68 research outputs found
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
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
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
"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"
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
Spin Channels in Functionalized Graphene Nanoribbons
We characterize the transport properties of functionalized graphene
nanoribbons using extensive first-principles calculations based on density
functional theory (DFT) that encompass both monovalent and divalent ligands,
hydrogenated defects and vacancies. We find that the edge metallic states are
preserved under a variety of chemical environments, while bulk conducting
channels can be easily destroyed by either hydrogenation or ion or electron
beams, resulting in devices that can exhibit spin conductance polarization
close to unity.Comment: 14 pages, 5 figure
Chemically-induced Mobility Gaps in Graphene Nanoribbons: A Route for Upscaling Device Performances
We report a first-principles based study of mesoscopic quantum transport in
chemically doped graphene nanoribbons with a width up to 10 nm. The occurrence
of quasibound states related to boron impurities results in mobility gaps as
large as 1 eV, driven by strong electron-hole asymmetrical backscattering
phenomena. This phenomenon opens new ways to overcome current limitations of
graphene-based devices through the fabrication of chemically-doped graphene
nanoribbons with sizes within the reach of conventional lithography.Comment: Nano Letters (in press
Longitudinal unzipping of carbon nanotubes to form graphene nanoribbons.
Published versio
Excitons, biexcitons, and phonons in ultrathin CdSe/ZnSe quantum structures
The optical properties of CdSe nanostructures grown by migration-enhanced epitaxy of CdSe on ZnSe are studied by time-, energy-, and temperature-dependent photoluminescence and excitation spectroscopy, as well as by polarization-dependent four-wave mixing and two-photon absorption experiments. The nanostructures consist of a coherently strained Zn1−xCdxSe/ZnSe quantum well with embedded islands of higher Cd content with sizes of a few nanometer due to strain-induced CdSe accumulation. The local increase in CdSe concentration results in a strong localization of the excitonic wave function, in an increase in radiative lifetime, and a decrease of the dephasing rate. Local LO-phonon modes caused by the strong modulation of the Cd concentration profile are found in phonon-assisted relaxation processes. Confined biexcitons with large binding energies between 20 and 24 meV are observed, indicating the important role of biexcitons even at room temperature
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