Voltage increases of up to 25 percent in two barely separated nanowires should help designers of next generation phones, handheld computers, batteries and solar arrays.
“People have been working on nanowires for 20 years,” said Sandia National Laboratories Lead Researcher Mike Lilly. “At first, you study such wires individually or all together, but eventually you want a systematic way of studying the integration of nanowires into nanocircuitry. That’s what’s happening now. It’s important to know how nanowires interact with each other rather than with regular wires.”
Though the gallium-arsenide nanowire structures used by Lilly’s team are fragile, nanowires in general have very practical characteristics. They may crack less than their bigger cousins, they’re cheaper to produce and they offer better electronic control.
For years, the best available test method required researchers to put a charged piece of material called a gate between two nanowires on a single shelf. The gate, flooded with electrons, acted as a barrier: It maintained the integrity of the wires on either side of it by repelling any electrons attempting to escape across it. But the smallest wire separation allowed by the gate was 80 nanometers. Nanowires in future devices will pack together much more closely, so a much smaller gap was necessary for testing.
The current test design is simple. What Lilly and co-workers at McGill University in Montreal envisioned was to put the nanowires one above the other, rather than side by side, by separating them with a few atomic layers of extremely pure, home-grown crystal. This allowed them to test nanowires separated vertically by only 15 nanometers — about the distance next-generation devices will require. Because each wire sits on its own independent platform, electrical inputs varied by the researchers can independently feed and control the wires.
While applications for technical devices interest Lilly, it’s the characteristics of nanowires as a problem in one-dimensional (1-D) basic science that fascinates him.
A 1-D wire is not your common, thick-waisted, 3-D household wire, which allows current to move horizontally, vertically, and forward; nor is it your smaller, flattened micron-sized 2-D wires in typical electronic devices that allow electrons to move forward and across but not up and down. In 1-D wires, the electrons can only move in one direction: One behind the other.
“In the long run, our test device will allow us to probe how 1-D conductors are different from 2-D and 3-D conductors,” Lilly said. “They are expected to be very different, but there are relatively few experimental techniques that have been used to study the 1-D ground state.”
One reason for the difference is the Coulomb force, responsible for the Coulomb “drag” effect, regardless of whether the force hastens or retards currents. Operating between wires, the force is inversely proportional to the square of the distance; that is, in ordinary microelectronics, the force is practically unnoticeable, but at nanodistances, the force is large enough that electrons in one wire can “feel” the individual electrons moving in another placed nearby.
The drag means the first wire needs more energy because the Coulomb force creates, in effect, increased resistance. “The amount is very small,” said Lilly, “and we can’t measure it. What we can measure is the voltage of the other wire.”
There are no straightforward answers as to why the Coulomb force creates negative or positive drag, but it does. The force got its name from 18th century scientist Charles August Coulomb.
What researchers do know is “enough electrons get knocked along that they provide positive source at one wire end, negative at the other,” Lilly said. “A voltage builds up in the opposite direction to keep electrons in place,” thus increasing drag.