Thermal and Electrical Properties of Multiwall Carbon Nanotubes

TL;DR

This dissertation experimentally investigates the thermal, electrical, and quantum transport properties of aligned multiwall carbon nanotubes, revealing insights into their thermal behavior, tunneling characteristics, and quantum interference effects.

Contribution

It provides comprehensive experimental data on MWNTs' thermal, electrical, and quantum properties, highlighting weak inter-wall coupling, Coulomb interactions, and quantum interference effects.

Findings

MWNTs show a linear specific heat from 10-300 K.

Thermal conductivity exhibits a crossover at ~120K.

Quantum interference effects are observed at low temperatures.

Abstract

In this dissertation, thermal and electrical properties of aligned multiwall carbon nanotubes (MWNTs) prepared by thermal decomposition of hydrocarbons have been experimentally studied. The thesis consists of six chapters. Ch1 is an introduction. In Ch2, sample preparation and characterizations are described. In Ch3, by using a self-heating 3-Omega method, the specific heat, thermal diffusivity and thermal conductivity of MWNTs are measured. MWNTs of 20-40 nm diameter show a linear specific heat over a temperature range of 10-300 K, suggesting that inter-wall coupling in MWNTs is rather weak compared with that of graphite. The thermal conductivity shows a crossover from linear temperature dependence to a square law at ~120K, with a rather low room-temperature amplitude which may have resulted from structural defects. In Ch4, four-wire tunneling spectroscopy of junctions between MWNTs and a normal metal is measured. The Coulomb interactions in the MWNTs give rise to a strong zero-bias suppression of tunneling density of states that can be fitted numerically with the environmental quantum-fluctuation theory. At low temperatures, an asymmetric conductance anomaly near zero bias is observed, which is interpreted as Fano resonance in the strong tunneling regime. In Ch5, the thermoelectric power (TEP) and longitudinal magnetoresistance (MR) of MWNTs are measured. A moderate positive TEP with metallic-like linear temperature dependence is found, suggesting that the electron-hole symmetry in metallic MWNTs is broken. Periodic oscillations in MR are observed at 20 mK when a longitudinal magnetic field is applied. The period of oscillation agrees well with the period h/2e of Altshuler-Aronov-Spivak (AAS) effect if only the outermost graphene wall contributes to conductance, clearly indicating quantum-interference effects at low temperatures. Ch6 presents the main conclusions.