Nanoscale silicon: A really cool hot spot

Posted on 04. Oct, 2011 by in Electrical and Computer Engineering, Issues, Research, Summer: Annual Report

Irena Knezevic and postdoctoral researchers

Associate Professor Irena Knezevic (pictured with postdoctoral research associate Edwin Ramayya, left, and postdoctoral fellow Zlatan Aksamija). Photo: Narayan Mahon.

Silicon’s ability to dissipate heat, a property called high thermal conductivity, is part of the reason it is a popular material for electronics applications.

Yet when silicon is reduced to the nanoscale, it displays a very different property, becoming an efficient thermoelectric material that can convert heat into electricity (a process called energy harvesting) or produce cooling via an electrical current.

Electrical and Computer Engineering Associate Professor Irena Knezevic (pictured with postdoctoral research associate Edwin Ramayya, left, and postdoctoral fellow Zlatan Aksamija), and Erwin W. Mueller Professor and Bascom Professor of Surface Science and Materials Science and Engineering Max Lagally, and Physics Professor Mark Eriksson are exploring ways to develop silicon nanostructures that best take advantage of the material’s thermoelectric properties. Knezevic is an expert in microscopic computer simulations, and her group helps guide experiments by Lagally, an expert in materials growth, and Eriksson, who measures the thermal and electrical properties of nanostructures.

Semiconductors with good thermoelectric properties typically are materials with a heavy atomic weight, such as bismuth telluride. However, tellurium-based materials are toxic and rare, which is a barrier to developing thermoelectric devices beyond research labs. Silicon is the most abundant semiconducting material.

Thermoelectric devices are designed to preserve heat at one end and cooler temperatures at the other; good thermal conductivity does exactly the opposite by dissipating heat evenly to both ends. When silicon is grown so that at least one dimension is at the nanoscale, its nano-boundary scatters the quantum lattice vibrations, or phonons, that serve as primary heat carriers. This scattering is an effective way to reduce a material’s thermal conductivity.

Knezevic has discovered that growing silicon nanowires to a range of 20 to 50 nanometers is the optimal size to scatter phonons but not charge-carrying electrons. This range drastically reduces silicon’s thermal conductivity and a quantity known as the thermoelectric figure of merit, making it the “sweet spot” for silicon to demonstrate its optimal thermoelectric properties. The researchers also are evaluating the thermoelectric behavior of various silicon nanomembranes with quantum dots inclusions and membrane-based heterostructures. “Nanostructuring enables you to get surprising performance out of household materials, and we’re using silicon in ways we wouldn’t normally envision,” Knezevic says.

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