Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session T5: Graphene: Transport and Optical Phenomena: Raman and Phonons |
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Sponsoring Units: DCMP Chair: Francois Peeters, University of Antwerp Room: 301 |
Thursday, March 21, 2013 8:00AM - 8:12AM |
T5.00001: Polarization dependence of Raman 2D band in bilayer graphene Jae-Ung Lee, Ngor Mbaye Seck, Duhee Yoon, Hyeonsik Cheong The Raman intensity of the double-resonance 2D band in sigle-layer graphene has a strong polarization dependence(Yoon et al. Nano Lett.). The intensity is maximum when the excitation and detection polarization are parallel and minimum when they are orthogonal, whereas that of G band is isotropic. This strong polariztion dependence is the consequence of inhomogeneous optical absorption and emission mediated by electron-phonon interactions. Here, the polarization dependence of the Raman 2D band in bilayer graphene. The 2D band of bilayer graphene can be decomposed to 4 Lorentzian peaks corresponding to different scattering process involving 2 conduction and 2 valence bands. The 2D band in bialyer graphene shows a similar polarization dependence as that of single layer. Futhermore, the excitation energy dependence was investigated by using 4 different excitation laser wavelenghths. The polarization ratio of each of the 4 Lorentizan peaks seems to reflect the features of the electronic band structure of bilayer graphene in the energy range of the excitation laser. [Preview Abstract] |
Thursday, March 21, 2013 8:12AM - 8:24AM |
T5.00002: Undetectable Raman Spectrum of Graphene on Platinum Surface A. Zettl, Qin Zhou Raman spectrometry is often used as a quick and convenient tool to evaluate the growth quality of graphene. Recently there has been growing interest in platinum mediated graphene CVD growth for producing high-quality, large grain size, and highly flat graphene layers. Surprisingly, no Raman signal of graphene can be detected in the as-grown state on platinum substrates, despite using different laser wavelengths from 488 nm to 785nm. This phenomenon is briefly mentioned in earlier literature and has been attributed to strong platinum-graphene interaction. We investigate the disappearance of graphene Raman signatures on metal substrates, by performing Raman spectrum measurements on graphene layers transferred onto various substrates. [Preview Abstract] |
Thursday, March 21, 2013 8:24AM - 8:36AM |
T5.00003: Raman spectroscopy of single layer graphitic carbon nitride Joel Therrien, Yancen Li, Daniel Schmidt, Adam Collard, Daniel Finkenstadt, Taylor Yust Single layer graphitic carbon nitride (referred to as melon) has been synthesized by our group in sizes up to 50 $\mu $m across. Raman spectroscopy has been performed on single layer melon and multi layer samples. Much like graphene, melon shows a unique raman spectrum when in single layer form. These experimental results have been compared to theoretical calculations for possible melon structures. Bond counts for feasible structures of hexagonal carbon nitride have been calculated and some possible structures have been eliminated from consideration based on these efforts. Periodic supercells have been built to make sheets based on structures to be modeled via density-functional theory, as implemented using VASP, to calculate thermodynamic and structural stability and frequencies of IR and Raman active modes. [Preview Abstract] |
Thursday, March 21, 2013 8:36AM - 8:48AM |
T5.00004: Magneto-Raman experiments in single- and multi-layer graphene F.M. Ardito, T.G. Mendes de Sa, P.F. Gomes, E. Nery, D.L. Mafra, F. Iikawa, M.J.S.P. Brasil, L.M. Malard, F. Plentz, M.A. Pimenta, R.G. Lacerda, E. Granado Micro-Raman experiments as a function of magnetic field up to 15 T were performed on a set of natural graphene flakes on Si/SiO$_2$ substrates and multilayer epitaxial graphene grown on a carbon face of SiC. Pronounced oscillations of the $G$-band position and linewidth attributed to crossings of this mode with Landau levels were observed in epitaxial graphene. Calculated phonon energy and broadening oscillations obtained from the phonon's Green function show good agreement with the results obtained for SiC samples, in line with a previous report [1]. For graphene flakes, the field evolution of the G-band is strongly sample-dependent, and may also depend on the position of the focal spot. A splitting of $G$-band in two peaks was observed in some cases for $B>12$ T. Our results suggest the large sensitivity of graphene electron-phonon interaction to both magnetic field and local conditions. [1] C. Faugeras {\it et al.}, Phys. Rev. Lett. {\bf 103}, 186803 (2009). [Preview Abstract] |
Thursday, March 21, 2013 8:48AM - 9:00AM |
T5.00005: Transport and Raman measurements in Graphene: Interaction strength and scattering mechanisms Sebastian Remi, Anna Swan, Bennett Goldberg Among the most common techniques for characterization of Graphene materials have been electronic transport and Raman measurements, for instance both can be easily tuned by changing the charge carrier density and electronic screening. In each situation the underlying physics is connected to the interactions and relaxation mechanisms in the material. However it is well known that the electronic scattering time does not necessarily describe the broadening observed in Raman measurements. Here we present micro Raman and transport measurements of single layer graphene field effect devices. We discuss interaction and scattering mechanisms and how these are connected in the different measurements. [Preview Abstract] |
Thursday, March 21, 2013 9:00AM - 9:12AM |
T5.00006: Raman scattering of 2D materials Ting Yu, Riichiro Saito, Mildred Dresselhaus Motivated by graphene, two-dimensional (2D) materials become the center of current Nanoscience and Nanotechnology. In this talk, I will report our recent works on Raman scattering study of 2D materials such as graphene and MoS2. In detail: the in-plane and out-of-plane arrangement of carbons in graphene layers are identified by both Raman and HRTEM with atomic resolution; the structure evolution of molecules anchored on the surface of graphene is studied by Raman; the behavior of Dirac Fermions of graphene in a magnetic field is probed; the strain effects on MoS2 and the identification of crystallographic orientation of MoS2 are also discussed. The results presented here are highly relevant to the fundamental and applications of graphene and other 2D transition metal dichalcogenides (TMDs).. [Preview Abstract] |
Thursday, March 21, 2013 9:12AM - 9:24AM |
T5.00007: Temperature-dependent photoluminescence and Raman spectroscopy of single-layer MoS$_2$ J.R. Simpson, R. Yan, S. Bertolazzi, A. Kis, J. Brivio, M. Watson, H.G. Xing, A.R. Hight Walker We report the temperature-dependent photoluminescence (PL) and Raman spectra of single-layer MoS$_2$. Mechanical exfoliation from bulk MoS$_2$ provides single-layer flakes which are then transferred to either sapphire (with and without ALD HfO$_2$ overcoating) or suspended over holes in a Si/Si$_3$N$_4$ substrate. We measure the temperature dependence of PL and Raman spectra from (100 to 400)\,K using HeNe 632.8\,nm (PL) and Ar$^+$-ion 514.5\,nm (Raman) laser excitations coupled to a microscope and grating spectrometer. PL shows a single, narrow peak corresponding to a direct-band transition approximately centered at 1.9\,eV with a width of 50\,meV. The PL peak redshifts and broadens with increasing temperature. Raman spectra reveal two strong phonon vibrational modes, the planar $E^1_{2g}$ and out-of-plane $A_{1g}$, both of which soften linearly with increasing temperature as a result of anharmonic effects. We extract a linear temperature coefficient for both Raman modes comparable to the G-mode of graphene. A comparison with the dependence of phonon peak position on incident optical power for the suspended sample shows moderate heat flux efficiency. The impact of dielectric and substrate environment on extraction of thermal conductivity will be discussed. [Preview Abstract] |
Thursday, March 21, 2013 9:24AM - 9:36AM |
T5.00008: Observation of polaronic effects in electron transport in graphene by infrared spectroscopy Keliang He, Liang Zhao, Jie Shan, Kin Fai Mak, Nick Petron, James Hone, Tony F. Heinz, G. Larry Carr Polarons, quasi-particles consisting of electrons and the accompanying lattice polarization, are generally considered to be unimportant for the electrical transport properties of nonpolar crystals such as graphene. The distinctive linear dispersion relation found in graphene and the drastically reduced screening of Coulomb interactions associated with the material's reduced dimensionality, however, lead to strong coupling between Dirac electrons and high-energy optical phonons in graphene. In this work, we apply the infrared absorption spectroscopy to investigate the optical conductivity of graphene as a function of electrostatic doping density. We have observed a phonon side band in the intraband optical conductivity with a significant spectral weight transfer from the Drude response, indicating the importance of the polaronic effects. The effects can also be tuned by doping. The conductivity spectra have been analyzed in the framework of the extended Drude model to yield the spectral dependence of the mass enhancement factor (band structure renormalization) and the scattering rate (with an onset for phonon scattering) at different doping levels. Our results are in good agreement with many-body calculations for graphene conductivity with polaronic corrections. [Preview Abstract] |
Thursday, March 21, 2013 9:36AM - 9:48AM |
T5.00009: Electron-phonon bound states in graphene Justin Zhu, Samvel M. Badalyan, Francois Peeters We investigate the fine structure of the energy spectrum in graphene induced by electron-optical phonon coupling. Despite the small electron-phonon coupling, perturbation theory is inapplicable in the part of spectrum near the optical phonon emission threshold. In zero magnetic field [1] we derive new dispersion equation, which in the immediate neighborhood below the threshold describes an electron-phonon bound state. We find that the singular vertex corrections beyond perturbation theory strongly inhance the electron-phonon binding energy scale. In quantizing magnetic fields [2], our findings beyond perturbation theory show that the true spectrum near the phonon emission threshold is completely governed by new branches of the spectrum, corresponding to bound states of an electron and an optical phonon with a binding energy of the order of $\alpha\omega_{0}$ where $\alpha$ is the electron-phonon coupling and $\omega_{0}$ the phonon energy. \\[4pt] [1] S. M. Badalyan and F. M. Peeters, Phys. Rev. B {\bf 85}, 205453 (2012).\\[0pt] [2] J. Zhu, S. M. Badalyan and F. M. Peeters, arXiv:{\bf 1206.5107}. [Preview Abstract] |
Thursday, March 21, 2013 9:48AM - 10:00AM |
T5.00010: Graphene thermal conductivity from first principles Lucas Lindsay, Tom Reinecke, David Broido Previous theoretical work based on an optimized Tersoff interatomic potential found that the thermal conductivity of graphene is dominated by out-of-plane phonons in part due to reflection symmetry of the graphene sheet [1,2]. Since empirical potentials can have questionable predictive power, here we present calculations of the thermal conductivity of graphene using interatomic forces determined from \textit{first principles} coupled with a numerical solution to the Peierls-Boltzmann transport equation. We find good agreement with experiment for the calculated phonon dispersion and thermal conductivity of graphene and validate earlier theoretical results which used the optimized empirical potential. [1] J.H. Seol, \textit{et al}, \textit{Science} 328, 213 (2010). [2] L. Lindsay, \textit{et al}, \textit{Phys. Rev. B} 82, 115427 (2010). [Preview Abstract] |
Thursday, March 21, 2013 10:00AM - 10:12AM |
T5.00011: Thermal transport in low-dimensional systems: the case of Graphene and single layer Boron Nitride Luiz Felipe Pereira, Davide Donadio Low-dimensional systems present unusual transport properties in comparison to bulk materials. In contrast with the three-dimensional case, in one- and two-dimensions heat transport models predict a divergence of the thermal conductivity with system size. In reality, in a low-dimensional system the mean-free-path of heat carriers (phonons) becomes comparable to the micrometer size of experimental samples. Recent developments in nanostructure fabrication allow a direct comparison between theory and experiments for such low-dimensional systems. We perform extensive molecular dynamics simulations of heat transport in graphene and single layer BN, in order to clarify the behavior of the thermal conductivity in realistic low-dimensional systems. In particular, we address the influence of system size on the simulation results. Equilibrium molecular dynamics predicts a convergence of the thermal conductivity with system size, even for systems with less than one hundred nanometers and thousands of atoms. Meanwhile, large scale non-equilibrium molecular dynamics shows a divergence of the thermal conductivity with system size up to the micrometer scale. We analyse the discrepancy between methods in terms of perturbations in phonon populations induced by the non-equilibrium regime. [Preview Abstract] |
Thursday, March 21, 2013 10:12AM - 10:24AM |
T5.00012: Phonon-limited transport coefficients in extrinsic graphene Enrique Munoz The effect of electron-phonon scattering processes over the thermoelectric properties of extrinsic graphene was studied. Electron-phonon interaction is formulated in the second quantization language, for chiral Dirac spinor fields and phonon Bose fields, within the deformation potential approximation. Electrical and thermal resistivity, as well as the thermopower, were calculated within the Bloch theory approximations. Analytical expressions for the different transport coefficients were obtained from a variational solution of the Boltzmann transport equation. The phonon-limited electrical resistivity $\rho_{e-ph}$ shows a linear in temperature dependence at high temperatures, and follows a $\rho_{e-ph}\sim T^{4}$ at low temperatures, in agreement with experiments. The phonon-limited thermal resistivity at low temperatures exhibits a $\sim T$ dependence and achieves a nearly constant value at high temperatures. The predicted Seebeck coefficient at very low temperature is $Q(T ) \sim \pi^{2} k_{B} T /(3e E_{F} )$, which shows a $n^{-1/2}$ dependence with the carrier density, in agreement with experiments.\\[4pt] [1] E. Mu\~noz, Journal of Physics: Condensed Matter {\bf{24}} (2012) 195302.\\[0pt] [2] E. Mu\~noz, J. Lu and B. I. Yakobson, Nano Letters {\bf{10}} (2010) 1652. [Preview Abstract] |
Thursday, March 21, 2013 10:24AM - 10:36AM |
T5.00013: Thermal Conductivity Measurements in Sub-micron Graphene Crystals Serap Yigen, Vahid Tayari, James Porter, Joshua O. Island, A. R. Champagne Heat conductivity measurements in graphene using optical spectroscopy have been limited to micron-scale devices, and mostly room temperature and uncontrolled charge densities. We present an electron transport method to measure thermal conductivity, $\kappa$, in sub-micron suspended graphene, over a broad range of temperature (50K - 350K), and as a function of charge density. We study suspended two-point graphene devices whose length ranges from 350 nm up to 1.2 micron. We show that the there can be good thermalization of electrons and acoustic phonons in these devices. This enables us to use electron resistivity as a thermometer for electrons or phonons. Our devices are in the near-diffusive regime, permitting Joule heating of the samples and modelling heat transport using a heat equation. We measure an increase of two orders of magnitude in $\kappa$ over the studied temperature range and crystal lengths. $\kappa$ is dominated by the electronic heat conductivity in sub-micron devices, and phononic heat conductivity in longer devices. In short devices, we can tune $\kappa$ by more than a factor of two with charge density, opening the possibility of creating room temperature heat transistors. [Preview Abstract] |
Thursday, March 21, 2013 10:36AM - 10:48AM |
T5.00014: Electron-Phonon Coupling in Silicene Jia-An Yan, Mei-Yin Chou We report here a first-principles study of the electron-phonon coupling (EPC) in silicene and compare the results to graphene. The $E_g$ mode at $\Gamma$ and the $A_1$ mode at $K$ of the first Brillouin zone are shown to exhibit Kohn anomalies, similar to that in graphene. Detailed calculations show that although the EPC matrix elements are much smaller than in graphene, the linear band with smaller slope compensate this effect, resulting in a slightly larger phonon linewidth. Finally, the phonon frequency shift and the linewidth of the $E_g$ mode as a function of the Fermi level $E_F$ have been calculated. [Preview Abstract] |
Thursday, March 21, 2013 10:48AM - 11:00AM |
T5.00015: Out-of-equilibrium current-induced forces on a suspended graphene sheet Silvia Viola Kusminskiy We have recently developed a formalism that allows to obtain the current-induced forces that act on the vibrational degrees of freedom of a nanoelectromechanical system, purely from scattering matrix theory [{\it cf} N. Bode, S. {Viola Kusminskiy}, R. Egger, F. {von Oppen}, {\it Phys. Rev. Lett.} {\bf 107}, 036804 (2011) and {\it Beilstein J. Nanotechnol.} {\bf 3}, 144 (2012), and M. Thomas, T. Karzig, S. {Viola Kusminskiy}, G. Zar\'and, F. {von Oppen}, arXiv:1209.0620 (2012)].The forces are expressed in terms of the frozen electronic scattering matrix and its first non-adiabatic correction, the A-matrix, and the expressions are valid both in and out of thermal equilibrium. We apply our results to study the effects of transport currents on the dynamics of the flexural modes of a suspended graphene sheet. We pay particular attention to the non-equilibrium contributions to the force which occur in the presence of a finite applied bias voltage. [Preview Abstract] |
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