Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session W21: Focus Session: Graphene: Strain |
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Sponsoring Units: DMP Chair: Michael Fuhrer, University of Maryland Room: Portland Ballroom 251 |
Thursday, March 18, 2010 11:15AM - 11:27AM |
W21.00001: Effects of Strain on Electronic Properties of Graphene Young-Woo Son, Seon-Myeong Choi, Seung-Hoon Jhi We present first-principles calculations of electronic properties of graphene under uniaxial and isotropic strains, respectively [1]. The semi-metallic nature is shown to persist up to a very large uniaxial strain of 30\% except a very narrow strain range where a tiny energy gap opens. As the uniaxial strain increases along a certain direction, the Fermi velocity parallel to it decreases quickly and vanishes eventually, whereas the Fermi velocity perpendicular to it increases by as much as 25\%. Thus, the low energy properties with small uniaxial strains can be described by the generalized Weyl's equation while massless and massive electrons coexist with large ones. The work function is also predicted to increase substantially as both the uniaxial and isotropic strain increases. Hence, the homogeneous strain in graphene can be regarded as the effective electronic scalar potential. [1] S.-M. Choi, S.-H. Jhi and Y.-W. Son, arXiv.org:0908.0977. [Preview Abstract] |
Thursday, March 18, 2010 11:27AM - 11:39AM |
W21.00002: Probing strain-induced changes in the electronic structure of graphene by Raman spectroscopy Mingyuan Huang, Hugen Yan, Tony Heinz, James Hone Two-phonon Raman scattering in graphitic materials provides a distinctive approach to probing the material's electronic structure through the spectroscopy of phonons. This sensitivity arises from the role of resonant electronic transitions in two-phonon Raman scattering process. The 2D mode, for instance, is known to display a strong variation with thickness in few-layer graphene. This behavior is a consequence the change of the electronic structure of the material with thickness. Here we report studies of Raman scattering of the 2D mode of single-layer graphene under uniaxial stress and the implications of these measurements for the electronic structure of anisotropically strained graphene. Under anisotropic strain, two types of modification of the low-energy electronic structure of graphene are present: a deformation of the Dirac cone and its displacement away from the K point. By analyzing the 2D Raman spectrum for excitation at different photon energies, we have identified both effects. The direct influence of strain on the 2D phonons will also be discussed. [Preview Abstract] |
Thursday, March 18, 2010 11:39AM - 11:51AM |
W21.00003: Effect of uniaxial strain on graphene Duhee Yoon, Hyerim Moon, Seoungwoo Woo, Young-Woo Son, Hyeonsik Cheong Strain plays an important role in crystalline materials since it can change the electronic or mechanical properties of materials. Recently, several experimental and theoretical studies have shown the effect of strain on single layer graphene. Experimentally, Gr\"uneisen parameters and crystallographic orientation of graphene were studied by polarized micro-Raman spectroscopy. Band gap opening and electron beam collimation of graphene were theoretically predicted. Those proposed the possibility to control the properties of graphene by strain. In this work, we investigate the effect of uniaxial strain on single layer graphene by using micro-Raman spectroscopy. We deposited graphene samples on acrylic substrates by the mechanical cleaving method. Strain was applied by bending the flexible substrates and Raman spectra were obtained as a function of the uniaxial strain. The peak position and the shape of G and 2D bands vary significantly as the strain increases. We will discuss deep into the variation of the G and 2D bands. The implications of the strain effect in terms of the band structure will be discussed. [Preview Abstract] |
Thursday, March 18, 2010 11:51AM - 12:27PM |
W21.00004: Supercurrent and Interference in Carbon Nanotube Josephson Transistors Invited Speaker: Carbon nanotubes have emerged as a new model system for quantum dots as well as ballistic one-dimensional conductors. For instance, dissipation is ubiquitous in quantum systems, and its interplay with fluctuations is critical to maintaining quantum coherence. We experimentally investigate the dissipation dynamics in single-walled carbon nanotubes coupled to superconductors, and show that the junction undergoes a periodic modulation between underdamped and overdamped regimes. Finally, I will discuss the observation of Fano-like features in transport conductance spectroscopy on carbon nanotubes, which arise from quantum interference of electron waves in two different channels. [Preview Abstract] |
Thursday, March 18, 2010 12:27PM - 12:39PM |
W21.00005: How wrinkling singularities impact graphene's electronic transport Vitor Pereira, Antonio Castro Neto Wrinkles seem ubiquitous in graphene. They appear most noticeably in exfoliated samples (on account of the shear and strains involved in the cleavage process and subsequent transfer to the silica substrates) and also on suspended samples (in which case they can be controlled by tuning the amount of shear). It is also known that wrinkling in 2D membranes (of which graphene is the ultimate microscopic model) is intimately associated with the presence of certain conical singularities in the membrane profile. These arise from the necessity of the sheared/strained system to relieve as much in-plane stain as possible, while still conforming to flat boundary conditions. Such regions of singular strain and curvature will directly affect the motion of the Dirac electrons in graphene. We will show how that impacts the transport and local electronic properties of system. In particular, we will show that a quasi-linear conductivity is obtained, which might allow the utilization of wrinkling as a means to tune the electronic response of graphene, constituting another example of the concept of strain-engineered transport in graphene first introduced in [1]. \\[4pt] [1] V. M. Pereira and A. H. Castro Neto, Phys. Rev. Lett. 103, 046801 (2009). [Preview Abstract] |
Thursday, March 18, 2010 12:39PM - 12:51PM |
W21.00006: Singular elastic strains and magnetoresistance of suspended graphene G. Leon, E. Prada, P. San-Jose, M. M. Fogler, F. Guinea Graphene membranes suspended off electric contacts or other rigid supports are prone to elastic strain, which is concentrated at the edges and corners of the samples. Such a strain leads to an algebraically varying effective magnetic field that can reach a few Tesla in sub-micron wide flakes. In the quantum Hall effect regime the interplay of the effective and the physical magnetic fields causes backscattering of the chiral edge channels, which can result in overshoots or even complete destruction of the quantized resistance plateaus. [Preview Abstract] |
Thursday, March 18, 2010 12:51PM - 1:03PM |
W21.00007: Probing strain-induced changes of phonon dispersion in graphene by Raman spectroscopy Hugen Yan, Mingyuan Huang, James Hone, Tony Heinz The Raman spectra of graphene include prominent features associated with two-phonon modes. This response is important because it permits us to probe the nature of phonons away from the zone center by optical spectroscopy. In graphene, these two-phonon processes are enhanced by electronic resonances and are associated with well-defined phonon momentum. In this paper, we examine the effect of the application of uniaxial stress on the doubly resonant intravalley scattering (2D') mode. The frequency and line-shape of the 2D' mode show a strong dependence on the angle between the incident light polarization and the stress axis. The response reveals the induced anisotropy of the phonon dispersion relation around the zone center, as well as the inherent light absorption anisotropy in k-space. As a result, the polarization dependence of the Raman response in strained graphene can be used to determine the principal axes of the strain. The phonon softens under tensile strain, exhibiting a softening rate similar to that of G mode. [Preview Abstract] |
Thursday, March 18, 2010 1:03PM - 1:15PM |
W21.00008: Theoretical STM maps of curved graphene G.S. Diniz, S.E. Ulloa We calculate the effect of curvature on the electronic states in monolayer graphene, considering a local ripple along different directions on the plane. The curvature introduces hybridization between $\sigma$ and $\pi$ bands and affects the electronic structure and associated wave functions, even opening a gap of the order of few meV at the Dirac point. Our calculation uses a non-orthogonal four-orbital tight-binding representation up to nearest-neighbors, which fully describes the electronic states of the structure. We focus this study on the analysis of theoretical STM maps for different ripple directions and radius of curvature. We discuss the appearance of a well defined pattern in the STM map near the Dirac point, which is strongly modified when the graphene is rippled along different directions, and discuss this effect in terms of a geometric Berry phase. Although it is difficult experimentally to produce and control a well defined direction for the graphene ripple, recent experiments suggest that this structure may be possible in suspended samples with stressors along the edges [1].\\[4pt] [1] W. Bao {\em et al.}, Nature Nanotech. {\bf 4}, 562 (2009). [Preview Abstract] |
Thursday, March 18, 2010 1:15PM - 1:27PM |
W21.00009: Focused Ion Beam Etching of Suspended Graphene Devices Britton Baugher, Pablo Jarillo-Herrero In this talk we will present our achievements etching nanostructured devices into suspended graphene using a focused ion beam (FIB). Nanoscale devices were recently etched into graphene on a substrate using an FIB, but unfortunately, severe contamination from the beam and the substrate all but completely masked graphene's unique electrical properties in those devices. Suspended devices, however, may be able to escape this fate. Their separation from the substrate keeps the largest source of impurities at a distance and makes annealing far more effective. Annealed, suspended devices have been shown to survive a myriad of fabrication procedures while still achieving the highest mobilities found in graphene. Here we elucidate the effects of etchings in graphene by two different ion beams: a standard gallium focused ion beam and the recently developed helium ion beam. We will be presenting extensive electrical measurements on the devices we created, in addition to characterizations of the graphene after etching down to the atomic scale, as seen by TEM measurements. [Preview Abstract] |
Thursday, March 18, 2010 1:27PM - 1:39PM |
W21.00010: Morphology and Transport in Suspended graphene Membrane Wenzhong Bao, Feng Miao, Zhen Chen, Hang Zhang, Wanyoung Jiang, Chris Dames, Chun Ning Lau Graphene is the nature's thinnest elastic member. Using in-situ SEM imaging, we examine the response of graphene's morphology to temperature and strain, which has important implications towards future applications in mechanical or thermal engineering. Finally, electrical transport data from ultra clean suspended graphene devices will also be discussed. [Preview Abstract] |
Thursday, March 18, 2010 1:39PM - 1:51PM |
W21.00011: Interplay between electronic excitations and flexural phonons in graphene Jose Gonzalez, Pablo San-Jose We investigate the mutual influence of electronic excitations and flexural phonons in graphene by analyzing the many-body corrections that they induce on each other. Thus we show that the interaction with the electron-hole excitations leads to a drastic softening of the acoustic branch of out-of-plane phonons, which become unstable for wavelengths longer than a critical value. In terms of the effective potential of flexural phonons, this is interpreted as the onset of spontaneous symmetry breaking, with the development of a nonvanishing expectation value for the field of vertical displacement in the graphene sheet. Conversely, we show that the flexural phonons induce an effective density-density interaction that is logarithmically divergent in the infrared, potentially balancing the repulsive Coulomb interaction as this is renormalized away by the logarithmic divergence of the Fermi velocity in the low-energy regime. [Preview Abstract] |
Thursday, March 18, 2010 1:51PM - 2:03PM |
W21.00012: Corrugation and energetics of graphene on SiO$_{2}$ William Cullen, Mahito Yamamoto, Kristen Burson, Jianhao Chen, Ellen Williams, Michael Fuhrer Measurements of the topographic structure of graphene on SiO$_{2}$ are seen to vary considerably between different published reports, particularly with respect to the amplitude of corrugation observed at the few-nm length scale. Intrinsic rippling of the graphene has been reported, in addition to evidence of tip interaction effects. In spite of recent high-resolution STM measurements of exfoliated graphene by several groups, the structure of graphene on SiO$_{2}$ remains controversial, necessitating a better understanding of the interaction between graphene and the substrate, and clarification of assumptions of equilibrium in assessing the structure. Our measurements incorporate a unique combination of UHV high-resolution non-contact AFM and STM to characterize both the graphene and the underlying SiO$_{2}$ substrate. We measure monolayer and few-layer graphene with complementary STM and NC-AFM, and assess the observed structures in terms of the energy balance between elastic energy of the graphene membrane and adhesion energy of the substrate. [Preview Abstract] |
Thursday, March 18, 2010 2:03PM - 2:15PM |
W21.00013: Graphene wettability engineering Young Jun Shin, Yingying Wang, Han Huang, Gopinadhan Kalon, Andrew Thye Shen Wee, Zexiang Shen, Charanjit Singh Bhatia, Hyunsoo Yang Graphene has attracted much attention due to its superior characteristics. In order to fabricate useful devices, understanding the surface property of graphene is very important since the contact deposition is critical for the device performance and functionality. However, there have been few studies investigating the surface property of graphene. We investigated the wettability of graphene on SiC by contact angle measurements. Mono layer epitaxial graphene showed a hydrophobic characteristic similar to HOPG and no correlation was found between different layers of graphene and wettability. Upon oxygen plasma treatment, defects are introduced into graphene and the level of damage was investigated by Raman spectroscopy. There exists a correlation between the level of defects and the contact angle. As more defects are induced, surface energy of graphene is increased leading to hydrophilic nature. Oxygen plasma treatment with an optimized power and duration has been proposed to control the adhesion property for contact fabrication. [Preview Abstract] |
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