Rod-shaped nanocrystals

To date, experimental nanocrystals fashioned from semiconductors have all been shaped like dots or spheres, but now, researchers with the US Department of Energy’s Berkeley Lab have learned to make semiconductor nanocrystals shaped like rods.

‘We have demonstrated that controlling the kinetics of semiconductor nanocrystal growth can be used to vary the shapes of the resulting particle from a nearly spherical morphology to a rod-like one,’ said Paul Alivisatos, the leader of the experimental team. Alivisatos claims that these rod-like nanocrystals may prove advantageous in biological labelling experiments and as chromophores in light-emitting diodes.

Optical and other properties of nanocrystals are partially dependent upon shape and until now, all non-metal nanocrystals have been dot-shaped, meaning they are essentially one-dimensional.

However, by carefully maintaining a relatively fast rate of growth in the right mix of surfactant, the Berkeley researchers have been able to induce crystals of a selected size to assume an elongated rod-like faceted shape that maximises crystal surface area. Subsequent tests have shown that these rod-shaped nanocrystals emit light that is polarised along their long-axis in contrast to the non-polarised light fluoresced by cadmium-selenide nanocrystal dots.

Other tests showed that the gap between emission and absorption energies is larger for nanocrystal rods than for nanocrystal dots which Alivisatos says should be an advantage in applications such as Light-Emitting Diodes (LEDs) where the re-absorption of light can be a problem. It was also shown that the multiple rods could be packed and aligned, another advantage for both LEDs and for the use of these rods in photovoltaic cells.