Wednesday, September 07, 2011

Single-molecule electric motor

As a nano person, I feel like I'm practically obligated to comment on this paper, which has gotten a good deal of media attention. In this experiment, the authors have anchored a single small molecule down to a single-crystal copper surface, in such a way that the molecule can pivot about the single anchoring atom, rotating in the plane of the copper surface. Because of the surface atom arrangement and its interactions with the molecule, the molecule has six energetically equivalent ways that it can be oriented on the metal surface. It's experimentally impressive that the authors came up with a way to track the rotation of the molecule one discrete hop between orientations at a time. This is only do-able when the temperature is sufficiently low that thermally driven orientational diffusion is suppressed. When a current of electrons is properly directed at the molecule, the electrons can dump enough energy into the molecule (inelastically) to kick the molecule around rotationally. In that sense, this is an electric motor. (Of course, while the rotor is a single small molecule, the metal substrate and scanning tunneling microscope tip are macroscopic in size.) The requirements for this particular scheme to work include cryogenic temperatures, ultrahigh vacuum, and ultraclean surfaces. In that sense, talk in the press release about how this will be useful for pushing things around and so forth in, e.g., medical devices is a bit ridiculous. Still a nice experiment, though.  I continue to find the whole problem of nanoscale systems driven out of thermal equilibrium (e.g., by the flow of "hot" electrons) to be fascinating - how is a steady state established, where does the energy go, where does irreversibility come into play, etc.