Shape control of CdSe nanocrystals

Nanometre-size inorganic dots, tubes and wires exhibit a wide range of elec trical and optical properties(1,2) that depend sensitively on both size and shape(3,4), and are of both fundamental and technological interest. In con trast to the syntheses of zero-dimensional systems, existing preparations o f one-dimensional systems often yield networks of tubes or rods which are d ifficult to separate(5-12). And, in the case of optically active II-YI and III-V semiconductors, the resulting rod diameters are too large to exhibit quantum confinement effects(6,8-10). Thus, except for some metal nanocrysta ls(13), there are no methods of preparation that yield soluble and monodisp erse particles that are quantum-confined in two of their dimensions. For se miconductors, a benchmark preparation is the growth of nearly spherical II- VI and III-V nanocrystals by injection of precursor molecules into a hot su rfactant(14,15). 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 o ne, 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 obtain ing particles with spectroscopic properties that could prove advantageous i n biological labelling experiments(16,17) and as chromophores in light-emit ting diodes(18,19).