We report on a method for direct fabrication of integrated carbon nanotube circuits. The circuits consist of a multi wall carbon nanotube bridge between two electrodes, forming a metal/carbon nanotube/metal structure. Electron-beam lithography is used to pattern electrode sets separated by a desired distance consisting of catalysts for nanotube growth. Following metal evaporation and lift-off, carbon nanotubes are grown by chemical vapor deposition. Carbon nanotubes selectively grow on the catalyst leads and eventually form a bridge between nearby electrodes consisting of one, or in some cases more than one multi wall nanotubes. The number of nanotubes connecting each electrode set is determined by scanning electron microscopy. Transmission electron microscopy reveals that the nanotubes are multi-walled, and that catalyst particles exist at both ends of the nanotubes. Hence, we believe that the contacts between the carbon nanotubes and electrodes are top-contacts instead of side-contacts. The resistance at room temperature for most of the samples is less than 100 kohm, which agrees with values reported by others. For high resistance samples (>>100 kohm), transport properties were measured in a temperature range from room temperature to 2K. At room temperature the I-V is linear. With decreasing temperature, the I-V becomes non-linear, and a gap appears around V=0 suggesting semiconducting behavior. However, no field effect has been observed. One of the samples with two nanotubes bridging the electrodes exhibits what appears to be negative differential resistance.
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