Wednesday, July 23, 2008

Two papers in Nano Letters

Two new papers in Nano Letters caught my eye.

Danilov et al., "Nanoelectromechanical switch operating by tunneling of an entire C60 molecule"
This is a single-molecule electronic experiment, and it's a pretty neat example of using careful measurements to deduce a fair bit of information about a nanoscale system without a direct microscopic imaging probe. In their experiment the authors find bistable switching between two different conducting configurations of a junction thought to contain a single C60 molecule (as inferred by a signature in the electronic conduction of a well-known vibrational mode of the fullerene). Now, telegraph-like switching isn't new by any stretch, and many nanoscale systems exhibit discrete changes in their properties due to motion of one or a few atoms or molecules. Here, by carefully analyzing the voltage and temperature dependence of the switching, they are able to deduce that the most likely mechanism for the change in configuration is motion of the fullerene between two different sites. I'm not sure that I'm 100% convinced by their interpretation, but the data are quite pretty and the analysis is clever.

Stampfer et al., "Tunable graphene single-electron transistor"
In this work, Klaus Ensslin's group starts from a graphene flake (the now-usual scotch-tape approach) and uses standard patterning methods to make a little graphene region connected by narrow graphene constrictions to source and drain electrodes. Using gate electrodes near the constrictions, they could electrostatically shift the local chemical potential there, shifting the graphene from having electrons charge carriers to holes as charge carriers. By further tuning with an additional gate, they could make a complete single-electron transistor - essentially a puddle of confined charges weakly connected by tunnel barriers to larger reservoirs, with the confinement sufficiently strong that the electrostatic energy cost of changing the puddle population by one charge exceeded the available thermal energy. (This was all done at cryogenic temperatures, since the charging energy of the dot was around 3 meV, or about 35 K.). This is a particularly nice, clean example of using graphene and its relatively unique properties as a platform for nanoscale device fabrication. Several groups are getting very good at this, and if efforts to grow large area, high quality, single-layer graphene succeed, there could be some genuine technological implications.