189 research outputs found
Observation of Magnetic Edge State and Dangling Bond State on Nanographene in Activated Carbon Fibers
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 non-bonding edge states of the \pi-electron, a pre-peak assigned to
the edge state was observed below the conduction electron {\pi}* peak close to
the Fermi level in NEXAFS. Via the fluorination of the ACFs, an extra peak,
which was assigned to the \sigma-dangling bond state, was observed between the
pre-peak of the edge state and the {\pi}* 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
\pi-electrons and dangling bond states of \sigma-electrons on fluorinated
nanographene sheets.Comment: 4 figures, to appear in Phys. Rev.
STM observation of the quantum interference effect in finite-sized graphite
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 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.Comment: 11 pages; 2 figures; accepted for publication in J. Phys. Chem.
Solids; ISIC1
Novel electronic wave interference patterns in nanographene sheets
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 band structure of the two-dimensional
graphene sheet.Comment: Published as a LETTER in J. Phys.: Condens. Matter; 8 pages; 6
figures; Online version at
http://www.iop.org/EJ/S/3/1256/0hJAmc5sCL6d.7sOO.BtLw/abstract/0953-8984/14/3
6/10
Theoretical study on novel electronic properties in nanographite materials
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-graphene distance, as experiments on adsorption of molecules suggest.
Next, possible charge-separated states are considered using the extended
Hubbard model with nearest-neighbor 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.Comment: 8 pages; 6 figures; accepted for publication in J. Phys. Chem.
Solids; related Web site: http://staff.aist.go.jp/k.harigaya/index_E.htm
Kohn anomalies in graphene nanoribbons
The quantum corrections to the energies of the point optical phonon
modes (Kohn anomalies) in graphene nanoribbons are investigated. We show
theoretically that the longitudinal optical modes undergo a Kohn anomaly
effect, while the transverse optical modes do not. In relation to Raman
spectroscopy, we show that the longitudinal modes are not Raman active near the
zigzag edge, while the transverse optical modes are not Raman active near the
armchair edge. These results are useful for identifying the orientation of the
edge of graphene nanoribbons by G band Raman spectroscopy, as is demonstrated
experimentally. The differences in the Kohn anomalies for nanoribbons and for
metallic single wall nanotubes are pointed out, and our results are explained
in terms of pseudospin effects.Comment: 11 pages, 6 figure
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