Dye J-aggregate—semiconductor nanocrystal hybrid nanostructures in reverse micelles: an experimental study

Reverse micelle solutions can be used for the assembly of hybrid nanostructures of the composition dye monomer—Ag₂S nanocrystal, dye J-aggregate—CuI nanocrystal, and dye J-aggregate—PbI₂ nanocrystal. The assembly is effected by means of adsorption of the dye (3,3′-di-(γ-sulfopropyl)-4,5,4′,5′-dibenzo-9-ethylthiacarbocyanine betaine pyridinium salt) onto the nanocrystal surface. Factors responsible for the dye adsorption onto semiconductor nanocrystals in reverse micelle solutions are analyzed. It is suggested that adsorption can be the outcome of forces induced by both van der Waals and chemical interactions. The surfactants used for stabilization of reverse micelle solutions also influence the assembly of the hybrid nanostructures.     

Homogeneously alloyed CdSxSe1-x nanocrystals: synthesis, characterization, and composition/size-dependent band gap

Alloy nanocrystals provide an additional degree of freedom in selecting desirable properties for nanoscale engineering because their physical and optical properties depend on both size and composition. We report the pyrolytic synthesis of homogeneously alloyed CdS(x)Se(1-x) nanocrystals in all proportions. The nanocrystals are characterized using UV-visible absorption spectroscopy, transmission electron microscopy, X-ray diffractrometry, and Rutherford backscattering spectrometry to determine precisely structure, size, and composition. The dependence of band gap on nanocrystal size and composition is elucidated, yielding a bowing constant of 0.29, in agreement with bulk values. In addition, the morphology of the resultant nanocrystals can be altered by changing the reaction conditions, generating structures ranging from homogeneous, spherical nanocrystals to one-dimensional gradient nanorods.     

Electrochemical probing of thiol-capped nanocrystals

Electrochemical studies of thiol-capped semiconductor nanocrystals have demonstrated several distinct oxidation and reduction peaks in the voltammograms with the peak positions being nanocrystal size dependent. It is demonstrated that the method is very sensitive to the nanocrystal surface states, providing complimentary information for better understanding the optical properties of semiconductor nanocrystals. Correspondence: Alexander Eychmüller, Physical Chemistry, TU Dresden, Bergstr. 66b, D-01062 Dresden, Germany     

溶剂热合成单分散硫化镉纳米晶

在双表面活性剂十八胺和油酸存在条件下, 以氯化镉和硫粉作为反应前驱物, 通过简单的溶剂热方法合成单分散性闪锌矿硫化镉纳米晶, 粒径大小在13 nm. 用X射线衍射(XRD)、透射电子显微镜(TEM)对产物的结构和形貌进行了表征, 同时对硫化镉纳米晶的紫外吸收谱和光致发光谱(PL)性能进行了表征. 实验结果表明合成的样品具有很好的发光性能, 此外溶剂热反应的温度对纳米晶的单分散性有重要影响. 并对硫化镉纳米晶的形成机理做了初步的研究.     

Colloidal Hybrid Nanostructures: A New Type of Functional Materials

One key goal of nanocrystal research is the development of experimental methods to selectively control the composition and shape of nanocrystals over a wide range of material combinations. The ability to selectively arrange nanosized domains of metallic, semiconducting, and magnetic materials into a single hybrid nanoparticle offers an intriguing route to engineer nanomaterials with multiple functionalities or the enhanced properties of one domain. In this Review, we focus on recent strategies used to create semiconductor–metal hybrid nanoparticles, present the emergent properties of these multicomponent materials, and discuss their potential applicability in different technologies.     

Bottom-up assembly of colloidal gold and silver nanostructures for designable plasmonic structures and metamaterials

We report on bottom-up assembly routes for fabricating plasmonic structures and metamaterials composed of colloidal gold and silver nanostructures, such as nanoparticles ("metatoms") and shape-controlled nanocrystals. Owing to their well-controlled sizes/shapes, facile surface functionalization, and excellent plasmonic properties in the visible and near-infrared regions, these nanoparticles and nanocrystals are excellent building blocks of plasmonic structures and metamaterials for optical applications. Recently, we have utilized two kinds of bottom-up techniques (i.e., multiple-probe-based nanomanipulation and layer-by-layer self-assembly) to fabricate strongly coupled plasmonic dimers, one-dimensional (1D) chains, and large-scale two-dimensional/three-dimensional (2D/3D) nanoparticle supercrystals. These coupled nanoparticle/nanocrystal assemblies exhibit unique and tunable plasmonic properties, depending on the material composition, size/shape, intergap distance, the number of composing nanoparticles/nanocrystals (1D chains), and the nanoparticle layer number in the case of 3D nanoparticle supercrystals. By studying these coupled nanoparticle/nanocrystal assemblies, the fundamental plasmonic metamaterial effects could be investigated in detail under well-prepared and previously unexplored experimental settings.     

Electrode nanomaterials self-assembled from thiolate-encapsulated gold nanocrystals

This paper describes the preliminary findings of an investigation of thin film assembly from monolayer-encapsulated gold nanocrystals and 1,9-nonanedithols. The creation of novel electrode nanomaterials derived from intriguing combinations of the encapsulating shells and the particle cores constitutes the motivation of this work. Narrow-sized, shaped and encapsulated nanocrystals were assembled as thin films on different substrates via an exchange reaction between alkanethiolates on the nanocrystal shells and dithiols in the solution. Both microscopic and spectroscopic data have confirmed the formation of dithiol-linked nanocrystals in the thin films. The electrochemical study has revealed interesting parallels and differences between monolayers on planar and nanocrystal gold surfaces, which have important implications to the correlation between binding properties at nanocrystal facets and the electrode properties of this interesting class of composite nanomaterials.     

Bioactivation and cell targeting of semiconductor CdSe/ZnS nanocrystals with phytochelatin-related peptides

Synthetic phytochelatin-related peptides are used as an organic coat on the surface of colloidal CdSe/ZnS semiconductor nanocrystals synthesized from hydrophobic coordinating trioctyl phosphine oxide (TOPO) solvents. The peptides are designed to bind to the nanocrystals via a C-terminal adhesive domain. This adhesive domain, composed of multiple repeats of cysteines pairs flanked by hydrophobic 3-cyclohexylalanines, is followed by a flexible hydrophilic linker domain to which various bio-affinity tags can be attached. This surface coating chemistry results in small, buffer soluble, monodisperse peptide-coated nanoparticles with high colloidal stability and ensemble photophysical properties similar to those of TOPO-coated nanocrystals. Various peptide coatings are used to modulate the nanocrystal surface properties and to bioactivate the nanoparticles. CdSe/ZnS nanocrystals coated with biotinylated peptides efficiently bind to streptavidin and are specifically targeted to GPI-anchored avidin-CD14 chimeric proteins expressed on the membranes of live HeLa cells. This peptide coating surface chemistry provides a novel approach for the production of biocompatible photoluminescent nanocrystal probes.     

EuF3 纳米晶/PNIPAm复合水凝胶的制备及荧光温敏性能

以十一烯酸为表面活性剂, 采用液体-固体-溶液法(LSS)制备了EuF3纳米晶; 将其用CCl4处理, 得到表面修饰有C-Cl基团的功能化EuF3纳米晶; 通过原子转移自由基聚合(ATRP)制备EuF3 /聚N-异丙基丙烯酰胺(EuF3/PNIPAm)复合温敏水凝胶. 采用HRTEM, XRD, FTIR, DSC及PL等对EuF3 纳米晶及EuF3/ PNIPAm 复合凝胶的微观结构与性能进行了表征, 用变温荧光光谱研究了环境温度对复合凝胶荧光性能的影响. 结果表明, EuF3纳米晶呈六方相晶型; 粒径呈多分散分布, 且相对集中于10, 20和50 nm. 该复合凝胶的较低临界溶解温度(LCST)随纳米晶含量的增加而下降, 环境温度与纳米晶含量对复合凝胶的荧光特性产生明显影响.     

Constraints in post-synthesis ligand exchange for hybrid organic (MEH-PPV)–inorganic (CdSe) nanocomposites

For an optimum charge/energy transfer performance of hybrid organic–inorganic colloidal nanocrystals for applications such as photonic devices and solar cells, the determining factors are the distance between the nanocrystal and polymer which greatly depends upon nanocrystal size/nanocrystal ligands. Short chain ligands are preferred to ensure a close contact between the donor and acceptor as a result of the tunnelling probability of the charges and the insulating nature of long alkyl chain molecules. Short distances increase the probability for tunnelling to occur as compared to long distances induced by long alkyl chains of bulky ligands which inhibit tunnelling altogether. The ligands on the as-synthesized nanocrystals can be exchanged for various other ligands to achieve desirable charge/energy transfer properties depending on the bond strength of the ligand on the nanocrystal compared to the replacement ligand. In this work, the constraints involved in post-synthesis ligand exchange process have been evaluated, and these factors have been tuned via wet chemistry to tailor the hybrid material properties via appropriate selection of the nanocrystal capping ligands. It has been found that both oleic acid and oleylamine (OLA)-capped cadmium selenide (CdSe) quantum dots (QDs) as compared with trioctylphosphine oxide (TOPO)-passivated CdSe QDs are of high quality, and they provide better steric stability against coagulation, homogeneity, and photostability to their respective polymer:CdSe nanocomposites. CdSe QDs particularly with OLA capping have relatively smaller surface energies, and thus, lesser quenching capabilities show dominance of photoinduced Forster energy transfer between donors (polymer) and acceptors (CdSe nanocrystals) as compared to charge transfer mechanism as observed in polymer:CdSe (TOPO) composites. It is conjectured that size quantization effects, stereochemical compatibility of ligands (TOPO, oleic acid, and oleyl amine), and polymer MEH-PPV stability greatly influence the photophysics and photochemistry of hybrid polymer–semiconductor nanocomposites.     

Luminescence in colloidal Mn-doped semiconductor nanocrystals

Recent advances in nanocrystal doping chemistries have substantially broadened the variety of photophysical properties that can be observed in colloidal Mn²⁺-doped semiconductor nanocrystals. A brief overview is provided, focusing on Mn²⁺-doped II–VI semiconductor nanocrystals prepared by direct chemical synthesis and capped with coordinating surface ligands. These Mn²⁺-doped semiconductor nanocrystals are organized into three major groups according to the location of various Mn²⁺-related excited states relative to the energy gap of the host semiconductor nanocrystals. The positioning of these excited states gives rise to three distinct relaxation scenarios following photoexcitation. A brief outlook on future research directions is provided.     

Electrochemical properties of CdSe and CdTe quantum dots

Semiconductor nanocrystal quantum dots (QDs), owing to their unique opto-electronic properties determined by quantum confinement effects, have been the subject of extensive investigations in different areas of science and technology in the past two decades. The electrochemical behaviour of QDs, particularly for CdSe and CdTe nanocrystals, has also been explored, although to a lesser extent compared to the optical properties. Voltammetric measurements can be used to probe the redox levels available for the nanocrystals, which is an invaluable piece of information if these systems are involved in electron transfer processes. Electrochemical data can also foster the interpretation of the spectroscopic properties of QDs, and give insightful information on their chemical composition, dimension, and surface properties. Hence, electrochemical methods constitute in principle an effective tool to probe the quality of QD samples in terms of purity, size dispersion, and surface defects. The scope of this critical review is to discuss the results of electrochemical studies carried out on CdSe and CdTe core and core-shell semiconductor nanocrystals of spherical shape. Examples of emerging or potential applications that exploit electroactive quantum dot-based systems will also be illustrated.     

Electron-conducting quantum dot solids: novel materials based on colloidal semiconductor nanocrystals

We review the optical and electrical properties of solids that are composed of semiconductor nanocrystals. Crystals, with dimensions in the nanometre range, of II-VI, IV-VI and III-V compound semiconductors, can be prepared by wet-chemical methods with a remarkable control of their size and shape, and surface chemistry. In the uncharged ground state, such nanocrystals are insulators. Electrons can be added, one by one, to the conduction orbitals, forming artificial atoms strongly confined in the nanocrystal. Semiconductor nanocrystals form the building blocks for larger architectures, which self-assemble due to van der Waals interactions. The electronic structure of the quantum dot solids prepared in such a way is determined by the orbital set of the nanocrystal building blocks and the electronic coupling between them. The opto-electronic properties are dramatically altered by electron injection into the orbitals. We discuss the optical and electrical properties of quantum dot solids in which the electron occupation of the orbitals is controlled by the electrochemical potential.     

Second harmonic generation and confined acoustic phonons in highly excited semiconductor nanocrystals

The photo-induced enhancement of second harmonic generation and the effect of nanocrystal shape and pump intensity on confined acoustic phonons in semiconductor nanocrystals have been investigated with time-resolved scattering and absorption measurements. The second harmonic signal showed a sublinear increase of the second-order susceptibility with respect to the pump pulse energy, indicating a reduction of the effective one-electron second-order nonlinearity with increasing electron-hole density in the nanocrystals. The coherent acoustic phonons in spherical and rod-shaped semiconductor nanocrystals were detected in a time-resolved absorption measurement. Both nanocrystal morphologies exhibited oscillatory modulation of the absorption cross section, the frequency of which corresponded to their coherent radial breathing modes. The amplitude of the oscillation also increased with the level of photoexcitation, suggesting an increase in the amplitude of the lattice displacement as well.     

从纳米晶到三维超晶格结构

纳米晶的有序组装对未来纳米材料的应用拓展具有重要意义. 本综述介绍了纳米晶三维超晶格的研究价值以及制备方法,着重对“胶体溶液蒸发”、“不良溶剂扩散”、“胶束引导集聚”、“氢键连接”、“静电聚集”、“DNA导向”、“外场辅助”和“水－油界面辅助”的组装机制进行了总结与评述,同时也探讨了这一新兴领域中仍然存在的挑战.     

Symmetry-controlled colloidal nanocrystals: nonhydrolytic chemical synthesis and shape determining parameters

Since inorganic nanocrystals exhibit unique shape-dependent nanoscale properties and can be utilized as basic building blocks for futuristic nanodevices, a systematic study on the shape control of these nanocrystals remains an important subject in materials and physical chemistry. In this feature article, we overview the recent progress on the synthetic development of symmetry-controlled colloidal nanocrystals of semiconductor and metal oxide, which are prepared through nonhydrolytic chemical routes. We describe their shape-guiding processes and illustrate the detailed key factors controlling their growth by examining various case studies of zero-dimensional spheres and cubes, one-dimensional rods, and quasi multidimensional structures such as disks, multipods, and stars. Specifically, the crystalline phase of nucleating seeds, surface energy, kinetic vs thermodynamic growth, and selective adhesion processes of capping ligands are found to be most crucial for the determination of the nanocrystal shape.     

Enhancing the photoluminescence of peptide-coated nanocrystals with shell composition and UV irradiation

The composition and structure of inorganic shells grown over CdSe semiconductor nanocrystal dots and rods were optimized to yield enhanced photoluminescence properties after ligand exchange followed by coating with phytochelatin-related peptides. We show that, in addition to the peptides imparting superior colloidal properties and providing biofunctionality in a single-step reaction, the improved shells and pretreatment with UV irradiation resulted in high quantum yields for the nanocrystals in water. Moreover, peptide coating caused a noticeable red-shift in the absorption and emission spectra for one of the tested shells, suggesting that exciton-molecular orbital (X-MO) coupling might take place in these hybrid inorganic-organic composite materials.     

Fabrication of all-inorganic nanocrystal solids through matrix encapsulation of nanocrystal arrays

A general strategy for low-temperature processing of colloidal nanocrystals into all-inorganic films is reported. The present methodology goes beyond the traditional ligand-interlinking scheme and relies on encapsulation of morphologically defined nanocrystal arrays into a matrix of a wide-band gap semiconductor, which preserves optoelectronic properties of individual nanoparticles while rendering the nanocrystal film photoconductive. Fabricated solids exhibit excellent thermal stability, which is attributed to the heteroepitaxial structure of nanocrystal-matrix interfaces, and show compelling light-harvesting performance in prototype solar cells.     

Colloidal gallium indium oxide nanocrystals: a multifunctional light-emitting phosphor broadly tunable by alloy composition

We demonstrate compositionally tunable photoluminescence in complex transparent conducting oxide nanocrystals. Alloyed gallium indium oxide (GIO) nanocrystals with variable crystal structures are prepared by a colloidal method throughout the full composition range and studied by different structural and spectroscopic methods, including photoluminescence and X-ray absorption. The structures and sizes of the GIO nanocrystals can be simultaneously controlled, owing to the difference in the growth kinetics of In(2)O(3) and Ga(2)O(3) nanocrystals and the polymorphic nature of both materials. Using the synthesized nanocrystal series, we demonstrate the structural and compositional dependences of the photoluminescence of GIO nanocrystals. These dependences, induced by the interactions between specific defect sites acting as electron donors and acceptors, are used to achieve broad emission tunability in the visible spectral range at room temperature. The nature of the photoluminescence is identified as donor-acceptor pair recombination and changes with increasing indium content owing to the changes in the energy states of, and interactions between, donors and acceptors. Structural analysis of GIO nanocrystals by extended X-ray absorption fine structure spectroscopy reveals that In(3+) occupies only octahedral, rather than tetrahedral, sites in the spinel-type γ-Ga(2)O(3) nanocrystal host lattice, until reaching the substitutional incorporation limit of ca. 25%. The emission decay dynamics is also strongly influenced by the nanocrystal structure and composition. The oxygen vacancy defects, responsible for the observed photoluminescence properties, are also implicated in other functional properties, particularly conductivity, enabling the application of colloidal GIO nanocrystals as integrated optoelectronic materials.