Quasi-ballistic electron transport in double-wall carbon nanotubes

Room-temperature quasi-ballistic electron transport in double-wall carbon nanotubes (DWNT) is demonstrated. Conductance dependence on the length was measured by submerging DWNTs into liquid mercury. The conductance plots show plateaus, indicating weak dependence of the electrode–tube–electrode electrical resistance on the length of the connecting nanotube. We infer a mean free path between 0.6 and 10 lm for 80% of the DWNTs, which is in good agreement with calculations based on the electron scattering by acoustic phonons and by disorder. 2004 Elsevier B.V. All rights reserved. Double-wall carbon nanotubes (DWNTs) [1] have recently attracted much attention because of their unique structure and theoretically predicted peculiar physical and chemical properties, not found in either single-wall carbon nanotubes or multi-wall carbon nanotubes (MWNTs) [2–5]. However, due to the difficulties in producing DWNT samples, their fundamental characteristics, such as ballistic transport properties [6], have not been well established [7–10]. In order to investigate electronic properties of DWNTs, we have developed a new synthesis method [11], which enables us to prepare suitable DWNT samples for transport measurements. Using these DWNT samples, we studied their ballistic transport properties by submerging them into liquid mercury (Hg), following the method pioneered by the de Heer group [12]. The experiments were carried out at room temperature. The results show that the electrons in DWNTs can propagate without scattering over a distance of a few microns, which is comparable to the typical length of the nanotubes. We compare these results to the calculated mean free path limited by scattering of electrons by acoustic phonons [13] and find a 0009-2614/$ see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.09.115 * Corresponding author. Fax: +81 46 226 2453. E-mail address: [email protected] (H. Kajiura). good agreement with the measured values. Some tubes showed a shorter mean free path, compared to what is expected if only electron–phonon scattering is involved. In these tubes, there may be some positional disorder of as much as 2%. DWNT samples, which appear as threadlike soot, were produced on the surface of a bowl-shaped cathode using arc-discharge in helium (purity >99.9999%) [11]. An image of the as-produced DWNT soot sample obtained with a scanning electron microscopy (SEM) Hitachi-S4700 is shown in Fig. 1a. The micrograph shows DWNT bundles protruding at the edges. Images made using a transmission electron microscope (TEM) Hitachi-HF2000 show DWNTs with the outer diameter in the range 2–7 nm (Fig. 1b). The as-produced soot sample contains catalytic metal particles up to 60% [11]. Transport measurements were made using a piezodriven nanopositioning system (Fig. 2a). A piezopositioner allowed gentle and reproducible contact between the nanotube sample and the Hg electrode [12]. The as-produced soot sample was attached to a mobile metallic electrode ( probe ) using conducting silver paste. The probe was then attached to a piezo-positioner with a displacement range of 20 lm (17PAZ005, MELLES GRIOT). The liquid Hg electrode was positioned below Fig. 1. (a) SEM image showing that DWNT bundles protruding from the edges of the threadlike soot sample. (b) TEM image showing the outer diameter of the DWNTs to range 2–7 nm. Piezo (a) (b) positioner