365 research outputs found
Electric Field Effects on Graphene Materials
Understanding the effect of electric fields on the physical and chemical
properties of two-dimensional (2D) nanostructures is instrumental in the design
of novel electronic and optoelectronic devices. Several of those properties are
characterized in terms of the dielectric constant which play an important role
on capacitance, conductivity, screening, dielectric losses and refractive
index. Here we review our recent theoretical studies using density functional
calculations including van der Waals interactions on two types of layered
materials of similar two-dimensional molecular geometry but remarkably
different electronic structures, that is, graphene and molybdenum disulphide
(MoS). We focus on such two-dimensional crystals because of they
complementary physical and chemical properties, and the appealing interest to
incorporate them in the next generation of electronic and optoelectronic
devices. We predict that the effective dielectric constant () of
few-layer graphene and MoS is tunable by external electric fields (). We show that at low fields ( V/\AA)
assumes a nearly constant value 4 for both materials, but increases at
higher fields to values that depend on the layer thickness. The thicker the
structure the stronger is the modulation of with the electric
field. Increasing of the external field perpendicular to the layer surface
above a critical value can drive the systems to an unstable state where the
layers are weakly coupled and can be easily separated. The observed dependence
of on the external field is due to charge polarization driven by
the bias, which show several similar characteristics despite of the layer
considered.Comment: Invited book chapter on Exotic Properties of Carbon Nanomatter:
Advances in Physics and Chemistry, Springer Series on Carbon Materials.
Editors: Mihai V. Putz and Ottorino Ori (11 pages, 4 figures, 30 references
Atomic Hole Doping of Graphene
Graphene is an excellent candidate for the next generation of electronic
materials due to the strict two-dimensionality of its electronic structure as
well as the extremely high carrier mobility. A prerequisite for the development
of graphene based electronics is the reliable control of the type and density
of the charge carriers by external (gate) and internal (doping) means. While
gating has been successfully demonstrated for graphene flakes and epitaxial
graphene on silicon carbide, the development of reliable chemical doping
methods turns out to be a real challenge. In particular hole doping is an
unsolved issue. So far it has only been achieved with reactive molecular
adsorbates, which are largely incompatible with any device technology. Here we
show by angle-resolved photoemission spectroscopy that atomic doping of an
epitaxial graphene layer on a silicon carbide substrate with bismuth, antimony
or gold presents effective means of p-type doping. Not only is the atomic
doping the method of choice for the internal control of the carrier density. In
combination with the intrinsic n-type character of epitaxial graphene on SiC,
the charge carriers can be tuned from electrons to holes, without affecting the
conical band structure
Graphene for spintronics: giant Rashba splitting due to hybridization with Au
Graphene in spintronics has so far primarily meant spin current leads of high
performance because the intrinsic spin-orbit coupling of its pi-electrons is
very weak. If a large spin-orbit coupling could be created by a proximity
effect, the material could also form active elements of a spintronic device
such as the Das-Datta spin field-effect transistor, however, metal interfaces
often compromise the band dispersion of massless Dirac fermions. Our
measurements show that Au intercalation at the graphene-Ni interface creates a
giant spin-orbit splitting (~100 meV) in the graphene Dirac cone up to the
Fermi energy. Photoelectron spectroscopy reveals hybridization with Au-5d
states as the source for the giant spin-orbit splitting. An ab initio model of
the system shows a Rashba-split dispersion with the analytically predicted
gapless band topology around the Dirac point of graphene and indicates that a
sharp graphene-Au interface at equilibrium distance will account for only ~10
meV spin-orbit splitting. The ab initio calculations suggest an enhancement due
to Au atoms that get closer to the graphene and do not violate the sublattice
symmetry.Comment: 16 pages (3 figures) + supplementary information 16 pages (14
figures
Kinks in the dispersion of strongly correlated electrons
The properties of condensed matter are determined by single-particle and
collective excitations and their interactions. These quantum-mechanical
excitations are characterized by an energy E and a momentum \hbar k which are
related through their dispersion E_k. The coupling of two excitations may lead
to abrupt changes (kinks) in the slope of the dispersion. Such kinks thus carry
important information about interactions in a many-body system. For example,
kinks detected at 40-70 meV below the Fermi level in the electronic dispersion
of high-temperature superconductors are taken as evidence for phonon or
spin-fluctuation based pairing mechanisms. Kinks in the electronic dispersion
at binding energies ranging from 30 to 800 meV are also found in various other
metals posing questions about their origins. Here we report a novel, purely
electronic mechanism yielding kinks in the electron dispersions. It applies to
strongly correlated metals whose spectral function shows well separated Hubbard
subbands and central peak as, for example, in transition metal-oxides. The
position of the kinks and the energy range of validity of Fermi-liquid (FL)
theory is determined solely by the FL renormalization factor and the bare,
uncorrelated band structure. Angle-resolved photoemission spectroscopy (ARPES)
experiments at binding energies outside the FL regime can thus provide new,
previously unexpected information about strongly correlated electronic systems.Comment: 8 pages, 5 figure
Substrate-induced band gap opening in epitaxial graphene
Graphene has shown great application potentials as the host material for next
generation electronic devices. However, despite its intriguing properties, one
of the biggest hurdles for graphene to be useful as an electronic material is
its lacking of an energy gap in the electronic spectra. This, for example,
prevents the use of graphene in making transistors. Although several proposals
have been made to open a gap in graphene's electronic spectra, they all require
complex engineering of the graphene layer. Here we show that when graphene is
epitaxially grown on the SiC substrate, a gap of ~ 0.26 is produced. This gap
decreases as the sample thickness increases and eventually approaches zero when
the number of layers exceeds four. We propose that the origin of this gap is
the breaking of sublattice symmetry owing to the graphene-substrate
interaction. We believe our results highlight a promising direction for band
gap engineering of graphene.Comment: 10 pages, 4 figures; updated reference
Efficacy of ImageJ in the assessment of apoptosis
<p>Abstract</p> <p>Objective</p> <p>To verify the efficacy of ImageJ 1.43 n in determining the extent of apoptosis which is a complex and multistep process.</p> <p>Study Design</p> <p>Cisplatin in different concentrations was used to induce apoptosis in cultured Hep2 cells. Cell viability assay and nuclear image analysis of stained Hep2 cells were used to discriminate apoptotic cells and cells suspected to be undergoing apoptosis from control cells based on parameters such as nuclear area, circularity, perimeter and nuclear area factor (NAF), in association with visual morphology.</p> <p>Results</p> <p>Image analysis revealed a progressive and highly significant decrease in nuclear area factor detected in apoptotic cells and in cells suspected of undergoing apoptosis compared to the control cells (P-values < 0.01). Some of the other studied parameters showed also the same trend. This decrease was assumed to indicate DNA loss. Image analysis results correlated positively and significantly with the results obtained by cell viability assay (R = 0.958, P-value = 0.042). NAF was the most reliable parameter in assessment of apoptosis.</p> <p>Conclusion</p> <p>Nuclear area factor can be calculated using powerful free and open-source software. Consequently, a quantitative measure of apoptosis can be obtained that is linked to morphological changes. ImageJ 1.43 n may therefore provide a useful tool for the assessment and discrimination of apoptotic cells.</p> <p>Virtual slides</p> <p>The virtual slide(s) for this article can be found here:</p> <p><url>http://www.diagnosticpathology.diagnomx.eu/vs/5929043086367338</url></p
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
Electronic properties of bilayer and multilayer graphene
We study the effects of site dilution disorder on the electronic properties
in graphene multilayers, in particular the bilayer and the infinite stack. The
simplicity of the model allows for an easy implementation of the coherent
potential approximation and some analytical results. Within the model we
compute the self-energies, the density of states and the spectral functions.
Moreover, we obtain the frequency and temperature dependence of the
conductivity as well as the DC conductivity. The c-axis response is
unconventional in the sense that impurities increase the response for low
enough doping. We also study the problem of impurities in the biased graphene
bilayer.Comment: 36 pages, 42 figures, references adde
Dirac electrons in graphene-based quantum wires and quantum dots
In this paper we analyse the electronic properties of Dirac electrons in
finite-size ribbons and in circular and hexagonal quantum dots made of
graphene.Comment: Contribution for J. Phys.: Cond. Mat. special issue on graphene
physic
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