Accelerating global energy consumption makes the development of clean and renewable alternative energy sources indispensable. Nanotechnology opens up new frontiers in materials science and engineering to meet this energy challenge by creating new materials, particularly carbon nanomaterials, for efficient energy conversion and storage. Since the Nobel Prize winning research on graphene by Geim and Novoselov, considerable efforts have been made to exploit graphene as an energy material, and tremendous progress has been achieved in developing high-performance devices for energy conversion and energy storage. This article reviews recent progress in the research and development of graphene materials for advanced energy-conversion devices, including solar cells and fuel cells, and energy-storage devices, including supercapacitors and lithium-ion batteries, and discusses some challenges in this exciting field.

We present an introduction to the rapidly growing field of epitaxial graphene on silicon carbide, tracing its development from the original proof-of-concept experiments a decade ago to its present, highly evolved state. The potential of epitaxial graphene as a new electronic material is now being recognized. Whether the ultimate promise of graphene-based electronics will ever be realized remains an open question. Silicon electronics is based on single-crystal substrates that allow reliable patterning on the nanoscale, which is an absolute requirement for any new electronic material. That is why epitaxial graphene is based on single-crystal silicon carbide. We also present recent results on nanopatterned graphene produced by etching the silicon carbide before annealing so that the graphene structures are produced in their final shapes. This avoids postannealing patterning, which is known to greatly affect transport properties on the nanoscale. Creating such structured graphene is an elegant method for avoiding pervasive patterning problems.

Recently, transmission electron microscopy (TEM) and related techniques have brought unique insights to graphene research, demonstrating remarkable flexibility in characterizations ranging from atomic ordering to charge distribution. Such TEM studies have helped advance areas including the understanding of graphene growth and the effects of defects and dopants on the mechanical and electrical properties of graphene. Electron microscopy has proved particularly useful in determining the structure of crystals and grain boundaries across six orders of magnitude—from the shapes, arrangements, and stacking sequences of grains to the atomic arrangements at grain boundaries. Meanwhile, graphene is becoming a promising two-dimensional laboratory bench for electron microscopy, for example, turning graphene into a medium for nanosculpting by transforming buckyballs into graphene and vice versa. Finally, graphene has been used as an ultrathin support membrane for TEM, enabling studies of the motion of single atoms, direct imaging of two-dimensional amorphous materials, and even formation of nano-aquaria for imaging bacteria or nanoparticles in liquid media. Rapid developments in the fields of both electron microscopy and graphene will continue to provide a rich ground for future insights.

As graphene technologies progress to commercialization and large-scale manufacturing, issues of material and processing safety will need to be more seriously considered. The single word “graphene” actually represents a family of related materials with large variations in number of layers, surface area, lateral dimensions, stiffness, and surface chemistry. Many of these materials have aerodynamic diameters below 5 μm and can potentially be inhaled into the human lung. Graphene materials show several unique modes of interaction with biological molecules, tissues, and cells. The limited literature suggests that graphene materials can be either benign or harmful and that the biological response varies according to a material’s physicochemical properties and biologically effective dose. The present article reviews the current literature on the graphene–biological interface with an emphasis on the mechanisms and fundamental biological responses relevant to material safety and also to potential biomedical applications

The optical properties of graphene-based nanomaterials have attracted much recent attention. This article provides an overview of recent advances in the study of linear and nonlinear optical transitions associated mostly with tailored energy bandgaps. In particular, the optical absorption characteristics and photoluminescence emissions due to various induced bandgaps and, in some cases, the formation of graphene quantum dots are highlighted. Nonlinear optical properties of these materials are reviewed with an emphasis on optical limiting through both nonlinear absorption and scattering mechanisms.

Graphene is a two-dimensional (2D) material with over 100-fold anisotropy of heat flow between the in-plane and out-of-plane directions. High in-plane thermal conductivity is due to covalent sp2bonding between carbon atoms, whereas out-of-plane heat flow is limited by weak van der Waals coupling. Herein, we review the thermal properties of graphene, including its specific heat and thermal conductivity (from diffusive to ballistic limits) and the influence of substrates, defects, and other atomic modifications. We also highlight practical applications in which the thermal properties of graphene play a role. For instance, graphene transistors and interconnects benefit from the high in-plane thermal conductivity, up to a certain channel length. However, weak thermal coupling with substrates implies that interfaces and contacts remain significant dissipation bottlenecks. Heat flow in graphene or graphene composites could also be tunable through a variety of means, including phonon scattering by substrates, edges, or interfaces. Ultimately, the unusual thermal properties of graphene stem from its 2D nature, forming a rich playground for new discoveries of heat-flow physics and potentially leading to novel thermal management applications.

Graphene is a material with outstanding properties that make it an excellent candidate for advanced applications in future electronics and photonics. The potential of graphene in high-speed analog electronics is currently being explored extensively because of its high carrier mobility, its high carrier saturation velocity, and the insensitivity of its electrical-transport behavior to temperature variations. Herein, we review some of the key material and carrier-transport physics of graphene, then focus on high-frequency graphene field-effect transistors, and finally discuss graphene monolithically integrated circuits (ICs). These high-frequency graphene transistors and ICs could become essential elements in the blossoming fields of wireless communications, sensing, and imaging. After discussing graphene electronics, we describe the impressive photonic properties of graphene. Graphene interacts strongly with light over a very wide spectral range from microwaves to ultraviolet radiation. Most importantly, the light–graphene interaction can be adjusted using an electric field or chemical dopant, making graphene-based photonic devices tunable. Single-particle interband transitions lead to a universal optical absorption of about 2% per layer, whereas intraband free-carrier transitions dominate in the microwave and terahertz wavelength range. The tunable plasmonic absorption of patterned graphene adds yet another dimension to graphene photonics. We show that these unique photonic properties of graphene over a broad wavelength range make it promising for many photonic applications such as fast photodetectors, optical modulators, far-infrared filters, polarizers, and electromagnetic wave shields. These graphene photonic devices could find various applications in optical communications, infrared imaging, and national security.

The superlative electronic, optical, mechanical, and chemical properties of graphene suggest broad technological opportunities for graphene-based materials and composites. However, the transition from the research laboratory to widespread commercial utilization requires economical methods for the mass production of graphene and graphene-based materials. Among the emerging methods for synthesizing graphene, solution-based processing holds particular promise because of its low cost, high throughput, chemical versatility, and scalability to large quantities. Furthermore, solution-processed graphene can be seamlessly integrated with other nanomaterials or polymers to yield composites for a wide array of applications such as energy conversion and storage, catalysis, electronics, and high-strength materials. This article highlights the range of techniques being developed for processing graphene in solution with a specific emphasis on solution-based methods for realizing graphene-based composites. In addition to fundamental principles, representative applications for these materials are presented.

Graphene came to the forefront in the nanosciences in the early 2000s, in particular, when high-quality graphene with atomic thickness and two-dimensional extension in the micrometer range was isolated and the resulting novel electronic properties were demonstrated. Graphene has two unique features: lateral size up to tens of micrometers or larger and quantum confinement in an atomically thin sheet. It provides an excellent platform for exploring novel material properties, designing new materials, and enhancing material performance. Now, after extensive research for nearly a decade, graphene research has moved well beyond electronic applications and has begun to extend into a wide variety of disciplines. This expanded issue of MRS Bulletin is focused on graphene and consists of 20 articles and three commentaries that collectively address the major impact of graphene on materials science, highlight the newest advances, discuss challenging issues, explore applications, and reveal future directions.

Whereas efforts toward graphene commercialization are still in their early stages, lessons from the commercialization of carbon nanotubes (CNTs) might be applicable, given the similarities between the two materials (specifically, a single-walled CNT can be thought of as a monolayer of graphene wrapped into a cylinder). This article reviews the commercialization of CNT materials (with a special emphasis on single-walled CNTs) in selected electronics applications, including specific examples of successes, failures, and promising opportunities. Two application areas are reviewed: (1) alternatives to silicon for fabricating transistors used in display backplanes, radio-frequency identification, and smart cards, for example, and (2) alternatives to indium tin oxide for transparent conductive films used in displays, electronic paper for e-readers, touch sensors, light-emitting diode lighting, photovoltaics, and electrochromic windows. Some important lessons learned from these commercialization experiences can potentially help accelerate the commercialization of other exciting nanomaterials such as graphene.

In this article, I describe my early interest in graphene and contributions that I and my co-authors, in particular, have made to the field, along with a brief history of the experimental discovery of graphene. I then turn to new carbon materials whose experimental syntheses might be on the horizon. One example involves using graphene as a template to generate large-area ultrathin sp3-bonded carbon sheets that could also be substitutionally doped with, for example, nitrogen atoms, as one approach to making materials of interest for quantum computing. Such large-area sp3-bonded carbon sheets hold tremendous promise for use in thermal management; as a new material for electronics and photonics; and as ultrahigh-strength components in various structures including those used in aerospace, among other applications. Another example is the class of negative-curvature carbons (NCCs) that have atom-thick walls and carbon atoms trivalently bonded to other carbon atoms. Such NCCs have a nanoscale pore structure, atom-thick walls, and exceptionally high specific surface areas, and they fill three-dimensional space in ways that suggest their use as electrode materials for ultracapacitors and batteries, as adsorbents, as support material for catalysts, and for other applications.

From the earliest days of graphene electronics, epitaxial graphene grown on SiC has been the focus of both academic and industrial research because it is potentially scalable to large electronic systems. Yet, epitaxial graphene electronics is still in its infancy. In the race to demonstrate devices, the fundamental work of understanding and controlling this material has just begun. It is entirely possible that graphene’s potential for electronics lies in new ways of thinking about electronics. In that case, significant advances will come only after serious materials physics and engineering research. One of the arguably most important properties of epitaxial graphene is that it can be studied with a variety of analytical probes beyond electron-transport measurements. Synchrotron studies have been key to understanding a wide variety of properties, including the role of the graphene–SiC interface in graphene’s transport properties, how the films are doped, whether and how a bandgap (critical to digital electronics) can be formed, and how metals and insulators can be grown on graphene for critical ohmic contacts and gate structures. These important studies are discussed in this review.

The exceptional properties of graphene originate from its two-dimensional polymeric structure of sp2-bonded carbon. This feature also causes graphene to grow on metal substrates through mechanisms that are strikingly different from those of conventional heteroepitaxy. We provide here a brief review of graphene growth on metals, a subject with a rich history even before the recent explosion of interest in graphene. The current activities related to graphene growth on metals have been motivated by the need to develop low-cost, scalable processes for graphene synthesis and to understand how graphene–metal interfaces behave in devices. In this article, we examine the current state of the art, emphasizing the basic processes that distinguish graphene growth from normal crystal growth.

Homogeneous graphene layers can be grown epitaxially on SiC(0001), promising scalable graphene technology. However, covalent bonds at the SiC–graphene interface induce strong n-doping of the graphene. This doping can be compensated by functionalizing the graphene surface with electronegative molecules. Alternatively, the influence of the substrate can be largely suppressed by breaking the covalent bonds through atomic intercalation. Hydrogen atoms migrate under the graphene, passivate the underlying SiC layer, and decouple the graphene from the substrate. In this way, large-scale, homogeneous, quasi-free-standing graphene layers can be achieved. By intercalation of germanium, the electronic structure of the decoupled graphene can be tailored. Two symmetrically doped, namely, n- and p-type, phases are stabilized, depending on the amount of intercalated germanium. This is achieved by annealing a germanium film at various temperatures after it is initially deposited on the covalently bonded carbon layer. In an intermediate temperature regime, lateral p–n junctions between the two phases can be formed, size-tailored on a mesoscopic scale.