Transfer of a cluster or single atom at nano gap has been investigated by many groups. The critical voltage of transfer is a few volts, when the STM tip was within tunneling range. This has been explained mainly by field evaporation of tip atoms, which is enhanced by the close proximity of surface. Transfer of a single molecule, however, has not been observed yet. It is still an open question whether a single molecule can be transferred or not at nano-gap. Among molecules, we have an interest in a carbon fullerene, since it is stable mechanically and chemically.
In this study, we investigate transfer of a single carbon fullerene using transmission electron microscope – scanning tunneling microscope system. In this system, we can obtain geometrical relationship among both electrodes and a single carbon fullerene simultaneously with applying a bias voltage between both electrodes. In this study, a single carbon fullerene was synthesized at nano gap of both gold electrodes [1].
In experiment, we brought the one side of gold electrode close to a single carbon fullerene adsorbed on another side while a bias voltage was applied. We observed that a single carbon fullerene was transferred back and forth between both electrodes when the gap distance almost had a long 1nm than the fullerene diameter. In this condition, the fullerene did not bond with the opposite electrode, since the bond length of a carbon and gold atom was about 0.3 nm. Such a transfer was never observed when the bias voltage was lower than 0.1 V.
The conductance of a single carbon fullerene was 1G0 (=2e2/h; e is elementary charge and h is Plank constant) or less. We found that the conductance showed the order of 10-1G0 in pulse at the same time when the fullerene was transferred. This fact suggested that the fullerene was expanded to have a contact with the opposite electrode when it was transferred. We, furthermore, considered that the expansion of the fullerene resulted from high electric field at nano gap by a simply calculation.
[1] M. Yoshida et al., Jpn. J. Appl. Phys. 46, L67 (2007) |