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Abstract: The electronic structure of nanographene depends on the shape of their edges. Around the zigzag-shaped edges, nonbonding edge state of π-electron origin is created, which is the origin of electronic and chemical activities of nanographene. In addition, the strongly spin polarized edge state has localized spin, giving magnetic activity to nanographene. We investigate the magnetic properties of nanoporous activated carbon fibers consisting of a three dimensional disordered network of nanographite domains, each of which is a stack of 3–4 nanographene sheets, in relation to host–guest interaction. Chemically inert guest species such as water, organic solvent, argon physisorbed into the nanopores mechanically compress the nanographite domains, resulting in a high spin/low spin magnetic switching phenomenon induced by the physisorption. The presence of an energy gap and the nanographene edges decorated by oxygen-containing functional groups make nanographene less amphoteric, modifying charge transfer mechanism in the host–guest interaction. HNO3 shows a two-step charge transfer process. In the Br adsorption, physisorbed Br species cooperate with charge transfer Br species, giving an interesting interplay of charge transfer and magnetic switching. In the K adsorption, physisorbed K atoms form antiferromagnetic K clusters in the nanopores, while a part of K species is responsible for charge transfer. [Copyright &y& Elsevier]

Abstract: Edges of finite size graphene, particularly nanographene, play an important role in giving unconventional electronic structure owing to the massless Dirac fermion features. This emerges as the creation of nonbonding edge state localized in the zigzag-shaped edges and electron wave interference in the armchair-shaped ones. We fabricated zigzag-edged and armchair-edged graphene nanoribbons, and investigated the Raman G-band and STM superlattice which are governed by the electron wave interference at the armchair edges. [Copyright &y& Elsevier]

Abstract: Nanographene has unique edge-shape dependence of the electronic structure with non-bonding edge states being created in its zigzag edges. The presence of the edge state is experimentally confirmed in well-defined hydrogen-terminated zigzag edges by scanning tunneling microscopy/spectroscopy (STM/STS) observations. In the three-dimensional (3D) disordered network of nanographite domains in nanoporous carbon (activated carbon fibers), the localized edge-state spins are in a spin-glass-like ordered state at low temperatures with the aid of exchange interactions whose strengths varies randomly in space, when the strengths of inter-nanographene and nanographite interactions are tuned. Chemical and structural modifications of nanographene edges change the magnetism of edge-state spins through covalent bond formation and charge transfer. [Copyright &y& Elsevier]

The electronic structure of nanographene having open edges around its circumference crucially depends on its edge shape. The circumference of an arbitrary shaped nanographene sheet is described in terms of a combination of zigzag and armchair edges. According to theoretical suggestions, nanographene has a non-bonding π-electron state (edge state) localized in zigzag edges. This is reminiscent of the non-bonding π -electron state appearing in non-Kekulé-type aromatic molecules. The localized spins of the edge states can give rise to unconventional magnetism in nanographene such as carbon-only ferromagnetism, magnetic switching phenomenon, spin glass state, etc. Nanographene can be prepared by heat-induced conversion of nanodiamond particles. Nanographene ribbons are found by chance around step edges of graphite. The detailed structures of individual nanographene ribbons thus found can be characterized by resonance Raman experiments in which the graphitic G-band is used as a fingerprint. A nanographene sheet inclined along a direction is found to show an interference superperiodic pattern with a varying periodicity. The stacking of sheets also gives an interference effect on the dislocation network created by rhombohedral stacking faults. STM/STS investigations of well defined graphene edges which are hydrogen terminated in ultra-high vacuum condition confirm the presence of edge states around zigzag edges in good agreement with theoretical works. Armchair edges are generally long and defect free whereas zigzag edges tend to be short and defective. This suggests that the armchair edge is energetically more stable than the zigzag edge that has an edge state at the Fermi level. The feature of the edge state depends on the detailed geometry of the edge structures. The edge state in a short zigzag edge embedded between armchair edges becomes less localized due to state mixing with the adjacent armchair edges. The intersheet interaction modifies the spatial distribution of the local density of states of the edge states. The electrons in the edge state in a finite-length zigzag edge are subjected to an electron confinement effect. Nanographene sheets are tailored by cutting along the direction which is chosen intentionally for designing functionality. Well defined edges can be prepared by chemical modifications with foreign atoms or functional groups. A combination of an atomic-resolution electron lithography technique and chemical modifications of the nanographene edges is expected to give nanographene-based molecular devices in the development of nanotechnology. Recent works on the preparations structural and electronic characterizations of graphene edges and nanographene are reviewed. [ABSTRACT FROM AUTHOR]

Abstract: We have studied the electronic structure of the surface of graphite irradiated by Ar+ ions with various kinetic energies using in situ near-edge X-ray absorption fine structure spectroscopy in order to verify the appearance of an edge state over a macroscopic area and the variation of the edge state feature by severely damaging a sample. The disruption of the π bonding network of graphite is manifested by the reduction of the intensity of the π∗ peak upon Ar+ irradiation. The increase of the lower energy shoulder of the π∗ peak can be clearly attributed to the edge state of π-electron origin at vacancies. High energy Ar+ irradiation strongly induces an amorphous-like structure at the surface where carbon atoms have a random coordination. As a consequence, sp 3-hybridized carbon atoms become involved in bonding, resulting in the mixing of π and σ states that then broadens the observed π∗ peak. In addition, the reduction in concentration of π electrons that screen core holes results in a blue shift of the central π∗ peak energy. Moreover, the edge state, which is the signature of π conjugation, survives even at the severely damaged Ar+ irradiated graphite surface having an amorphous-like structure. [Copyright &y& Elsevier]

The relation between electronic structure and hydrogen content at the edges of nanosized holes (nanoholes) in graphite single layer is studied by means of atomically resolved scanning tunneling microscopy and first-principles calculations. Repeatable production of nanoholes with predominantly zigzag hydrogenated edges was realized by bombardment of the top graphene layer of graphite with low-energy (100 eV) Ar+ ions and its further treatment (etching) in an atomic hydrogen environment. Two main types of nanohole zigzag edge, with a striking contrast to each other, are identified: (i) monohydrogenated zigzag edge supporting edge-localized π state on its boundary; (ii) zigzag edge with repeating two monohydrogenated sites and one dihydrogenated site, without features of the edge-localized state and characterized by a prominent standing wave pattern. Absence of the localized state at a general (chiral) hydrogenated graphitic edge consisting of a mixture of zigzag and armchair fragments is also discussed. [ABSTRACT FROM AUTHOR]

Abstract: Nonbonding π-electron states (edge-state) localized at the graphene edges are the origin of electronic/chemical activities of nanographene, and the localized spin of this edge state gives unconventional magnetic features to nanographene. The magnetic properties of nanoporous carbon consisting of a 3D disordered network of nanographite domains (each consists of a stack of 3–4 nanographene sheets) are investigated in relation to the host-guest interaction with nitric acid molecules possessing significant oxidizing power. The adsorption of HNO3 molecules induces a two-step reduction in the edge-state spin concentration due to the charge transfer from nanographene to HNO3 molecules; charge transfer with the exterior nanographene sheets of the nanographite domain in the first step, and subsequent intercalation into the interior nanographene sheets in the second step. Besides the charge transfer reaction, it is found that the paramagnetic NO molecules are produced owing to the decomposition of HNO3 and they are dimerized to form nonmagnetic (NO)2 at low temperatures. This finding suggests that the chemically active nanographene having edge states plays an important role in enhancing charge transfer and the concomitant decomposition of the guest HNO3 molecules. [Copyright &y& Elsevier]

The electronic structure of nanographene in pristine and fluorinated activated carbon fibers (ACFs) have been investigated with near-edge x-ray absorption fine structure (NEXAFS) and compared with magnetic properties we reported on previously. In pristine ACFs in which magnetic properties are governed by nonbonding edge states of the π electron, a prepeak assigned to the edge state was observed below the conduction electron π* peak close to the Fermi level in NEXAFS. Via the fluorination of the ACFs, an extra peak, which was assigned to the s-dangling bond state, was observed between the prepeak of the edge state and the π* peak in the NEXAFS profile. The intensities of the extra peak correlate closely with the spin concentration created upon fluorination. The combination of the NEXAFS and magnetic measurement results confirms the coexistence of the magnetic edge states of π electrons and dangling bond states of s electrons on fluorinated nanographene sheets. [ABSTRACT FROM AUTHOR]

Antiferromagnetism in stacked nanographite is investigated with using the Hubbard-type models. The A–B stacking or the stacking near to that of A–B type is favorable for the hexagonal nanographite with zigzag edges, in order that magnetism appears. We also find that the open shell electronic structure can be an origin of the decreasing magnetic moment with the decrease of the inter-graphene distance, as experiments on adsorption of molecules suggest. Next, superperiodic patterns with a long distance in a nanographene sheet observed by STM are discussed in terms of the interference of electronic wave functions. The period and the amplitude of the oscillations decrease spatially in one direction. We explain the superperiodic patterns with a static linear potential theoretically. In the k·p model, the oscillation period decreases, and agrees with experiments. The spatial difference of the static potential is estimated as 1.3 eV for 200 nm in distance, and this value seems to be reasonable in order that the potential difference remains against perturbations, for example, by phonon fluctuations and impurity scatterings. It turns out that the long-distance oscillations come from the electronic structure of the two-dimensional graphene sheet. [Copyright &y& Elsevier]

Superperiodic patterns were observed by STM on two kinds of finite-sized graphene sheets. One is nanographene sheets inclined from a highly oriented pyrolitic graphite (HOPG) substrate and the other is a several-layer-thick graphene sheets with dislocation-network structures against a HOPG substrate. As for the former, the in-plane periodicity increased gradually in the direction of inclination, and it is easily changed by attachment of a nanographite flake on the nanographene sheets. The oscillation pattern can be explained by the interference of electron waves confined in the inclined nanographene sheets. As for the latter, patterns and their corrugation amplitudes depended on the bias voltage and on the terrace height from the HOPG substrate. The interference effect by the perturbed and unperturbed waves in the overlayer is responsible for the patterns whose local density of states varies in space. [Copyright &y& Elsevier]

Antiferromagnetism in stacked nanographite is investigated with using the Hubbard-type model. We find that the open shell electronic structure can be an origin of the decreasing magnetic moment with the decrease of the inter-layer distance, as experiments on adsorption of molecules suggest. Next, possible charge-separated states are considered using the extended Hubbard model with nearest-neighbor repulsive interactions. The charge-polarized state could appear, when a static electric field is present in the graphene plane for example. Finally, superperiodic patterns with a long distance in a nanographene sheet observed by STM are discussed in terms of the interference of electronic wave functions with a static linear potential theoretically. In the analysis by the k·p model, the oscillation period decreases spatially in agreement with experiments. [Copyright &y& Elsevier]