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SIZE MATTERS A LOT IN THE WORLD OF ELECTRONICS
Size matters a lot in the world of electronics and will matter even more in the upcoming age of nanotechnology where devices may be a thousand times smaller than the microchips of today. But shape matters too. To date, experimental nanocrystals fashioned from semiconductors have all been shaped like dots or spheres. No longer. Researchers with the U.S. Department of Energy's Lawrence Berkeley National Laboratory and the University of California at Berkeley have learned to make semiconductor nanocrystals that are shaped like rods. "We have demonstrated that controlling the kinetics of semiconductor nanocrystal growth can be used to vary the shapes of the resulting particles from a nearly spherical morphology to a rod-like one," says Paul Alivisatos, the leader of the experimental team who holds a joint appointment with Berkeley Lab's Materials Sciences Division, and with the UC Berkeley Chemistry Department. "These rod-like semiconductor nanocrystals may prove advantageous in biological labeling experiments and as chromophores in light-emitting diodes." An earlier discovery by Alivisatos and his research group that nanometer-sized crystal dots (spheres a few billions of a meter in size) made from semiconductors such as cadmium selenide can emit multiple colors of light depending upon the size of the crystal opened the door to a number of potential applications including their use as fluorescent probes for the study of biological materials. However, optical and other properties of nanocrystals are also dependent upon shape. Until now, all non-metal nanocrystals have been dot-shaped, meaning they are essentially one-dimensional. No techniques had been reported for making two-dimensional or rod-shaped semiconductor nanocrystals that would also be of uniform size. However, in a paper that appeared in the March 2 issue of the journal Nature, Alivisatos and his colleagues reported on techniques they used to select the size but vary the shapes of the cadmium-selenide nanocrystals they produced. By carefully maintaining a relatively fast rate of growth in the right mix of surfactant, the Berkeley researchers could induce crystals of a selected size to assume an elongated rod-like faceted shape that maximized crystal surface area. Subsequent tests showed that these rod-shaped nanocrystals emit light that is polarized along their long-axis in contrast to the non-polarized light fluoresced by cadmium-selenide nanocrystal dots. "Polarized emission along the long axis of these rods should be helpful in biological tagging experiments where the orientation of the tag needs to be determined," says Alivisatos. 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 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.
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