We have developed a noncontact, scanning nonlinear dielectric microscopy (NC-SNDM) system operated under ultrahigh vacuum and have used it to observe the surface of graphite. By using the NC-SNDM technique ($2\omega$ amplitude feedback mode), we succeeded in obtaining clear SNDM images of the graphite surface. In the SNDM image patterns (inverted contrast $\omega$ amplitude image), a number of convex spots, with threefold symmetry, positioned at the corners of hexagons were observed when the probe tip was near the graphite surface. In contrast, a number of convex spots, with threefold symmetry, were also observed in the normal contrast $\omega$ amplitude image when the distance between the probe tip and graphite was large. Current images originating from tunneling were also observed in NC-SNDM and were similar to the $\omega$ amplitude images. Electrochemical capacitance between the probe tip and graphite surface with tunneling was introduced to investigate the origin of the SNDM signal. Applying this model to the NC-SNDM measurement, we found that the $\omega$ amplitude signal is dependent on the ratio of the slope of the local density of states (LDOS) and the energy curve, and the LDOS at the Fermi energy in the graphite surface. The convex spots and hollow spots in the hexagons on the graphite surface also originate from the LDOS in the NC-SNDM measurements. This means that the SNDM signal for the graphite surface reflects the LDOS on the graphite surface. Considering the relationship between the $\omega$ amplitude signal and LDOS on the graphite surface, the current signal observed in the NC-SNDM measurement originates from the tunneling effect between the probe tip and graphite surface. The proposed technique can potentially detect not only a unique carbon surface but also organic molecules on a metallic surface.

• Received 1 June 2010

DOI:https://doi.org/10.1103/PhysRevB.82.245427