Temperature dependence of electric resistance and magnetoresistance of pressed nanocomposites of multilayer nanotubes with the structure of nested cones

Bulk samples of carbon multilayer nanotubes with the structure of nested cones (fishbone structure) suitable for transport measurements, were prepared by compressing under high pressure (∼ 25 kbar) a nanotube precursor synthesized through thermal decomposition of polyethylene catalyzed by nickel. The structure of the initial nanotube material was studied using high-resolution transmission electron microscopy. In the low-temperature range (4.2–100 K) the electric resistance of the samples changes according to the law ln ρ ∝ (T0/T ) 1/3, where T0 ∼ 7 K. The measured magnetoresistance is quadratic in the magnetic field and linear in the reciprocal temperature. The measurements have been interpreted in terms of two-dimensional variable-range hopping conductivity. It is suggested that the space between the inside and outside walls of nanotubes acts as a two-dimensional conducting medium. Estimates suggest a high value of the density of electron states at the Fermi level of about 5× 10 eVcm. Investigations of electric transport properties of carbon nanotubes has attracted great attention recently. According to theoretical concepts [1], an isolated nanotube can be either a metal, or semimetal, or insulator, depending on such structural parameters as its diameter, chirality, and the number of concentric layers in it. Despite enormous difficulties in measurements of electric parameters of isolated nanotubes or nanotube bundles, several attempts 1 Author for correspondence ([email protected]). Preprint submitted to Elsevier Preprint 1 February 2008 undertaken recently have been successful [2–4]. The latest published measurements [4] clearly indicate the presence of both metallic and insulating nanotubes in a single set of samples prepared in the same conditions. The authors emphasized that each multilayer nanotube manifested its specific conducting properties, thus indicating a strong correlation between structural and electric parameters. In this connection, it is interesting to study, in addition to the transport properties of isolated carbon nanotubes, the conducting properties of bulk nanotube materials, in which contacts between nanotubes and/or their sections are randomly distributed. In our previous publication [5] we reported on the conductivity temperature dependence and structure (see also Ref. [6]) of carbon nanotube films fabricated by evaporating graphite in an electron beam. The data of those experiments were interpreted in terms of a three-dimensional model of hopping conductivity with a Coulomb gap about the Fermi level (the resistivity was described by the law ln ρ ∝ (T0/T ) ). The density of states at the Fermi level for films that contained, as shown by structural investigations, mostly one-layer carbon nanotubes (isolated or assembled in bundles) and had a relatively high conductivity was estimated to be g(μ) ∼ 10 eVcm. On the other hand, films containing multilayer carbon nanotubes were characterized by fairly large values of resistivity, which changed with temperature to Mott’s law, ln ρ ∝ (T0/T ) . In this case, estimates of the density of states, g(μ) ∼ 10 eVcm, corresponded to g(μ) for amorphous carbon. Amorphous carbon in significant quantities was detected on the outside surfaces of multilayer nanotubes in such films by electron microscopy [5,6], and it seems that the conductivity of such films can be attributed to the presence of carbon. It is well known that, in addition to one-layer and multilayer nanotubes with walls made of coaxial carbon layers, there are nanotubes whose walls consist of nested truncated cones (these are the so-called fishbone-type structures [7]). Such nanocones are usually detected at the ends of carbon nanotubes, but can also exist in the form of independent objects among products of arc discharges in a helium atmosphere [8], commonly used in synthesizing carbon nanotubes. In our recent work [9–11] we demonstrated that thermal decomposition of polyethylene with nickel used as a catalyst is a fairly efficient technique for fabrication of large quantities of fishbone nanotubes. This technique allows one to manufacture in a relatively short time considerable quantities (several grams) of fairly homogeneous nanotube material. According to the data of thermal analysis in oxiding atmosphere, the nickel content in this material is less then 15% by mass. Nickel is present in the material in the form of nanoparticles, which can be eliminated completely by thermal processing of the nanocomposite in vacuum at temperatures of up to 2800◦C [10]. In this paper we present our measurements of electric resistance versus tem-