chiral inorganic compounds with significant enantiomeric excess has been demonstrated. This is achieved through the use of chiral molecular ligands, which bind to the atomic/ionic components of the crystals, preferentially forming one crystal enantiomorph. Here, recent progress on several aspects of these NCs is described, including the connection between ligand structure and its ability to direct NC handedness, chiral amplification in the synthesis leading to enantiopure NC samples, spontaneous symmetry breaking, the formation of NCs with chiral shapes, the connection between lattice and shape chirality and mixed contributions of atomic-scale and shape chirality to the chiroptical properties.     

离子注入氧化锌纳米晶体光致发光特性(英文)

为进一步了解离子注入过程对纳米晶体发光响应的损伤效果,研究了注入氢离子和氦离子对氧化锌纳米晶体光致发光(PL)特性的影响.首先在二氧化硅衬底上制备了直径为4 nm和9 nm的氧化锌纳米晶体薄膜,接着在室温下向生成的薄膜中注入氢离子和氦离子,并研究其PL响应变化.结果表明,与未注入离子的纳米晶体相比,注入氦离子导致缺陷PL发光增加,而当注入氢离子之后,缺陷发光大幅降低,表明氢原子对注入损伤有钝化作用.比较近谱带边沿(NBE)与缺陷发光之比发现,较大颗粒的纳米晶体其近谱带边沿与缺陷发光比在注入氢离子后增大,而在注入氦离子后降低.该规律同样适用于较小颗粒的纳米晶体,但其近谱带边沿与缺陷发光比在注入氢离子之后的增幅比大颗粒晶体的小得多.     

魔力尺寸硒化镉纳米晶的压力稳定性研究

在极端条件下应用的耐压材料必须具有极高的结构和性能稳定性。魔力尺寸纳米晶体具有良好的结构、超小的粒子尺寸和精确的原子组成,逐渐成为研究的热点。采用胶体化学方法合成了硒化镉(CdSe)魔力尺寸纳米晶,其第一激子吸收峰位于463nm处,相应的光致发光光谱表现出窄的半高宽,仅为13nm。进一步利用金刚石对顶砧压机研究了所制备的CdSe魔力尺寸纳米晶在压力下的光学响应和稳定性。研究结果表明:随着压力的增大,CdSe魔力尺寸纳米晶的吸收和发光峰位保持不变,这种压力下的稳定行为与传统纳米材料在外界压力下所表现的敏感特性相反;压力卸下后,样品仍然保持CdSe魔力尺寸纳米晶的原始结构和形态。CdSe魔力尺寸纳米晶在加压过程中表现出的良好耐压性能,有助于极端压缩条件下的魔力尺寸纳米材料的研究。