Origin of conductivity cross over in entangled multi-walled carbon nanotube network filled by iron

TL;DR

This study presents a transport model for iron-filled multi-walled carbon nanotubes, revealing a conductivity crossover from variable range hopping to weak localization driven by iron filling percentage and nanowire formation.

Contribution

It introduces a realistic model linking hopping and diffusive transport in MWNTs with iron filling, supported by low-temperature measurements and analysis of quantum effects.

Findings

Conductivity crossover from Efros-Shklovski to Mott VRH with increased iron content.

Observation of positive and negative magneto-resistance linked to quantum interference.

Formation of long iron nanowires influences dephasing length and transport behavior.

Abstract

A realistic transport model showing the interplay of the hopping transport between the outer shells of iron filled entangled multi-walled carbon nanotubes (MWNT) and the diffusive transport through the inner part of the tubes, as a function of the filling percentage, is developed. This model is based on low-temperature electrical resistivity and magneto-resistance (MR) measurements. The conductivity at low temperatures showed a crossover from Efros-Shklovski (E-S) variable range hopping (VRH) to Mott VRH in 3 dimensions (3D) between the neighboring tubes as the iron weight percentage is increased from 11% to 19% in the MWNTs. The MR in the hopping regime is strongly dependent on temperature as well as magnetic field and shows both positive and negative signs, which are discussed in terms of wave function shrinkage and quantum interference effects, respectively. A further increase of the iron percentage from 19% to 31% gives a conductivity crossover from Mott VRH to 3D weak localization (WL). This change is ascribed to the formation of long iron nanowires at the core of the nanotubes, which yields a long dephasing length (e.g. 30 nm) at the lowest measured temperature. Although the overall transport in this network is described by a 3D WL model, the weak temperature dependence of inelastic scattering length expressed as L_phi ~T^-0.3 suggests the possibility for the presence of one-dimensional channels in the network due to the formation of long Fe nanowires inside the tubes, which might introduce an alignment in the random structure.