84 research outputs found
Low-energy electron diffraction study of potassium adsorbed on single-crystal graphite and highly oriented pyrolytic graphite
Potassium adsorption on graphite has been a model system for the understanding of the interaction of alkali
metals with surfaces. The geometries of the s232d structure of potassium on both single-crystal graphite
(SCG) and highly oriented pyrolytic graphite (HOPG) were investigated for various preparation conditions for
graphite temperatures between 55 and 140 K. In all cases, the geometry was found to consist of K atoms in the
hollow sites on top of the surface. The K-graphite average perpendicular spacing is 2.79±0.03 Å, corresponding
to an average C-K distance of 3.13±0.03 Å, and the spacing between graphite planes is consistent with the
bulk spacing of 3.35 Å. No evidence was observed for a sublayer of potassium. The results of dynamical LEED studies for the clean SCG and HOPG surfaces indicate that the surface structures of both are consistent with the truncated bulk structure of graphite
Noble gas films on a decagonal AlNiCo quasicrystal
Thermodynamic properties of Ne, Ar, Kr, and Xe adsorbed on an Al-Ni-Co
quasicrystalline surface (QC) are studied with Grand Canonical Monte Carlo by
employing Lennard-Jones interactions with parameter values derived from
experiments and traditional combining rules. In all the gas/QC systems, a
layer-by-layer film growth is observed at low temperature. The monolayers have
regular epitaxial fivefold arrangements which evolve toward sixfold
close-packed structures as the pressure is increased. The final states can
contain either considerable or negligible amounts of defects. In the latter
case, there occurs a structural transition from five to sixfold symmetry which
can be described by introducing an order parameter, whose evolution
characterizes the transition to be continuous or discontinuous as in the case
of Xe/QC (first-order transition with associated latent heat). By simulating
fictitious noble gases, we find that the existence of the transition is
correlated with the size mismatch between adsorbate and substrate's
characteristic lengths. A simple rule is proposed to predict the phenomenon.Comment: 19 pages. 8 figures. (color figures can be seen at
http://alpha.mems.duke.edu/wahyu/ or
http://www.iop.org/EJ/abstract/0953-8984/19/1/016007/
Probing Mechanical Properties of Graphene with Raman Spectroscopy
The use of Raman scattering techniques to study the mechanical properties of
graphene films is reviewed here. The determination of Gruneisen parameters of
suspended graphene sheets under uni- and bi-axial strain is discussed and the
values are compared to theoretical predictions. The effects of the
graphene-substrate interaction on strain and to the temperature evolution of
the graphene Raman spectra are discussed. Finally, the relation between
mechanical and thermal properties is presented along with the characterization
of thermal properties of graphene with Raman spectroscopy.Comment: To appear in the Journal of Materials Scienc
Negative Thermal Expansion Coefficient of Graphene Measured by Raman Spectroscopy
The thermal expansion coefficient (TEC) of single-layer graphene is estimated
with temperature-dependent Raman spectroscopy in the temperature range between
200 and 400 K. It is found to be strongly dependent on temperature but remains
negative in the whole temperature range, with a room temperature value of
-8.0x10^{-6} K^{-1}. The strain caused by the TEC mismatch between graphene and
the substrate plays a crucial role in determining the physical properties of
graphene, and hence its effect must be accounted for in the interpretation of
experimental data taken at cryogenic or elevated temperatures.Comment: 17 pagese, 3 figures, and supporting information (4 pages, 3
figures); Nano Letters, 201
Micro-Raman and micro-transmission imaging of epitaxial graphene grown on the Si and C faces of 6H-SiC
Micro-Raman and micro-transmission imaging experiments have been done on epitaxial graphene grown on the C- and Si-faces of on-axis 6H-SiC substrates. On the C-face it is shown that the SiC sublimation process results in the growth of long and isolated graphene ribbons (up to 600 μm) that are strain-relaxed and lightly p-type doped. In this case, combining the results of micro-Raman spectroscopy with micro-transmission measurements, we were able to ascertain that uniform monolayer ribbons were grown and found also Bernal stacked and misoriented bilayer ribbons. On the Si-face, the situation is completely different. A full graphene coverage of the SiC surface is achieved but anisotropic growth still occurs, because of the step-bunched SiC surface reconstruction. While in the middle of reconstructed terraces thin graphene stacks (up to 5 layers) are grown, thicker graphene stripes appear at step edges. In both the cases, the strong interaction between the graphene layers and the underlying SiC substrate induces a high compressive thermal strain and n-type doping
Correlating Raman Spectral Signatures with Carrier Mobility in Epitaxial Graphene: A Guide to Achieving High Mobility on the Wafer Scale
We report a direct correlation between carrier mobility and Raman topography
of epitaxial graphene (EG) grown on silicon carbide (SiC). We show the Hall
mobility of material on the Si-face of SiC [SiC(0001)] is not only highly
dependent on thickness uniformity but also on monolayer strain uniformity. Only
when both thickness and strain are uniform over a significant fraction (> 40%)
of the device active area does the mobility exceed 1000 cm2/V-s. Additionally,
we achieve high mobility epitaxial graphene (18,100 cm2/V-s at room
temperature) on the C-face of SiC [SiC(000-1)] and show that carrier mobility
depends strongly on the graphene layer stacking. These findings provide a means
to rapidly estimate carrier mobility and provide a guide to achieve very high
mobility in epitaxial graphene. Our results suggest that ultra-high mobilities
(>50,000 cm2/V-s) are achievable via the controlled formation of uniform,
rotationally faulted epitaxial graphene.Comment: 13 pages including supplimental material. Submitted to Nature
Materials 2/23/200
Compression Behavior of Single-layer Graphene
Central to most applications involving monolayer graphene is its mechanical
response under various stress states. To date most of the work reported is of
theoretical nature and refers to tension and compression loading of model
graphene. Most of the experimental work is indeed limited to bending of single
flakes in air and the stretching of flakes up to typically ~1% using plastic
substrates. Recently we have shown that by employing a cantilever beam we can
subject single graphene into various degrees of axial compression. Here we
extend this work much further by measuring in detail both stress uptake and
compression buckling strain in single flakes of different geometries. In all
cases the mechanical response is monitored by simultaneous Raman measurements
through the shift of either the G or 2D phonons of graphene. In spite of the
infinitely small thickness of the monolayers, the results show that graphene
embedded in plastic beams exhibit remarkable compression buckling strains. For
large length (l)-to-width (w) ratios (> 0.2) the buckling strain is of the
order of -0.5% to -0.6%. However, for l/w <0.2 no failure is observed for
strains even higher than -1%. Calculations based on classical Euler analysis
show that the buckling strain enhancement provided by the polymer lateral
support is more than six orders of magnitude compared to suspended graphene in
air
Rapid, direct and non-destructive assessment of fossil organic matter via microRaman spectroscopy
Raman spectroscopy is widely used to evaluate the nature and potential origins of carbonaceous matter in Earth's oldest rocks and minerals. It is also the tool that will be used for organic detection on the next vehicles to remotely explore the surface of Mars. Here we present, for the first time, a novel quantitative method in which previously neglected Raman spectral features are correlated directly, linearly, and with excellent accuracy, to the microchemistry of carbonaceous materials through the elemental H:C ratio, regardless of contamination. We show applicability and predictive capabilities of this methodology in evaluating H:C ratios between 0.01 and 0.65 in Archean and type III kerogens. We demonstrate its application to chemical microRaman mapping by statistical analysis of a 750Ma microfossil and its encompassing sediments. Raman-derived H:C data can also be used to estimate the degree to which kerogen C-isotopic data has been shifted from its original values due to the effects of metamorphism. The new methodology directly and non-invasively affords spatially resolved assessments of organic matter preservation and microscale chemical diversity within any geologically preserved terrestrial or extraterrestrial sample, including in the use of organic matter in technological applications
Dynamical low-energy electron diffraction study of graphite (0001)-(√3×√3)R30°-Xe
Dynamical low-energy electron diffraction (LEED) was used to study the structures of graphite (0 0 0 1) and graphite
(0 0 0 1)-ðp3 " p3ÞR30!-Xe. Natural single crystals of graphite were used in these studies, and their surface structure was found to be the same as the bulk structure. The adsorbed Xe was found to occupy the hollow sites of the honeycomb substrate at a distance of 3.59± 0.04 !AA above the graphite plane
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