Controlled growth of tetrapod-branched inorganic nanocrystals.

Nanoscale materials are currently being exploited as active components in a wide range of technological applications in various fields, such as composite materials, chemical sensing, biomedicine, optoelectronics and nanoelectronics. Colloidal nanocrystals are promising candidates in these fields, due to their ease of fabrication and processibility. Even more applications and new functional materials might emerge if nanocrystals could be synthesized in shapes of higher complexity than the ones produced by current methods (spheres, rods, discs). Here, we demonstrate that polytypism, or the existence of two or more crystal structures in different domains of the same crystal, coupled with the manipulation of surface energy at the nanoscale, can be exploited to produce branched inorganic nanostructures controllably. For the case of CdTe, we designed a high yield, reproducible synthesis of soluble, tetrapod-shaped nanocrystals through which we can independently control the width and length of the four arms.

Shape control and applications of nanocrystals.

Inorganic nanocrystals with well-defined shapes are important for understanding basic size-dependent scaling laws, and may be useful in a wide range of applications. Methods for controlling the shapes of inorganic nanocrystals are evolving rapidly. This paper will focus on how we currently control the shape of nanocrystals and this will be illustrated using CdSe and Co nanocrystals as examples for semiconductors and for metals. These materials show a more pronounced variation of fundamental properties with aspect ratio. However, to take advantage of these shape-dependent properties in possible applications, several challenges need to be overcome. Issues such as alignment, high quantum yield and photostability and precise control of three-dimensional structures need to be addressed. These challenges, as well as several potential applications, will be described briefly.

Epitaxial growth and photochemical annealing of graded CdS/ZnS shells on colloidal CdSe nanorods.

We report the preparation and structural characterization of core/shell CdSe/CdS/ZnS nanorods. A graded shell of larger band gap is grown around CdSe rods using trioctylphosphine oxide as a surfactant. Interfacial segregation is used to preferentially deposit CdS near the core, providing relaxation of the strain at the core/shell interface. The reported synthesis allows for variation of the shell thickness between one and six monolayers, on core nanorods ranging from aspect ratios of 2:1 to 10:1. After an irreversible photochemical annealing process, the core/shell nanorods have increased quantum efficiencies and are stable in air under visible or UV excitation. In addition to their robust optical properties, these samples provide an opportunity for the study of the evolution of epitaxial strain as the shape of the core varies from nearly spherical to nearly cylindrical.