175 research outputs found
Shot Noise Probing of Magnetic Ordering in Zigzag Graphene Nanoribbons
The nonequilibrium time-dependent fluctuations of charge current have
recently emerged as a sensitive experimental tool to probe ballistic transport
through evanescent wave functions introduced into clean wide and short graphene
strips by the attached metallic electrodes. We demonstrate that such
"pseudo-diffusive" shot noise can be substantially modified in zigzag graphene
nanoribbon (ZGNR) due to the topology of its edges responsible for localized
states that facilitate ferromagnetic ordering along the edge when Coulomb
interaction is taken into account. Thus, the shot noise enhancement of
unpolarized, and even more sensitively of spin-polarized, charge currents
injected into ZGNR will act as an all-electrical and edge-sensitive probe of
such low-dimensional magnetism.Comment: 5 pages, 3 color figures; references update
Electron density and transport in top-gated graphene nanoribbon devices: First-principles Green function algorithms for systems containing large number of atoms
The recent fabrication of graphene nanoribbon (GNR) field-effect transistors
poses a challenge for first-principles modeling of carbon nanoelectronics due
to many thousand atoms present in the device. The state of the art quantum
transport algorithms, based on the nonequilibrium Green function formalism
combined with the density functional theory (NEGF-DFT), were originally
developed to calculate self-consistent electron density in equilibrium and at
finite bias voltage (as a prerequisite to obtain conductance or current-voltage
characteristics, respectively) for small molecules attached to metallic
electrodes where only a few hundred atoms are typically simulated. Here we
introduce combination of two numerically efficient algorithms which make it
possible to extend the NEGF-DFT framework to device simulations involving large
number of atoms. We illustrate fusion of these two algorithms into the
NEGF-DFT-type code by computing charge transfer, charge redistribution and
conductance in zigzag-GNR/variable-width-armchair-GNR/zigzag-GNR two-terminal
device covered with a gate electrode made of graphene layer as well. The total
number of carbon and edge-passivating hydrogen atoms within the simulated
central region of this device is ~7000. Our self-consistent modeling of the
gate voltage effect suggests that rather large gate voltage might be required
to shift the band gap of the proposed AGNR interconnect and switch the
transport from insulating into the regime of a single open conducting channel.Comment: 19 pages, 8 PDF figures, PDFLaTe
Characterization of nanometer-sized, mechanically exfoliated graphene on the H-passivated Si(100) surface using scanning tunnelling microscopy
We have developed a method for depositing graphene monolayers and bilayers
with minimum lateral dimensions of 2-10 nm by the mechanical exfoliation of
graphite onto the Si(100)-2x1:H surface. Room temperature, ultra-high vacuum
(UHV) tunnelling spectroscopy measurements of nanometer-sized single-layer
graphene reveal a size dependent energy gap ranging from 0.1-1 eV. Furthermore,
the number of graphene layers can be directly determined from scanning
tunnelling microscopy (STM) topographic contours. This atomistic study provides
an experimental basis for probing the electronic structure of nanometer-sized
graphene which can assist the development of graphene-based nanoelectronics.Comment: Accepted for publication in Nanotechnolog
Electron transport through honeycomb lattice ribbons with armchair edges
We address electron transport in honeycomb lattice ribbons with armchair
edges attached to two semi-infinite one-dimensional metallic electrodes within
the tight-binding framework. Here we present numerically the conductance-energy
and current-voltage characteristics as functions of the length and width of the
ribbons. Our theoretical results predict that for a ribbon with much smaller
length and width, so-called a nanoribbon, a gap in the conductance spectrum
appears across the energy E=0. While, this gap decreases gradually with the
increase of the size of the ribbon, and eventually it almost vanishes. This
reveals a transformation from the semiconducting to the conducting material,
and it becomes much more clearly visible from our presented current-voltage
characteristics.Comment: 8 pages, 6 figure
DNA nucleotide-specific modulation of \mu A transverse edge currents through a metallic graphene nanoribbon with a nanopore
We propose two-terminal devices for DNA sequencing which consist of a
metallic graphene nanoribbon with zigzag edges (ZGNR) and a nanopore in its
interior through which the DNA molecule is translocated. Using the
nonequilibrium Green functions combined with density functional theory, we
demonstrate that each of the four DNA nucleotides inserted into the nanopore,
whose edge carbon atoms are passivated by either hydrogen or nitrogen, will
lead to a unique change in the device conductance. Unlike other recent
biosensors based on transverse electronic transport through DNA nucleotides,
which utilize small (of the order of pA) tunneling current across a nanogap or
a nanopore yielding a poor signal-to-noise ratio, our device concept relies on
the fact that in ZGNRs local current density is peaked around the edges so that
drilling a nanopore away from the edges will not diminish the conductance.
Inserting a DNA nucleotide into the nanopore affects the charge density in the
surrounding area, thereby modulating edge conduction currents whose magnitude
is of the order of \mu A at bias voltage ~ 0.1 V. The proposed biosensor is not
limited to ZGNRs and it could be realized with other nanowires supporting
transverse edge currents, such as chiral GNRs or wires made of two-dimensional
topological insulators.Comment: 6 pages, 6 figures, PDFLaTe
Tailoring the atomic structure of graphene nanoribbons by STM lithography
The practical realization of nano-scale electronics faces two major
challenges: the precise engineering of the building blocks and their assembly
into functional circuits. In spite of the exceptional electronic properties of
carbon nanotubes only basic demonstration-devices have been realized by
time-consuming processes. This is mainly due to the lack of selective growth
and reliable assembly processes for nanotubes. However, graphene offers an
attractive alternative. Here we report the patterning of graphene nanoribbons
(GNRs) and bent junctions with nanometer precision, well-defined widths and
predetermined crystallographic orientations allowing us to fully engineer their
electronic structure using scanning tunneling microscope (STM) lithography. The
atomic structure and electronic properties of the ribbons have been
investigated by STM and tunneling spectroscopy measurements. Opening of
confinement gaps up to 0.5 eV, allowing room temperature operation of GNR-based
devices, is reported. This method avoids the difficulties of assembling
nano-scale components and allows the realization of complete integrated
circuits, operating as room temperature ballistic electronic devices.Comment: 8 pages text, 5 figures, Nature Nanotechnology, in pres
Towards Graphene Nanoribbon-based Electronics
The successful fabrication of single layer graphene has greatly stimulated
the progress of the research on graphene. In this article, focusing on the
basic electronic and transport properties of graphene nanoribbons (GNRs), we
review the recent progress of experimental fabrication of GNRs, and the
theoretical and experimental investigations of physical properties and device
applications of GNRs. We also briefly discuss the research efforts on the spin
polarization of GNRs in relation to the edge states.Comment: 9pages,10figure
Electron Wave Function in Armchair Graphene Nanoribbons
By using analytical solution of a tight-binding model for armchair
nanoribbons, it is confirmed that the solution represents the standing wave
formed by intervalley scattering and that pseudospin is invariant under the
scattering. The phase space of armchair nanoribbon which includes a single
Dirac singularity is specified. By examining the effects of boundary
perturbations on the wave function, we suggest that the existance of a strong
boundary potential is inconsistent with the observation in a recent scanning
tunneling microscopy. Some of the possible electron-density superstructure
patterns near a step armchair edge located on top of graphite are presented. It
is demonstrated that a selection rule for the G band in Raman spectroscopy can
be most easily reproduced with the analytical solution.Comment: 7 pages, 4 figure
Electromagnetic properties of graphene junctions
A resonant chiral tunneling (CT) across a graphene junction (GJ) induced by
an external electromagnetic field (EF) is studied. Modulation of the electron
and hole wavefunction phases by the external EF during the CT
processes strongly impacts the CT directional diagram. Therefore the a.c.
transport characteristics of GJs depend on the EF polarization and frequency
considerably. The GJ shows great promises for various nanoelectronic
applications working in the THz diapason.Comment: 4 pages 3 figure
"Narrow" Graphene Nanoribbons Made Easier by Partial Hydrogenation
It is a challenge to synthesize graphene nanoribbons (GNRs) with narrow
widths and smooth edges in large scale. Our first principles study on the
hydrogenation of GNRs shows that the hydrogenation starts from the edges of
GNRs and proceeds gradually toward the middle of the GNRs so as to maximize the
number of carbon-carbon - bonds. Furthermore, the partially
hydrogenated wide GNRs have similar electronic and magnetic properties as those
of narrow GNRs. Therefore, it is not necessary to directly produce narrow GNRs
for realistic applications because partial hydrogenation could make wide GNRs
"narrower"
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