Formation of Colloidal Semiconductor Nanocrystals

The present work describes different techniques to control some ma jor parameters of colloidal nanocrystals. The individual techniques rely on the manipulation of the nucleation event. The sensitive control of the nanocrystals’ size and shape is discussed. Furthermore the formation of hybrid nanocrystals composed of different materials is presented. The synthesis technique for the production of the different samples involves organic solvents and surfactants and reactions at elevated temperatures. The presence of magic size clusters offers a possibility to control the size of the nanocrystals even at very small dimensions. The clusters produced comprise ca. 100 atoms. In the case of CdSe, nanocrystals of this size emit a blue fluorescence and therefore extend the routinely accessible spectrum for this material over the whole visible range. Samples fluorescing in the spectral range from green to red are produced with standard recipes. In this work a reaction scheme for magic size clusters is presented and a theoretical model to explain the particular behaviour of their growth dynamics is discussed. The samples are investigated by optical spectroscopy, transmission electron microscopy, X-ray diffraction and elemental analysis. Shape controlled nanocrystals might be of interest for a variety of applications. The size dependent properties of nanocrystals are dominated by their smallest dimension. Therefore anisotropically shaped nanocrystals exhibit similar optical and electronic properties as spherical nanocrystals with a compatible diameter. This makes nanorods and nanowires an appealing object for electronics. Another possible application for these materials is to incorporate them into synthetic materials to influence their mechanical stability. Here, a method to form branched nanocrystals is discussed. It turned out that the presence of small impurities in the reaction vessel triggers the formation of branching points. Furthermore this synthesis technique offers some insights into the architecture of the branching point. The branching point is analysed by high resolution transmission electron microscopy and proves for the occurrence of a multiple twinned structure are strengthened by simulation of the observed patterns. Incorporation of a second material into a nanocrystal adds different functionality to the entire ob ject. Ideally both materials contribute with their own functionality and they are not affected by the presence of the other material. Two different techniques to generate nanocrystals of this type are presented. The first relies on a seeded growth approach in which the nucleation of the second material is allowed only on defined sites of the seeds. Anisotropic nanorods show a reactivity that varies for the individual facets. Using such nanorods as seeds dumbbell structures are formed. The second technique uses the tips of pre-formed nano-dumbbells as sacrificial domains. The material on the tips is replaced by gold. In any of the processes a different aspect of the nucleation event or the earliest stage of the growth is of relevance. In the growth of the magic size clusters the nucleation event itself is slowed down to a pace at which the experimenter can follow any step. The occurrence of branching can be traced down to the emergence of defects in the crystalline structure in the earliest stage of the growth. Hybrid materials are formed by a seeded-growth mechanism. Pre-formed nanocrystals provide the nucleation sites for the second material

Synthesis and perspectives of complex crystalline nano‐structures

Research on inorganic colloidal nanocrystals has moved from the synthesis of simple structures, such as spherical nanoparticles, to more elaborate particles with shapes such as rods, stars, discs, and branched nanocrystals, and recently to nanoparticles that are composed out of sections of different materials. Nanocrystal heterostructures represent a convenient approach to the development of nanoscale building blocks, as they merge sections with different functionality in the same particle, without the need of inorganic cross-linkers. The present article gives an overview of synthesis strategies to complex nanocrystals and will highlight their structural properties, as well as discuss some envisaged applications

Selective reactions on the tips of colloidal semiconductor nanorods

A strategy to access several types of Au-tipped dumbbell-like nanocrystal heterostructures is presented, which involves the selective oxidation of either PbSe or CdTe sacrificial domains, initially grown on CdSe and CdS nanorods, with a Au(III) : surfactant complex. The formation of gold patches is supported by TEM, XRD and elemental analysis. This approach has allowed us to grow Au domains onto specific locations of anisotropically shaped nanocrystals for which direct metal deposition is unfeasible, as for the case of CdS nanorods. We believe that this strategy may be of general utility to create other types of complex colloidal nanoheterostructures, provided that a suitable sacrificial material can be grown on top of the starting nanocrystal seeds

Multiple wurtzite twinning in CdTe nanocrystals induced by methylphosphonic acid.

Branching in semiconductor nanocrystals, which leads to tetrapods and to more complex architectures, is the subject of intensive investigation. Here we support the model according to which branching in CdTe nanocrystals is driven by the formation of multiple wurtzite twins. This is in contrast to previous models for this material. We found that twinning, as well as anisotropic growth, can be triggered by the presence of suitable molecules, such as for instance methylphosphonic acid. In the case of CdTe nanocrystals, we designed a robust growth scheme in which the variation of a single parameter (the concentration of methylphosphonic acid in solution) leads to the controlled formation of nanocrystals with shapes ranging from spheres to anisotropic structures with varying level of branching, as both twinning and anisotropic growth are progressively favored. We believe that these concepts can be extended to other nanocrystal systems

Exciton transitions in tetrapod-shaped CdTe nanocrystals investigated by photomodulated transmittance spectroscopy

The excitonic nature of the optical transitions in tetrapod-shaped colloidal CdTe nanocrystals is assessed by means of photomodulated transmittance spectroscopy. The line-shape analysis of the photomodulation transmittance spectra indicates the photoinduced Stark effect as the dominant modulation mechanism, and the presence of excitonic transitions even at room temperature, with an exciton binding energy of about 25meV, larger than the bulk value

Selective growth of PbSe on one or both tips of colloidal semiconductor nanorods.

PbSe nanocrystals with rock-salt structure are grown on the tips of colloidal CdS and CdSe nanorods. The facets of wurtzite rods provide a substrate with various degrees of reactivity for the growth of PbSe. The presence of dangling Cd bonds may explain subtle differences between nonequivalent facets resulting in the selective nucleation of PbSe only on one of the two tips of each CdS rod. This approach has the potential to facilitate the fabrication of heterostructures with tailored optical and electronic properties

Blue light emitting diodes based on fluorescent CdSe∕ZnS nanocrystals

The authors report on the blue electroluminescence from CdSe∕ZnS core/shell nanocrystals prepared from ultrasmall, magic size CdSe clusters that have a diameter of less than 2nm. The light emitting device consists of an active layer of nanocrystals blended with 4,4′,N,N′- diphenylcarbazole and an evaporated electron transporting/hole blocking layer made of 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline. A blue, stable electroluminescence at 485nm from the hybrid device was observed, in good agreement with the photoluminescence spectra of a solid film of the same nanocrystals used for the device

Confinement effects on optical phonons in spherical, rod-, and tetrapod-shaped nanocrystals detected by Raman spectroscopy

Spherical, rod- and tetrapod shaped CdSe nanocrystals are investigated by Raman spectroscopy and the longitudinal-optical and surface optical phonons are observed. We find that the position of the longitudinal-optical phonon slightly red-shifts with decreasing diameter, whereas the position of the surface optical phonon depends significantly on diameter and length of the rods or the tetrapod arms