Nanometre-size inorganic dots, tubes and wires exhibit a wide range of electrical and optical properties that depend sensitively on both size and shape, and are of both fundamental and technological interest. In contrast to the syntheses of zero-dimensional systems, existing preparations of one-dimensional systems often yield networks of tubes or rods which are difficult to separate. And, in the case of optically active II-VI and III-V semiconductors, the resulting rod diameters are too large to exhibit quantum confinement effects. Thus, except for some metal nanocrystals, there are no methods of preparation that yield soluble and monodisperse particles that are quantum-confined in two of their dimensions. For semiconductors, a benchmark preparation is the growth of nearly spherical II-VI and III-V nanocrystals by injection of precursor molecules into a hot surfactant. Here we demonstrate that control of the growth kinetics of the II-VI semiconductor cadmium selenide can be used to vary the shapes of the resulting particles from a nearly spherical morphology to a rod-like one, with aspect ratios as large as ten to one. This method should be useful, not only for testing theories of quantum confinement, but also for obtaining particles with spectroscopic properties that could prove advantageous in biological labelling experiments and as chromophores in light-emitting diodes.

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We demonstrate that hollow nanocrystals can be synthesized through a mechanism analogous to the Kirkendall Effect, in which pores form due to the difference in diffusion rates between two components in a diffusion couple. Cobalt nanocrystals are chosen as a primary example to show that their reaction in solution with oxygen, sulfur or selenium leads to the formation of hollow nanocrystals of the resulting oxide and chalcogenides. This process provides a general route to the synthesis of hollow nanostructures of large numbers of compounds. A simple extension of this process yields platinum-cobalt oxide yolk-shell nanostructures which may serve as nanoscale reactors in catalytic applications.