1,292 research outputs found
Energy Gaps in Graphene Nanoribbons
Based on a first-principles approach, we present scaling rules for the band
gaps of graphene nanoribbons (GNRs) as a function of their widths. The GNRs
considered have either armchair or zigzag shaped edges on both sides with
hydrogen passivation. Both varieties of ribbons are shown to have band gaps.
This differs from the results of simple tight-binding calculations or solutions
of the Dirac's equation based on them. Our {\it ab initio} calculations show
that the origin of energy gaps for GNRs with armchair shaped edges arises from
both quantum confinement and the crucial effect of the edges. For GNRs with
zigzag shaped edges, gaps appear because of a staggered sublattice potential on
the hexagonal lattice due to edge magnetization. The rich gap structure for
ribbons with armchair shaped edges is further obtained analytically including
edge effects. These results reproduce our {\it ab initio} calculation results
very well
Electron Beam Supercollimation in Graphene Superlattices
Although electrons and photons are intrinsically different, importing useful
concepts in optics to electronics performing similar functions has been
actively pursued over the last two decades. In particular, collimation of an
electron beam is a long-standing goal. We show that ballistic propagation of an
electron beam with virtual no spatial spreading or diffraction, without a
waveguide or external magnetic field, can be achieved in graphene under an
appropriate class of experimentally feasible one-dimensional external periodic
potentials. The novel chiral quasi-one-dimensional metallic state that the
charge carriers are in originates from a collapse of the intrinsic helical
nature of the charge carriers in graphene owing to the superlattice potential.
Beyond providing a new way to constructing chiral one-dimensional states in two
dimensions, our findings should be useful in graphene-based electronic devices
(e.g., for information processing) utilizing some of the highly developed
concepts in optics.Comment: 7 pages, 4 figures (including supporting online material), published
online in Nano Letter
New Generation of Massless Dirac Fermions in Graphene under External Periodic Potentials
We show that new massless Dirac fermions are generated when a slowly varying
periodic potential is applied to graphene. These quasiparticles, generated near
the supercell Brillouin zone boundaries with anisotropic group velocity, are
different from the original massless Dirac fermions. The quasiparticle
wavevector (measured from the new Dirac point), the generalized pseudospin
vector, and the group velocity are not collinear. We further show that with an
appropriate periodic potential of triangular symmetry, there exists an energy
window over which the only available states are these quasiparticles, thus,
providing a good system to probe experimentally the new massless Dirac
fermions. The required parameters of external potentials are within the realm
of laboratory conditions.Comment: 4 pages, 4 figure
Electrical Switching in Metallic Carbon Nanotubes
We present first-principles calculations of quantum transport which show that
the resistance of metallic carbon nanotubes can be changed dramatically with
homogeneous transverse electric fields if the nanotubes have impurities or
defects. The change of the resistance is predicted to range over more than two
orders of magnitude with experimentally attainable electric fields. This novel
property has its origin that backscattering of conduction electrons by
impurities or defects in the nanotubes is strongly dependent on the strength
and/or direction of the applied electric fields. We expect this property to
open a path to new device applications of metallic carbon nanotubes.Comment: 4 pages and 4 figure
Half-Metallic Graphene Nanoribbons
Electrical current can be completely spin polarized in a class of materials
known as half-metals, as a result of the coexistence of metallic nature for
electrons with one spin orientation and insulating for electrons with the
other. Such asymmetric electronic states for the different spins have been
predicted for some ferromagnetic metals - for example, the Heusler compounds-
and were first observed in a manganese perovskite. In view of the potential for
use of this property in realizing spin-based electronics, substantial efforts
have been made to search for half-metallic materials. However, organic
materials have hardly been investigated in this context even though
carbon-based nanostructures hold significant promise for future electronic
device. Here we predict half-metallicity in nanometre-scale graphene ribbons by
using first-principles calculations. We show that this phenomenon is realizable
if in-plane homogeneous electric fields are applied across the zigzag-shaped
edges of the graphene nanoribbons, and that their magnetic property can be
controlled by the external electric fields. The results are not only of
scientific interests in the interplay between electric fields and electronic
spin degree of freedom in solids but may also open a new path to explore
spintronics at nanometre scale, based on graphene
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The Value of End-Use Energy Efficiency in Mitigation of U.S. Carbon Emissions
This report documents a scenario analysis exploring the value of advanced technologies in the U.S. buildings, industrial, and transportation sectors in stabilizing atmospheric greenhouse gas concentrations. The analysis was conducted by staff members of Pacific Northwest National Laboratory (PNNL), working at the Joint Global Change Research Institute (JGCRI) in support of the strategic planning process of the U.S. Department of Energy (U.S. DOE) Office of Energy Efficiency and Renewable Energy (EERE). The conceptual framework for the analysis is an integration of detailed buildings, industrial, and transportation modules into MiniCAM, a global integrated assessment model. The analysis is based on three technology scenarios, which differ in their assumed rates of deployment of new or presently available energy-saving technologies in the end-use sectors. These technology scenarios are explored with no carbon policy, and under two CO2 stabilization policies, in which an economic price on carbon is applied such that emissions follow prescribed trajectories leading to long-term stabilization of CO2 at roughly 450 and 550 parts per million by volume (ppmv). The costs of meeting the emissions targets prescribed by these policies are examined, and compared between technology scenarios. Relative to the reference technology scenario, advanced technologies in all three sectors reduce costs by 50% and 85% for the 450 and 550 ppmv policies, respectively. The 450 ppmv policy is more stringent and imposes higher costs than the 550 ppmv policy; as a result, the magnitude of the economic value of energy efficiency is four times greater for the 450 ppmv policy than the 550 ppmv policy. While they substantially reduce the costs of meeting emissions requirements, advanced end-use technologies do not lead to greenhouse gas stabilization without a carbon policy. This is due mostly to the effects of increasing service demands over time, the high consumption of fossil fuels in the electricity sector, and the use of unconventional feedstocks in the liquid fuel refining sector. Of the three end-use sectors, advanced transportation technologies have the greatest potential to reduce costs of meeting carbon policy requirements. Services in the buildings and industrial sectors can often be supplied by technologies that consume low-emissions fuels such as biomass or, in policy cases, electricity. Passenger transportation, in contrast, is especially unresponsive to climate policies, as the fuel costs are small compared to the time value of transportation and vehicle capital and operating costs. Delaying the transition from reference to advanced technologies by 15 years increases the costs of meeting 450 ppmv stabilization emissions requirements by 21%, but the costs are still 39% lower than the costs assuming reference technology. The report provides a detailed description of the end-use technology scenarios and provides a thorough analysis of the results. Assumptions are documented in the Appendix
Experimentally Engineering the Edge Termination of Graphene Nanoribbons
The edges of graphene nanoribbons (GNRs) have attracted much interest due to
their potentially strong influence on GNR electronic and magnetic properties.
Here we report the ability to engineer the microscopic edge termination of high
quality GNRs via hydrogen plasma etching. Using a combination of
high-resolution scanning tunneling microscopy and first-principles
calculations, we have determined the exact atomic structure of plasma-etched
GNR edges and established the chemical nature of terminating functional groups
for zigzag, armchair and chiral edge orientations. We find that the edges of
hydrogen-plasma-etched GNRs are generally flat, free of structural
reconstructions and are terminated by hydrogen atoms with no rehybridization of
the outermost carbon edge atoms. Both zigzag and chiral edges show the presence
of edge states.Comment: 16+9 pages, 3+4 figure
Anisotropic behaviors of massless Dirac fermions in graphene under periodic potential
Charge carriers of graphene show neutrino-like linear energy dispersions as
well as chiral behavior near the Dirac point. Here we report highly unusual and
unexpected behaviors of these carriers in applied external periodic potentials,
i.e., in graphene superlattices. The group velocity renormalizes highly
anisotropically even to a degree that it is not changed at all for states with
wavevector in one direction but is reduced to zero in another, implying the
possibility that one can make nanoscale electronic circuits out of graphene not
by cutting it but by drawing on it in a non-destructive way. Also, the type of
charge carrier species (e.g. electron, hole or open orbit) and their density of
states vary drastically with the Fermi energy, enabling one to tune the Fermi
surface-dominant properties significantly with gate voltage. These results
address the fundamental question of how chiral massless Dirac fermions
propagate in periodic potentials and point to a new possible path for nanoscale
electronics.Comment: 10 pages, 9 figure
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