Mesoscopic thermal and thermoelectric measurements of individual carbon nanotubes

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Abstract

We discuss the mesoscopic experimental measurements of electron energy dissipation, phonon thermal transport, and thermoelectric phenomena in individual carbon nanotubes. The temperature distributions in electrically heated individual multiwalled carbon nanotubes have been measured with a scanning thermal microscope. The temperature profiles along the tube axis in nanotubes indicate the bulk dissipation of electronic energy to phonons. In addition, thermal conductivity of an individual multiwalled nanotube has been measured using a microfabricated suspended device. The observed thermal conductivity is two orders of magnitude higher than the estimation from previous experiments that used macroscopic mat samples. Finally, we present thermoelectric power (TEP) of individual single walled carbon nanotubes using a novel mesoscopic device. A strong modulation of TEP as a function of the gate electrode was observed.

Introduction

The intriguing 1-dimensional (1D) nature of electrons and phonons in carbon nanotubes have attracted considerable amount of theoretical and experimental work [1]. One of the fundamental issues that are also important for the device applications is how electrons and phonons transport through the nanotubes, i.e. how charges and energy are transported through these materials. Recent electrical transport experiments suggest that metallic single walled nanotubes (SWNTs) are remarkably good ballistic conductors [2], [3], [4] due to the strong suppression of back scatterings in the special symmetric 1D band [5]. In contrast, most of multiwalled nanotubes (MWNTs) experiments indicate diffusive electron transport [6], [7], [8], although others under a different experimental condition show ballistic transport [9]. These interesting electron transport characteristics are often related to the energy dissipation and disturbance of local quasi equilibrium. Therefore, local temperature profile along the current-carrying nanotubes elucidates the characteristics of electron transport mechanism in these 1D materials.

In addition to unusual electronic properties, carbon nanotubes are expected to have high thermal conductivity [10] and can conduct heat efficiently, thus preventing structure damage while used as current-carrying wires in micro/nano devices. Previous electronic transport measurements at high electric field regime [11], [12] suggested that carbon nanotubes can carry substantial amounts of current before their structural failure. To investigate these intriguing electrical and thermal properties, mesoscopic experimental methods at a single nanotube level are desirable.

On small length scales, however, the thermal transport quantities are more difficult to measure than their electrical counterparts, and thus have attracted less attention. There have been a number of experimental efforts to measure thermal transport properties such as thermal conductivity and thermoelectric power (TEP) of nanotubes using macroscopic ‘mat’ samples [13], [14], [15], [16], [17], [18], [19], [20]. However, these macroscopic scale measurements inherently include artifacts resulting from numerous unknown tube–tube junctions in the sample. In addition, the ensemble average over different tube species blurs the intrinsic properties of individual nanotubes. Therefore, the mesoscopic scale thermal transport measurements are necessary to investigate the intrinsic thermal properties of carbon nanotubes.

In this paper, we will discuss our recent mesoscopic experimental efforts to study electron energy dissipation, phonon thermal transport, and thermoelectric phenomena in individual carbon nanotubes.

Section snippets

Energy dissipation in MWNTs

Electron transport in mesoscopic conductors is closely related to the energy dissipation mechanism, and is often correlated with the temperature distribution along them. Recent transport studies of SWNTs [11], [12] and MWNTs [21] under a high bias voltage regime have suggested that the energy dissipation is largely due to the increased coupling of these energetic electrons to optical and zone boundary phonons, and hence the increase of lattice temperature should be expected. We have

Thermal transport in MWNTs

The thermal conductivity of carbon nanotubes have been measured by several groups using a millimeter sized mat sample [13], [14], [15], [16]. Although these studies demonstrated qualitative understanding of the low dimensional nature of these materials, it is difficult to extract absolute values of thermal conductivity in a mat sample due to the presence of numerous uncertain tube–tube junctions that might be the dominant barriers to the thermal transport through the sample.

We have recently

Mesoscopic thermoelectricpower measurements

There have been a number of experimental efforts to measure TEP of single walled [26], [27], and mutliwalled nanotubes [17] using macroscopic ‘mat’ samples. These studies showed interesting unique thermoelectric phenomena, such as, sign change of TEP upon absorption and desorption of gas molecules on the sample and a logarithmic temperature dependence of TEP. However, mesoscopic measurements of single nanotubes are required to understand the intrinsic TEP of nanotube materials as we already

Prospect

The TEP, thermal and electrical conductivity of the materials are of interest for thermoelectric device applications, such as heat pumps and power generators [30]. The performance of a thermoelectric device is quantified by a figure of merit, ZT=S2GT/κ. ZT is a dimensionless number which measures the thermoelectric efficiency of thermoelectric energy conversion. Higher value correspond higher thermoelectric energy conversion efficiency. Recent work on superlattice semiconducting device

Acknowledgements

The authors wish to thank A. Majumdar and P.L. McEuen for helpful discussion. This work is supported primarily by the Nanoscale Science and Engineering Initiative of the National Science Foundation under NSF Award Number CHE-011752.

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