Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session Q31: Graphene: Raman Spectroscopy and Phonons |
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Sponsoring Units: DMP DCMP Chair: Tony Heinz, Columbia University Room: 607 |
Wednesday, March 5, 2014 2:30PM - 2:42PM |
Q31.00001: How strain affect graphene's optical contrast on SiO2/Si gratings Xuanye Wang, Khwanchai Tantiwanichapan, Roberto Paiella, Anna Swan Optical contrast interference provides a fast and easy method for identifying graphene layer number the widely used silicon dioxide/silicon substrates. Uniaxial strain deforms the band structure and consequently it has been suggested that optical absorption in visible range varies with strain.\footnote{Many-Electron Effects on Optical Absorption Spectra of Strained Graphene, Liang et al, arXiv:1110.0212 [cond-mat.mtrl-sci]} Here we explore how uniaxial strain affects the optical contrast.~ Mechanically cleaved single layer graphene is deposited onto sinusoidal shaped SiO2 optical diffraction grating. Graphene strains as it conforms to the corrugated surface. We observe a dramatic optical contrast change for the graphene on the grating under white light and different color channels. To quantitatively analyze this optical response, we map graphene's strain distribution by analyzing redshift of G and 2D peaks positions in Raman line scan measurement and~ compare with~ AFM measurements~ to compare optical and Raman results with how well~ graphene conforms to the corrugated surface.~ We explore different surface treatments that vary the friction between the graphene and the corrugated oxide in order to control the strain and the conformation. [Preview Abstract] |
Wednesday, March 5, 2014 2:42PM - 2:54PM |
Q31.00002: Th Q1 e thermal stability of graphene in air investigated by Raman spectroscopy Haiyan Nan, Zhenhua Ni, Jun Wang, Zainab Zafar, Zhixiang Shi, Yingying Wang The thermal stability in air of graphene synthesized by either chemical vapor deposition or mechanical cleavage is studied. It is found that single layer graphene prepared by both methods starts to show defects at $\sim$500 $^{\circ}$C, indicated by the appearance of a disorder-induced Raman D peak. The defects are initially sp3 type and become vacancy like at higher temperature. On the other hand, bilayer graphene shows better thermal stability, and the D peak appears at $\sim$600 $^{\circ}$C. These results are quite different from those annealing in vacuum and controlled atmosphere. Raman images show that the defects in chemical vapor deposition graphene are not homogeneous, whereas those in mechanical cleavage graphene are uniformly distributed across the whole sample. The factors that affect the thermal stability of graphene are discussed. Our results could be important for guiding the future electronics process and chemical decoration of graphene. [Preview Abstract] |
Wednesday, March 5, 2014 2:54PM - 3:06PM |
Q31.00003: Relationship between Transport Properties and Raman Spectra in Electron Beam Irradiated Graphene Hikari Tomori, Rineka Hiraide, Hirokazu Tanaka, Youiti Ootuka, Akinobu Kanda Raman spectroscopy is commonly used to characterize disorder in graphene. Increase of structural defects leads to raise in intensity of the Raman D band for low defect densities. Defects also cause degradation of graphene transport properties. Thus, a certain relationship is expected between the Raman spectra and transport properties in graphene. Here, we investigate Raman spectra and transport properties of graphene as a function of the amount of electron beam irradiation. The electron beam irradiation leads to generation of the Raman D band and decrease of carrier mobility. We find that the intensity ratio of Raman D to G peaks is inversely proportional to square of the carrier mean free path. The proportionality coefficient is proportional to the carrier density. This kind of relationship has not been reported so far. Our result may pave the way for evaluating graphene transport properties with Raman spectroscopy. [Preview Abstract] |
Wednesday, March 5, 2014 3:06PM - 3:18PM |
Q31.00004: Theory of topological Raman band in graphene Ken-ichi Sasaki, Kouta Tateno, Hideki Gotoh In the Raman spectrum measured at the edge of graphene, the $D$ band appears as a prominent peak. The edge $D$ band is useful in the characterization of the armchair edge because it is a result of an intervalley scattering of the electron by the armchair edge. The intervalley scattering is not unique to the armchair edge, but common to topological defects, such as pentagons and heptagons. Since topological defects are found throughout the honeycomb network of carbon, the characterization is an important issue. Based on our recent work on the activation mechanism of the edge $D$ band [1], we found a novel type of the $D$ band induced by a topological defect, which we call a topological $D$ band [2]. A photo-excited carrier with a non-zero winding number is the key to activating a topological $D$ band. A topological $D$ band can be distinguished from the conventional edge $D$ band by its peak position and non-dispersive nature, because the selection rules for electron-phonon matrix elements are altered in an essential way by the presence of the topological defect. [1] Sasaki et al., Crystals 3(1) 120 (2013). [2] Sasaki et al., Phys. Rev. Lett. 111, 116801 (2013). [Preview Abstract] |
Wednesday, March 5, 2014 3:18PM - 3:30PM |
Q31.00005: Electronic structure modification of graphene on d-band metal surfaces and its Raman signature Sinisa Coh, Qin Zhou, Alex Zettl, Marvin L. Cohen, Steven G. Louie We find strong modifications of the graphene electronic structure when it is placed on a platinum surface. Additionally, these modifications strongly depend on the relative orientation of the graphene and platinum lattices. We expect that the same will occur whenever graphene is brought in contact with a surface of a material that has d-orbital close to the Fermi level. We demonstrate experimentally and theoretically that these modifications leave a distinct signature in the Raman spectrum of graphene. Out of two prominent graphene Raman peaks, one is unaffected (the G peak) while the other (the 2D peak) is severely affected, in proportion with the modification of the graphene electronic structure. [Preview Abstract] |
Wednesday, March 5, 2014 3:30PM - 3:42PM |
Q31.00006: Influence of dopants on carrier dynamics and low-frequency phonon modes in bilayer graphene Ramakrishna Podila, Benoy Anand, Ajay Sood, Reji Philip, Apparao Rao Controlling the electronic structure of graphene with substitutional doping is central to many fascinating applications. For example, graphene's unique band structure coupled with its nonlinear optical properties (NLO) allows it to serve as a saturable absorber in an all-carbon optical diode . Although dopant effects are often correlated to the dopant concentration in the graphene lattice, the role of dopant's local bonding environment has not been explored in sufficient detail (R. Podila et al., Appl. Phys. Lett., 101, 123108 (2012)). Here, we present the effect of substitutional nitrogen (N) doping on the saturable absorption characteristics and carrier dynamics of chemical vapor deposited bi-layer graphene. We find that the saturation depth and carrier relaxation times are greatly influenced by the dopant density, and the N bonding configurations. The latter is inferred from the low-frequency phonon modes which are derived from the Fourier transform pump-probe spectra of N-doped bilayer graphene. Understanding the role of dopants on the NLO properties of graphene offers the possibility of tailoring graphene for opto-electronic applications \textit{via} defect engineering. [Preview Abstract] |
Wednesday, March 5, 2014 3:42PM - 3:54PM |
Q31.00007: Raman Spectroscopy as an Accurate Probe of Defects in Graphene Joaquin Rodriguez-Nieva, Eduardo Barros, Riichiro Saito, Mildred Dresselhaus Raman Spectroscopy has proved to be an invaluable non-destructive technique that allows us to obtain intrinsic information about graphene. Furthermore, defect-induced Raman features, namely the $D$ and $D^{\prime}$ bands, have previously been used to assess the purity of graphitic samples. However, quantitative studies of the signatures of the different types of defects on the Raman spectra is still an open problem. Experimental results [1] already suggest that the Raman intensity ratio $I_{D}/I_{D^{\prime}}$ may allow us to identify the nature of the defects. We study from a theoretical point of view the power and limitations of Raman spectroscopy in the study of defects in graphene. We derive an analytic model that describes the Double Resonance Raman process of disordered graphene samples, and which explicitly shows the role played by both the defect-dependent parameters as well as the experimentally-controlled variables. We compare our model with previous Raman experiments, and use it to guide new ways in which defects in graphene can be accurately probed with Raman spectroscopy. [1] A. Eckmann, \textit {et al}., Nano Lett 12,3925 (2012) [Preview Abstract] |
Wednesday, March 5, 2014 3:54PM - 4:06PM |
Q31.00008: Investigation of 2D materials by wide-field Raman imaging Jae-Ung Lee, Hyeonsik Cheong Raman spectroscopy is a very useful tool to investigate 2D materials such as graphene, hBN, and MoS2. Due to the uniqueness of the Raman spectrum of each material, we can use various Raman features to distinguish the number of layers, and other external effects (strain, doping, and temperature) on the sample. To study the spatial variations of the Raman features, confocal Raman imaging technique have been used conventionally. But due to limited beam size ($\sim$ 1 $\mu$m) of confocal Raman systems, investigating a large area of the samples would consume a lot of time. In contrast to conventional confocal Raman systems, the wide-field Raman system has advantages for fast and large area investigation. A shaped laser beam with the size of few-hundred microns is shone onto a sample, and only a specific wavelength is transmitted through a tunable band pass filter and directly imaged onto an EMCCD. We exfoliated three most common 2D materials (graphene, hBN, and MoS2) on the same SiO2/Si substrates. The optical contrast images of these materials are difficult to distinguish. But in wide-field Raman system, we can characterize the samples within a few seconds. This demonstrates that the wide-field Raman system provides a useful platform to characterize 2D materials. [Preview Abstract] |
Wednesday, March 5, 2014 4:06PM - 4:18PM |
Q31.00009: The imprint of transition metal d-orbitals on a graphene Dirac cone: A Raman investigation Qin Zhou, Sinisa Coh, Marvin Cohen, Steven Louie, A. Zettl We investigate the influence of SiO2, Au, Ag, Cu, and Pt substrates on the Raman spectrum of graphene. Experiments reveal particularly strong modifications to the intensity, position, width, and shape of the Raman signal of graphene on platinum, compared to that of suspended graphene. The modifications also strongly depend on the relative orientation of the graphene and platinum lattices. Surprisingly, the interaction between graphene and platinum is often considered as weak Van der Waals interaction. We theoretically investigates the observations from electromagnetic shielding, charge transferring and from hybridization of electronic states in graphene and platinum. [Preview Abstract] |
Wednesday, March 5, 2014 4:18PM - 4:30PM |
Q31.00010: ABSTRACT WITHDRAWN |
Wednesday, March 5, 2014 4:30PM - 4:42PM |
Q31.00011: Tunable Infrared Phonon Anomalies in Trilayer Graphene Zhiqiang Li, Chun Hung Lui, Emmanuele Cappelluti, Tony F. Heinz Trilayer graphene in both ABA (Bernal) and ABC (rhombohedral) stacking sequences is shown to exhibit intense infrared absorption from in-plane optical phonons. The phonon feature, lying at 1580 cm$^{\mathrm{-1}}$, changes strongly with electrostatic gating. For ABC-stacked graphene trilayers, we observed a large enhancement in phonon absorption amplitude, as well as softening of the phonon mode, as the Fermi level is tuned away from charge neutrality. A similar, but substantially weaker, effect is seen in samples with the more common ABA stacking order. The strong infrared response of the optical phonons and the pronounced variation with electrostatic gating and stacking order reflect the interactions of the phonons and electronic excitations in the two systems. The key experimental findings can be reproduced within a simplified charged-phonon model that considers the influence of charging through Pauli blocking of the electronic transitions. [Preview Abstract] |
Wednesday, March 5, 2014 4:42PM - 4:54PM |
Q31.00012: Unusual ultra-low frequency fluctuations in freestanding graphene Steven Barber, Peng Xu, Mehdi Neek-Amal, Matthew Ackerman, James Schoelz, Paul Thibado, Ali Sadeghi, Francois Peeters Intrinsic ripples in freestanding graphene have been difficult to study with common experimental methods. In notable breakthroughs, ripple geometry was recently imaged using scanning electron microscopy as well as scanning tunneling microscopy (STM), but these measurements are thus far limited to static configurations. Thermally-activated flexural phonon modes could generate dynamic changes in curvature which would be of great interest to observe. Here, we present how to track the vertical movement of a one-square-angstrom region of suspended graphene using STM. This allows a direct measurement of the out-of-plane trajectory at one point in space over long periods of time. Based on these data, we present a model from elasticity theory to explain the very-low frequency oscillations that are observed. Unexpectedly, we sometimes detect a sudden colossal jump, which we interpret as due to mirror buckling. This innovative technique provides a much needed atomic-scale probe for the time-dependent behaviors of intrinsic ripples in freestanding graphene, and it represents a fundamental advance in the use of STM. [Preview Abstract] |
Wednesday, March 5, 2014 4:54PM - 5:06PM |
Q31.00013: Strong enhancement of electron-phonon coupling in doped-graphene Choongyu Hwang, Duck Young Kim, D.A. Siegel, Kevin T. Chan, J. Noffsinger, A.V. Fedorov, Marvin L. Cohen, J.B. Neaton, B. Johansson, A. Lanzara Fundamental physical properties of a material are affected by many-body interactions. Among them, the interactions of electrons to phonon modes not only govern transport properties of the material, but also play an important role in realizing novel phenomena, when such an electron-phonon coupling is strongly enhanced. By using angle-resolved photoemission spectroscopy, we study strong enhancement of electron-phonon coupling of doped graphene. Our finding provides a viable route to realize strongly correlated electron phases in graphene. [Preview Abstract] |
Wednesday, March 5, 2014 5:06PM - 5:18PM |
Q31.00014: Scattering of flexural acoustic phonons at graphene grain boundaries Edit Helgee, Andreas Isacsson We have studied the scattering of long-wavelength flexural phonons against grain boundaries in graphene using molecular dynamics. The grain boundaries consist of arrays of dislocations, where the size of each dislocation is of the order of magnitude of the lattice constant. The small size of the dislocations suggests that long-wavelength phonons should be unaffected by the boundary. However, dislocations cause out-of-plane buckling of the graphene sheet. The width of the buckles can be on the order of nanometers, large enough to interact with long-wavelength vibrations. Of the two grain boundaries considered here, one shows no buckling while the other displays an out-of-plane buckling 0.5 nm high and approximately 1.5 nm wide. For the flat grain boundary, the phonon transmission approaches unity at long wavelengths. The buckled grain boundary, on the other hand, yields transmission coefficients between 0.4 and 0.6 for wavelengths exceeding 1 nm. Also, the flexural vibrations couple to longitudinal modes at the buckled grain boundary. This indicates that grain boundaries scatter long-wavelength flexural phonons, provided that the boundary causes out of plane buckling of the graphene sheet. [Preview Abstract] |
Wednesday, March 5, 2014 5:18PM - 5:30PM |
Q31.00015: Interplay between electron-phonon and Coulomb interactions in the honeycomb lattice Laura Classen, Michael M. Scherer, Carsten Honerkamp We study the impact of electron-phonon interactions on the many-body instabilities of electrons in the honeycomb lattice and their interplay with local and non-local short-ranged Coulomb interactions at charge neutrality. Therefore, we consider the in-plane optical phonon branches giving the most important contribution to the electron-phonon coupling and calculate the effective phonon-mediated electron-electron interaction by integrating out the phonon modes. The ordering tendencies are studied by means of a momentum-resolved functional renormalization group approach allowing for an unbiased investigation of the appearing instabilities. In the case of an exclusive and supercritical phonon-mediated interaction, we find a nematic ground state being favored over the s-wave superconducting state conjectured from a simple mean-field treatment. We further discuss the influence of phonon-mediated interactions on the instabilities induced by onsite, nearest neighbor and next-nearest neighbor density-density interactions. We find an extension of the parameter regime of the spin density wave order going along with an increase of the critical scales where ordering occurs. [Preview Abstract] |
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