Characterization of transport properties of multiwalled carbon nanotube networks by microwave plasma chemical vapor deposition

We report the synthesis of multiwalled carbon nanotubes (MWCNTs) and the characterization of temperature-dependent electrical transport properties of MWCNT networks by using a two-point configuration without the lithographical technique. MWCNTs were grown by microwave plasma chemical vapor deposition with the bias enhanced growth technique. The Raman intensity ratio between the D- (∼1360 cm^{- 1}) and G- (∼1590 cm^{- 1}) peaks (I_D / I_G) as well as the full width at half maximum of the G-peak decreased from 1.03 to 0.03 and 18 to 13 cm^{- 1}, respectively, with the increase in the oxidative purification time. This indicates that the crystallinity of graphite sheets is improved by the oxidative purification process and burn-off of the defects in MWCNT networks. The metal electrodes were attached on both the top and the bottom of the insulating thin films, and the as-grown and oxidative-purified MWCNT networks were connected between the electrodes for I-V measurements at various temperatures. At room temperature, the conductance for the MWNT networks at around zero bias was 0.65 G₀ (G₀: fundamental conductance unit), which was less than the value of 1 G₀ for metallic MWCNTs. Further, the conductance increased linearly with the bias voltage until it attained its peak. In the 190-390 K range, the temperature characteristic of the I-V shows that the electron transport of the as-grown MWCNT networks was activated by a lower activation energy than that in oxidative-purified MWCNT networks.