153 research outputs found
Ab initio study of reflectance anisotropy spectra of a sub-monolayer oxidized Si(100) surface
The effects of oxygen adsorption on the reflectance anisotropy spectrum (RAS)
of reconstructed Si(100):O surfaces at sub-monolayer coverage (first stages of
oxidation) have been studied by an ab initio DFT-LDA scheme within a
plane-wave, norm-conserving pseudopotential approach. Dangling bonds and the
main features of the characteristic RAS of the clean Si(100) surface are mostly
preserved after oxidation of 50% of the surface dimers, with some visible
changes: a small red shift of the first peak, and the appearance of a distinct
spectral structure at about 1.5 eV. The electronic transitions involved in the
latter have been analyzed through state-by-state and layer-by-layer
decompositions of the RAS. We suggest that new interplay between present
theoretical results and reflectance anisotropy spectroscopy experiments could
lead to further clarification of structural and kinetic details of the Si(100)
oxidation process in the sub-monolayer range.Comment: 21 pages, 8 figures. To be published in Physical Rev.
Electrostatically confined monolayer graphene quantum dots with orbital and valley splittings
The electrostatic confinement of massless charge carriers is hampered by
Klein tunneling. Circumventing this problem in graphene mainly relies on
carving out nanostructures or applying electric displacement fields to open a
band gap in bilayer graphene. So far, these approaches suffer from edge
disorder or insufficiently controlled localization of electrons. Here we
realize an alternative strategy in monolayer graphene, by combining a
homogeneous magnetic field and electrostatic confinement. Using the tip of a
scanning tunneling microscope, we induce a confining potential in the Landau
gaps of bulk graphene without the need for physical edges. Gating the localized
states towards the Fermi energy leads to regular charging sequences with more
than 40 Coulomb peaks exhibiting typical addition energies of 7-20 meV. Orbital
splittings of 4-10 meV and a valley splitting of about 3 meV for the first
orbital state can be deduced. These experimental observations are
quantitatively reproduced by tight binding calculations, which include the
interactions of the graphene with the aligned hexagonal boron nitride
substrate. The demonstrated confinement approach appears suitable to create
quantum dots with well-defined wave function properties beyond the reach of
traditional techniques
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
Mapping of functionalized regions on carbon nanotubes by scanning tunneling microscopy
Scanning tunneling microscopy (STM) gives us the opportunity to map the
surface of functionalized carbon nanotubes in an energy resolved manner and
with atomic precision. But this potential is largely untapped, mainly due to
sample stability issues which inhibit reliable measurements. Here we present a
simple and straightforward solution that makes away with this difficulty, by
incorporating the functionalized multiwalled carbon nanotubes (MWCNT) into a
few layer graphene - nanotube composite. This enabled us to measure energy
resolved tunneling conductance maps on the nanotubes, which shed light on the
level of doping, charge transfer between tube and functional groups and the
dependence of defect creation or functionalization on crystallographic
orientation.Comment: Keywords: functionalization, carbon nanotubes, few layer graphene,
STM, CITS, ST
Biodiversity and Ecosystem Function in the Gulf of Maine: Pattern and Role of Zooplankton and Pelagic Nekton
This paper forms part of a broader overview of biodiversity of marine life in the Gulf of Maine area (GoMA), facilitated by the GoMA Census of Marine Life program. It synthesizes current data on species diversity of zooplankton and pelagic nekton, including compilation of observed species and descriptions of seasonal, regional and cross-shelf diversity patterns. Zooplankton diversity in the GoMA is characterized by spatial differences in community composition among the neritic environment, the coastal shelf, and deep offshore waters. Copepod diversity increased with depth on the Scotian Shelf. On the coastal shelf of the western Gulf of Maine, the number of higher-level taxonomic groups declined with distance from shore, reflecting more nearshore meroplankton. Copepod diversity increased in late summer, and interdecadal diversity shifts were observed, including a period of higher diversity in the 1990s. Changes in species diversity were greatest on interannual scales, intermediate on seasonal scales, and smallest across regions, in contrast to abundance patterns, suggesting that zooplankton diversity may be a more sensitive indicator of ecosystem response to interannual climate variation than zooplankton abundance. Local factors such as bathymetry, proximity of the coast, and advection probably drive zooplankton and pelagic nekton diversity patterns in the GoMA, while ocean-basin-scale diversity patterns probably contribute to the increase in diversity at the Scotian Shelf break, a zone of mixing between the cold-temperate community of the shelf and the warm-water community offshore. Pressing research needs include establishment of a comprehensive system for observing change in zooplankton and pelagic nekton diversity, enhanced observations of underknown\u27\u27 but important functional components of the ecosystem, population and metapopulation studies, and development of analytical modeling tools to enhance understanding of diversity patterns and drivers. Ultimately, sustained observations and modeling analysis of biodiversity must be effectively communicated to managers and incorporated into ecosystem approaches for management of GoMA living marine resources
Anomalous Lattice Vibrations of Single and Few-Layer MoS2
Molybdenum disulfide (MoS2) of single and few-layer thickness was exfoliated
on SiO2/Si substrate and characterized by Raman spectroscopy. The number of
S-Mo-S layers of the samples was independently determined by contact-mode
atomic-force microscopy. Two Raman modes, E12g and A1g, exhibited sensitive
thickness dependence, with the frequency of the former decreasing and that of
the latter increasing with thickness. The results provide a convenient and
reliable means for determining layer thickness with atomic-level precision. The
opposite direction of the frequency shifts, which cannot be explained solely by
van der Waals interlayer coupling, is attributed to Coulombic interactions and
possible stacking-induced changes of the intralayer bonding. This work
exemplifies the evolution of structural parameters in layered materials in
changing from the 3-dimensional to the 2-dimensional regime.Comment: 14 pages, 4 figure
Mechanical properties of freely suspended atomically thin dielectric layers of mica
We have studied the elastic deformation of freely suspended atomically thin
sheets of muscovite mica, a widely used electrical insulator in its bulk form.
Using an atomic force microscope, we carried out bending test experiments to
determine the Young's modulus and the initial pre-tension of mica nanosheets
with thicknesses ranging from 14 layers down to just one bilayer. We found that
their Young's modulus is high (190 GPa), in agreement with the bulk value,
which indicates that the exfoliation procedure employed to fabricate these
nanolayers does not introduce a noticeable amount of defects. Additionally,
ultrathin mica shows low pre-strain and can withstand reversible deformations
up to tens of nanometers without breaking. The low pre-tension and high Young's
modulus and breaking force found in these ultrathin mica layers demonstrates
their prospective use as a complement for graphene in applications requiring
flexible insulating materials or as reinforcement in nanocomposites.Comment: 9 pages, 5 figures, selected as cover of Nano Research, Volume 5,
Number 8 (2012
Prevalence of Listeria species in camel sausages from retail markets in Aydin province in Turkey and RAPD analysis of Listeria monocytogenes isolates
Samples were taken from 100 camel sausages from the different retail markets in Aydin province in the south-west of Turkey and they were tested for the presence of Listeria spp by biochemical methods. Samples were enriched using Listeria Enrichment Broth and they were inoculated onto Listeria Selective Agar. Listeria monocytogenes was isolated from nine samples (9%), Listeria innocua from 14 samples (14%) and Listeria welshimeri from two samples(2%). A 701 bp fragment of listeriolysin O sequence for L. monocytogenes was amplified using specific primers by polymerase chain reaction (PCR) for confirmation of the identification. A random primer (OPA-11) was used in a random amplified polymorphic DNA (RAPD) assay. This detected five different band profiles amongst the L. monocytogenes isolates, indicating a relatively large amount of genetic heterogeneity amongst the nine isolates. The study has highlighted the need for improved strategies for food safety, in particular appropriate hygienic precautions to avoid contamination of sausage during the manufacturing process and appropriate preservation techniques during storage and transport, to prevent transmission of Listeria spp to consumers at home and abroad
Large tunable valley splitting in edge-free graphene quantum dots on boron nitride
Coherent manipulation of binary degrees of freedom is at the heart of modern
quantum technologies. Graphene offers two binary degrees: the electron spin and
the valley. Efficient spin control has been demonstrated in many solid state
systems, while exploitation of the valley has only recently been started, yet
without control on the single electron level. Here, we show that van-der Waals
stacking of graphene onto hexagonal boron nitride offers a natural platform for
valley control. We use a graphene quantum dot induced by the tip of a scanning
tunneling microscope and demonstrate valley splitting that is tunable from -5
to +10 meV (including valley inversion) by sub-10-nm displacements of the
quantum dot position. This boosts the range of controlled valley splitting by
about one order of magnitude. The tunable inversion of spin and valley states
should enable coherent superposition of these degrees of freedom as a first
step towards graphene-based qubits
Theory and computation of electronic excitations in condensed matter systems : the ETSF project
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