114 research outputs found
Electronic transport in polycrystalline graphene
Most materials in available macroscopic quantities are polycrystalline.
Graphene, a recently discovered two-dimensional form of carbon with strong
potential for replacing silicon in future electronics, is no exception. There
is growing evidence of the polycrystalline nature of graphene samples obtained
using various techniques. Grain boundaries, intrinsic topological defects of
polycrystalline materials, are expected to dramatically alter the electronic
transport in graphene. Here, we develop a theory of charge carrier transmission
through grain boundaries composed of a periodic array of dislocations in
graphene based on the momentum conservation principle. Depending on the grain
boundary structure we find two distinct transport behaviours - either high
transparency, or perfect reflection of charge carriers over remarkably large
energy ranges. First-principles quantum transport calculations are used to
verify and further investigate this striking behaviour. Our study sheds light
on the transport properties of large-area graphene samples. Furthermore,
purposeful engineering of periodic grain boundaries with tunable transport gaps
would allow for controlling charge currents without the need of introducing
bulk band gaps in otherwise semimetallic graphene. The proposed approach can be
regarded as a means towards building practical graphene electronics.Comment: accepted in Nature Material
First-Principles Study of the Electronic and Magnetic Properties of Defects in Carbon Nanostructures
Understanding the magnetic properties of graphenic nanostructures is
instrumental in future spintronics applications. These magnetic properties are
known to depend crucially on the presence of defects. Here we review our recent
theoretical studies using density functional calculations on two types of
defects in carbon nanostructures: Substitutional doping with transition metals,
and sp-type defects created by covalent functionalization with organic and
inorganic molecules. We focus on such defects because they can be used to
create and control magnetism in graphene-based materials. Our main results are
summarized as follows: i)Substitutional metal impurities are fully understood
using a model based on the hybridization between the states of the metal
atom and the defect levels associated with an unreconstructed D carbon
vacancy. We identify three different regimes, associated with the occupation of
distinct hybridization levels, which determine the magnetic properties obtained
with this type of doping; ii) A spin moment of 1.0 is always induced by
chemical functionalization when a molecule chemisorbs on a graphene layer via a
single C-C (or other weakly polar) covalent bond. The magnetic coupling between
adsorbates shows a key dependence on the sublattice adsorption site. This
effect is similar to that of H adsorption, however, with universal character;
iii) The spin moment of substitutional metal impurities can be controlled using
strain. In particular, we show that although Ni substitutionals are
non-magnetic in flat and unstrained graphene, the magnetism of these defects
can be activated by applying either uniaxial strain or curvature to the
graphene layer. All these results provide key information about formation and
control of defect-induced magnetism in graphene and related materials.Comment: 40 pages, 17 Figures, 62 References; Chapter 2 in Topological
Modelling of Nanostructures and Extended Systems (2013) - Springer, edited by
A. R. Ashrafi, F. Cataldo, A. Iranmanesh, and O. Or
Higher-order renormalization of graphene many-body theory
We study the many-body theory of graphene Dirac quasiparticles interacting
via the long-range Coulomb potential, taking as a starting point the ladder
approximation to different vertex functions. We test in this way the low-energy
behavior of the electron system beyond the simple logarithmic dependence of
electronic correlators on the high-energy cutoff, which is characteristic of
the large-N approximation. We show that the graphene many-body theory is
perfectly renormalizable in the ladder approximation, as all higher powers in
the cutoff dependence can be absorbed into the redefinition of a finite number
of parameters (namely, the Fermi velocity and the weight of the fields) that
remain free of infrared divergences even at the charge neutrality point. We
illustrate this fact in the case of the vertex for the current density, where a
complete cancellation between the cutoff dependences of vertex and electron
self-energy corrections becomes crucial for the preservation of the gauge
invariance of the theory. The other potentially divergent vertex corresponds to
the staggered (sublattice odd) charge density, which is made cutoff independent
by a redefinition in the scale of the density operator. This allows to compute
a well-defined, scale invariant anomalous dimension to all orders in the ladder
series, which becomes singular at a value of the interaction strength marking
the onset of chiral symmetry breaking (and gap opening) in the Dirac field
theory. The critical coupling we obtain in this way matches with great accuracy
the value found with a quite different method, based on the resolution of the
gap equation, thus reassuring the predictability of our renormalization
approach.Comment: 27 pages, 7 figures, references adde
Control and Characterization of Individual Grains and Grain Boundaries in Graphene Grown by Chemical Vapor Deposition
The strong interest in graphene has motivated the scalable production of high
quality graphene and graphene devices. Since large-scale graphene films
synthesized to date are typically polycrystalline, it is important to
characterize and control grain boundaries, generally believed to degrade
graphene quality. Here we study single-crystal graphene grains synthesized by
ambient CVD on polycrystalline Cu, and show how individual boundaries between
coalescing grains affect graphene's electronic properties. The graphene grains
show no definite epitaxial relationship with the Cu substrate, and can cross Cu
grain boundaries. The edges of these grains are found to be predominantly
parallel to zigzag directions. We show that grain boundaries give a significant
Raman "D" peak, impede electrical transport, and induce prominent weak
localization indicative of intervalley scattering in graphene. Finally, we
demonstrate an approach using pre-patterned growth seeds to control graphene
nucleation, opening a route towards scalable fabrication of single-crystal
graphene devices without grain boundaries.Comment: New version with additional data. Accepted by Nature Material
Spin-half paramagnetism in graphene induced by point defects
Using magnetization measurements, we show that point defects in graphene -
fluorine adatoms and irradiation defects (vacancies) - carry magnetic moments
with spin 1/2. Both types of defects lead to notable paramagnetism but no
magnetic ordering could be detected down to liquid helium temperatures. The
induced paramagnetism dominates graphene's low-temperature magnetic properties
despite the fact that maximum response we could achieve was limited to one
moment per approximately 1000 carbon atoms. This limitation is explained by
clustering of adatoms and, for the case of vacancies, by losing graphene's
structural stability.Comment: 14 pages, 14 figure
Visualizing chemical states and defects induced magnetism of graphene oxide by spatially-resolved-X-ray microscopy and spectroscopy
[[abstract]]This investigation studies the various magnetic behaviors of graphene oxide (GO) and reduced
graphene oxides (rGOs) and elucidates the relationship between the chemical states that involve
defects therein and their magnetic behaviors in GO sheets. Magnetic hysteresis loop reveals that the
GO is ferromagnetic whereas photo-thermal moderately reduced graphene oxide (M-rGO) and heavily
reduced graphene oxide (H-rGO) gradually become paramagnetic behavior at room temperature.
Scanning transmission X-ray microscopy and corresponding X-ray absorption near-edge structure
spectroscopy were utilized to investigate thoroughly the variation of the C 2p(π*) states that are
bound with oxygen-containing and hydroxyl groups, as well as the C 2p(σ*)-derived states in flat
and wrinkle regions to clarify the relationship between the spatially-resolved chemical states and
the magnetism of GO, M-rGO and H-rGO. The results of X-ray magnetic circular dichroism further
support the finding that C 2p(σ*)-derived states are the main origin of the magnetism of GO. Based
on experimental results and first-principles calculations, the variation in magnetic behavior from GO
to M-rGO and to H-rGO is interpreted, and the origin of ferromagnetism is identified as the C 2p(σ*)-
derived states that involve defects/vacancies rather than the C 2p(π*) states that are bound with
oxygen-containing and hydroxyl groups on GO sheets.[[notice]]補正完
Properties of Graphene: A Theoretical Perspective
In this review, we provide an in-depth description of the physics of
monolayer and bilayer graphene from a theorist's perspective. We discuss the
physical properties of graphene in an external magnetic field, reflecting the
chiral nature of the quasiparticles near the Dirac point with a Landau level at
zero energy. We address the unique integer quantum Hall effects, the role of
electron correlations, and the recent observation of the fractional quantum
Hall effect in the monolayer graphene. The quantum Hall effect in bilayer
graphene is fundamentally different from that of a monolayer, reflecting the
unique band structure of this system. The theory of transport in the absence of
an external magnetic field is discussed in detail, along with the role of
disorder studied in various theoretical models. We highlight the differences
and similarities between monolayer and bilayer graphene, and focus on
thermodynamic properties such as the compressibility, the plasmon spectra, the
weak localization correction, quantum Hall effect, and optical properties.
Confinement of electrons in graphene is nontrivial due to Klein tunneling. We
review various theoretical and experimental studies of quantum confined
structures made from graphene. The band structure of graphene nanoribbons and
the role of the sublattice symmetry, edge geometry and the size of the
nanoribbon on the electronic and magnetic properties are very active areas of
research, and a detailed review of these topics is presented. Also, the effects
of substrate interactions, adsorbed atoms, lattice defects and doping on the
band structure of finite-sized graphene systems are discussed. We also include
a brief description of graphane -- gapped material obtained from graphene by
attaching hydrogen atoms to each carbon atom in the lattice.Comment: 189 pages. submitted in Advances in Physic
Testicular Dysgenesis Syndrome and the Estrogen Hypothesis: A Quantitative Meta-Analysis
BACKGROUND: Male reproductive tract abnormalities such as hypospadias and cryptorchidism, and testicular cancer have been proposed to comprise a common syndrome together with impaired spermatogenesis with a common etiology resulting from the disruption of gonadal development during fetal life, the testicular dysgenesis syndrome (TDS). The hypothesis that in utero exposure to estrogenic agents could induce these disorders was first proposed in 1993. The only quantitative summary estimate of the association between prenatal exposure to estrogenic agents and testicular cancer was published over 10 years ago, and other systematic reviews of the association between estrogenic compounds, other than the potent pharmaceutical estrogen diethylstilbestrol (DES), and
TDS end points have remained inconclusive.
OBJECTIVES: We conducted a quantitative meta-analysis of the association between the end points related to TDS and prenatal exposure to estrogenic agents. Inclusion in this analysis was based on mechanistic criteria, and the plausibility of an estrogen receptor (ER)-–mediated mode of action was specifically explored.
RESULTS: We included in this meta-analysis eight studies investigating the etiology of hypospadias and/or cryptorchidism that had not been identified in previous systematic reviews. Four additional studies of pharmaceutical estrogens yielded a statistically significant updated summary estimate for testicular cancer.
CONCLUSIONS: The doubling of the risk ratios for all three end points investigated after DES exposure is consistent with a shared etiology and the TDS hypothesis but does not constitute evidence of an estrogenic mode of action. Results of the subset analyses point to the existence of unidentified sources of heterogeneity between studies or within the study population
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