Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session L30: Graphene: Thermal Conduction and Phonons |
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Sponsoring Units: DCMP Chair: Vitor Pereira, National University of Singapore Room: C147/154 |
Tuesday, March 22, 2011 2:30PM - 2:42PM |
L30.00001: Thermal transport in suspended and supported monolayer graphene grown by chemical vapor deposition Weiwei Cai, Arden Moore, Shanshan Chen, Yanwu Zho, Li Shi, Rodney S. Ruoff Although electron transport in graphene has been studied extensively and graphene is predicted to have very high thermal conductivity near room temperature, there is only limited experimental data in the literature on phonon transport in graphene because of experimental challenges. We report results based on micro-Raman spectroscopy for the measurement of the thermal conductivity of large-area, monolayer graphene grown by CVD on copper and subsequently suspended over a circular hole. The obtained optical absorption is measured directly by measuring the transmission through the graphene covered hole. Based on the thermal interface conductance of (28+2.8/-3.8) MW/m$^{2}$ K, the contact thermal resistance is determined to be considerably smaller than the measured thermal resistance of the suspended graphene. The obtained thermal conductivity of the supported graphene is (370 +490/-300) W/m K, which is considerably smaller than that of suspended graphene in agreement with recent measurements of mechanically exfoliated graphene supported on SiO$_{2}$. [Preview Abstract] |
Tuesday, March 22, 2011 2:42PM - 2:54PM |
L30.00002: Surprising Effects of Substrate on Thermal Transport in Supported Graphene Zhun-Yong Ong, Eric Pop We study thermal transport in graphene ``supported'' on SiO2 using molecular dynamics (MD) simulations. We find that coupling to the substrate leads to an order of magnitude decrease in the apparent thermal conductivity (TC), explaining recent experiments [1]. This reduction is due to the substrate damping of flexural acoustic (ZA) phonons, which implies that the high TC of isolated graphene is due to the large mean free path of long-wavelength ZA modes [2]. However, we find that by increasing the strength of the interfacial interaction, the apparent TC is enhanced by up to a factor of four. Using a continuum model [3], we relate the apparent TC enhancement to the ZA modes coupling with the substrate Rayleigh waves. In the weak coupling limit, the ZA modes have a quadratic dispersion and small group velocities at long wavelengths; in the strong coupling limit, the hybridized interfacial modes have a linear dispersion and larger group velocities. This finding suggests that the TC of supported graphene may be tunable through interfacial interaction. \\[4pt] [1] J. H. Seol et al., Science 328, 213 (2010)\\[0pt] [2] L. Lindsay et al., PRB 82, 115427 (2010).\\[0pt] [3] B. N. J. Persson et al., EuroPhys. Lett. 91, 56001 (2010) [Preview Abstract] |
Tuesday, March 22, 2011 2:54PM - 3:06PM |
L30.00003: Two Dimensional Phonon Transport in Graphene Insun Jo, Jae Hun Seol, Arden L. Moore, Michael T. Pettes, Lucas Lindsay, Natalio Mingo, David Broido, Zhen Yao, Li Shi We present thermal conductivity measurements of monolayer graphene exfoliated on a silicon dioxide substrate at different temperatures. A nanofabricated resistance thermometer device is developed to measure the thermal conductance of graphene and supporting 300nm thick SiO$_{2}$ layer, which allows us to extract the thermal conductivity of graphene while supported on this layer. The measured value is as high as 600 W/mK near room temperature, which is lower than that of suspended graphene, 1500-5800 W/mK, but still higher than those of metal interconnects. Theoretical calculations show that the strong interface-scattering of flexural modes across the graphene-oxide interface is responsible for the decreased value. [Preview Abstract] |
Tuesday, March 22, 2011 3:06PM - 3:18PM |
L30.00004: Influence of Polymeric Residue on the Thermal Conductivity of Suspended Bi-Layer Graphene Michael Pettes, Insun Jo, Zhen Yao, Li Shi The thermal conductivity (\textit{$\kappa $}) of two bi-layer graphene samples suspended between two micro-resistance thermometers was measured to be close to 600 W m$^{-1}$ K$^{-1}$ at room-temperature and exhibits a \textit{$\kappa $} $\propto \quad T^{1.5}$ behavior at temperature ($T)$ between 50 -- 125 K. The lower thermal conductivity than the basal plane values of graphite and the temperature dependence are attributed to scattering of phonons in the bi-layer graphene by a residual polymeric layer that was clearly observed by transmission electron microscopy. [Preview Abstract] |
Tuesday, March 22, 2011 3:18PM - 3:30PM |
L30.00005: Phonon thermal conductivities of multi-layered graphene Lucas Lindsay, David Broido Using an exact numerical solution of the phonon Boltzmann equation, we show that the intrinsic lattice thermal conductivities, $\kappa $, of $N$-layer graphene ($N$=1-5) are dominated by contributions from out-of-plane, flexural (ZA) phonon modes contrary to previous theories based on the relaxation time approximation, which assumed this contribution to be negligible [1, 2]. We find a reduction of $\kappa $ with increasing $N$ due to interlayer coupling, which: 1) lifts the degeneracy of the flexural acoustic mode frequencies, 2) makes the ZA phonon branch become linear near the zone-center, and 3) breaks a selection rule for anharmonic phonon-phonon scattering in two-dimensional systems. \\[4pt] [1] P. G. Klemens and D. F. Pedraza, Carbon vol. 32, pp. 735-741 (1994). \\[0pt] [2] B. D. Kong, S. Paul, M. B. Nardelli and K. W. Kim, Phys. Rev. B 80, 033406 (2009). [Preview Abstract] |
Tuesday, March 22, 2011 3:30PM - 3:42PM |
L30.00006: Thermal properties of novel 2D hybrid graphene-BN nanostructures Nikhil Medhekar, Jun Song Graphene, a 2D honeycomb carbon crystal of one-atom thickness, has been widely recognized as a very promising material for next generation optoelectronic and NEMS applications. Recent developments have shown that it is possible to obtain hybrid 2D structures by combining sp2-graphene lattice with sp2-lattice of non-carbon materials such as hexagonal Boron Nitrides. The atomically thin sheets containing both hexagonal-Boron Nitride and graphene can result in new materials with properties complementary to their individual properties and further enrich the potential applications. Here, using molecular dynamics simulations, we elucidate the characteristics of thermal transport in 2D hybride h-BN and graphene materials. We find the thermal conductivity of the hybrid material is a strong function of the relative domain widths, interface type (e.g., zigzag and armchair) as well as the interface quality. Our results provide crucial insights on the role of the interfaces and defects in phonon scattering in the hybrid material and can potentially provide means to tailor its thermal properties. [Preview Abstract] |
Tuesday, March 22, 2011 3:42PM - 3:54PM |
L30.00007: Thermal Expansion in Graphene and Graphane: Role of Anharmonic and Harmonic Effects Arunima Singh, Richard G. Hennig As the practical application of graphene nears realization, knowledge of effects of temperature on mechanical properties of graphene becomes important. In this study we use empirical potentials and density-functional perturbation theory (DFPT) to determine the thermal expansion of free-standing graphene, graphene on substrates, and its hydrogenated derivative graphane. Comparisons of MD simulations with calculations using the quasi- harmonic approximation using an empirical potential show that anharmonic effects are negligible at temperatures below 2200K. In contrast to the DFPT calculations using the quasi-harmonic approximation, MD results show that free-standing graphene has a positive thermal expansion coefficient above 600K. For graphene on a substrate our DFPT results agree with those of Jiang et. al [1] and show that the substrate suppresses the negative thermal expansion coefficient with increasing strength of the substrate- graphene interaction. We also investigate the thermal expansion of the thermodynamically stable conformers of graphane using DFPT.\\[4pt] [1] J. W. Jiang, J. S. Wang, B. Li, Phys. Rev. B 80, 205429 (2009). [Preview Abstract] |
Tuesday, March 22, 2011 3:54PM - 4:06PM |
L30.00008: Observation of coherent G-mode phonon oscillations in graphene films J.-H. Kim, M.H. Jung, B.H. Hong, E.H. Haroz, J. Kono, K.J. Yee We have observed coherent G-mode lattice vibrations in three stacked-mono layer and multi-layer graphene films by using ultrashort pulses from a Ti:Sapphire laser. The degenerated $E_{2g} $modes were excited through the impulsive stimulated Raman scattering process, and detected through induced reflectivity modulations. The G-mode frequency from the stacked-mono layer graphene is shifted toward higher energy compared with that of the multi-layer graphene. A dephasing time of abound 0.6 ps for the stacked-mono layer graphene was found to be shorter than that of semiconducting single-walled carbon nanotubes (1.48 ps) and slower than that of metallic single-walled carbon nanotubes, due to stronger electron-phonon interactions, where the phonon energy can be dissipated by exciting electrons between a linear bands of graphene. Through the strong polarization dependence of coherent G-mode lattice vibrations, we confirmed that the $E_{2g}^{(2)} $ symmetry is dominant. [Preview Abstract] |
Tuesday, March 22, 2011 4:06PM - 4:18PM |
L30.00009: Theory of coherent phonons in graphene G.D. Sanders, C.J. Stanton, J.-H. Kim, K.-J. Yee, M.H. Jung, B.H. Hong, E.H. Haroz, J. Kono We develop a theory for the generation and detection of coherent phonons in graphene. Coherent phonons are generated via the deformation potential electron-phonon interaction with photogenerated carriers. In our theory the electronic states are treated in a third nearest neighbor extended tight binding formalism which gives a good description of the states over the entire graphene Brillouin zone while the phonon states are treated in a valence force field model. The equations of motion for the coherent phonon amplitudes are obtained in a density matrix formalism and we find that the coherent phonon amplitudes satisfy driven oscillator equations for each value of the phonon wavevector. Comparison is made with recent experimental measurements. [Preview Abstract] |
Tuesday, March 22, 2011 4:18PM - 4:30PM |
L30.00010: Coherent phonon spectroscopy of the shearing mode in bilayer and few-layer graphene Davide Boschetto, Lenadro Malard, Chun Hung Lui, Kin Fai Mak, Hugen Yan, Zhiqiang Li, Tony F. Heinz The interlayer shearing vibration in graphite, a low-energy optical phonon, is known to consist of adjacent atomic planes moving laterally in opposite directions with respect to one another. We have applied coherent phonon spectroscopy, based on a sensitive femtosecond pump-probe measurement, to investigate the corresponding mode in few-layer graphene samples down to bilayer thickness. Here we report on the evolution of the frequency and lifetime of this mode with thickness. To model the expected behavior, we have analyzed a model of identical nearest-neighbour couplings. We find that this model predicts most of the observed reduction in frequency with decreasing layer thickness. We consider to the remaining deviations between the model and our experimental data in terms of a slight increase in the interlayer spacing, leading to a reduced restoring force, with decreasing graphene layer thickness. This decrease in lattice spacing with thickness is expected for layered materials governed by van der Waals forces. We also show experimentally that the shearing mode frequency is robust against external perturbations, such as different substrates and the presence of adlayers. [Preview Abstract] |
Tuesday, March 22, 2011 4:30PM - 4:42PM |
L30.00011: Tuning the Kohn Anomaly in the Phonon Dispersion of Graphene by Interaction with the Substrate and by Doping Ludger Wirtz, Adrien Allard, Claudio Attaccalite, Michele Lazzeri, Francesco Mauri, Angel Rubio The phonon dispersion of graphene displays two strong Kohn Anomalies (kinks) in the highest optical branch (HOB) at the high-symmetry points G and K. The slope of the HOB around K is a measure of the electron-phonon coupling (EPC) and determines the dispersion of the Raman D and 2D lines as a function of the laser energy. We show that the EPC can be strongly modified both due to interaction with a metallic substrate and due to doping. For graphene grown on a Ni(111) surface, a total suppression of the Kohn anomaly occurs: the HOB around K becomes completely flat. This is due to the strong hybridization of the graphene p-bands with the Nickel d-bands which lifts the linear crossing of the p-bands at K. From experimental phonon dispersions one can therefore draw conclusions about the interaction strength between graphene and its different substrates. Furthermore, we present a new way to tune the EPC in graphene through electron/hole doping. We show that for the highest optical branch at K, the EPC is strongly dependent on the doping level. This dependency influences the dispersion of the Raman D and 2D lines and makes it possible to measure the charge state of graphene via resonant Raman spectroscopy. [Preview Abstract] |
Tuesday, March 22, 2011 4:42PM - 4:54PM |
L30.00012: Thermoelectric effect in high mobility single layer epitaxial graphene Xiaosong Wu, Yike Hu, Ming Ruan, Nerasoa K. Madiomanana, Claire Berger, Walt A. de Heer The thermoelectric response of high mobility single layer epitaxial graphene on silicon carbide substrates as a function of temperature and magnetic field have been investigated. For the thermopower, a strong deviation from the Mott relation, i.e. a quardratic correction to the linear temperature dependence, has been observed even when the carrier density is high. In the quantum Hall regime, the amplitude of the TEP peaks is lower than a quantum value predicted by theories, despite the high mobility of the sample. A systematic reduction of the amplitude with decreasing temperature suggests that the suppression of the TEP is intrinsic to Dirac electrons in graphene. [Preview Abstract] |
Tuesday, March 22, 2011 4:54PM - 5:06PM |
L30.00013: Thermoelectric transport in graphene with tunable mobility Xinfei Liu, Deqi Wang, Jing Shi Thermoelectric transport properties of single layer graphene have recently been studied both experimentally and theoretically. The unique band structure of graphene leads to unusual thermoelectric properties which are very sensitive to the carrier mobility. However, all previous experiments were carried out in graphene devices with different mobility values and comparisons were drawn among different devices. Recently, we have shown that by controlling the charge state of the ligand-bound nanoparticles on graphene it is possible to tune the mobility of the same graphene device over a wide range, e.g. 5000-19000cm$^2$/Vs. In this work, we adopted this method and successfully tuned the mobility of graphene while systematically studied the Seebeck and Nernst effects in a magnetic field up to 14 Tesla for each fixed mobility value. Our results show that at zero magnetic field, the width of the transition region near the Dirac point decreases sharply and the diverging behavior in the Seebeck coefficient becomes more pronounced as the mobility is tuned from low to high. At high magnetic fields, the Seebeck coefficient in the high mobility state clearly reveals additional features that are related to the splitting of the zeroth Landau level near the Dirac point. Moreover, we demonstrate that the Nernst peak height at the Dirac point depends linearly on the carrier mobility in graphene. [Preview Abstract] |
Tuesday, March 22, 2011 5:06PM - 5:18PM |
L30.00014: Thermoelectric Properties of Graphene Ribbons Enrique Munoz Several theoretical and experimental studies have been recently concerned with electric and thermal transport in graphene layers and ribbons, where propagation of electrons [1] and phonons [2] seems to be dominated by a ballistic mechanism. Of particular interest in this context is the identification and characterization of thermoelectric effects [3], which represent a promising alternative for energy recovery in technological applications. In the present work, the effect of the electron- phonon interaction over a predominantly ballistic transport mechanism in graphene ribbons is studied in the context of thermoelectricity. Theoretical estimations of the thermopower S, and the corresponding figure of merit ZT, are presented for this system as a function of temperature. \\[4pt] [1] K. Saito, J. Nakamura, and A. Natori, ``Ballistic thermal conductance of a graphene sheet,'' Phys. Rev. B 76, 115409 (2007).\\[0pt] [2] E. Munoz, J. Lu, and B. I. Yakobson, ``Ballistic thermal conductance of graphene ribbons,'' Nano Lett. 10, 1652 (2010).\\[0pt] [3] Y. Ouyang and J. Guo, ``A theoretical study on thermoelectric properties of graphene nanoribbons,'' Appl. Phys. Lett. 94, 263107 (2009). [Preview Abstract] |
Tuesday, March 22, 2011 5:18PM - 5:30PM |
L30.00015: Theromelectricity in Graphene: Effects of a gap and magnetic fields Subroto Mukerjee, Aavishkar Patel We calculate the thermopower of monolayer graphene in various circumstances. First we show that experiments on the thermopower of graphene can be understood quantitatively with a very simple model of screening in the semiclassical limit. We can calculate the energy dependent scattering time for this model exactly. We then consider acoustic phonon scattering which might be the operative scattering mechanism in free standing films, and predict that the thermopower will be linear in any induced gap in the system. Further, the thermopower peaks at the same value of chemical potential (tunable by gate voltage) independent of the gap. Finally, we show that in the semiclassical approximation, the thermopower in a magnetic field saturates at high field to a value which can be calculated exactly and is independent of the details of the scattering. This effect might be observable experimentally. [Preview Abstract] |
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