in Fig- 3 b) and going horizontally across the surface. The line scan clearly shows the nanotube structure The fact that the above nanotubes were imaged using low tip-sample voltages of 75 mv, typically used for metals, is evidence that these nanotubes are metallic. To directly test this hypothesis, we measured the I-V curves of individual nanotubes. From the I-V curves, one can deterrmine the con ductivity di/d L' and electronic density of states CDOSD p from the expression pox(di/d VO/(1/v). 13 It is well known that for a metal one has a linear I-V curve about the Fermi evel at a tip-sample voltage of o V, whereas the DOS of a semiconductor leads to the absence of tunneling current about the Fermi level due to the band gap 1-r spectros copy has been used to study the band structure of metal and quantum well nanostructures and the electronic structure of passivated and nonpassivated Si 16 and GaAs 17 surfaces in air. curves were measured for all the nanotubes in Fig 30b, and the 11OPG substrate. Figure 4(a) shows the 1. curve for a carbon nanotube with diameter of 2.O nm mnea sured at the location indicated by the in Fig. 30b). A linear relationship for tip-sample voltage from 80 to 80 m V about the Ferren i level at O V is observed, indicating a curves for all nanotubes in Fig. 30b) metallic DOS.