1/23/2009 Charlie Johnson, ECE Illinois
Imagine spending six years trying to engineer strained films only a few nanometers thick, to prevent them from curling up and destroying themselves. Now, imagine receiving a five year grant from the National Science Foundation to exploit that exact same phenomenon. For ECE Assistant Professor and Beckman Institute affiliate Xiuling Li, that's exactly what happened.
Written by Charlie Johnson, ECE Illinois
“The last thing I wanted on a large semiconductor device wafer is edge buckling, and now we’re asking ourselves, ‘what can we do with it?’ if we allow the buckling to occur on a nanometer scale. It’s really thinking outside the box,” said Li.
Li’s NSF Career Award is funding her research on semiconductor nanotubes, which are elongated, microscopic structures with a hollow center and a diameter as small as three nanometers. The tubes are formed when a thin film of material with a smaller lattice constant is placed on top of an outer layer with a larger lattice constant. The two layers pull and push in opposite directions, creating a natural buckling effect and forming a tube.
While other forms of nanotubes, specifically carbon nanotubes already exist, Li’s method combines both “bottom up” and “top down” methodology of construction to produce this new type of nanotubes. “Bottom up” construction allows scientists to build a nanostructure atom by atom or layer by layer, but with very little control over the assembly and placement. The “top-down” method allows scientists to build a more orderly nanostructure array, but it places limitations on how small they can make the nanostructures. By taking advantage of the two aspects, “bottom-up” to determine the diameter and “top-down” to determine the nanotubes placement, Li can build smaller, better organized nanotubes. The inner wall of the tube surface can be made functional with chemical and biological molecules, as well as metals and insulators. And, because of the nature of compound semiconductors, active device structures, such as quantum dots, nanowires, or quantum wells can be embedded in the tube wall to provide the nanotube with far more functionality and versatility. “It helps that we have the best facility for compound semiconductor research here at Illinois,” said Li.
Li’s research, while groundbreaking, is still in an intermediate stage. “My group has made significant advancement to this field. We are at a stage where we know how to fabricate, and we know how to embed these active structures. We are actively studying the fundamental physics of these active nanotubes and exploring the application side of how we can use these for something that’s not possible with a two dimensional structure,” said Li.
And according to her, the potential applications of the nanotubes are endless. Nanotube resonators could be useful in quantum information processing and high-sensitivity biological sensors. Because of the unique characteristics of the semiconductor nanotube, molecules could be attached onto the walls of passive or active nanotubes and released in a controlled manner when excited by a light source, which could have potential medical applications. Creating tiny fluid channel assemblies for biological applications have been studied as well.
In just one year, Li’s research on semiconductor nanotubes, with her graduate student Ik Su Chun as the lead author, has been published in the IEEE Transactions on Nanotechnology, the Journal of Crystal Growth, and Journal of Physics D, as well as MRS proceedings. With all the progress being made, Li is excited that there are vast discoveries to be made, even when dealing with the smallest of structures.