Volume 4, Issue 31 September 2007
A Giant in a Small, Small World
Professor Steven Louie. Photo credit: Roy Kaltschmidt, Lawrence Berkeley National Laboratory
For those who know how to read it, the Periodic Table of the Elements contains a wealth of information. From the numbers scattered about each square of that traditional piece of chemistry classroom wall decor, you can look up an element's boiling point, its atomic weight-even whether it will float in water.
To Steven Louie, professor of physics, the most important of these is an element's atomic number. From this simple count of an atom's protons, he can forecast the physical properties of any material.
How electrons quiver in a crystal lattice, lock two atoms together in a mutual embrace, or skitter along a length of wire can explain why silicon is a semiconductor, why iron rusts, why gold is yellow and not blue. Over the last 30 years, Louie has developed theories and computer algorithms that can calculate these behaviors with great accuracy. For example, he developed the elaborate equations and numerical techniques needed to predict how much energy is needed to excite an electron into conducting electricity for any material. This information is critical for developing new materials and designs for electronics.
A metal-semiconductor junction of carbon nanotubes. Image credit: Vin Crespi
More recently, Louie's methods have become indispensable in a much smaller arena-nanoscience, the study of very, very small, objects 100,000 times smaller than the diameter of a human hair. Louie's discoveries and predictions are the foundation upon which much of the field now stands.
"Shrinking objects to the nanoscale makes their properties change," Louie says. "As the dimensions of an object get small enough to be comparable to the wavelength of an electron, its geometry starts affecting the properties of the system."
One of Louie's specialties is the study of nanotubes. Made of atomic mesh rolled into a seamless, hollow cylinder just one nanometer in diameter, they bear an uncanny resemblance to a roll of ordinary chicken wire. But nothing is ordinary about nanotube physics. The very first nanotubes discovered were made of carbon. They are the strongest fibers known to man, can conduct electrons with ballistic speed, and serve as wires for electronic devices too small to see with the naked eye.
A view down the center of a boron nitride nanotube. Image credit: Vin Crespi
Louie has been instrumental in forecasting how such molecule-sized components might behave. Together with fellow Berkeley physics professor Marvin Cohen, Louie predicted the existence of an entirely new class of nanotubes made of boron, carbon, and nitrogen atoms. Their colleague, physics professor Alex Zettl, fabricated the new materials, which were just as Louie and Cohen had predicted.
Louie has also predicted that one nanotube could be joined to another of a slightly different weave to form a metal-semiconductor junction. The structure can serve as a diode, an undirectional barrier to electrons flowing down its length. This and Louie's many other predictions of the nanoworld, such as the highly unconventional optical behavior of nanotubes, have been confirmed by experiments.
Louie also directs the Theory Facility of the Molecular Foundry, a Department of Energy center at the Lawrence Berkeley National Laboratory near campus, where he interacts with scientists from many other fields who are working on nanoscience. The physicists, chemists, material scientists, biologists, and other experimentalists there both help spur his creativity and usher his ideas into being. "The idea is to have a firm grounding of your theories with experiments," he says, "but sometimes you also like to let your imagination take over."