The I-V curve for all nanotubes in Fig. 3(b) with diameters from 2.O to 5.1 nm were similar in shape and slope. Figure 4(b) shows the 1- V curve for the HOPG sub strate measured at the location indicated by the empty circle in Fig- 3 b A linear relationship is observed, as expected Semiconducting nanotubes were not observed. However, since I spectroscopy measures surface rather than bulk electronic structure, the presence of surface states on the nanotubes may produce 1 curves similar to those dis played here. In addition, energy band bending induced by the tunnel tip may yield a spectrum similar to that shown here Recently, it was reported, using STM imaging that carbon nanotubes with diameters of 5 nm deposited on a thin poly crystalline gold film on Si were semiconducting and had a band gap of 200 mv. 10 However it should be pointed out that thin polycrystalline gold films on Si have very poor conductivity for STM imaging at such low voltages and that p curves for the nanotubes were not measured Also observed in Fig. 3(a), near the regions labeled A, B, and C, arc nanometer-size rectangular structures which we dentity as graphene sheets. These structures are approxi mately 50 nm in width and several hundred nanometers in ength The heights of all of these structures were approxi with the thickness of four