122 research outputs found
Changes in copepod distributions associated with increased turbulence from wind stress
Vertical profiles of turbulent kinetic energy dissipation rate (ε), current velocity, temperature, salinity, chlorophyll fluorescence, and copepods were sampled for 4 d at an anchor station on the southern flank of Georges Bank when the water column was stratified in early June 1995. Copepodite stages of Temora spp., Oithona spp., Pseudocalanus spp., and Calanus finmarchicus, and all of their naupliar stages except for Temora spp., were found deeper in the water column when turbulent dissipation rates in the surface mixed layer increased in response to increasing wind stress. Taxa that initially occurred at the bottom of the surface mixed layer at 10 to 15 m depth ( ε ¾ 10-8 W kg-1) before the wind event were located in the pycnocline at 20 to 25 m depth when dissipation rates at 10 m increased up to 10-6 W kg-1. Dissipation rates in the pycnocline were similar to those experienced at shallower depths before the wind event. After passage of the wind event and with relaxation of dissipation rates in the surface layer, all stages returned to prior depths above the pycnocline. Temora spp. nauplii did not change depth during this period. Our results indicate that turbulence from a moderate wind event can influence the vertical distribution of copepods in the surface mixed layer. Changes in the vertical distribution of copepods can impact trophic interactions, and movements related to turbulence would affect the application of turbulence theory to encounter and feeding rates
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
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
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
Effects of temperature and ammonia flow rate on the chemical vapour deposition growth of nitrogen-doped graphene
We doped graphene in situ during synthesis from methane and ammonia on copper in a low-pressure
chemical vapour deposition system, and investigated the effect of the synthesis temperature and
ammonia concentration on the growth. Raman and X-ray photoelectron spectroscopy was used to
investigate the quality and nitrogen content of the graphene and demonstrated that decreasing the
synthesis temperature and increasing the ammonia flow rate results in an increase in the concentration
of nitrogen dopants up to ca. 2.1% overall. However, concurrent scanning electron microscopy studies
demonstrate that decreasing both the growth temperature from 1000 to 900 1C and increasing the N/C
precursor ratio from 1/50 to 1/10 significantly decreased the growth rate by a factor of six overall. Using
scanning tunnelling microscopy we show that the nitrogen was incorporated mainly in substitutional
configuration, while current imaging tunnelling spectroscopy showed that the effect of the nitrogen on
the density of states was visible only over a few atom distances
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
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
Observation of competing, correlated ground states in the flat band of rhombohedral graphite
In crystalline solids, the interactions of charge and spin can result in a variety of emergent quantum ground states, especially in partially filled, topological flat bands such as Landau levels or in magic angle graphene layers. Much less explored is rhombohedral graphite RG , perhaps the simplest and structurally most perfect condensed matter system to host a flat band protected by symmetry. By scanning tunneling microscopy, we map the flat band charge density of 8, 10, 14, and 17 layers and identify a domain structure emerging from a competition between a sublattice antiferromagnetic insulator and a gapless correlated paramagnet. Our density matrix renormalization group calculations explain the observed features and demonstrate that the correlations are fundamentally different from graphene based magnetism identified until now, forming the ground state of a quantum magnet. Our work establishes RG as a platform to study many body interactions beyond the mean field approach, where quantum fluctuations and entanglement dominat
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
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