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

There is intense interest in graphene in fields such as physics, chemistry, and materials science, among others. Interest in graphene's exceptional physical properties, chemical tunability, and potential for applications has generated thousands of publications and an accelerating pace of research, making review of such research timely. Here is an overview of the synthesis, properties, and applications of graphene and related materials (primarily, graphite oxide and its colloidal suspensions and materials made from them), from a materials science perspective.

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Graphene and Graphene Oxide: Synthesis, Properties, and Applications

We present a comprehensive, up-to-date compilation of band parameters for the technologically important III–V zinc blende and wurtzite compound semiconductors: GaAs, GaSb, GaP, GaN, AlAs, AlSb, AlP, AlN, InAs, InSb, InP, and InN, along with their ternary and quaternary alloys. Based on a review of the existing literature, complete and consistent parameter sets are given for all materials. Emphasizing the quantities required for band structure calculations, we tabulate the direct and indirect energy gaps, spin-orbit, and crystal-field splittings, alloy bowing parameters, effective masses for electrons, heavy, light, and split-off holes, Luttinger parameters, interband momentum matrix elements, and deformation potentials, including temperature and alloy-composition dependences where available. Heterostructure band offsets are also given, on an absolute scale that allows any material to be aligned relative to any other.

Band parameters for III–V compound semiconductors and their alloys

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

Graphene’s success has shown that it is possible to create stable, single and few-atom-thick layers of van der Waals materials, and also that these materials can exhibit fascinating and technologically useful properties. Here we review the state-of-the-art of 2D materials beyond graphene. Initially, we will outline the different chemical classes of 2D materials and discuss the various strategies to prepare single-layer, few-layer, and multilayer assembly materials in solution, on substrates, and on the wafer scale. Additionally, we present an experimental guide for identifying and characterizing single-layer-thick materials, as well as outlining emerging techniques that yield both local and global information. We describe the differences that occur in the electronic structure between the bulk and the single layer and discuss various methods of tuning their electronic properties by manipulating the surface. Finally, we highlight the properties and advantages of single-, few-, and many-layer 2D materials in...

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Progress, Challenges, and Opportunities in Two-Dimensional Materials Beyond Graphene

Photocurrent measurements are incisive probes of crystal symmetry, electronic band structure, and material interfaces. However, conventional scanning photocurrent microscopy (SPCM) con-volves the processes for photocurrent generation and collection, which can obscure the intrinsic light-matter interaction. Here, by using ac magnetometry with a nitrogen-vacancy center spin ensemble, we demonstrate the high-sensitivity, sub-micron resolved imaging of vector photocurrent ﬂow. Our imaging reveals that in anisotropic semimetals WTe 2 and TaIrTe 4 , the photoexcited electron carriers propagate outward along the zigzag chains and inward perpendicular to the chains. This circulating pattern is explained by our theoretical modeling to emerge from an anisotropic photothermoelectric eﬀect (APTE) caused by a direction-dependent thermopower. Through simultaneous SPCM and magnetic imaging, we directly visualize how local APTE photocurrents stim-ulate long-range photocurrents at symmetry-breaking edges and contacts. These results uniquely validate the Shockley-Ramo process for photocurrent collection in gapless materials and identify the overlooked APTE as the primary origin of robust photocurrents in anisotropic semimetal devices. Our work highlights quantum-enabled photocurrent ﬂow microscopy as a clarifying perspective for complex optoelectronic

Visualizing the thermoelectric origin of photocurrent ﬂow in anisotropic semimetals

Tailoring the electron-phonon interaction with metallic plasmonic structures

It has been well established that (i) the thermopower of semiconductors can be enhanced through a phenomenon known as the drag effect, and (ii) the drag enhancement involves only low-frequency acoustic phonons and benefits from low electron densities and low temperatures. Using first-principles calculations we show that large drag enhancements to the thermopower are possible at high carrier density even at room temperature and arise from high-frequency acoustic phonons. A fascinating example is cubic boron arsenide (BAs) for which the calculated room temperature drag enhancement of the thermopower exceeds an order of magnitude at a high hole density of ${10}^{21}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}3}$. This remarkable behavior stems from the simultaneously weak phonon-phonon and phonon-hole scattering of the high-frequency phonons in BAs that become drag active at high carrier densities through electron-phonon interactions. This work advances our understanding of coupled electron-phonon nanoscale transport and introduces an unexpected paradigm for achieving large thermopowers. 

High-frequency phonons drive large phonon-drag thermopower in semiconductors at high carrier density

The 2D electron system in GaAs quantum wells was investigated using optical absorption and photoluminescence measurements in the bandgap region, for magnetic fields B≦30 T and temperatures T≧0.4 K. Several optical anomalies appear as spectral blue-shifts for both increasing field and decreasing temperature, and are related to electron-electron interactions. The anomalies observed at the 2D Landau filling factors ν⋍1 and ν<2/3 have very different field and temperature dependencies. The one near ν=1 is associated with changes in population of the lowest spin-split Landau level. The ones at higher fields are observed only for T<3 K, concurrent with the fractional quantum Hall effect.

Spectral Blue-Shifts in Optical Absorption and Emission of the 2D Electron System in the Magnetic Quantum Limit

http://absimage.aps.org/image/MAR15/MWS_MAR15-2014-005838.pdf

David Broido

Papers

Visualizing the thermoelectric origin of photocurrent ﬂow in anisotropic semimetals

Tailoring the electron-phonon interaction with metallic plasmonic structures

High-frequency phonons drive large phonon-drag thermopower in semiconductors at high carrier density

Spectral Blue-Shifts in Optical Absorption and Emission of the 2D Electron System in the Magnetic Quantum Limit

High Thermal Conducting Boron Arsenide: Crystal Growth and Characterization