Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session D17: Focus Session: Graphene Devices: Thermal and Mechanical Properties |
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Sponsoring Units: DMP Chair: Kirill Bolotin, Vannderbilt University Room: 102AB |
Monday, March 2, 2015 2:30PM - 2:42PM |
D17.00001: Graphene Electrostatic Microphone Qin Zhou, Seita Onishi, A. Zettl We demonstrate a wideband electrostatic graphene microphone displaying flat frequency response over the entire human audible region as well as into the ultrasonic regime. Using the microphone, low-level ultrasonic bat calls are successfully recorded. The microphone can be paired with a similarly constructed electrostatic graphene loudspeaker to create a wideband ultrasonic radio. [Preview Abstract] |
Monday, March 2, 2015 2:42PM - 2:54PM |
D17.00002: Nonlinear dynamics in tunable graphene nanoelectromechanical systems Fen Guan, Piranavan Kumaravadivel, Dmitri Averin, Xu Du We report the fabrication and characterization of graphene nanoelectromechanical resonators (GNEMR) on flexible substrates. The intrinsic stain in graphene is tuned by bending the substrate, during which a transition from hardening to softening resonance behavior and a minimum resonance frequency are observed. To explain these observations, a resonator model taking into account the intrinsic strain and electrostatic force is developed. Including higher-order nonlinear terms, a minimum frequency is obtained analytically from the model and matches with experimental data. Results from numerical simulation demonstrate also the transition in the nonlinear behavior. Additionally, the model-based fittings determine the intrinsic strain and mass of graphene samples accurately. Our devices allow thorough exploration of the nonlinear dynamics in GNEMR and may help further study of the intrinsic electrical properties of the materials under strain. [Preview Abstract] |
Monday, March 2, 2015 2:54PM - 3:06PM |
D17.00003: High frequency nanomechanical resonators in ultraclean suspended graphene pn junctions Minkyung Jung, Peter Rickhaus, Simon Zihmann, Peter Makk, Alexander Eichler, Markus Weiss, Christian Sch\"onenberger Here, we demonstrate high frequency nanomechanical resonators in ultraclean suspended graphene pn junctions. The suspended graphene resonators are fabricated on two bottom gates (left and right) covered with lift-off resist (LOR) by using a mechanical transfer technique. After current annealing, the device exhibits a clear charge neutrality point around zero gate voltage. Depending on the left and right bottom gate voltages, the device shows four different conductance regimes: pp, nn, np and pn corresponding to two different carrier types in the two sides of the sample. At pn and np regimes, the clear Fabry-Perot interference pattern is observed, indicating ballistic transport behavior over 1$\mu$m-long channel. Then, the mechanical resonance is measured in the same device with a frequency modulation (FM) mixing technique at 4.2 K in the vacuum chamber. The resonance frequency is about 405 MHz. By fitting resonance frequency, we deduce both the mass density and the built-in tension in the graphene sheet. In a similar device structure with different strain environment, we observe a resonance frequency as high as 1.17 GHz for the fundamental mode. [Preview Abstract] |
Monday, March 2, 2015 3:06PM - 3:18PM |
D17.00004: Angle dependent phonon spectra and thermal properties of misoriented bilayer graphene Mahesh Neupane, Pankaj Ramnani, Supeng Ge, Ashok Mulchandani, Roger Lake The Raman spectra of misoriented bilayer graphene (MBG) show angle dependent signatures of the misorientation angle ($\theta )$ in the low frequency breathing modes. We investigate these low frequency modes using molecular dynamics including temperature dependent phonon anharmonicity. The calculated vibrational and thermal properties are compared against our experimental data. Our theoretical investigations reveal that the layer breathing mode (LBM) frequencies at 100 $\pm$ 10 cm$^{-1}$ for angles 6$^{\circ} \le \theta \le $ 30$^{\circ}$ are consistent with the observed frequencies of ZO modes in the Raman spectrum. For the smaller $\theta $ (or larger L), the reduced BZ leads to the zone-folding of the phonon spectrum at the zone center, and leads to broadened optical phonons width in the vibrational density of states. Finally, increasing $\theta $ in the MBG leads to a reduction in the lattice specific heat capacity. [Preview Abstract] |
Monday, March 2, 2015 3:18PM - 3:30PM |
D17.00005: Transport in Strained Graphene at Low Temperatures Juan Aguilera-Servin, Adrian Nosek, Cheng Pan, Marc Bockrath Strain in graphene layers produces synthetic gauge fields that may be used to modify the properties of its electron system [1,2]. We study single layers of graphene transferred over Ti/Au electrical contacts on oxidized Si wafers with etched triangular holes in the oxide. The layers are strained by applying pressure electrostatically using a gate voltage and hydrostatically using an external inert gas. We investigate electronic transport in this suspended variable-strain graphene system at low temperatures. We will discuss our latest results. [1] Guinea, F., Katsnelson, M. I., Geim, A. K. Energy gaps and a zero-field quantum Hall effect in graphene by strain engineering. Nat. Phys. 6, 30-33 (2009). [2] Levy, N., et al. Strain-induced pseudo--magnetic fields greater than 300 tesla in graphene nanobubbles. Science, 329 544-547 (2010). [Preview Abstract] |
Monday, March 2, 2015 3:30PM - 3:42PM |
D17.00006: Control Over the Adhesion and Strain on Graphene Using Arrays of Mesoscale Pyramids Stephen Gill, Shuze Zhu, J. Henry Hinnefeld, William Swanson, Teng Li, Nadya Mason Applying non-uniform shear strain to graphene can lead to new electronic states. For example, strain having triangular symmetry has been shown theoretically and experimentally to generate a nearly uniform pseudo-magnetic field [1,2]. However, the lack of methods to control non-uniform strain in graphene devices has limited the ability to explore transport phenomena tuned by strain. In this talk, we demonstrate that the adhesion and strain of graphene can be controlled by using arrays of mesoscale pyramids. By manipulating the arrangement of pyramids and the aspect ratio of the array, graphene's adhesion to the array ranges from conformal to suspended between pyramids. Strain in graphene adhered to pyramids is revealed by Raman spectroscopy, and the amount of strain experienced is shown to depend on the adhesion to the pyramids. Supporting calculations demonstrate the pseudo-magnetic field profile for graphene adhered to pyramids for different strains. These results indicate a potential route for exploring strain-controlled transport phenomena in graphene. \newline 1. F. Guinea, M. I. Katsnelson, and A. K. Geim, Nat. Phys. \textbf{6, }30 (2009). \newline 2. L. Levy et al., Science \textbf{329}, 544 (2010). [Preview Abstract] |
Monday, March 2, 2015 3:42PM - 3:54PM |
D17.00007: Electron-phonon interactions in bilayer graphene Heli Vora, Xu Du We report measurements on electron thermal conductance in bilayer graphene due to cooling via phonons. The measurements were carried out using bilayer graphene-superconductor tunnel junctions, where the superconducting contacts effectively confine the hot electrons inside the graphene channel, allowing access to phonon cooling at low temperatures. We show results on the temperature and doping dependence of the cooling power in bilayer graphene. Contrary to what was observed in monolayer graphene, the phonon cooling power decreases with increasing carrier density in bilayer graphene. The temperature dependence of the phonon cooling power can be described by a power law with a power factor $\sim 5$, again, qualitatively different from the $\mathrm{T^3}$ temperature dependence observed in the disordered monolayer graphene. These new results may shed light on the dominating mechanisms for hot electron cooling bilayer graphene. [Preview Abstract] |
Monday, March 2, 2015 3:54PM - 4:06PM |
D17.00008: Dissipation and feedback cooling of graphene and MoS$_{2}$ nanomechanical resonators Ronald Van Leeuwen, Gary Steele, Warner Venstra, Herre van der Zant The interesting mechanical and electronic properties of 2-dimensional materials make them candidates for nano-electromechanical systems. Remarkably, the mechanical resonance linewidth of such suspended structures is found to be invariably low at room temperature. Time-domain measurements were used as a tool to differentiate between dissipation and non-dissipative frequency fluctuations, thus providing more insight in the origin of the low linewidth. We perform time-domain measurements on MoS$_{2}$ resonators with thickness down to a single layer, and compare the relaxation times obtained from ringdown measurements to the resonance linewidth obtained from Brownian motion and driven frequency responses. We conclude that dephasing plays a negligible role at room temperature, and that the spectral line-width is bounded by dissipation. To modify the damping we introduce an optical feedback technique. We demonstrate continuous tuning of the oscillator linewidth, by modifying the linewidth over 2 orders of magnitude. Feedback also enables cooling of the fundamental mode of graphene and MoS$_{2}$ drum resonators down to an effective temperature of 100 K. [Preview Abstract] |
Monday, March 2, 2015 4:06PM - 4:18PM |
D17.00009: Electron transport in graphene with uniaxial local strain Hikari Tomori, Rineka Hiraide, Hirokazu Tanaka, Yu Itou, Kenta Katakura, Youiti Ootuka, Akinobu Kanda Strain engineering is a promising method for controlling electron transport in graphene; Spatial variation of pseudo-vector potential and pseudo-scalar potential induced by lattice strain modulate transport property of graphene. We have succeed in fabricating a graphene FET with uniaxial local strain, and observed clear deformation in gate voltage dependence of conductivity ($\sigma-V_g$ curve). From a comparison with numerical calculation, we conclude that strain-induced scalar potential is responsible for the deformation of the $\sigma-V_g$ curve. [Preview Abstract] |
Monday, March 2, 2015 4:18PM - 4:30PM |
D17.00010: Probing Mechanics of Rippled Two-Dimensional Materials Ryan Nicholl, Hiram Conely, Nickolay Lavrik, Ivan Vlassiouk, Kirill Bolotin Two-dimensional materials such as graphene tend to ripple in the out of plane direction. These ripples arise both due to thermal fluctuations and uneven stress forces at the boundary. In this work, we study the effect of the rippling on the effective mechanical properties of graphene: Young's modulus and bending rigidity. To accomplish this, we developed a non-contact technique that allows probing mechanical properties of graphene at temperatures between 4K and 400K. We use a high voltage electrostatic force to pull on graphene and high-resolution optical interferometric profilometery to measure its mechanical response. We find that the effective Young's modulus of graphene is significantly softened and the bending rigidity is increased due to rippling. [Preview Abstract] |
(Author Not Attending)
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D17.00011: Thermal Conductance of Epitaxial and Transferred CVD-Grown Graphene Huang Bin, Koh Yee Kan The knowledge of how heat is carried across graphene-copper interface is crucial for the development of graphene devices with hybrid graphene-copper interconnects. Time-domain thermoreflectance (TDTR) is used to measure the interfacial thermal conductance of epitaxial grown single layer graphene (SLG) on copper foil and after it is transferred to a deposited copper substrate. It is found out that the thermal conductance of un-annealed transferred SLG on deposited copper is around 20 MW/m$^{\mathrm{2}}$K, much lower than that of SLG grown on copper foil which is approximately 30 MW/m$^{\mathrm{2}}$K. Annealing in forming gas/vacuum causes the thermal conductance of transferred SLG to increase to 31 MW/m$^{\mathrm{2}}$K. X-ray spectroscopy (XPS) and Atomic force microscopy (AFM) are then employed to investigate the various factors, (i.e., copper oxide, polycarbonate (PC) residue, roughness and conformity) that may cause a difference in thermal conductance after the transfer. XPS measurement results show an absence of PC residue, even before annealing. The results also reveal that annealing in forming gas reduces the copper oxide thickness by about 2.5nm, and such a small reduction in oxide thickness is not sufficient to cause a drastic increase of approximately 10MW/m$^{\mathrm{2}}$K in thermal conductance. AFM results show that before annealing, the SLG has elongated ridges-like morphology. This morphology is different from that of copper which has circular-like features. After annealing, the SLG morphology becomes very similar to that of copper -- both exhibiting circular-like features. This shows that the SLG can conform better to the copper surface after annealing. [Preview Abstract] |
Monday, March 2, 2015 4:42PM - 4:54PM |
D17.00012: Graphene Mechanical Resonators under Large Strain Seita Onishi, Qin Zhou, Alex Zettl Graphene has shown promise as a high frequency mechanical resonator due to its high Young's modulus and light mass [1]. With large strains, theoretical predictions anticipate even changes to graphene's band structure [2]. We developed an integrated platform to apply large strains on suspended graphene with a MEMS based actuator. We will show preliminary results on the optical detection of the change in resonance frequency as the graphene mechanical resonator is strained. \\[4pt] [1] J. Scott. Bunch, et al. Science 315, 490 (2007)\\[0pt] [2] Vitor M. Pereira, A.H. Castro Neto and N. M. R. Peres. Phys. Rev. B 80, 045401 (2009) [Preview Abstract] |
Monday, March 2, 2015 4:54PM - 5:06PM |
D17.00013: Thermal and Thermoelectric Transport across Graphene/BN and Graphene/BN/Graphene Heterostructures Nirakar Poudel We report thermal and thermoelectric transport measurements across graphene/hexagonal boron nitride (h-BN)$^{1}$ and graphene/hexagonal boron nitride (h-BN)/graphene$^{2}$ heterostructure devices. Using an AC lock-in technique, we are able to separate the thermoelectric contribution to the $I-V$ characteristics of these important device structures. The temperature gradient is measured optically using Raman spectroscopy, which enables us to explore thermoelectric transport produced at material interfaces, across length scales of just 1-2 nm. A temperature drop of 60 K can be achieved across this junction at high electrical powers (14 mW). Based on the temperature difference and the applied power data, we determine the thermal interface conductance of this junction to be 7.4 x10$^{6}$ W/m$^{2}\cdot $K, which is below the 10$^{7}$-10$^{8}$ W/m$^{2}\cdot $K values previously reported for graphene/SiO$_{2}$ interface. Based on the observed thermoelectric voltage ($\Delta V)$ and temperature gradient ($\Delta T)$, a Seebeck coefficient of 99.3 $\mu$ V/K is ascertained for the heterostructure device. [Preview Abstract] |
Monday, March 2, 2015 5:06PM - 5:18PM |
D17.00014: Development of a quadrupole trap apparatus for UHV measurements of levitated graphene Joyce Coppock, Pavel Nagornykh, Ian McAdams, Bruce Kane Completely decoupling graphene from any substrate opens up new possibilities for measurement of its electrical and mechanical properties as well as the exploration of novel methods of crystal growth and fabrication of 2D materials. We levitate a charged micron-scale few-layer graphene-like flake in an electrical AC quadrupole trap and induce rotation using a circularly polarized laser beam [1]. We aim to achieve an ultra-high vacuum (UHV) environment (\textless 10$^{\mathrm{-9}}$ Torr), which will allow us to conduct experiments on graphene lattice stretching (via rotation at frequencies greater than 100 MHz), to perform thermodynamic measurements on the particles as they are heated by the laser, and to avoid chemical contamination of the particles. Measurements of particles in UHV require two technologies: (1) the reliable capture of particles and their introduction into a UHV environment, and (2) a center-of-mass cooling method to prevent particle loss. This talk will focus on the first challenge. We will discuss improvements to the sample preparation and to the trapping procedure, describe a method of transferring particles from the initial capture trap to a second trap in a UHV chamber, and present a model of the trap potential. Finally, we will discuss preliminary work on the deposition of particles onto a conducting substrate after they have been cooled and oriented parallel to the substrate. [1] Kane, B.E. \textit{Phys. Rev. B.}, \textbf{82}, 115441 (2010). [Preview Abstract] |
Monday, March 2, 2015 5:18PM - 5:30PM |
D17.00015: Effect of stray electric fields on cooling of center of mass motion of levitated graphite flakes Pavel Nagornykh, Joyce Coppock, Bruce Kane Levitation of charged multilayer graphene flakes in a quadrupole ion trap provides a unique way to study graphene in isolated conditions. Cooling of a flake in such a setup is necessary for high vacuum measurements of the flake and is achieved by using a parametric feedback scheme [1]. We present data showing the strong dependence of the cooling of the flake's center of mass motion on the stray electric fields. We achieve this by using auxiliary electrodes to shift the position of the trap center in space. Once the point of minimum interaction between the stray fields and the particle is found (leading to cooling of the flake motion to temperatures below 20K at pressure of $10^{-7}$ Torr), we can estimate charge and mass of the flake by observing quantized discharge of the particle and measure transient dynamics of the center of mass motion by turning the cooling off and on. As an additional benefit, the behavior of the flake away from the optimum trap position can be used to quantify stray fields' effect on the particle motion by measuring its spinning orientation and frequency dependence on offset from the optimum position. \\[0pt][1] J. Gieseler et al., Phys. Rev. Lett. \textbf{109}, 103603 (2012) [Preview Abstract] |
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