727 research outputs found
The Stability of Polar Oxide Surfaces
The structures of the polar surfaces of ZnO are studied using ab initio calculations and surface x-ray diffraction. The experimental and theoretical relaxations are in good agreement. The polar surfaces are shown to be very stable; the cleavage energy for the (0001)-Zn and (0001Ì… )-O surfaces is 4.0J/m2 comparable to 2.32J/m2 for the most stable nonpolar (1010) surface. The surfaces are stabilized by an electronic mechanism involving the transfer of 0.17 electrons between them. This leads to 2D metallic surface states, which has implications for the use of the material in gas sensing and catalytic applications
Graphene Spin Valve Devices
Graphene - a single atomic layer of graphite - is a recently-found
two-dimensional form of carbon, which exhibits high crystal quality and
ballistic electron transport at room temperature. Soft magnetic NiFe electrodes
have been used to inject polarized spins into graphene and a 10% change in
resistance has been observed as the electrodes switch from the parallel to the
antiparallel state. This coupled with the fact that a field effect electrode
can modulate the conductivity of these graphene films makes them exciting
potential candidates for spin electronic devices.Comment: 3 pages, 6 figure
On resonant scatterers as a factor limiting carrier mobility in graphene
We show that graphene deposited on a substrate has a non-negligible density
of atomic scale defects. This is evidenced by a previously unnoticed D peak in
the Raman spectra with intensity of about 1% with respect to the G peak. We
evaluated the effect of such impurities on electron transport by mimicking them
with hydrogen adsorbates and measuring the induced changes in both mobility and
Raman intensity. If the intervalley scatterers responsible for the D peak are
monovalent, their concentration is sufficient to account for the limited
mobilities achievable in graphene on a substrate.Comment: version 2: several comments are taken into account and refs adde
Anomalies in thickness measurements of graphene and few layer graphite crystals by tapping mode atomic force microscopy
Atomic Force Microscopy (AFM) in the tapping (intermittent contact) mode is a
commonly used tool to measure the thickness of graphene and few layer graphene
(FLG) flakes on silicon oxide surfaces. It is a convenient tool to quickly
determine the thickness of individual FLG films. However, reports from
literature show a large variation of the measured thickness of graphene layers.
This paper is focused on the imaging mechanism of tapping mode AFM (TAFM) when
measuring graphene and FLG thickness and we show that at certain measurement
parameters significant deviations can be introduced in the measured thickness
of FLG flakes. An increase of as much as 1 nm can be observed in the measured
height of FLG crystallites, when using an improperly chosen range of free
amplitude values of the tapping cantilever. We present comparative Raman
spectroscopy and TAFM measurements on selected single and multilayer graphene
films, based on which we suggest ways to correctly measure graphene and FLG
thickness using TAFM
Employment Among People With Spinal Cord Injury in 22 Countries Across the World:Results From the International Spinal Cord Injury Community Survey
Objectives: To describe the employment situation of individuals with spinal cord injury (SCI) in 22 countries participating in the International Spinal Cord Injury community survey, to compare observed and predicted employment rates, to estimate gaps in employment rates among people with SCI compared with the general population, and to study differences in employment between men and women. Design: Cross-sectional survey. Setting: Community. Participants: People of employable age (N=9875; 18-64 y) with traumatic or non-traumatic SCI (including cauda equina syndrome) who were at least 18 years of age at the time of the survey, living in the community, and able to respond to one of the available language versions of the questionnaire. Interventions: Not applicable Main Outcome Measures: The observed employment rate was defined as performing paid work for at least 1 hour a week, and predicted employment rate was adjusted for sample composition from mixed logistic regression analysis. Results: A total of 9875 participants were included (165-1174 per country). Considerable differences in sample composition were found. The observed worldwide employment rate was 38%. A wide variation was found across countries, ranging from 10.3% to 61.4%. Some countries showed substantially higher or lower employment rates than predicted based on the composition of their sample. Gaps between the observed employment rates among participants with SCI and the general population ranged from 14.8% to 54.8%. On average, employment rates were slightly higher among men compared with women, but with large variation across countries. Employment gaps, however, were smaller among women for most countries. Conclusions: This first worldwide survey among people with SCI shows an average employment rate of 38%. Differences between observed and predicted employment rates across countries point at country-specific factors that warrant further investigation. Gaps with employment rates in the general population were considerable and call for actions for more inclusive labor market policies in most of the countries investigated. (C) 2020 by the American Congress of Rehabilitation Medicin
Cascaded Optical Field Enhancement in Composite Plasmonic Nanostructures
Copyright © 2010 The American Physical SocietyWe present composite plasmonic nanostructures designed to achieve cascaded enhancement of electromagnetic fields at optical frequencies. Our structures were made with the help of electron-beam lithography and comprise a set of metallic nanodisks placed one above another. The optical properties of reproducible arrays of these structures were studied by using scanning confocal Raman spectroscopy. We show that our composite nanostructures robustly demonstrate dramatic enhancement of the Raman signals when compared to those measured from constituent elements
Impurity-assisted tunneling in graphene
The electric conductance of a strip of undoped graphene increases in the
presence of a disorder potential, which is smooth on atomic scales. The
phenomenon is attributed to impurity-assisted resonant tunneling of massless
Dirac fermions. Employing the transfer matrix approach we demonstrate the
resonant character of the conductivity enhancement in the presence of a single
impurity. We also calculate the two-terminal conductivity for the model with
one-dimensional fluctuations of disorder potential by a mapping onto a problem
of Anderson localization.Comment: 6 pages, 3 figures, final version, typos corrected, references adde
Strong Suppression of Electrical Noise in Bilayer Graphene Nano Devices
Low-frequency 1/f noise is ubiquitous, and dominates the signal-to-noise
performance in nanodevices. Here we investigate the noise characteristics of
single-layer and bilayer graphene nano-devices, and uncover an unexpected 1/f
noise behavior for bilayer devices. Graphene is a single layer of graphite,
where carbon atoms form a 2D honeycomb lattice. Despite the similar
composition, bilayer graphene (two graphene monolayers stacked in the natural
graphite order) is a distinct 2D system with a different band structure and
electrical properties. In graphene monolayers, the 1/f noise is found to follow
Hooge's empirical relation with a noise parameter comparable to that of bulk
semiconductors. However, this 1/f noise is strongly suppressed in bilayer
graphene devices, and exhibits an unusual dependence on the carrier density,
different from most other materials. The unexpected noise behavior in graphene
bilayers is associated with its unique band structure that varies with the
charge distribution among the two layers, resulting in an effective screening
of potential fluctuations due to external impurity charges. The findings here
point to exciting opportunities for graphene bilayers in low-noise
applications
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