For the first time, armed with the ability to measure the heat transfer through a single molecule, a team of international researchers from the University of Michigan and elsewhere believe the discovery could be a step toward molecular computing.
In essence, building circuits that better dissipate heat at the molecular level could make computers more powerful and efficient.
“Heat is a problem in molecular computing because the electronic components are essentially strings of atoms bridging two electrodes,” says Edgar Meyhofer, professor of mechanical engineering at U-M. “As the molecule gets hot, the atoms vibrate very rapidly, and the string can break.”
The transfer of heat along these molecules previously could not be measured or controlled. Meyhofer and Pramod Reddy, also a professor or mechanical engineering at U-M, have led the first experiment observing the rate at which heat flows through a molecular chain. Other researchers on the team were from Japan, Germany, and South Korea.
“While electronic aspects of molecular computing have been studied for the past 15 or 20 years, heat flows have been impossible to study experimentally,” says Reddy. “The faster heat can dissipate from molecular junctions, the more reliable future molecular computing devices could be.”
Meyhofer and Reddy have been building the capacity to conduct this experiment for nearly 10 years and have developed a heat-measuring device, or calorimeter, that is almost totally isolated from the rest of the room, giving it extreme thermal sensitivity. It is heated to about 20-40 degrees Celsius above room temperature.
The device is equipped with a gold electrode with a nanometer-sized tip, roughly one thousandth the thickness of a hair. The team put a coating of molecules – chains of carbon atoms – on another electrode at room temperature.
The two electrodes were then brought together until they just barely touched, attaching some chains of carbon atoms to the calorimeter’s electrode. With the electrodes intact, heat flowed from the calorimeter, along with an electrical current. The researchers then drew the electrodes apart slowly so that only the chains of carbon atoms connected them.
As they separated, the chain ripped or dropped away. The team used the amount of electrical current flowing across the electrodes to deduce how many molecules remained and calculated the current expected when just one molecule remained.
When a single molecule remained between the electrodes, the team held the them at that separation point until the molecule broke away on its own. This caused a sudden, tiny rise in the temperature of the calorimeter, which the team used to calculate how much heat had been flowing through the single-molecule carbon chain.
They conducted heat flow experiments with carbon chains between two and 10 atoms long, but the length of the chain did not seem to affect the rate at which heat moved through it. This is different than what happens in the macroscopic world, where the thermal conductance falls as the length of the material increases.
Predictions suggest that heat’s east of movement at the nanoscale holds up even as the molecular chains get much longer. The team is exploring how to test this theory.
The study was published in the journal Nature and was funded by the U.S. Office of Naval Research, Department of Energy, National Science Foundation, Korean National Research Foundation, and German Research Foundation.