INDUSTRY NEWS

Spare the rod

In the infinitesimal world of nanotechnology it has been virtually impossible to make experimental semiconductor nanocrystals in shapes other than dots or spheres. Dots made of semiconductors such as cadmium selenide emit light of multiple colors and are effective for a variety of potential uses, including fluorescent probes for studying biological materials, according to researchers at Lawrence Berkeley National Laboratory.

However, the optical properties of the crystals depend on shape. The problem was that all nonmetal nanocrystals were essentially one-dimensional dots, says Paul Alivisatos, head of a research team at the lab's materials sciences division. And those limited shapes limited the uses of the nanocrystals. All that changed when Alivisatos and his team invented a method to make semiconductor nanocrystals shaped like rods.

By controlling the kinetics of semiconductor nanocrystal growth the researchers are able to change the shapes of the resulting particles, Alivisatos says. To grow crystals shaped like elongated rods that maximize the crystals' surface area, the researchers maintain a relatively fast growth rate using the right mix of surfactant. Tests show that the rod-shaped nanocrystals emit light that is polarized along their long axis.

In contrast, cadmium-selenide nanocrystal dots emit nonpolarized light. The polarized emissions along the rods' long axis can be used in biological tagging experiments "where the orientation of the tag needs to be determined," he points out. Packed and aligned just so, the rods can also be used in both LEDs and photovoltaic cells.

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