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Edge state of Nanographene and its Electronic and Magnetic Properties The study of carbon nanomaterials has been one of the central issues in nanoscience/nanotechnology since the discovery of fullerenes and carbon nanotubes. In particular, the successful isolation of graphene (single layer graphite) and discovery of its unconventional electronic properties in 2004 have led to wide recognition of its importance: Being a two dimensional single-atom-thick sheet, it represents the ultimate system and thus forms the basis for studies on other carbon nanomaterials such as fullerenes and carbon nanotubes. The electronic properties of graphene, which are described using massless Dirac fermions, not only constitute an important issue in fundamental physics but also hold a promise to be used in future device applications.
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Prof. Toshiaki Enoki
Graduate School of Science and Engineering
Tokyo Institute of Technology
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From the viewpoint of chemistry, graphene, consisting of an infinite number of hexagonal rings, can be thought of as an extreme version of an aromatic hydrocarbon molecule. Therefore, the study of this material has implications across a wide variety of fundamental issues in chemistry. The chemical and electronic activities of graphene are considered to originate particularly from the edges of graphene sheets and from nanosized graphene flakes (nanographene). Prof. Toshiaki Enoki has been engaged in the study of the physical chemistry of nanographene and graphene edges since the mid-1990s. He has successfully prepared nanographene sheets and discovered their unconventional electronic and magnetic structures. His pioneering work has significantly contributed toward broadening the scope of graphene research and making it into an interdisciplinary field, extending from organic structural chemistry to condensed matter physics.
His major contributions to the progress in the physical chemistry of graphene edges and nanographene are summarized as follows:
1. In 2001, he successfully prepared single-layer nanographene sheets with a size of ∼10 nm through heat-induced conversion of nanodiamond particles. He determined the structure of the obtained nanographene sheets from atomic images obtained by scanning tunneling microscopy (STM). In 2004, he obtained single-layer nanographene ribbons with a width of ∼ 8 nm by heat-treatment of graphite step edges and confirmed the structure of individual nanographene ribbons by resonance Raman experiments with a polarized laser beam. These studies were conducted just before the report on the discovery of graphene in 2004.
2. There are two types of graphene edges: zigzag and armchair. The periphery of an arbitrary-shaped nanographene sheet is described in terms of a combination of zigzag and armchair edges. The electronic structure of graphene edges depends crucially on the geometry of the edges. In 1987, Stein and Brown theoretically predicted the presence of an unconventional electronic state localized at the zigzag edges. In 1996, Fujita, Wakabayashi, and their coworkers theoretically confirmed the origin of this state to be a nonbonding π-electron state, which they named the gedge state.h In physics, the presence of the edge state is a consequence of symmetrical breaking of the massless Dirac fermions, whereas in chemistry, it is related to the degradation of the aromaticity. In 2005-2006, Prof. Enoki experimentally confirmed the presence of the edge state at the zigzag edges by using STM/STS (scanning tunneling spectroscopy) with well-defined hydrogen-terminated graphene zigzag edges. His systematic and comprehensive STM/STS investigations clarified the correlation between the electronic structure and geometrical details of the graphene edges.
3. The edge-state well localized at the zigzag-type graphene edges is strongly spin polarized and gives rise to the magnetic properties of graphene- and graphite-based materials. It has been theoretically predicted that the edge-state spins mutually interact through strong ferromagnetic exchange interactions. Since the mid-1990s, he has been investigating the magnetic properties of nanographene and related materials. In 1998, he experimentally confirmed the important role of the edge-state spins in the magnetism of nanographene. He discovered the spin glass state of the edge-state spins in the three-dimensional random network of nanographene sheets in 2000. Later on, he experimentally confirmed that the edge-state spins in a nanograpehene sheet form a ferrimagnetic structure under the competition between the strong intra-zigzag-edge ferromagnetic interaction and intermediate-strength inter-zigzag-edge ferromagnetic/antiferromagnetic interaction. The magnetism of carbon-based materials has been the subject of scientific investigation for the last five decades, but its origin is still not been well understood. His experimental investigations on the magnetism of nanographene have opened up the way toward finally concluding the long-lasting issue of the origin of carbon magnetism. His studies have also revealed that edge-state spins can be a useful ingredient in carbon-based molecular magnetism and spintronics device applications.
The study of nanographene and graphene edges, which was pioneered by Prof. Enoki, constitutes one of the most important topics in current research on graphene. His achievements represent a great step, not only in bridging the chemistry and physics of graphene, but also in developing nanographene-based applications such as electronics/spintronics devices, catalysts, batteries, and fuel cells.
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