Professor James Kakalios is a condensed matter experimental physicist whose research involves the growth and study of materials that combine amorphous and nanocrystalline semiconductors. Kakalios and his colleague and collaborator, Prof. Uwe Kortshagen in Mechanical Engineering at the University of Minnesota are hoping to get the best of both worlds out of these entirely new, hybrid materials. The composites combine the ease of deposition over large areas at low cost provided by amorphous silicon with the superior electronic properties of crystalline silicon.
The group has constructed a unique dual chamber co-deposition system to synthesize nanocrystals of one type of semiconductor material and inject them into a second chamber where a different surrounding matrix amorphous material can be deposited. “We aren’t married to only putting silicon nanocrystals in amorphous silicon,” Kakalios says. “We have synthesized amorphous silicon with germanium nanocrystals, sprinkling [the nanocrystals] in like chocolate chips.” Kakalios and his group then examine the properties of these composite materials as the nanocrystal type, diameter and concentration are systematically varied.
The group has found that in the amorphous silicon/germanium hybrid material, as the concentration of germanium crystals is increased, there is a change from electronic conduction by negatively charged electrons to positively charged holes at higher nanocrystal densities. Normally, to change the polarity of the charge carriers one needs to add new elements, and introduce chemical impurities. “The advantage of adding nanocrystalsis that we can tune their properties based on their size. It frees us from the tyranny of chemistry.” Kakalios uses the analogy of stained glass to explain further. “Window glass is transparent to visible light. If you want to make color the glass red, green, or blue, you have to add specific chemical impurities. With the nanocrystals you can actually change their color just by changing their size.” Kakalios says the change in color comes about thanks to quantum mechanics, but the effect can be seen with the naked eye.
These materials have potential applications in solving real world problems such as making cheap and efficient solar cells. Another possible application is in particle detectors used in high-energy physics. The group could potentially create materials that combine radiation-hard amorphous silicon with efficient nanocrystal semiconductors. Kakalios and his group are also trying to understand the electronic and optical properties of these entirely new materials. “We’re just scratching the surface of what we can do and what properties we can explore.”
The group has adapted the process used to grow amorphous silicon for growing nanocrystals as well. This allows them to tune the nanocrystals, giving them more control over the various properties of the final hybrid. “There are two reasons there is interest in nano-materials: they’re small and they’re very small.” He explains that a one nanometer particle has 200 atoms, 140 of which are on the surface. “It’s all surface, which makes it ideal for so many applications.” The small size also allows physicists to manipulate the quantum mechanical properties of the materials. “You have electrons inside this tiny box. The box is so small the electrons know they’re in a box and quantum mechanical effects come into play. In a wire that’s 12 inches long you wouldn’t see these effects. If your wire is only a nanometer long you can’t ignore them."