Investigation of electrical transport in hydrogenated multiwalled carbon nanotubes

Highly disordered multiwalled carbon nanotubes of large outer diameter ( 60 nm) fabricated by means of chemical vapor deposition process inside porous alumina templates exhibit ferromagnetism when annealed in a H2/Ar atmosphere. In the presence of an applied magnetic field, there is a transition from positive to negative magnetoresistance. The transition may be explained in terms of the Bright model for ordered and disordered carbon structures. Additionally, temperature dependent electrical transport experiments exhibit a zero-bias anomaly at low temperature. & 2010 Elsevier B.V. All rights reserved. There are many reports on the electrical transport properties of carbon nanotubes (CNTs). Most of them exhibit a zero-bias anomaly (ZBA) associated with Luttinger liquid (LL) behavior in highly ordered CNTs [1,2]. However, some have suggested that apart from a LL explanation, for quasi-ballistic single electron junctions in disordered conductors, a ZBA can arise due to either electron or plasmon scattering near the tunnel barrier and finite size effects or due to extremely high resistance contacts or transmission lines [3–6]. However, there are very few studies reported to date on the transport properties of disordered multiwall CNTs (MWCNTs). Initial measurements indicate that the electrical transport properties of disordered MWCNTs can be explained in terms of weak localization theory [7,8]. Weak localization or Anderson localization also predicts that in the presence of an applied magnetic field a sufficiently disordered nanotube will exhibit negative magnetoresistance (MR) [9]. However, the magnetotransport behavior, even for well-ordered nanotubes, appears to be dominated by weak localization effects rather than the LL [9]. This indicates that an applied magnetic field effectively destroys the LL. However, in order to observe suchweak localization effects, the temperature must be sufficiently low. In most previous studies, negative MR was only observed for temperatures below 5 K for ordered CNTs [9]. In this work, we report results of electrical transport measurements on highly disordered MWCNTs fabricated by means of chemical vapor deposition (CVD) inside nanoporous alumina templates. Earlier we reported that ferromagnetism in such nanotubes can be induced by annealing in hydrogen [12]. The nature of this ferromagnetism has been discussed in the literature [13–16]. In this work, we study temperature dependent transport properties of ferromagnetic nanotubes. We show that at low temperatures and in zero magnetic field, the nanotubes exhibit a ZBA. MR measurements for the ferromagnetic MWCNTs show positive MR at low temperatures, in some cases up to 40 K, beyond which they exhibit negative MR. Such a transition in MR is not seen in non-ferromagnetic MWCNTs annealed without hydrogen. This transition may be attributed to increased disorder and may possibly be explained in terms of the Bright model [10]. The MWCNTs are synthesized inside nanoporous alumina templates fabricated by anodization of aluminum foil [17]. The template acts as a deposition substrate for the MWCNTs yielding nanotubes with outer diameter ( 60 nm for our study) corresponding with the pore diameter of the template. MWCNTs are synthesized without catalyst in the templates in a CVD process at 660 1C with acetylene acting as the precursor gas. By adjusting the CVD time, we control the inner diameter of the MWCNTs. Here, we make MWCNTs with four different inner diameters using CVD times of 60, 75, 90, and 100 min. The right inset of Fig. 1 shows the inner diameter of MWCNTs as a function of CVD time. The nanotubes are then removed from the templates by dissolving the alumina in 15% sulfuric acid heated to 100 1C. The acid is removed from the solution and replaced with 99.5% ethanol. The nanotubes are then dispersed in the solution by sonicating for 3 s in an ultrasonic bath. TEM studies show that the tubes are polycrystalline and extremely disordered (left inset of Fig. 1). However, the nanotubes retain the SP2 bonding characteristic of a carbon nanotube, as evidenced byXPS studies [12]. A drop of nanotube solution is