Single-layer metal dichalcogenides are two-dimensional semiconductors that present strong potential for electronic and sensing applications complementary to that of graphene.

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Electronics and optoelectronics of two-dimensional transition metal dichalcogenides.

Graphene is a wonder material with many superlatives to its name. It is the thinnest known material in the universe and the strongest ever measured. Its charge carriers exhibit giant intrinsic mobility, have zero effective mass, and can travel for micrometers without scattering at room temperature. Graphene can sustain current densities six orders of magnitude higher than that of copper, shows record thermal conductivity and stiffness, is impermeable to gases, and reconciles such conflicting qualities as brittleness and ductility. Electron transport in graphene is described by a Dirac-like equation, which allows the investigation of relativistic quantum phenomena in a benchtop experiment. This review analyzes recent trends in graphene research and applications, and attempts to identify future directions in which the field is likely to develop.

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Graphene: Status and Prospects

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

Interest in graphene centres on its excellent mechanical, electrical, thermal and optical properties, its very high specific surface area, and our ability to influence these properties through chemical functionalization. There are a number of methods for generating graphene and chemically modified graphene from graphite and derivatives of graphite, each with different advantages and disadvantages. Here we review the use of colloidal suspensions to produce new materials composed of graphene and chemically modified graphene. This approach is both versatile and scalable, and is adaptable to a wide variety of applications.

Chemical methods for the production of graphenes.

Raman spectroscopy is an integral part of graphene research. It is used to determine the number and orientation of layers, the quality and types of edge, and the effects of perturbations, such as electric and magnetic fields, strain, doping, disorder and functional groups. This, in turn, provides insight into all sp(2)-bonded carbon allotropes, because graphene is their fundamental building block. Here we review the state of the art, future directions and open questions in Raman spectroscopy of graphene. We describe essential physical processes whose importance has only recently been recognized, such as the various types of resonance at play, and the role of quantum interference. We update all basic concepts and notations, and propose a terminology that is able to describe any result in literature. We finally highlight the potential of Raman spectroscopy for layered materials other than graphene.

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Raman spectroscopy as a versatile tool for studying the properties of graphene

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

The authors reported on investigation of the thermal conductivity of graphene suspended across trenches in Si∕SiO2 wafer. The measurements were performed using a noncontact technique based on micro-Raman spectroscopy. The amount of power dissipated in graphene and corresponding temperature rise were determined from the spectral position and integrated intensity of graphene’s G mode. The extremely high thermal conductivity in the range of ∼3080–5150W∕mK and phonon mean free path of ∼775nm near room temperature were extracted for a set of graphene flakes. The obtained results suggest graphene’s applications as thermal management material in future nanoelectronic circuits.

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Extremely high thermal conductivity of graphene: Prospects for thermal management applications in nanoelectronic circuits

The ability to propagate heat in a film should improve with increasing thickness. However, graphene has a higher thermal conductivity than graphite, despite having a smaller thickness. The crossover from two-dimensional to bulk graphite is now studied experimentally and explained theoretically. The results may pave the way to thermal management applications in nanoelectronics.

Dimensional crossover of thermal transport in few-layer graphene

Graphene, in addition to its unique electronic and optical properties, revealed unusually high thermal conductivity. The fact that thermal conductivity of large enough graphene sheets should be higher than that of basal planes of bulk graphite was predicted theoretically by Klemens. However, the exact mechanisms behind drastic alteration of material's intrinsic ability to conduct heat as its dimensionality changes from 2-D to 3-D remain elusive. Recent availability of high-quality few-layer graphene materials allowed us to study dimensional crossover experimentally. Here we show that the room-temperature thermal conductivity changes from K~3000 W/mK to 1500 W/mK as the number of atomic plains in few-layer graphene increases from 2 to 4. We explained the observed evolution from 2-D to bulk by the cross-plane coupling of the low-energy phonons and corresponding changes in the phonon Umklapp scattering. The obtained results shed light on heat conduction in low-dimensional materials and may open up few-layer graphene applications in thermal management of nanoelectronics.

Suchismita Ghosh

Papers

Superior Thermal Conductivity of Single-Layer Graphene

Extremely high thermal conductivity of graphene: Prospects for thermal management applications in nanoelectronic circuits

Dimensional crossover of thermal transport in few-layer graphene

Dimensional crossover of thermal transport in few-layer graphene materials

Raman nanometrology of graphene: Temperature and substrate effects