Quasi-ballistic electron transport in as-produced and annealed multiwall carbon nanotubes

Abstract

The electronic properties of multiwall carbon nanotubes (MWNTs) [1] have attracted much attention because they could lead to nano-sized devices [2–6]. To obtain optimal performance, it is desirable to utilize MWNTs of the highest purity and the best quality. Using chemical vapor deposition (CVD), MWNTs can be produced in large quantities at a low cost [7]. CVDproduced MWNTs, however, contain a considerable amount of structural defects due to their low temperature synthesis [7,8]. Since these defects act as scattering centers in electron transport and thus limit the electronic mean free path (EMFP), high-temperature annealing should lengthen the EMFP. This expectation is based on the previous experiments in which annealing eliminated structural defects in various carbon materials including nanotubes [8–10]. In this study we investigate how annealing affects the transport properties of CVDgrown MWNTs. To measure the EMFP of individual nanotubes, we submerged them into liquid mercury (Hg) and measured the variation in conductance [4]. We found that the EMFP is much longer in annealed nanotubes compared with that of as-produced ones. The annealed nanotubes show quasi-ballistic electronic transport with the EMFP reaching a few microns, even at room temperature. We started with soot consisting of CVD-produced MWNTs (10–20 nm diameter, 1–5 lm length, >95% purity; NanoLab, Inc.). Scanning electron microscopy (SEM, JSM-6700F, JEOL) showed nanotubes protruding from the edges of the soot sample (Fig. 1A). The annealing was done in argon (>99.9999% purity) at 2700 C for 30 min. The concentration of defects was estimated using thermogravimetric analysis (TGA, Pylis 1 TGA, Perkin–Elmer). Oxidation in carbon nanotubes begins at structural defects [11], and when nanotubes with many defects are subjected to an oxidative atmosphere, they begin to lose weight at a lower temperature than tubes with fewer defects. For the TGA measurements, the samples were dried at 105 C for 1 h, and heated to 800 C at 5 C/min in dry air at 30 ml/min. The oxidation starting point, defined as the temperature at which the weight is reduced by 5%, increased from 360 C to 560 C with annealing (Fig. 1B). This indicates that certain types of defects are eliminated by annealing at 2700 C. The conduction properties were measured in air at room temperature using a piezo-driven nanopositioning system (Fig. 2A) [12], which allowed gentle and reproducible contact between the sample, attached to the probe using silver paste, and the Hg counter electrode [4]. The mobile electrode (‘‘probe’’) was attached to the piezo-positioner with a displacement range of 20 lm (17PAZ005, MELLES GRIOT). The Hg was positioned below the mobile electrode. To make electrical contact between the sample and Hg, the probe was driven cyclically up and down with a peak-to-peak