Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session T26: Focus Session: Graphene XII: Synthesis and Growth |
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Sponsoring Units: DMP Chair: Taisuke Ohta, Sandia National Laboratories Room: 328 |
Wednesday, March 18, 2009 2:30PM - 2:42PM |
T26.00001: Evidence for graphene growth by C cluster attachment Elena Loginova, Norman C. Bartelt, Peter J. Feibelman, Kevin F. McCarty Until now the detailed mechanisms of graphene growth have not been experimentally determined, owing to limitations of the available experimental techniques. We study the epitaxial growth of graphene on Ru(0001) measuring simultaneously the growth rate of individual graphene islands and the local, absolute concentration of vapor-deposited, mobile carbon adatoms. We have learned what controls the nucleation and growth rate of graphene, and what species transport carbon over the metal surface. We find that the growth rate is limited by C-atom attachment, not by C-atom diffusion, and that the absolute value of the supersaturation required for appreciable growth rates is comparable to that required to nucleate new islands. Thus, a large barrier must exist for monomers to attach to the graphene step edge. The growth rate as a function of supersaturation is highly nonlinear. Such behavior can be explained if carbon clusters must form as precursors to carbon attachment. As experiment and theory reveal, this could arise from strong bonding of individual monomers to the metal substrate. We will discuss a model that explains and provides insight into the molecular processes by which graphene grows. This research is supported by the Office of Basic Energy Sciences, Division of Materials Sciences, U. S. D. O. E. under Contract No. DE-AC04-94AL85000. [Preview Abstract] |
Wednesday, March 18, 2009 2:42PM - 2:54PM |
T26.00002: Large Scale Graphene Synthesized on Metal and Transferred to Insulators: Material and Electronic Properties Yong P. Chen, H. Cao, D. Pandey, I. Childres, D. Zemlyanov, V. Drachev, R. Reifenberger, Q. Yu, S. Siripongert, S. Pei, J. Lian, H. Li We report a systematic study of the material and electronic properties of large scale graphene films grown on metal then transferred to insulator substrates. Few-layer graphene films as large as several cm's in size are grown by cooling-induced surface segregation on Ni under ambient pressure (Q. Yu \textit{et al}., APL \textbf{93}, 113103 (2008)). The Ni is subsequently etched by acid and graphene film transferred on thin SiO$_{2 }$ on doped Si wafer. TEM and STM images show the expected graphitic lattice structure locally with atomic resolution. XPS and Raman spectroscopies further confirm the high quality of transferred graphene films. At larger scale, various SPM and optical imaging reveal non-uniform thickness and considerable height fluctuation, with the film consisting with domains ($\sim $1 $\mu $m in size) separated by elevated ridges. Using the doped Si as backgate, we observe moderate field effect in such transferred graphene films. Magnetotransport at variable temperatures show negative magnetoresistance at low magnetic field and characteristic features of weak localization in graphene, allowing us to extract information on carrier scattering in such large scale graphene. [Preview Abstract] |
Wednesday, March 18, 2009 2:54PM - 3:06PM |
T26.00003: Investigation of the early stages of graphene formation on 6H-SiC J. R. Skuza, C. Clavero, K. Yang, B. Wincheski, R. A. Lukaszew The predicted and/or observed unique properties of graphene have sparked tremendous research efforts to develop graphene-based ultra-high speed electronic and optical devices. The most promising technique to fabricate epitaxial graphene to date is via high temperature sublimation of atomic layers of Si from monocrystalline SiC substrates [1,2]. However, this approach leads to rough surfaces and little work has been done to investigate graphene nucleation during the early stages of growth. We have used atomic force microscopy, scanning electron microscopy, and Raman spectroscopy to investigate the early stages of graphene nucleation and surface evolution when annealing semi-insulating and n-type doped 6H-SiC substrates under low vacuum ($\sim $ 10$^{-3}$ Torr) and ultra-high vacuum (10$^{-9}$ Torr) regimes. Scaling laws applied to the surface evolution in these two cases will be compared. [1] I. Forbeaux \textit{et al.}, Phys. Rev. B \textbf{58}, 16396 (1998). [2] C. Berger \textit{et al.}, J. Phys. Chem. B \textbf{108}, 19912 (2004). [Preview Abstract] |
Wednesday, March 18, 2009 3:06PM - 3:18PM |
T26.00004: High-yield production of graphene sheets by chemical exfoliation of graphite Xiaohong An, Swastik Kar, Morris Washington, Saroj Nayak Graphene, a single atomic layer of graphite, has attracted vast interest recently owing to its perfect two-dimensional crystallographic nature, which have resulted in intensive investigations of fundamental physics and promising applications. Up to now, several techniques have been used to produce small areas of graphene, such as mechanical methods, exfoliation, epitaxial growth method and reduced graphene from graphene oxide. However, chemical approaches for high-yield production of graphene sheets is still absent. Here, we report that graphene dispersion produced by chemical exfoliation of graphite in solvent of 1-pyrenecarboxilic acid in water. We confirm the presence of monolayer graphene sheets by Scanning transmission electron microscopy and Raman spectroscopy. Large area of graphene sheets on SiO$_{2}$/Si substrate can be obtained by evaporating the graphene dispersion in oven and rinsing with methanol. We demonstrate the high-yield production of graphene sheets by optical microscopy and scanning electron microscopy. Electrical and other applications of graphene developed this way are currently being investigated. This new graphene processing of chemical exfoliation of graphite could lead to applications in future scalable graphene nano-electronics devices. [Preview Abstract] |
Wednesday, March 18, 2009 3:18PM - 3:30PM |
T26.00005: Scalable chemical vapor deposition of single- and few-layer graphene Lewis Gomez De Arco, Yi Zhang, Akshay Kumar, Chongwu Zhou We report the implementation of a simple and scalable method to prepare single and few-layer graphene films by chemical vapour deposition. Micro Raman spectroscopy analysis of the synthesized films revealed the presence of single and few-layer graphene domains throughout the substrate. Synthesized graphene films were recovered on Si/SiO$_{2}$ substrates where back-gated FETs were fabricated. Four-probe measurements revealed sheet resistance of $\sim $68 k$\Omega $/sq for the recovered films. I$_{DS}$-V$_{DS}$ and transfer characteristics indicate a weak p-type behavior in the films and weak modulation of the drain current by the gate bias. [Preview Abstract] |
Wednesday, March 18, 2009 3:30PM - 3:42PM |
T26.00006: Temperature-dependence of Epitaxial Graphene Formation on SiC(0001) Luxmi Luxmi, Nishtha Srivastava, Patrick Fisher, Randall Feenstra, Jakub Kedzierski, Yugang Sun, Gong Gu The formation of epitaxial graphene on SiC(0001) (the \textit{Si-face}) is studied using atomic force microscopy, Auger electron spectroscopy, low energy electron diffraction/microscopy, Raman spectroscopy, and electrical measurements. Starting from hydrogen-etched surfaces, graphene formation by vacuum annealing is observed to begin at about 1150\r{ }C, with the overall step-terrace arrangement of the H-etched surface being preserved but with significant roughness (pit formation) on the terraces. At temperatures near 1350\r{ }C, the surface morphology changes into relatively large flat terraces covered with several layers of graphene and containing a few large pits, with the terraces separated by step bunches. On the terraces the graphene thickness varies by typically $\pm $1 monolayer. At higher temperatures the graphene film is observed to buckle and break up, presumably due to thermal mismatch with the SiC. Field-effect mobilities as high as 4200 cm$^{2}$/Vs for few-layer graphene films are found. [Preview Abstract] |
Wednesday, March 18, 2009 3:42PM - 3:54PM |
T26.00007: Epitaxial Graphene Formation on SiC(000$\bar {1})$ Nishtha Srivastava, Luxmi Luxmi, Patrick Fisher, Randall Feenstra, Jakub Kedzierski, Yugang Sun, Gong Gu The formation of epitaxial graphene on SiC(000$\bar {1})$ (the \textit{C-face}) is studied using atomic force microscopy, spatially resolved Auger electron spectroscopy, low energy electron diffraction, Raman spectroscopy, and electrical measurements. Starting from hydrogen-etched surfaces, graphene formation by vacuum annealing is observed over the temperature range 1200-1400\r{ }C. Unlike the situation for the Si-face, it is found for the C-face that the initial graphene formation is three-dimensional. Micron-size islands with height of several nm are formed, with the graphene being \underline {thinner} on these islands than between the islands. At higher formation temperatures the graphene layer becomes relatively flat, and has typical thickness of $>$10 monolayers. Electron diffraction indicates rotational disorder, with $\pm $15\r{ }-oriented spots observed in addition to the known $\pm $2.2\r{ }-spots.$^{2}$ Field-effect mobilities as high as 4400 cm$^{2}$/Vs for multi-layer graphene films are found, with relatively good homogeneity over the wafer. $^{2}$J. Hass et al., Phys. Rev. Lett. 100, 125504 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 3:54PM - 4:06PM |
T26.00008: Band-gap tuning through progressive oxidation of graphene Tanesh Bansal, Aditya Mohite, Bruce Alphenaar, Jacek Jasinski, Mahendra Sunkara Graphene has a high electron mobility at room temperature, making it attractive for device applications. Because graphene is a zero-gap semiconductor, it is challenging to modulate its conductance using a field effect gate. Oxidation of graphene opens up a band gap, transforming oxidized graphene into an insulator. However, theory also suggests that there are a range of stable oxidation states corresponding to different oxygen coverage on the surface. Here, we demonstrate that it is possible to tune the band-gap of oxidized graphene by varying the surface oxygen concentration. Commercially obtained KISH graphite was converted to graphite oxide by treatment with a mixture of sulfuric acid and nitric acid. Oxidized graphene sheets were dispersed on quartz substrates following sonication and centrifugation of the graphite oxide. Using photocurrent spectroscopy the energy gap of individual oxidized graphene flakes were observed to increase from 0.62 eV to 0.69 eV with increasing oxidation time. Band-gap measurements were correlated with the surface oxygen concentration using XPS, UPS and FTIR. \textit{ONR N00014-06-1-0228} [Preview Abstract] |
Wednesday, March 18, 2009 4:06PM - 4:18PM |
T26.00009: Morphology and Electrical Characterization of Reduced Epitaxial Graphene Oxide Yike Hu, Xiaosong Wu, Michael Sprinkle, Nerasoa Madiomanana, Ming Ruan, Claire Berger, Walter de Heer We present results for on-chip oxidation of epitaxial graphene and sequential reduction of the insulating graphene oxide layers. In our previous work , we have used the Hummer's method to oxidize epitaxial graphene and used electron beam exposure and heat treatment to reduce the epitaxial graphene oxide (EGO) band gap by changing the degree of oxidation. Here we further explore various oxidation and reduction methods on epitaxial graphene. EGO is characterized by atomic force microscopy, low-energy electron diffraction, ellipsometry, and Raman Spectrometry. Mobility measurements of patterned structures are presented where epitaxial graphene layers pads are seamlessly connected to EGO ribbons. [Preview Abstract] |
Wednesday, March 18, 2009 4:18PM - 4:30PM |
T26.00010: Preparation of macroscopic graphene oxide membranes Zhengtang Luo, Ye Lu, Luke Somers, A.T. Charlie Johnson Graphene oxide membranes up to 2000 square micrometers in size can be synthesized with 90 {\%} yield in bulk quantities through a microwave assisted chemical method. Membranes are readily visualized on oxidized silicon substrate, which enables efficient fabrication of electronic devices and sensors. Field effect transistors made of the membrane show ambipolar behavior, and their conductivity is significantly higher than previously reported values. [Preview Abstract] |
Wednesday, March 18, 2009 4:30PM - 4:42PM |
T26.00011: Optical and diamagnetic anisotropy of graphene oxide A.L. Exarhos, P.M. Vora, Z. Lou, A.T. Johnson, J.M. Kikkawa We have recently shown that graphene oxide (GO) emits a broad photoluminescence (PL) band in both solid and aqueous preparations. The origin of this PL is not yet well understood, but for absorptive and emissive optical processes originating in the two dimensional GO plane, one expects an in-plane polarization. Studies of optical anisotropy can therefore help to clarify the origin of the PL. Here we use a method of optical nanomagnetometry (Torrens, et al, JACS 129, p. 252 (2007)) to extract these quantities, also determining the magnetic anisotropy. We find that when aqueous preparations of GO are placed in a magnetic field, diamagnetically induced alignment leads to marked linear polarization anisotropy of absorbance and photoluminescence. By taking six optical measurements at each magnetic field, we are able to extract the intrinsic polarization anisotropies of optical absorption and emission of GO flakes and to quantify the orbital diamagnetic anisotropy. We discuss how these quantities give insight into electronic delocalization in these systems. [Preview Abstract] |
Wednesday, March 18, 2009 4:42PM - 4:54PM |
T26.00012: Graphite oxide as a nanoscale dielectric Brian Standley, Anthony Mendez, Emma Schmidgall, Marc Bockrath Graphite oxide's ease of deposition and graphene-like properties when chemically reduced make it a promising electronic material. To complement this effort, we are studying graphite oxide as a potential dielectric for nanoscale devices. While unreduced graphite oxide is known to have a sheet resistance in the G$\Omega$ range, its out-of-plane conductivity has yet to be measured. We have fabricated ultrathin capacitors from graphite oxide sheets, and will present our efforts to measure its leakage current and breakdown electric field, thus providing an assessment of its potential as a gate insulator. [Preview Abstract] |
Wednesday, March 18, 2009 4:54PM - 5:06PM |
T26.00013: Diffusion and Self-alignment of Atomic Oxygens on the Graphene Surfaces: First Principles Calculations Takazumi Kawai, Yoshiyuki Miyamoto Graphene is attracting much attention for the application of nano-devices due to its interesting electronic properties, and its robustness. For the device applications, it is very important to know the behaviors of atmospheric molecules such as adsorption, diffusion, and desorption on the graphene surface since the reaction with such chemicals cause the significant change in the electronic properties at Fermi level and even break the $sp^2$ network. Here, the oxygen is one of the most important impurities that we want to know and control the behavior. In this paper, we performed density function calculations for the diffusion of atomic oxygens on a graphene sheet in a periodic boundary condition. The results for a single atomic oxygen in our calculations are consistent with the previous works with cluster models. However, the favorable adsorption site for the next oxygen atom and diffusion barriers are completely different from them. The atomic oxygens prefer to align along armchair direction but not zigzag one. We will further discuss the stability and diffusion of the next oxygen atom on the other side of the graphene. [Preview Abstract] |
Wednesday, March 18, 2009 5:06PM - 5:18PM |
T26.00014: Large area graphene growth on 6H-SiC(0001) L.I. Johansson, C. Virojanadara, M. Syv\"aj\"arvi, R. Yakimova, A.A. Zakharov, T. Balasubramanian Large area graphene growth on commercial Si-face on-axis 6H-SiC(0001) is demonstrated in this work. Samples were produced in a prototype of an inductively heated furnace. The growth was carried out in strongly isothermal conditions at a temperature of 2000 C and at an ambient argon pressure of 1 atm. The quality and thickness of the graphene layers grown, using this \textit{ex situ} method, were investigated using PES, ARPES), LEED as well as LEEM, PEEM micro-LEED and micro-PES at specifically defined small areas. Our results show that single layer graphene is formed over quite large areas on the sample but that two different domains can exist on some parts. A comparison with an \textit{in situ} graphene sample, prepared by resistive heating to 1275 C, was made. The results then obtained were similar to earlier findings [1-2] and showed that the size of the graphene flakes were very small compared to those obtained on the samples prepared with our \textit{ex situ }method. \\[3pt] [1]. T. Ohta, F. El Gabaly, A. Bostwick, J.L. McChesney, K.V. Emtsev, A.K. Schmid, Th. Seyller, K. Horn, E. Rotenberg, New. J. Phys. \textbf{10} 023034 (2008).\\[0pt] [2]. J.B. Hannon and R. M. Tromp, Phys. Rev. B\textbf{ 77} 241404 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 5:18PM - 5:30PM |
T26.00015: Epitaxial graphene: Structure, growth and molecular interactions Andrew Wee, Wei Chen, Siew Wai Poon, Han Huang, Shi Chen, Dongchen Qi, Eng Soon Tok, Kian Ping Loh The discovery of graphene has opened up a new paradigm in nanoelectronics that could offer better performance than conventional semiconductor devices. We used \textit{in situ} scanning tunnelling microscopy (STM), synchrotron synchrotron radiation techniques and density functional theory (DFT) calculations to investigate the structure of the various reconstructions of 6H-SiC(0001) prior to its thermal decomposition to form epitaxial graphene (EG). Using Co-decoration technique coupled with STM, the evolution of EG was found to preferentially begin at SiC step edges and occurs with the loss of Si and breakdown of the C-rich ($\surd $6$\times \surd $6)$R$30\r{ } template, which provides the C source for graphene growth. The C-rich phase that forms at the interface acts as a buffer layer for graphene from the underlying bulk SiC. We show that the transition from monolayer to trilayer EG adopts a bottom-up growth mechanism. With increasing annealing temperature, the fluorescence yield of Si $K$-edge NEXAFS indicates an increase in disorder of Si atoms in the SiC substrate beneath the surface due to out-diffusion of Si atoms to the surface forming increased Si vacancies. We also show that EG thermally grown on 6H-SiC(0001) can be p-type doped via a novel surface transfer doping scheme by modifying the surface with the electron acceptor, F4-TCNQ. [Preview Abstract] |
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