The electrical conduction mechanism of multiwalled carbon nanotubes film synthesized by microwave plasma chemical vapor deposition

Multiwalled carbon nanotubes (MWNTs) were synthesized by microwave plasma chemical vapor deposition (MPCVD) on Ti/Si substrate with Ni catalyst. Then MWNT films were annealed at 300 degreesC for 10 min. Micro-Raman spectroscopy was performed to analyze the micro structure of the samples. It was noted from the Raman spectra that the ratio of I-D/I-G decreased after the annealing process, indicating more MWNTs were transformed to graphite structure at the temperature of 300 degreesC. In this research, the electrical properties of MWNTs film were investigated before and after the annealing process, different from the past research that focused on single or bundle of MWNTs. It was found that the I-V traces of the unannealed MWNTs film exhibited abrupt current increase and decrease phenomena by sweeping up and down voltages, respectively, and with different traces. Moreover, the threshold voltages for the saturation region increased after some successive measurements during sweeping up, but the similar effect was not observed during sweeping down. Similar phenomena were observed for the annealed MWNTs film; however, the threshold voltages, compared to that of the unannealed MWNTs film, spread out more during sweeping up and became smaller during sweeping down. It suggests that the nanotubes have thermally activated defects in rectifying conduction contacts between tungsten probes and MWNTs at the higher temperature induced by high applied voltages. Therefore, the breakdown voltages of the rectified contacts would change due to various thermally activated defects at higher temperatures. The equivalent circuit for the W-probe/MWNT-films/W-probe structure will be further presented in this study. (C) 2004 Elsevier B.V. All rights reserved.