Graphene: Fundamentals and functionalities
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Introduction Nanomaterials have become an important part of materials research over the past 30 years. Carbon nanomaterials, in particular, have been at the forefront of nanomaterials developments in recent decades. Fullerenes (C60)—discovered by Smalley et al.1 in 1985—became one of the first recognized nanomaterials and played an important role in the early development of nanoscience and nanoengineering. In the 1990s, carbon nanotubes (CNTs), discovered by S. Iijima and colleagues, became one of the most intensively investigated nanomaterials.2 More recently, graphene has gained significant attention and has become one of the most widely investigated materials.3,4 Indeed, today, graphene science and technology is a broad research field. Graphene is a single-atomic-layer honeycomb lattice of carbon atoms in an sp 2 hexagonal bonding configuration (Figure 1). It is a two-dimensional allotrope of sp2 carbon in the form of a planar monolayer. Graphene can be considered to be the basic atomic structural unit for a wide range of carbon nanomaterials. Zero-dimensional C60 can be considered to be graphene with the introduction of a pentagonal arrangement of carbon atoms—and therefore curvature—to form a ball structure. One-dimensional carbon nanotubes (CNTs) are essentially graphene sheets rolled into cylinders with a variety of chiralities depending on the direction of rolling with respect
to the hexagonal lattice. The physical properties of carbon vary widely with the allotropic form. Low-dimensional carbon nanomaterials, including graphene, CNTs, and C60, have had a significant impact on the exploration of novel material properties, the design of new functional materials, and the enhancement of material performance. In 2004, Novoselov et al.3 discovered that a high-quality isolated graphene sheet exhibits a strong ambipolar electric field effect with a carrier concentration of 1013 cm–2 and a roomtemperature mobility of 10,000 cm2 V–1 s–1. The direct observation of isolated graphene monolayers that year sparked exponentially growing interest in this material and motivated research into the properties of this two-dimensional nanomaterial. The unique electron-transport properties of two-dimensional graphene from the atomic scale to the macroscale first attracted the attention of solid-state physicists and then of materials scientists, chemists, biologist, and engineers from various disciplines. These disparate research communities began exploring ways to incorporate graphene into devices for applications to exploit the novel properties of this unique nanomaterial, as highlighted in recent review articles.5–11 Although nanomaterials have had a significant recent impact on materials science and engineering, more conventional nanomaterials—such as quantum dots, carbon nanotubes, and oxide nanowires—lack structural integration in the scale
Weijie Lu, Air Force Research Laboratory, Wright-Patterson Air Force Base, USA; [email protected] Patrick Soukiassian, Université de Paris-Sud, Orsay and CEA/Saclay (Commissariat à l’E
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