41 research outputs found
Determination of substrate pinning in epitaxial and supported graphene layers via Raman scattering
The temperature-induced shift of the Raman G line in epitaxial graphene on
SiC and Ni surfaces, as well as in graphene supported on SiO2, is investigated
with Raman spectroscopy. The thermal shift rate of epitaxial graphene on
6H-SiC(0001) is found to be about three times that of freestanding graphene.
This result is explained quantitatively as a consequence of pinning by the
substrate. In contrast, graphene grown on polycrystalline Ni films is shown to
be unpinned, i.e., to behave elastically as freestanding, despite the
relatively strong interaction with the metal substrate. Moreover, it is shown
that the transfer of exfoliated graphene layers onto a supporting substrate can
result in pinned or unpinned layers, depending on the transfer protocol.Comment: 4 pages, 5 figures. To appear in Physical Review B, Brief
Communicatio
Genome-inspired molecular identification in organic matter via Raman spectroscopy
Rapid, non-destructive characterization of molecular level chemistry for
organic matter (OM) is experimentally challenging. Raman spectroscopy is one of
the most widely used techniques for non-destructive chemical characterization,
although it currently does not provide detailed identification of molecular
components in OM, due to the combination of diffraction-limited spatial
resolution and poor applicability of peak-fitting algorithms. Here, we develop
a genome-inspired collective molecular structure fingerprinting approach, which
utilizes ab initio calculations and data mining techniques to extract molecular
level chemistry from the Raman spectra of OM. We illustrate the power of such
an approach by identifying representative molecular fingerprints in OM, for
which the molecular chemistry is to date inaccessible using non-destructive
characterization techniques. Chemical properties such as aromatic cluster size
distribution and H/C ratio can now be quantified directly using the identified
molecular fingerprints. Our approach will enable non-destructive identification
of chemical signatures with their correlation to the preservation of
biosignatures in OM, accurate detection and quantification of environmental
contamination, as well as objective assessment of OM with respect to their
chemical contents
Evolution in Surface Morphology of Epitaxial Graphene Layers on SiC Induced by Controlled Structural Strain
The evolution in the surface morphology of epitaxial graphene films and
6H-SiC(0001) substrates is studied by electron channeling contrast imaging.
Whereas film thickness is determined by growth temperature only, increasing
growth times at constant temperature affect both internal stress and film
morphology. Annealing times in excess of 8-10 minutes lead to an increase in
the mean square roughness of SiC step edges to which graphene films are pinned,
resulting in compressively stressed films at room temperature. Shorter
annealing times produce minimal changes in the morphology of the terrace edges
and result in nearly stress-free films upon cooling to room temperature.Comment: 3 pages, 2 figures. Applied Physics Letters 93 (2008), 19191
Rethinking Coal: Thin Films of Solution Processed Natural Carbon Nanoparticles for Electronic Devices
Disordered carbon materials, both amorphous and with long-range order, have been used in a variety of applications, from conductive additives and contact materials to transistors and photovoltaics. Here we show a flexible solution-based method of preparing thin films with tunable electrical properties from suspensions of ball-milled coals following centrifugation. The as-prepared films retain the rich carbon chemistry of the starting coals with conductivities ranging over orders of magnitude, and thermal treatment of the resulting films further tunes the electrical conductivity in excess of 7 orders of magnitude. Optical absorption measurements demonstrate tunable optical gaps from 0 to 1.8 eV. Through low-temperature conductivity measurements and Raman spectroscopy, we demonstrate that variable range hopping controls the electrical properties in as-prepared and thermally treated films and that annealing increases the sp 2 content, localization length, and disorder. The measured hopping energies demonstrate electronic properties similar to amorphous carbon materials and reduced graphene oxide. Finally, Joule heating devices were fabricated from coal-based films, and temperatures as high as 285 °C with excellent stability were achieved
Unintended consequences: Why carbonation can dominate in microscale hydration of calcium silicates
The initial microscale mechanisms and materials interfacial process responsible for hydration of calcium silicates are poorly understood even in model systems. The lack of a measured microscale chemical signature has confounded understanding of growth mechanisms and kinetics for microreaction volumes. Here, we use Raman and optical spectroscopies to quantify hydration and environmental carbonation of tricalcium silicates across length and time scales. We show via spatially resolved chemical analysis that carbonate formation during the initial byproduct in microscale reaction volumes is significant, even for subambient CO2 levels. We propose that the competition between carbonation and hydration is enhanced strongly in microscale reaction volumes by increased surface-to-volume ratio relative to macroscale volumes, and by increased concentration of dissolved Ca2+ ions under poor hydration conditions that promote evaporation. This in situ analysis provides the first direct correlation between microscale interfacial hydration and carbonation environments and chemically defined reaction products in cementitious materials.United States. Department of Homeland Security. Science and Technology DirectorateMIT Concrete Sustainability HubPortland Cement AssociationReady Mixed Concrete (RMC) Research & Education Foundatio
Evolution of topological order in Xe films on a quasicrystal surface
We report results of the first computer simulation studies of a physically
adsorbed gas on a quasicrystalline surface, Xe on decagonal Al-Ni-Co. The grand
canonical Monte Carlo method is employed, using a semi-empirical gas-surface
interaction, based on conventional combining rules, and the usual Lennard-Jones
Xe-Xe interaction. The resulting adsorption isotherms and calculated structures
are consistent with the results of LEED experimental data. The evolution of the
bulk film begins in the second layer, while the low coverage behavior is
epitaxial. This transition from 5-fold to 6-fold ordering is temperature
dependent, occurring earlier (at lower coverage) for the higher temperatures
Adsorption of Xe and Ar on Quasicrystalline Al-Ni-Co
An interaction potential energy between and adsorbate (Xe and Ar) and the
10-fold Al-Ni-Co quasicrystal is computed by summing over all
adsorbate-substrate interatomic interactions. The quasicrystal atoms'
coordinates are obtained from LEED experiments and the Lennard-Jones parameters
of Xe-Al, Xe-Ni and Xe-Co are found using semiempirical combining rules. The
resulting potential energy function of position is highly corrugated.
Monolayer adsorption of Xe and Ar on the quasicrystal surface is investigated
in two cases: 1) in the limit of low coverage (Henry's law regime), and 2) at
somewhat larger coverage, when interactions between adatoms are considered
through the second virial coefficient, C_{AAS}. A comparison with adsorption on
a flat surface indicates that the corrugation enhances the effect on Xe-Xe
(Ar-Ar) interactions. The theoretical results for the low coverage adsorption
regime are compared to experimental (LEED isobar) data.Comment: 12 pages, 8figure
Xe films on a decagonal Al-Ni-Co quasicrystal surface
The grand canonical Monte Carlo method is employed to study the adsorption of
Xe on a quasicrystalline Al-Ni-Co surface. The calculation uses a semiempirical
gas-surface interaction, based on conventional combining rules and the usual
Lennard-Jones Xe-Xe interaction. The resulting adsorption isotherms and
calculated structures are consistent with the results of LEED experimental
data. In this paper we focus on five features not discussed earlier (Phys. Rev.
Lett. 95, 136104 (2005)): the range of the average density of the adsorbate,
the order of the transition, the orientational degeneracy of the ground state,
the isosteric heat of adsorption of the system, and the effect of the vertical
cell dimension.Comment: 6 pages, 5 pic
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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.Organismic and Evolutionary Biolog