Simultaneous imaging of tunneling current variation by noncontact atomic force microscopy in ultrahigh vacuum

The tunneling current between a sample surface and a tip on an oscillating cantilever is simultaneously sampled with noncontact atomic force microscopy (nc-AFM) images while changing the applied bias voltage. The tunneling current can be detected with the tip on an oscillating AFM cantilever, although it is reduced to less than 10% of the value at the closest separation, depending on the amplitude. The tunneling current and the chemical attractive force are large through the surface states at the same energy level on both the tip and the sample surface, the energy levels of which can be tuned by changing the bias voltage. By tuning them, the atomic contrast in a nc-AFM image is enhanced. On the other hand, by detuning them, the atomic contrast is weakened or can be inverted owing to the contribution of a short-range repulsive force.