Tufts researchers have built the smallest electric motor ever—it consists of just one molecule. Tufts research team has developed the world’s first single-molecule electric motor—which is a mere 1 nanometer across. They reported the results in a paper published in Nature Nanotechnology on Sept. 4. This development—made possible with a low-temperature scanning tunneling microscope at Tufts, one of only about 100 in the United States—may be the first step toward a new class of devices that could be used in applications ranging from medicine to engineering.
Sykes and his colleagues used the metal tip of the microscope to provide an electrical charge to a butyl methyl sulfide molecule that had been placed on a copper surface. The molecule had a sulfur atom at the center and carbon atoms radiating off to form two arms, so to speak: four carbons on one side, one on the other. In subsequent experiments, such arms could potentially act as interlocking cogs or gears, and as one molecule is powered, it could turn or rotate others in sequence.
In this illustration, the orange represents the copper surface on which the molecular motor is resting. The yellow ball is the molecule’s sulfur base, and the two arms are composed of carbon and hydrogen atoms. The power source above the device is the tip of a scanning tunneling microscope, which uses electricity to direct the molecule to rotate in one direction or another. Illustration: Sykes Laboratory
Sykes cautions that practical applications of the single-molecule electric motor are distant. But he imagines it could be used, for example, in sensing and medical test devices that involve tiny pipes. “At these small scales, friction of the fluid against the pipe walls increases, and covering the walls with motors may help drive the fluids along,” he says. Molecular electric motors may also be of use in nanoelectromechanical systems (NEMS). For instance, coupling molecular motion with electrical signals may allow scientists to build signal delay lines and nanoscale sensors.
Before realizing these potential applications, breakthroughs would have to be made in the temperatures at which electrically driven molecular motors operate. For the experiment that Sykes did at Tufts, the temperature surrounding the molecular motor had to be reduced to 5 Kelvin—a chilly minus 450 degrees F.
That’s because as temperatures rise, the motor spins much faster, far beyond the ability of the scientists to measure the rotations. At 100 Kelvin—or minus 279 degrees F—a molecular motor spins more than a million times per second. “It’s not that we couldn’t work at a higher temperature—it’s just that too much is happening,” says Sykes. “At that speed, it’s just a blur.”
At the colder temperature that the Sykes group used, the motor went through about 50 rotations per second, which were easily measured. But to prove that the motor was being driven by the electricity provided and that the movements were not just random, Sykes’ group had to track all those rotations. “For every single data point in the paper, about 5,000 of these [rotations] were counted,” Sykes says. “You collect data for five minutes, but it takes a week to analyze the data.”
For molecules to be used as components in molecular machines, methods that couple individual molecules to external energy sources and that selectively excite motion in a given direction are required. Significant progress has been made in the construction of molecular motors powered by light and by chemical reactions but electrically driven motors have not yet been built, despite several theoretical proposals for such motors. Here we report that a butyl methyl sulphide molecule adsorbed on a copper surface can be operated as a single-molecule electric motor. Electrons from a scanning tunnelling microscope are used to drive the directional motion of the molecule in a two-terminal setup. Moreover, the temperature and electron flux can be adjusted to allow each rotational event to be monitored at the molecular scale in real time. The direction and rate of the rotation are related to the chiralities of both the molecule and the tip of the microscope (which serves as the electrode), illustrating the importance of the symmetry of the metal contacts in atomic-scale electrical devices