Molecular Rotors

One of the ultimate aims of the nanoscience revolution is to build devices in which the role of each component is played by an individual molecule, over 10 million times smaller than classical counterparts. Such machines are ubiquitous in nature; they perform tasks as varied as powering the motion of cells and even driving whole body locomotion through muscle contraction. In stark contrast to nature current manmade devices, with the exception of liquid crystals, make no use of nanoscale molecular motion. This is due in part to a gap in the understanding of how individual molecular components behave in the face of opposing forces such as thermal fluctuations, friction cause by neighboring molecules, and lack of inertia.

STM image of three molecular rotors, just 1 nanometer wide, spinning at over 1,000,000 times per second when heated to a temperature of 78 Kelvin (-320 F).

By using scanning tunneling microscopy (STM), we are able to record atomic-scale images of individual molecular motion on surfaces. We are using low-temperature STM to investigate how small, simple molecules called thioethers function as molecular rotors and to devise methods for turning them into molecular motors. Thioether molecules are just 1 nm (one billionth of a meter) across and composed of two carbon chains connected to either side of a sulfur atom. These molecules rotate randomly in 3D space, but when anchored to a gold surface through the central sulfur atom, the molecules rotate like a propeller. At very low temperatures (-435 F), the molecules transition between a locked or “frozen” state to one in which they spin at over 1,000,000 times per second.

STM images showing how a spinning molecular rotor can be “braked” by physically moving it towards a chain of static molecules.

STM has allowed us to study how changing the chemistry of the molecule affects how easily it rotates. We have been able to set up simple arrays of the molecules, just like placing cogs on a pegboard, and understand how to mechanically “brake” them by pushing them towards other molecules that halt their motion. We are also learning how to “drive” the molecules electrically by exciting them with electrons supplied by the sharp tip of the microscope. This simple system is enabling us to study many important aspects of molecular rotation with unprecedented resolution. We have been able to unearth crucial details concerning the mechanism of rotation and we are now beginning to understand how we may control molecular motion so that one day molecular rotors and motors will have useful function in real nanoscale devices.

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