Single-molecule electric motor
First electric motor powered by quantum tunneling at molecular scale.
The world’s smallest electric motor is a single butyl methyl sulphide molecule, measuring just one nanometer across—roughly 60,000 times thinner than a human hair. This synthetic molecular motor, operated by electricity, was created by the Sykes group and researchers at the Tufts University School of Arts and Sciences. Unlike earlier single-molecule motors, which relied on chemical reactions or light for power, this one is the first to demonstrate that electrical energy can be directly converted into directed molecular rotation.
The motor works by adsorbing the butyl methyl sulphide molecule onto a copper (111) single-crystal surface through chemisorption. In the gas phase, this asymmetrical thioether is achiral, but when it binds to the surface via one of the sulfur’s two lone pairs, it gains surface-bound chirality. This asymmetry creates an uneven barrier to rotation around the sulfur-copper bond. Electrons quantum tunnel from the tip of a scanning tunneling microscope (STM), electrically exciting molecular vibrations that couple to rotational modes. The rotation rate and direction are controlled by adjusting the electron flux from the STM and the background temperature. The STM tip acts as an electrode, and the chirality of both the tip and the molecule influences how fast and in which direction the molecule spins.
At 5 K, under non-perturbative scanning conditions, the molecule appears as a crescent-shaped protrusion in STM images. When the temperature rises to 8 K, it begins rotating through six orientations, dictated by the hexagonal structure of the copper surface. Because the imaging timescale is much slower than the rotation, the molecule then appears as a hexagon in STM images. To determine the rate and direction of rotation, the STM tip is aligned asymmetrically on one lobe of the molecule during spectroscopy. The tunneling current is highest when the butyl tail is nearest the tip and lowest when it is farthest. By plotting the molecule’s position over time, researchers can track its rotation. At higher temperatures, such as 100 K, the motor spins too fast—up to one million rotations per second—to monitor directly.
Potential applications include engineering, nanotechnology, and medicine, where the motor could deliver drugs to precise locations.
- Size
- 1 nanometer (billionth of a meter) across
- Size comparison
- 60,000 times smaller than the thickness of a human hair
- Material
- butyl methyl sulphide molecule
- Substrate
- copper (111) single-crystal piece
- Adsorption method
- chemisorption
- Publication date
- September 4, 2011
- Developers
- Sykes group and scientists at the Tufts University School of Arts and Sciences
Lore & Background
Butyl methyl sulfide is an asymmetrical thioether which is achiral in the gas phase. When adsorbed onto the copper surface through either of the sulfur's two lone pairs, the molecule acquires surface-bound chirality. This asymmetry creates an asymmetrical barrier to rotation. The molecule rotates around the sulfur-copper bond. Electrons quantum tunneling from the scanning tunneling microscope (STM) tip electrically excite molecular vibrations, which couple to rotational modes. The rotation can be controlled by adjusting the electron flux from the STM and the background temperature. The tip of the STM acts as an electrode, and the chiralities of the tip and the molecule determine the rate and direction of rotation. At 5 K, non-perturbative scanning shows a crescent-shaped protrusion of the molecule. At 8 K, the molecule rotates along six orientations determined by the hexagonal structure of the copper, and the STM image appears as a hexagon because the imaging timescale is much slower than the rotation rate. The six rotational states can be determined by aligning the STM tip asymmetrically on one lobe of the molecule during spectroscopy; the tunneling current is maximum when the butyl tail is nearest the tip. By plotting position versus time, the rate and direction of rotation can be determined. At higher temperatures, the motor rotates too fast to monitor, reaching up to one million rotations per second at 100 K.
Reader's Guide
The single-molecule electric motor represents a significant step in molecular-scale electromechanical systems. According to the article, it can be efficiently used in engineering, nanotechnological applications, and medicinal applications, where drugs could be delivered to specified locations more accurately. By altering the chemical structure of the molecule, it could become a component of a nanoelectromechanical system (NEMS). It also has potential to be utilized to generate microwave radiation. The motor's operation via quantum tunneling of electrons from an STM tip, and its control through electron flux and temperature, establish a method for electrically driving directed molecular rotation. This contrasts with earlier molecular motors that relied on chemical reactions or light. The ability to monitor rotation at low temperatures and determine rate and direction via tunneling current provides a means to study and exploit molecular dynamics. The motor's small size—a nanometer across—and its operation on a copper surface highlight the feasibility of integrating such devices into nanoscale systems.
Did You Know?
- The single-molecule electric motor is made from a single butyl methyl sulphide molecule.
- It is 60,000 times smaller than the thickness of a human hair.
- At 100 K, the motor can rotate up to one million times per second.
- The direction and rate of rotation are determined by the chiralities of the STM tip and the molecule.
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