Researchers from the Lawrence Berkeley National Laboratory and the University of California at Berkeley are on a process working to make semiconductor nanocrystals shaped like rods. Until now, all nonmetal nano-crystals have been only one-dimensional, appearing as either dots or spheres. This new two-dimensional form may prove to be ideal for use as chromophores in light-emitting diodes (LEDs).
To achieve the uniform shape, scientists began with a cadmium-selenide sample. As a fast growth rate in the right mix of surfactant was realized, crystals of a selected size were forced into elongated rod-like shapes (see the figure). In turn, crystal surface area was maximized.
When the resulting rod-shaped crystals were tested, it was discovered that they emit polarized light along their long-axis. In comparison, light fluoresced by dot-shaped cadmium-selenide nanocrystals is nonpolarized.
Further testing showed that the gap between emission and absorption energies is larger for the nanocrystal rods than for nanocrystal dots. This is an ideal characteristic for applications such as LEDs, where the re-absorption of light can be a problem. Also benefitting LEDs is the fact that multiple rods can be packed and aligned. In addition, this feature enables the use of these rods in photovoltaic cells.
For more information on this project and results of subsequent testing, go to www.berkeley.edu.