We report the measurement of the thermal conductivity of a suspended single-layer graphene. The room temperature values of the thermal conductivity in the range ∼(4.84 ± 0.44) × 103 to (5.30 ± 0.48) × 103 W/mK were extracted for a single-layer graphene from the dependence of the Raman G peak frequency on the excitation laser power and independently measured G peak temperature coefficient. The extremely high value of the thermal conductivity suggests that graphene can outperform carbon nanotubes in heat conduction. The superb thermal conduction property of graphene is beneficial for the proposed electronic applications and establishes graphene as an excellent material for thermal management.

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Superior Thermal Conductivity of Single-Layer Graphene

Recent years have seen a rapid growth of interest by the scientific and engineering communities in the thermal properties of materials. Heat removal has become a crucial issue for continuing progress in the electronic industry, and thermal conduction in low-dimensional structures has revealed truly intriguing features. Carbon allotropes and their derivatives occupy a unique place in terms of their ability to conduct heat. The room-temperature thermal conductivity of carbon materials span an extraordinary large range--of over five orders of magnitude--from the lowest in amorphous carbons to the highest in graphene and carbon nanotubes. Here, I review the thermal properties of carbon materials focusing on recent results for graphene, carbon nanotubes and nanostructured carbon materials with different degrees of disorder. Special attention is given to the unusual size dependence of heat conduction in two-dimensional crystals and, specifically, in graphene. I also describe the prospects of applications of graphene and carbon materials for thermal management of electronics.

Thermal properties of graphene and nanostructured carbon materials

Recent years witnessed a rapid growth of interest of scientific and engineering communities to thermal properties of materials. Carbon allotropes and derivatives occupy a unique place in terms of their ability to conduct heat. The room-temperature thermal conductivity of carbon materials span an extraordinary large range – of over five orders of magnitude – from the lowest in amorphous carbons to the highest in graphene and carbon nanotubes. I review thermal and thermoelectric properties of carbon materials focusing on recent results for graphene, carbon nanotubes and nanostructured carbon materials with different degrees of disorder. A special attention is given to the unusual size dependence of heat conduction in two-dimensional crystals and, specifically, in graphene. I also describe prospects of applications of graphene and carbon materials for thermal management of electronics.

Thermal Properties of Graphene, Carbon Nanotubes and Nanostructured Carbon Materials

Recently discovered carbon nanotubes have exhibited many unique material properties including very high thermal conductivity. Strong sp 2 bonding configurations in carbon network and nearly perfect self-supporting atomic structure in nanotubes give unusually high phonon-dominated thermal conductivity along the tube axis, possibly even surpassing that of other carbon-based materials such as diamond and graphite (in plane). In this chapter, we explore theoretical and experimental investigations for the thermal-transport properties of these materials.

Unusually High Thermal Conductivity of Carbon Nanotubes

Rapid progress in the synthesis and processing of materials with structure on nanometer length scales has created a demand for greater scientific understanding of thermal transport in nanoscale devices, individual nanostructures, and nanostructured materials. This review emphasizes developments in experiment, theory, and computation that have occurred in the past ten years and summarizes the present status of the field. Interfaces between materials become increasingly important on small length scales. The thermal conductance of many solid–solid interfaces have been studied experimentally but the range of observed interface properties is much smaller than predicted by simple theory. Classical molecular dynamics simulations are emerging as a powerful tool for calculations of thermal conductance and phonon scattering, and may provide for a lively interplay of experiment and theory in the near term. Fundamental issues remain concerning the correct definitions of temperature in nonequilibrium nanoscale systems. Modern Si microelectronics are now firmly in the nanoscale regime—experiments have demonstrated that the close proximity of interfaces and the extremely small volume of heat dissipation strongly modifies thermal transport, thereby aggravating problems of thermal management. Microelectronic devices are too large to yield to atomic-level simulation in the foreseeable future and, therefore, calculations of thermal transport must rely on solutions of the Boltzmann transport equation; microscopic phonon scattering rates needed for predictive models are, even for Si, poorly known. Low-dimensional nanostructures, such as carbon nanotubes, are predicted to have novel transport properties; the first quantitative experiments of the thermal conductivity of nanotubes have recently been achieved using microfabricated measurement systems. Nanoscale porosity decreases the permittivity of amorphous dielectrics but porosity also strongly decreases the thermal conductivity. The promise of improved thermoelectric materials and problems of thermal management of optoelectronic devices have stimulated extensive studies of semiconductor superlattices; agreement between experiment and theory is generally poor. Advances in measurement methods, e.g., the 3ω method, time-domain thermoreflectance, sources of coherent phonons, microfabricated test structures, and the scanning thermal microscope, are enabling new capabilities for nanoscale thermal metrology.

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Nanoscale thermal transport

We have measured the temperature-dependent thermal conductivity $\ensuremath{\kappa}(T)$ of crystalline ropes of single-walled carbon nanotubes from 350 K to 8 K. $\ensuremath{\kappa}(T)$ decreases smoothly with decreasing temperature, and displays linear temperature dependence below 30 K. Comparison with electrical conductivity experiments indicates that the room-temperature thermal conductivity of a single nanotube may be comparable to that of diamond or in-plane graphite, while the ratio of thermal to electrical conductance for a given sample indicates that the thermal conductivity is dominated by phonons at all temperatures. Below 30 K, the linear temperature dependence and estimated magnitude of $\ensuremath{\kappa}(T)$ imply an energy-independent phonon mean free path of \ensuremath{\sim}0.5--1.5 \ensuremath{\mu}m.

Thermal conductivity of single-walled carbon nanotubes

Under appropriate non-equilibrium growth conditions, carbon atoms form relatively stable hollow clusters of well-defined mass number1, collectively known as fullerenes. The mass production, purification and condensation of such clusters into a molecular solid is generally essential to full experimental characterization: the initial discovery2 of C60, for example, had to await a bulk synthesis method3 six years later before detailed characterization of the molecule was possible. Gas-phase experiments1,4,5 have indicated the existence of a wide range of fullerene clusters, but beyond C60 only a few pure fullerene solids have been obtained6, most notably C70. Low-mass fullerenes are of particular interest because their high curvature and increased strain energy owing to adjacent pentagonal rings could lead to solids with unusual intermolecular bonding and electronic properties. Here we report the synthesis of the solid form of C36 by the arc-discharge method3. We have developed purification methods that separate C36 from amorphous carbon and other fullerenes, to yield saturated solutions, thin films and polycrystalline powders of the pure solid form. Solid-state NMR measurements suggest that the molecule has D6h symmetry, and electron-diffraction patterns are consistent with a tightly bound molecular solid with an intermolecular spacing of 6.68 A. We observe large increases in the electrical conductivity of the solid on doping with alkali metals.

C36, a new carbon solid

Thermal Conductivity of Single-Walled Carbon Nanotubes

The local density of electronic states N(E) has been measured on C{sub 36} thin films using scanning tunneling spectroscopy (STS). A 0.8thinspthinspeV energy gap and sharp features in N(E) with widths as small as 0.2thinspthinspeV can be ascribed to the discrete molecular energy levels predicted for the C{sub 36} molecule, broadened by intermolecular interactions. However, a more complete agreement between the experimental results and theoretical predictions occurs if the system is modeled as covalently bonded C{sub 36} dimers and trimers. These results confirm the molecular nature of C{sub 36} as well as its higher reactivity as compared to larger size fullerenes. {copyright} {ital 1998} {ital The American Physical Society}

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C. Piskoti

Papers

Thermal conductivity of single-walled carbon nanotubes

C36, a new carbon solid

Thermal Conductivity of Single-Walled Carbon Nanotubes

Scanning Tunneling Spectroscopy of C36

C36, a New Carbon Solid