technology can be considered commercially viable, such as the low operating temperature currently used, which will require further efforts in the development of high-temperature storage materials.

Conductivity measurements were performed on bundles of single-walled carbon nanotubes with the aid of a scanning tunneling microscope (STM). Semimetallic current-voltage (I-V) characteristics generally indicated the bundles to be electronically similar to graphite. However, by moving the STM tip along the length of the nanotubes, sharp deviations in the I-V characteristics could also be observed. Well-defined positions were found at which the nanotube transport current changed abruptly from a graphitic response to one that is highly nonlinear and asymmetric, including near-perfect rectification. This abrupt change in the nanotube transport suggests that the STM tip had passed a region of the nanotube which acts less like a wire than it does a Schottky barrier or other heterojunction. Similar on-tube nanodevices have been theoretically predicted for point defects in individual carbon nanotubes and are consistent with our observations.