Semiconductor nanowires have intrigued scientists for many years as they provide many opportunities to study and apply phenomena at the nanoscale. With diameters as small as a few billionths of a meter they hold promise for devices of the future, both in technology like light-emitting diodes and in new versions of transistors and circuits for next generation devices.

Because nanowires are so small, they also bring about continual challenges in terms of controlling synthesis, properties, and application. For instance researchers have still not been able to accurately determine just how much of the dopant gets into the nanowire during synthesis. Previously, researchers could not measure the amount of dopant and had to crudely judge the success of the synthesis based on indirect measurements of the conductivity of the nanowire devices. That meant that variations in device performance were not readily explained.

Using a technique called atom probe tomography, Lincoln Lauhon, assistant professor of materials science and engineering at Northwestern University's McCormick School of Engineering and Applied Science [Perea et al., DOI: 10.1038/NNANO.2009.51], has provided an atomic-level view of the composition of a nanowire. Now by being able to precisely measure the amount of dopant in a nanowire, researchers can finally understand the synthesis process on a quantitative level and better predict the electronic properties of nanowire devices.

“We simply mapped where all the atoms were in a single nanowire, and from the map we determined where the dopant atoms were,” said Lauhon. “The more dopant atoms you have, the higher the conductivity.”

Lauhon went on to say, “If we can understand the origin of the electrical properties of nanowires, and if we can rationally control the conductivity, then we can specify how a nanowire will perform in any type of device,” he says. “This fundamental scientific understanding establishes a basis for engineering.”

Lauhon and his group performed the research at Northwestern's Center for Atom Probe Tomography, which uses a Local Electrode Atom ProbeTM microscope to dissect single nanowires and identify their constituents. This instrumentation software allows 3-D images of the nanowire to be generated, so Lauhon could see from all angles just how the dopant atoms were distributed within the nanowire. They found that differences in precursor decomposition rates between the liquid catalyst and solid nanowire surface give rise to a heavily doped shell surrounding an underdoped core.

Professor Lauhon continues work in this area, “We would like to establish the general principles for doping semiconductor nanowires.”