The single molecule electric motor is an electrically operated motor which is made from a single butyl methyl sulphide molecule. The molecule is adsorbed onto a Copper

Copper

Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...

 (111) single crystal

Single crystal

A single crystal or monocrystalline solid is a material in which the crystal lattice of the entire sample is continuous and unbroken to the edges of the sample, with no grain boundaries...

 piece by chemisorption

Chemisorption

Chemisorption is a sub-class of adsorption, driven by a chemical reaction occurring at the exposed surface. A new chemical species is generated at the adsorbant surface...

. The motor, the world's smallest electric motor, is just a nanometer (billionth of a meter) across (60,000 times smaller than the thickness of a human hair). It was developed by chemists at the Tufts University School of Arts and Sciences

Tufts University School of Arts and Sciences

The School of Arts and Sciences is the largest of the eight schools and colleges that comprise Tufts University. Together with the School of Engineering, it offers undergraduate and graduate degrees in the liberal arts, sciences, and engineering...

 and published online September 4th, 2011.

Background

Single molecules motors have been demonstrated before. These motors were either powered by chemical reactions or by light. This is the first experimental demonstration of electrical energy successfully coupling to directed molecular rotation.

Mechanism of directed rotation

Butyl methyl sulfide ia an asymmetrical thioether

Thioether

A thioether is a functional group in organosulfur chemistry with the connectivity C-S-C as shown on right. Like many other sulfur-containing compounds, volatile thioethers have foul odors. A thioether is similar to an ether except that it contains a sulfur atom in place of the oxygen...

 which is achiral in the gas phase. The molecule can adsorbed to the surface through either of the sulfur's two lone pair

Lone pair

In chemistry, a lone pair is a valence electron pair without bonding or sharing with other atoms. They are found in the outermost electron shell of an atom, so lone pairs are a subset of a molecule's valence electrons...

. This gives rise to the surface bound chirality of the molecule. The asymmetry

Asymmetry

Asymmetry is the absence of, or a violation of, symmetry.-In organisms:Due to how cells divide in organisms, asymmetry in organisms is fairly usual in at least one dimension, with biological symmetry also being common in at least one dimension....

 of the molecular surface interface gives rise to an asymmetrical barrier to rotation. The molecule rotates around this sulfur-copper bond.Electrons quantum tunneling from the STM tip electrically excite molecular vibrations which couple to rotational modes. The rotation of the motor can be controlled by adjusting the electron flux from the scanning tunneling microscope and the background temperature. The tip of the scanning electron microscope acts as an electrode. The chiralities

Chirality (chemistry)

A chiral molecule is a type of molecule that lacks an internal plane of symmetry and thus has a non-superimposable mirror image. The feature that is most often the cause of chirality in molecules is the presence of an asymmetric carbon atom....

 of the tip of the STM and the molecule determine the rate and direction of rotation. Images taken of the molecule at 5 K

Kelvin

The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...

 and under non-perturbative scanning conditions show a crescent-shaped protrusion of the molecule. When the temperature is raised to 8 K, the molecule starts rotating along six orientations determined by the hexagonal structure of the copper it is adsorbed on. In this case, a STM image taken of the molecule appears as a hexagon as the timescale of the imaging is much slower than the rotation rate of the molecule.

Determination of rate and direction of rotation

The six states of rotation of the molecule can be determined by aligning the tip of the scanning electron microscope asymmetrically on the side of one of the lobes of the molecule during spectroscopy measurements. When the butyl tail is nearest to the tip of the microscope, the tunneling current would be maximum and vice-versa. The position of the molecule on the surface can be determined by the tunneling current. By plotting the position versus time the rate and direction of rotation can be determined. At higher temperatures, the single molecule motor rotates too fast (up to one million rotations per second at 100 K) to monitor.

Application

The single molecule electric motor 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) and it also has potential to be utilized to generate microwave radiation.

See also

• Single molecule electronics
  Single molecule electronics
  Molecular scale electronics, also called single molecule electronics, is a branch of nanotechnology that uses single molecules, or nanoscale collections of single molecules, as electronic components...
  
  
• Single molecule magnet
• Single molecule experiment