Speed Limits

 

When a beam of light impinges upon a semiconductor surface, some of the photons are absorbed in the material and “excite” it. Electrons inside the material that were previously fixed in their positions are now free to move. In doing so, they exert force on one another and on the material’s atomic nuclei. Dr. Dan Oron of the Faculty of Physics studies such interactions taking place in tiny crystals, less than ten nanometers in size, each containing no more than a few thousand atoms. Electron motion in such small bits of material is dramatically affected by the restricted volume. Thus, at this scale, the optical properties of the particle are directly linked to its size and shape. The dynamics of optical excitation are difficult to study since they are incredibly rapid. Oron overcomes this hurdle by using laser flashes lasting just a few millionths of a billionth of a second.

 

The insight gained through these studies can, among other things, aid in the development of the next generation of solar collectors. In photovoltaic cells based on organic dyes, electrons in the dye molecules, excited by solar radiation, can be harvested and used for electricity generation. Today, the stability and efficiency of such cells (measured in electricity production per unit of area) are insufficient for practical use. Semiconductor nanoparticles may remedy the situation. These could function as tiny “antennae” that absorb sunlight and funnel it to the layer of organic dye molecules, greatly improving the effectiveness of such devices.