Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session J8: Focus Session: Graphene: Raman, Strain, Thermal |
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Sponsoring Units: DMP Chair: Jun Yan , University of Massachusetts at Amhearst Room: 307 |
Tuesday, March 19, 2013 2:30PM - 2:42PM |
J8.00001: Identifying Few-Layer Graphene with Raman Spectroscopy David Tran, Nathaniel Gillgren, Kevin Myhro, Yongjin Lee, Jairo Velasco Jr., Lei Jing, Marc Bockrath, Jeanie Lau Few-layer graphene (FLG) exists in various crystallographic stacking sequences, which can strongly influence the material's electronic properties. We characterize stacking order in FLG using the distinctive features of the Raman 2D-mode's full-width at half-maximum (FWHM), relative peak size, and shape. Raman imaging allows us to visualize directly the spatial distribution of bilayer graphene, Bernal (ABA) trilayer graphene, and rhombohedral (ABC) trilayer graphene. [Preview Abstract] |
Tuesday, March 19, 2013 2:42PM - 2:54PM |
J8.00002: Optical Separation of Mechanical Strain from Charge Doping in Graphene Sunmin Ryu, Ji Eun Lee, Gwanghyun Ahn Graphene, due to its superior stretchability, exhibits rich structural deformation behaviors and its strain-engineering has proven useful in modifying its electronic and magnetic properties. Despite the strain-sensitivity of the Raman G and 2D modes, the optical characterization of the native strain in graphene on silica substrates has been hampered by excess charges interfering with both modes. Here we show that the effects of strain and charges can be optically separated from each other by correlation analysis of the two modes, enabling simple quantification of both. Graphene with in-plane strain randomly occurring between -0.2{\%} and 0.4{\%} undergoes modest compression (-0.3{\%}) and significant hole doping upon thermal treatments. This study suggests that substrate-mediated mechanical strain is a ubiquitous phenomenon in two-dimensional materials. The proposed analysis will be of great use in characterizing graphene-based materials and devices. [Preview Abstract] |
Tuesday, March 19, 2013 2:54PM - 3:06PM |
J8.00003: Folded Optical Phonons in Twisted Bilayer Graphene: Raman Signature of Graphene Superlattices Yanan Wang, Zhihua Su, Wei Wu, Sirui Xing, Xiaoxiang Lu, Xinghua Lu, Shin-shem Pei, Francisco Robles-Hernandez, Viktor Hadjiev, Jiming Bao In contrast to Bernal-stacked graphene exfoliated from HOPG, twisted bilayer graphene are widely observed in the samples prepared by silicon sublimation of SiC or chemical vapor deposition (CVD). However, many of its basic properties still remain unrevealed. In this work, hexagon-shaped bilayer graphene islands synthesized by CVD method were systematically studied using Raman spectroscopy. A series of folded phonons were observed in the range from 1375 cm$^{-1}$ to 1525 cm$^{-1}$. The frequency of folded phonon modes doesn't shift with laser excitation energy, but it is highly dependent on the rotational angle between two layers. In general, the frequency of folded phonon decreases with the increase of rotation angle. This rotation dependence can be qualitatively explained by the folding of phonon dispersion curve of single layer graphene into the reduced Brillouin zone of bilayer superlattice. The obseravtion of folded phonon is an important indication of superlattice band structure. [Preview Abstract] |
Tuesday, March 19, 2013 3:06PM - 3:42PM |
J8.00004: Large Physisorption Strain in Graphene Grown by Chemical Vapor Deposition on Copper Substrates Invited Speaker: Rui He Single layer graphene grown by chemical vapor deposition (CVD) on Cu substrates is subject to non-uniform physisorption strain that is dependent on the orientation of the Cu surface. The blue-shift and broadening of Raman bands of graphene on the Cu single crystal (111) surface reveal that the graphene layer is under compressive strain. This interpretation is consistent with Moire patterns seen in scanning tunneling microscopy. Graphene grown on the Cu (100) surface is subject to a highly non-uniform strain due to the mismatch between the graphene honeycomb lattice and the square lattice at this Cu surface. Molecular Dynamics simulations are in excellent agreement with experiment, predicting compressive strain on the order of 0.5 percent in graphene/Cu(111). In graphene/Cu(100) the simulated physisorption strain patterns show linear superstructures spaced about 1 nm apart and a highly non-uniform bond length distribution which leads to both compressive and tensile strains. CVD graphene grown on polycrystalline Cu foil is also studied for comparison. The strain in graphene is even more non-uniform for growth on Cu foil. However, this strain is greatly reduced after the graphene layer is removed and transferred onto a SiO2 substrate. Physisorption strain is thus revealed to be a major factor in the growth of CVD graphene on transition metals. [Preview Abstract] |
Tuesday, March 19, 2013 3:42PM - 3:54PM |
J8.00005: 2D line enhancement by quantum interference in graphene superlattice Zhihua Su, Yanan Wang, Wei Wu, Sirui Xing, Xiaoxiang Lu, Xinghua Lu, Shin-shem Pei, Francisco Robles-Hernandez, Viktor G. Hadjiev, Jiming Bao Raman scattering is used to study twisted bilayer graphene synthesized by chemical vapor deposition (CVD) method with rotation angle determined by relative edge misalignment. Degenerate Dirac band of twisted bilayer graphene is revealed by enhanced intensity of 2D line. This Raman signature is systematically studied and found to be correlated with G-line resonance and laser excitation energy. 2D enhancement only happens when the laser excitation energy is smaller than G-line resonance energy, while enhancement ratio increases as laser excitation energy decreases. The anomalous enhancement of 2D intensity is ascribed to the constructive quantum interference between two Raman paths enabled by degenerate Dirac cone. [Preview Abstract] |
Tuesday, March 19, 2013 3:54PM - 4:06PM |
J8.00006: Wrinkling instability in nanoparticle-supported graphene: implications for strain engineering William Cullen, Mahito Yamamoto, Olivier Pierre-Louis, Jia Huang, Michael Fuhrer, Theodore Einstein We have carried out a systematic study of the wrinkling instability of graphene membranes supported on SiO$_{2}$ substrates with randomly placed silica nanoparticles. At small nanoparticle density, monolayer graphene adheres to the substrate and is highly conformal over the nanoparticles. With increasing nanoparticle density, and decreasing nanoparticle separation to $\sim$100 nm, graphene's elastic response dominates substrate adhesion, and elastic stretching energy is reduced by the formation of wrinkles which connect protrusions. Above a critical nanoparticle density, the wrinkles form a percolating network through the sample. As the graphene membrane is made thicker, delamination from the substrate is observed. Since the wrinkling instability acts to remove inhomogeneous in-plane elastic strains through out-of-plane buckling, our results can be used to place limits on the possible in-plane strain magnitudes that may be created in graphene to realized strain-engineered electronic structures.\footnote{M. Yamamoto et al., ``Princess and the Pea at the nanoscale: Wrinkling and unbinding of graphene on nanoparticles,'' arXiv:1201.5667 (2012).} [Preview Abstract] |
Tuesday, March 19, 2013 4:06PM - 4:18PM |
J8.00007: Graphene slides over the substrate when you pull it: Direct measurement, theory, and frictional forces Alexander Kitt, Zenan Qi, Harold Park, Anna Swan, Bennett Goldberg Using graphene sealed cylindrical microchambers we characterize, for the first time, the sliding of graphene over a thermal oxide substrate. High spatially resolved Raman spectra recorded as external pressure is applied to the microchamber shows that as the graphene is pressed into the hole it drags some of the previously substrate-supported graphene with it. The well-understood strain response of the Raman G-band allows us to measure strains of less than .01{\%}, corresponding to 1nm stretching over a micron, with 500 nm lateral resolution as pressure is applied to the system. Our results are compared to both atomistic and continuum models, with interesting new conclusions, of the system in order to quantify the sliding for mono, bi, and tri layer graphenes over holes of radii between 1.25 and 5 $\mu$m with applied pressures between 0 and 100 psi. [Preview Abstract] |
Tuesday, March 19, 2013 4:18PM - 4:54PM |
J8.00008: Raman spectroscopy of graphene under strain Invited Speaker: Hyeonsik Cheong Raman spectroscopy is one of the most widely used characterization tools in graphene research. It has been used to estimate the number of layers, carrier concentration, edge types, thermal conductivity, etc. The effect of strain on the Raman spectrum of graphene is of particular interest, because the modification of the lattice due to strain is directly reflected on the Raman spectrum. Under uniaxial strain, the G and 2D bands of the Raman spectrum of single-layer graphene split due to the breaking of symmetry. From the polarization dependence of the split bands, one can determine the crystallographic orientation of the graphene sample. Furthermore, the splitting of the 2D band depends on the direction of the applied strain with respect to the crystallographic orientation, which provides critical information on the dominant scattering process responsible for this band. Bilayer graphene under uniaxial strain exhibits similar effects on the G and 2D bands, although the effect on the 2D band is rather complicated as each of the 4 components comprising the 2D band splits under strain. One can also estimate the thermal expansion coefficient and Young's modulus using Raman measurements under strain. Recent experimental results will be presented. [Preview Abstract] |
Tuesday, March 19, 2013 4:54PM - 5:06PM |
J8.00009: Experimental Measurement of Thermal Expansion Coefficient of few layer graphene Lei Jing, Wenzhong Bao, Hoon Cho, Fenglin Wang, Chun Ning(Jeanie) Lau In contrast to most materials, graphene has negative thermal expansion coefficient (TEC), which has important implications on device applications. We experimentally measure the TEC of single- and few-layer graphene by suspending them across predefined trenches, and monitoring their sagging arc lengths during cooling via in situ SEM imaging. Latest experimental data will be discussed and compared to theoretical models. [Preview Abstract] |
Tuesday, March 19, 2013 5:06PM - 5:18PM |
J8.00010: Lengthy logarithms and graphene's Debye-Waller factor B.C. Regan, Brian Shevitski, William A. Hubbard, E.R. White, Ben Dawson, M.S. Lodge, Masa Ishigami, Matthew Mecklenburg In an infinite, two-dimensional crystal, long wavelength thermal phonons create a divergence in the mean-square displacement $u_p^2$ of atoms from their ideal lattice positions, which has led some to infer that the existence of graphene might depend on the stabilizing influence of ripples in the third dimension. Using the Debye model to approximate graphene's phonon band structure, we calculate $u_p^2$ and the resulting Debye-Waller suppression of high order peaks in graphene's electron diffraction pattern. We find that at room temperature in a 10~$\mu$m sample $\sqrt{u_p^2}$ is less than 5\% of the carbon-carbon bond length, well below the Lindemann melting threshold. Our TEM measurements of the Debye-Waller factor in suspended, exfoliated graphene agree with the calculation. Finite size effects are sufficient to explain graphene's evident stability at room temperature. Surprisingly, in the case of graphene even $6\times 10^{23}$ carbon atoms, representing a sheet 126~m on a side, are not enough to approximate an infinitely large crystal. [Preview Abstract] |
Tuesday, March 19, 2013 5:18PM - 5:30PM |
J8.00011: In-Situ Thermal Mapping of Graphene via TEM Measurement of the Debye-Waller Factor William A. Hubbard, Matthew Mecklenburg, B.C. Regan Thermal motion of the constituent atoms attenuates high-order peaks in a crystal's electron diffraction pattern. Using TEM we measure this attenuation, parameterized by a Debye-Waller factor, in single-layer cleaved graphene that is Joule-heated \emph{in situ}. We find that the Debye-Waller factor, as probed with selected area electron diffraction, provides a reliable measure of the local temperature and thus allows for quantitative thermal mapping on the nanoscale. [Preview Abstract] |
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