Manish Chhowalla Rutgers University
John A. Rogers University of Illinois, Urbana-Champaign
Carey M. Tanner SRI International
Pagona Papakonstantinou University of Ulster
Andrea C. Ferrari University of Cambridge
L1: Chemically Derived Graphenes I
Monday AM, November 30, 2009
Room 310 (Hynes)
9:30 AM - **L1.1
R. Ruoff 1 Show Abstract
1 Mechanical Engineering, University of Texas, Austin, Texas, United States
Our top-down approaches [1,2] inspired physicists to study individual layers of graphite obtained by micromechanical exfoliation, and one of our current approaches has been to convert graphite to graphite oxide (GO), generate aqueous colloidal suspensions containing individual layers of GO (we call them ‘graphene oxide’), and to use these ‘graphene oxide sheets’ in a variety of ways. For example, we have embedded individual and reduced graphene oxide sheets in polymers such as polystyrene and evaluated their dispersion, morphology, and the electrical percolation and thus conductivity of the resulting composites. In parallel paths, we have: (i) undertaken studies of individual graphene oxide and reduced graphene oxide sheets, to elucidate their chemical, optical, and electrical properties, (ii) embedded graphene oxide sheets in glass by a sol-gel route and made electrically conductive and transparent glass coatings, and (iii) produced 'graphene oxide paper', a material with intriguing mechanical properties (iv) produced reduced graphene oxide powder with moderately high surface area and used this to study electrochemical double layer capacitance (v) made carbon-13 labeled graphite and thus carbon-13 labeled graphite oxide, and studied its detailed chemical structure with SS NMR. Finally, (vi) I will talk about recent work in our group on growing and characterizing graphene and few layer graphene films on metal substrates, and their transfer to other substrates for device fabrication and characterization. This survey talk about graphene and its chemical derivatives will present an overview of these various results. Support of our work by the NSF, ONR/NRL, NASA, and DARPA is appreciated. 1. Lu XK, Yu MF, Huang H, and Ruoff RS, Tailoring graphite with the goal of achieving single sheets, Nanotechnology, 10, 269-272 (1999).2. Lu XK, Huang H, Nemchuk N, and Ruoff RS, Patterning of highly oriented pyrolytic graphite by oxygen plasma etching, Applied Physics Letters, 75, 193-195 (1999).See also papers on http://bucky-central.me.utexas.edu/publications.htm such as #139,146, 150,155, 160, 164, 166, 168, 169, 174, 179, 180, 181, 182, etc.
10:00 AM - L1.2
Probing the De-oxidation and Electronic Structure of Graphene Oxide by in situ High Resolution X-ray Photoelectron Spectroscopy.
Surbhi Sharma 1 , Jeremy Hamilton 1 , Pagona Papakonstantinou 1 Show Abstract
1 School of Engineering, Nanotechnology and Integrated BioEngineering Centre, NIBEC, University of Ulster, Newtownabbey United Kingdom
Current interest in graphene oxide (GO) sheets has been sparkled by the recent discovery of graphene. Graphene oxide is the most promising precursor for bulk production of graphene. Graphene oxide sheets are heavily oxidised layers, which contain a large number of oxygen groups within the graphene structure. These oxygen functional groups can be partially removed by deoxidation either thermally or by chemical reduction, yielding a partially reduced structure, which is of interest as a filler to polymer composites; as transparent conducting films for low cost photovoltaics and energy –storage materials, liquid crystal devices as well as a potential component for biosensing and nanoelectronics devices. Although graphene derived from graphene oxide holds significant technological promise the evolution of its fundamental electronic structure at different reduction steps remains largely unexplored. There is also controversy in the literature which groups are dominant in graphene oxide or after its conversion to graphene. To address these points, we have studied the progressive loss of oxygen functional groups and the electronic structure of graphene oxide at various stages of ultra high vacuum thermal annealing process by in situ high resolution X ray photoelectron spectroscopy, XPS (conducted at the NCESS facility in Daresbury, UK). In particular temperature dependent XPS and valence band studies revealed that the majority of oxygen species in thermally deoxidized GO consist of hydroxyl C-OH groups; followed by contributions from carbonyl and carboxylic C=O, COOH groups, whereas the epoxy/ether C-O-C groups are the least thermally stable. Annealing up to 1000C is able to remove oxygen content from 32 at% to 3at% (C:O ratio 11.3). The valence band spectrum of the de-oxygenated film via heating to 1000 C resembles significantly that of graphite. After thermal treatment to 1000 C the intensity ratio of D to G Raman peaks slightly decreased and both peaks exhibited a reduction in FWHM suggesting improvement in the graphitization. The thermal treatment process can increase the sp2 bonding fraction up to ~70% and functionalize the surface to meet application demands. Our study shows that the graphene oxygen functional groups can be decreased in a controllable manner using high temperature annealing. Our findings provide useful information, critical to graphene device engineering and fabrication.
10:15 AM - L1.3
Evolution of Electrical, Chemical and Structural Properties of Graphene Oxide Upon Annealing.
Cecilia Mattevi 1 , Goki Eda 1 , Stefano Agnoli 2 , Steve Miller 1 , Andre Mkhoyan 3 , Ozgurd Celik 4 , Daniel Mastrogiovanni 4 , Gaetano Granozzi 2 , Eric Garfunkel 4 , Manish Chhowalla 1 Show Abstract
1 Materials Science and Engineering, Rutgers University, Piscataway, New Jersey, United States, 2 Department of Chemical Science, University of Padova, Padova Italy, 3 Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota, United States, 4 Department of Chemistry and Chemical Biology, Rutgers University, Piscataway, New Jersey, United States
Single sheets of graphite oxide are emerging as starting materials providing alternative path to graphene. The exfoliation of graphite oxide in aqueous solution allows deposition of single sheets, referred to as graphene oxide (GO), or multilayer films on virtually any substrate. The stoichiometry, atomic and electronic structures of GO are largely unknown. GO is an insulator but controlled reduction provides tunability of the electronic properties, allowing the possibility of accessing zero-band gap graphene. We present a detailed description of opto-electronic properties, chemical state  and structure  of single and few-layered of GO at different stages of reduction. Particular attention has been given to understanding the atomic structure of pristine GO before investigating its transformation upon annealing. The sp2/sp3 fraction for single and multilayered pristine GO have been elucidated using comparative X-ray photoelectron spectroscopy (XPS) and electron energy loss spectroscopy (EELS) measurements of the fine structure of C and O K-edges. The distortion of carbon basal plane induced by sp3 C-O bonds in pristine GO has been observed by annular dark filed imaging .We found that the electrical characteristics of reduced GO do not approach those of intrinsic graphene obtained by mechanical cleaving because the materials remains significantly oxidized. In fully reduced GO, the carbon-carbon sp2 bonding fraction is ~ 0.80 while the residual oxygen still forms sp3 bonds with carbon atoms in the basal plane. The oxygen disrupts the transport of carriers delocalized in the sp2 network, limiting the mobility and conductivity of reduced GO thin films. Our analysis reveals that removal of oxygen to achieve sp2 carbon fraction of > 0.95 in GO should lead to properties that are comparable to graphene. C. Mattevi et al.“Evolution of electrical, chemical, and structural properties of transparent and conducting chemically derived graphene thin films” Adv. Funct. Mater. Vol 19, (2009) p.1. K. A. Mkhoyan et al.” Atomic and electronic structure of graphene oxide” Nano Lett. vol.9, (2009), p 1058.
10:30 AM - L1.4
Photothermal Deoxygenation of Graphene Oxide for Patterning and Distributed Ignition Applications.
Scott Gilje 1 , Sergey Dubin 2 , Alireza Badakhshan 3 , Jabari Farrar 1 , Stephen Danczyk 3 , Richard Kaner 2 Show Abstract
1 Aerospace Research Laboratories, Northrop Grumman Aerospace Systems, Redondo Beach, California, United States, 2 Department of Chemistry and Biochemistry and California NanoSystems Institute, University of California Los Angeles, Los Angeles, California, United States, 3 AeroPhysics Branch, Combustion Devices Group, Air Force Reserach Laboratory, Edwards Air Force Base, California, United States
Exposing nanostructured graphene oxide (GO) to a camera flash results in a photothermally activated reaction. This process is accompanied by a pronounced photoacoustic effect along with a rapid temperature increase, which initiates a deoxygenation reaction to yield graphitic carbon. SEM images of the product reveal that an expanded graphite with an accordion type structure forms. Nitrogen absorption measurements (BET), carried out before and after exposure to the flash, yield a surface area that increases from 6 m2/g up to 980 m2/g. X-ray photoelectron spectroscopy (XPS) indicates a substantial increase in intensity of the C=C signal, while the oxygen content decreases markedly after flashing. XRD analysis shows a single peak at 26.4° 2θ, confirming conversion to graphitic carbon. Hydrogen gas uptake of 0.5 wt % was measured at 77 K. Thin films of GO can be patterned using a mask to selectively control exposure to the flash. This holds potential for future conducting graphene and insulating GO-based electronics. Another potential application involves using a photo-initiated reaction to achieve the simultaneous ignition of multiple nucleation sites. This type of “distributed ignition” has applications in liquid fuel rockets and in high efficiency homogeneous charge compression ignition (HCCI) engines.
10:45 AM - L1.5
Infrared Absorption Study of the Thermal Reduction of Graphene Oxide.
Muge Acik 1 , Cecilia Mattevi 2 , Geunsik Lee 1 , SeongYong Park 1 , Carlo Floresca 1 , Adam Pirkle 1 , Robert Wallace 1 , Moon Kim 1 , Kyeongjae Cho 1 , Manish Chhowalla 2 , Yves Chabal 1 Show Abstract
1 Materials Science and Engineering, The University of Texas at Dallas , Dallas, Texas, United States, 2 Materials Science and Engineering, Rutgers - the State University of New Jersey, Piscataway, New Jersey, United States
To characterize graphene oxide (GO) and understand how it is thermally reduced, we have carried out a series of experiments using in-situ IR absorption spectroscopy and ex-situ atomic force microscopy, Raman scattering, transmission electron microscopy, and x-ray photoelectron spectroscopy. The in-situ transmission Fourier Transform Infrared Spectroscopy (FTIR) measurements focus on the reduction of GO via thermal annealing to understand the deoxygenation process. Indeed, the manner in which oxygen might be incorporated into the structure (e.g. into the basal plane or the edges of graphene and graphite sheets) would help tailor the properties for optimum device performance. Therefore, GO samples of different thicknesses (single layer, three layers, five layers and multi-layers) were deposited on doped Si substrates coated with native SiO2 double sided polished, approximately 15Å thick layer). Ex-situ Raman scattering data and TEM images make it possible to determine the number of layers, and XPS is used to examine the binding configuration of carbon to oxygen. Thermal annealing of GO upon gradual heating was performed at various temperatures (30-900°C) under vacuum (10−3-10−4 Torr) in a reaction cell. As prepared, the absorbance spectrum of GO at room temperature shows a complex structure including a high concentration of functional groups, such as hydroxyls, carboxyls, sp2-hybridized C=C, epoxides and ethers. Using differential spectra, the reduction process was monitored in detail to determine the change in functional groups at each temperature with 25°C increments up to 850°C. Our observations reveal the formation of ketones, ethers and sp2-hybridized C=C as well as a loss of hydroxyls, carboxyls and epoxides upon gradual annealing. After high temperature annealing at 850°C, there is still oxygen remaining in the structure of GO. At that point the DC electrical conductivity as well as the AC conductivity (IR absorbance) increase, consistent with the presence of graphene domains. Yet, there is a clear incorporation of oxygen into the basal plane. *The authors acknowledge funding from the NRI SWAN program.
11:30 AM - **L1.6
Electronic Transport in Chemically Derived Graphene.
Klaus Kern 1 2 Show Abstract
1 Nanoscale Science Department, Max Planck Institute for Solid State Research, Stuttgart Germany, 2 , Ecole Polytechnique Federale de Lausanne , Lausanne Switzerland
Graphene, consisting of a layer of carbon atoms just one atom thick, is the ultimate in thin conducting sheets. The charge carriers can be tuned from electron-like to hole-like by the application of a gate voltage, and very high carrier mobilities have been reported. Future electronics applications envisage the creation of diverse nanoscale elements of electronic circuits on a single graphene sheet. However, progress in this direction is hampered by the limited availability of high-quality, large size graphene sheets. A very promising low-cost, up-scalable synthetic approach comprises the reduction of graphene oxide (GO) sheets, which can be deposited with controllable density onto a wide range of substrates. Chemical reduction converts the close-to-insulating GO into sheets with up to four orders of magnitude higher electrical conductivity. Such chemically derived graphene is a versatile basis for fabricating thin conductive films on solid support, thus opening access to transparent flexible electrodes. For the electrical conductivity of monolayers of reduced GO, only moderate values of 0.1-50 Scm-1 have been found. The observed temperature- and electric field-dependence of conductance can be consistently interpreted in the framework of two-dimensional variable-range hopping in parallel with electric field-driven tunneling. The latter mechanism is found to dominate the electrical transport at very low temperatures and high electric fields. Our results are consistent with a model of highly conducting graphene regions interspersed with disordered regions, across which charge carrier hopping and tunneling are promoted by strong local electric fields. Strategies to heal these defects are thus needed for more demanding device applications. We demonstrate that this task can be approached by a CVD process which enables substituting carbon atoms contained within the defective areas. In this manner, chemically derived graphene sheets of large dimensions and two orders of magnitude enhanced conductivity compared to the merely reduced GO can be obtained.
12:00 PM - L1.7
Ultra-Large Graphene Membranes as Flexible and Transparent Electronic Material.
Hisato Yamaguchi 1 , Goki Eda 1 , Cecilia Mattevi 1 , HoKwon Kim 1 , Manish Chhowalla 1 Show Abstract
1 Materials Science and Engineering, Rutgers University, Piscataway, New Jersey, United States
Preparation of single- to few-layered graphene thin films in ultra-large scale is one necessary step towards utilizing graphene as a practical electronic material. However, large-area deposition of graphene is still in the progress with deposition on the order of few cm [1-3] being demonstrated. Although chemical vapor deposition (CVD) process and annealing of SiC substrates are potentially promising routes for achieving pristine graphene films, solution-based process using chemically derived graphene has important advantage in the preparation of large-area thin films and membranes.We report the use of chemically derived graphene for deposition of single- to few-layered ultra-large graphene thin films, which are compatible with CMOS device manufacturing standards. The thin films were deposited using a modified version of the method proposed by Robinson et al. . By optimizing the conditions, we succeeded in deposition of uniform graphene thin films on 300 mm SiO2 on Si wafers. Single- to few-layered thin films were achieved. Furthermore, deposited films were transferred on to arbitrary substrates such as PET films or left as free-standing membranes. Detailed characterization using AFM, Raman spectroscopy, XPS, transmittance and electrical measurements as a function of reduction will be presented. We demonstrate that by using appropriate reduction conditions, it is possible to achieve mobility values of 10 – 15 cm2/Vs. Our results provide a pathway for integration of graphene into ultra-large area flexible electronics. K. S. Kim et al. “Large-scale pattern growth of graphene films for stretchable transparent electrodes” Nature 457, 706-710 (2009). X. Li et al. “Large-Area Synthesis of High-Quality and Uniform Graphene Films on Copper Foils” Science 324, 1312-1314 (2009). K. V. Emtsev et al. “Towards wafer-size graphene layers by atmospheric pressure graphitization of silicon carbide” Nature Materials 8, 203 - 207 (2009). J. T. Robinson et al. “Wafer-scale Reduced Graphene Oxide Films for Nanomechanical Devices” Nano Lett. 8, 3441–3445 (2008).
12:15 PM - L1.8
Graphene as a Transparent Electrode.
Ki-Bum Kim 1 , Chang-Mook Lee 1 , Jaewu Choi 1 Show Abstract
1 Information Display, Kyung Hee University, Seoul Korea (the Republic of)
In this talk, we will present the strength and the weakness of graphene as a transparent electrode for future electro-optic devices such as displays, solar cells, and light emitting diodes. The electro-optical properties of the graphene transparent electrodes, which were prepared by mechanical cleavage from a highly ordered pyrolytic graphite and chemical vapor deposition, are compared.
12:30 PM - L1.9
Fabrication of Large Area Graphene Sheets by Chemical Exfoliation for Transparent Conductive Films.
Takeshi Fujii 1 , Ryosuke Shimizu 1 , Yoshiyuki Yonezawa 1 , Yukimi Ichikawa 1 Show Abstract
1 Electron Device Technology Center, Fuji Electric Advenced Technology, Hino-city Japan
Graphene sheet is a promising candidate for a low-cost and rare-metal free transparent conductive film (TCF) with a high transparency in wide wavelength range. Recently, the TCF composed of densely stacked graphene sheets by chemical exfoliation have been reported . However, conductance of the graphene TCF was not high enough for electrical applications such as solar cells and flat panel displays, because the size of graphene sheet was as small as several micrometers, and carrier transport was limited by contact among adjacent graphene sheets. To improve the conductivity of graphene TCF, we have fabricated large area chemical exfoliated graphene sheets by using large size graphite oxide (GO) without coercive exfoliation by an ultrasonication process. Large size GO was prepared by modified Hummer’s method  using natural graphite flakes with a size of around 400 μm. Then, obtained GO was dispersed into methanol, which results in grahpene oxide naturally exfoliated to single layer. Graphene sheet was deposited on hydrophilically treated SiO2/Si substrate by casting the obtained graphene oxide dispersion and then reduced by hydrazine-monohydrate. From optical microscope image, the measured size of grahene sheet was over 100 μm, and, the thickness was about 1nm from AFM measurement. From both results, obtained graphene sheet was suggested to be mono-layer graphene with a dimension of over 100 μm. Electrical properties of the obtained graphene sheet will also be discussed in the presentation. This work was supported in part by the New Energy and Industrial Technology Development Organization (NEDO) under the Ministry of Economy, Trade and Industry (METI) of Japan. X. Wang et al., Nano Letters 8, 323 (2008).  M.Hirata et al., Carbon 42, 2929 (2004).
12:45 PM - L1.10
High Performance of Graphene-based Flexible Transparent Conducting Film by Chemical Doping.
Ki Kang Kim 1 , Alfonso Reina 1 , Hyesung Park 1 , Yumeng Shi 2 , Jing Kong 1 Show Abstract
1 Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 School of Materials Science and Engineering, Nanyang Technological UniVersity, Nanyang Singapore
Graphene based transparent conducting films were transferred to SiO2, glass, and PET substrates after their growth by ambient pressure chemical vapor deposition (CVD). Here, we show that the sheet resistance of the sample was significantly decreased after chemical doping with metal halide such as AuCl3, PtCl2 and PdCl2. Furthermore, the reduction of the sheet resistance was related to the reduction potential of cation of metal halides. Finally, we discuss the transmittance of those films in terms of doping and film morphology. The sheet resistance and transmittance obtained after doping of such films were measured to be between 150-1500 ohms/sq at 80-90% transmittance. Lastly, the adhesion energy between target substrates and the graphene was different due to the hydrophobicity of the substrates.
L2: Chemically Derived Graphenes II
Monday PM, November 30, 2009
Room 310 (Hynes)
2:30 PM - **L2.1
Large-Area Graphene Films for Sensor, MEMs and RF Applications.
Eric Snow 1 Show Abstract
1 , Naval Research Laboratory, Washington, District of Columbia, United States
The recent discovery of graphene and its extraordinary electrical, mechanical and chemical properties has stimulated an intensive effort to exploit these properties in potential applications. Recently, this effort has taken on increased interest and importance, because researchers have demonstrated the ability to form large-area sheets of single- to few-layer graphene and to deposit them on arbitrary substrates. In this presentation, we present initial results obtained at NRL on the formation and use of large-area films of graphene for sensor, microelectromechanical and radio-frequency device applications. We have investigated both transferred films of CVD-grown graphene and spin-on (or spray-deposited) films of chemically modified graphene. We find that the active surface of chemically modified graphene produces high-performance sensors with demonstrated part-per-billion detection of trace chemical vapors . We find that this same chemically active surface also improves the performance of micromechanical resonators where we have demonstrated ~ 100 MHz drum resonators with quality factors ~ 3,000, which is comparable to nanocrystalline diamond thin films . For high-frequency electronic devices pure graphene is optimal because of its high electron mobility and saturation velocity. Using this material we have demonstrated RF amplifiers with an fT*Lg product of 9 GHz-μm , which is comparable to Si NMOS. These initial promising results coupled with the low-cost and simplicity of graphene material growth indicate that graphene has a promising future for large-area electronics applications. JT Robinson, FK Perkins, ES Snow, ZQ Wei and PE Sheehan, NanoLetters 8, 3137 (2008). JT Robinson, M Zalalutdinov, JW Baldwin, ES Snow, ZQ Wei, PE Sheehan and BH Houston, Nanoletters 8, 3441 (2008). J.S. Moon, et al., IEEE Electr. Dev. Lett. 30, 650 (2009).
3:00 PM - L2.2
Development of Conductometric Sensors and Supercapacitors from Aqueous Suspensions of Functionalized Graphene.
Xiaohong An 1 , Trevor Simmons 2 , Rakesh Shah 3 , Morris Washington 1 , Saroj Nayak 1 , Saikat Talapatra 3 , Swastik Kar 1 Show Abstract
1 Physics, Applied Physics and Astronomy, Rensselaer Polytechnic Institute, Troy, New York, United States, 2 Department of Chemistry and Chemical Biology, Rensselaer Polytechnic Institute, Troy, New York, United States, 3 Department of Physics, Southern Illinois University Carbondale, Carbondale, Illinois, United States
We have developed a new method for producing large quantities of graphene flakes in aqueous suspensions. Using this, we present simple methods for large-scale applications of graphene such as conductometric sensors and supercapacitors. In both cases, the graphene flakes are transferred onto nanoporous membranes which then form thin films of large specific area. These films show large changes in resistance upon exposure to gases/vapors. In particular, the sensor films are extremely sensitive to the presence of alcohol vapors (dR/R >10,000%), which make them suitable for breathalyzers. Further, electrolytic double-layer capacitors fabricated from these graphene-film membrane structures show impressive specific capacitance (over 100 F/gm) at high energy densities (>9 Wh/kg) and are capable of delivering high power (operating at > 100 kW/kg) making them suitable for high surge-power applications.
3:15 PM - L2.3
Gas Sensors Based on Thermally Reduced Graphene Oxide.
Ganhua Lu 1 , Junhong Chen 1 , Leonidas Ocola 2 Show Abstract
1 Department of Mechanical Engineering, University of Wisconsin-Milwaukee, Milwaukee, Wisconsin, United States, 2 Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois, United States
Nanoscaled materials are attractive candidates for gas detection elements due to their unique and outstanding properties (e.g., extremely high surface-to-volume ratio) which can potentially lead to novel sensors with exceptional performance. Graphene is a two-dimensional monolayer of sp2-bonded carbon atoms and has been reported as a promising sensing material because of its excellent mechanical, thermal, and electrical properties. A variety of physical and chemical routes have been employed to produce graphene. A potential strategy to cost-effectively mass produce graphene-based devices is to first generate graphene oxide and then reduce it to obtain graphene for device applications. Here, we demonstrate a high-performance gas sensor using partially-reduced graphene oxide sheets. The sensing device was fabricated by dispersing the graphene oxide suspension onto gold interdigitated electrodes. The partial reduction of graphene oxide was achieved through low-temperature step annealing (300 °C at maximum) of the device in argon flow at atmospheric pressure. The electrical conductance of graphene oxide was measured after each heating cycle to evaluate the level of reduction. The thermally reduced graphene oxide showed p-type semiconducting property in ambient environment and were highly responsive to low-concentration NO2 and NH3 diluted in air at room temperature. The NO2 sensing mechanism of the fabricated sensor is attributed to the electron transfer from the reduced graphene oxide to adsorbed NO2 (electron acceptor), which leads to enriched hole concentration and enhanced electrical conduction in the reduced graphene oxide sheet. In the case of NH3 detection, the electron transfer is from adsorbed NH3 (electron donor) to the reduced graphene oxide, lowering hole concentration and electrical conduction in the reduced graphene oxide. The contact between the reduced graphene oxide and the electrode could also contribute to the sensing response. The simple and low-cost manufacturing process and the wide availability of graphene oxide could lead to cost-effective graphene-based gas sensors.
3:30 PM - **L2.4
Exfoliation of Graphene in Common Solvents and Other Systems: The Route to Useful Nano-structured Materials?
Jonathan Coleman 1 , Paul King 1 , Arlene O'Neill 1 , Mustafa Lotya 1 , Valeria Nicolosi 1 , Zhenyu Sun 1 , Shane Bergin 1 , Fiona Blighe 1 , Sukanta De 1 , Umar Khan 1 , Yenny Hernandez 1 Show Abstract
1 Physics, Trinity College Dublin, Dublin Ireland
We have built on our recent discovery of a small family of solvents to disperse and exfoliate graphite to give graphene. This simple process uses sonic energy to break up graphite powder in certain solvents. Due to the balance of graphene-graphene and graphene-solvent interactions, exfoliation occurs without any net energy cost. This process does not result in oxidation or defect formation and so no thermal or chemical reduction is required. Early results confirmed that this resulted in dispersions of well-exfoliated graphene at concentrations of up to 0.01 mg/ml. We have now demonstrated over 30 solvents for graphene. By measuring the concentration of graphene dispersed after centrifugation for all solvents we can estimate the Hansen solubility parameters for graphene. This allows us to show that the dispersability decreases as the enthalpy of mixing of the dispersion increases. In addition we have demonstrated a method to improve the concentration of graphene dispersed. We can attain up to 2 mg/ml of graphene dispersed in N-methylpyrrolidone. Even at this high concentration, the degree of exfoliation is excellent with 17% of flakes consisting of a single graphene layers and 83% of flakes containing <5 layers. These high concentration dispersions can be formed into mechanically strong, electrically conductive, free standing films. We have also developed methods to exfoliate graphene using surfactants which give graphene dispersions with concentration approaching 1 mg/ml. These dispersions can be used to prepare thin, transparent, conducting films with DC and optical conductivities of ~1.5*10^4 S/m and ~4*10^4 S/m respectively. This results in films with sheet resistance of <4 kOhm/Sq for transmittance ~75%. These films are electromechanically stable under flexing for at least 2000 bend cycles.
4:30 PM - **L2.5
Transparent Conducting Nanomaterials from Chemically Converted Graphene: Synthesis, Deposition and Selective Patterning.
Yang Yang 1 , Richard Kaner 2 , Vincent Tung 1 , Matthew Allen 2 , Steven Jonas 1 , Kitty Cha 1 , Jonathan Wassei 2 Show Abstract
1 Materials Science and Engineering, University of California, Los Angeles, Los Angeles, California, United States, 2 Chemistry and Biochemistry, University of California, Los Angeles, Los Angeles, California, United States
Graphene has been shown to exhibit extraordinary electrical characteristics that have caused a dramatic increase in interest and research into this material. Several methods have emerged for producing high quality large-area graphene for electrode applications. The advantages of chemically converted graphene (CCG) include the ability to be solution-cast, an essential property for roll-to-roll processing. Transparent electrodes comprised of a nano-composite of CCG and single-walled carbon nanotubes have been successfully fabricated from solution and applied to optoelectronic devices. Our method does not require surfactants, preserving the intrinsic electronic and mechanical properties of both components. This low temperature process is completely compatible with flexible substrates and does not require a sophisticated transfer process. Chemical doping further enhances the conductivity of the hybrid films. Recent progress in transfer patterning and selective area registration using straightforward soft lithographic methods is also presented.
5:00 PM - L2.6
Electrical and Materials Characterization of Large Area, Transparent, Conductive Graphene Film Networks and Their Potential for Gas Sensing.
Jason Johnson 1 , Ashkan Behnam 1 , Ant Ural 1 Show Abstract
1 Electrical and Computer Engineering, University of Florida, Gainesville, Florida, United States
We investigate the fundamental electrical and structural properties of room temperature processed, large area, transparent, conductive graphene film networks and demonstrate their ability to detect ammonia at low ppm levels. Several groups have obtained graphene in individual sheets or few-layers by either liquid exfoliation from expandable graphite, chemical conversion from graphene oxide (GO), or CVD grown graphene. In this talk, we show that expandable graphite (EG) can be treated with surfactant solutions and high energy sonication to obtain few-layer graphene sheets, which can then be filtered and transferred onto large area silicon substrates with excellent repeatability and with little time and cost. In order to investigate the electrical properties of the few-layer graphene (FLG) film networks fabricated using this method, we pattern the material into various structures using simple photolithography techniques and plasma etching; similar to what has been done for carbon nanotube films. We find that the graphene film networks have reasonable conductivity in the range from 25 to 65 S/cm at room temperature, and demonstrate excellent transmittance, greater than 80% in the visible regime and over 90% in the IR and near IR range. We determine the mechanisms responsible for transport in the FLG film network by measuring its four point probe resistance at various temperatures. The weak insulating behavior observed in the cryogenic regime suggests that 3D variable range hopping is the dominant transport mechanism at low temperatures. In addition to the above results, magneto-resistance measurements reveal a weak inverse dependence of resistivity on magnetic field at higher temperatures while at lower temperatures the dependence becomes stronger and changes sign as the magnetic field increases. Together with AFM and TEM analysis, these results enable us to better understand both the physical structure and the percolative nature of the transport in FLG film networks. Finally, we experimentally demonstrate the application of graphene film networks for gas sensing. Due to its large surface to volume ratio and large number of edge defects, graphene is highly sensitive to its environment. We fabricate a simple and efficient gas sensor based on FLG film networks and demonstrate large changes in its resistance upon exposure to both oxygen and ammonia. In addition, upon functionalizing the surface of our FLG film networks, we can achieve even higher changes in the film resistance. This works provides further insight into possible applications of large area graphene film networks and their potential in device applications.
5:15 PM - L2.7
Highly Sensitive Gas Sensors Using Graphene Thin Films as Sensing Materials.
Jinwoo Lee 1 , Byeonghyeon Kang 1 , Kyongsoo Lee 1 , Cheoljin Lee 1 , Byeongkwon Ju 1 Show Abstract
1 Electrical Engineering, Korea University, Seoul Korea (the Republic of)
After the 1st separation from graphites in 2004 by mechanical exfoliation method, graphene is regarded as one of promising new nano-materials required for future electronics. Graphene is an unfolded single-walled nanotube. Like carbon nanotubes, graphene is consisted of carbon honeycomb structure. However, it is a two dimensional material, while a nanotube is one dimensional. Both surface areas (upper and lower) of a graphene sheet have the same possibility for gas molecule adhesion, while the internal surface area inside a nanotube are limited for gas adhesion than the outer surface. Therefore, graphene is expected to offer wider surface area compared with nanotubes, and is more suitable for gas sensor applications.In this study, we obtained graphene sheets by chemical vapor deposition and also by mechanical exfoliation of a HOPG (Highly ordered pyrolitic graphite). In chemical vapor deposition method, graphene sheets were directly grown over Ni substrate using CH4 and H2 gas mixture and separated from the substrate to SiO2/Si surface using FeCl3 (aq). In mechanical exfoliation method, a HOPG mass was rubbed over SiO2/Si surface several times repeatedly until we obtain enough amounts of graphene thin layers for sensor applications. For each graphene sheet, the thickness was measured by Raman spectroscopy. Gas sensors were fabricated by E-beam deposition of electrodes over the pre-deposited graphene sheets. Several hazardous gases including NH3, NO2, and O3 were applied for the test of sensor performance in room temperature. NH3 and NO2 gases were purchased and ozone gas was obtained directly from an O3 generator. 5 ppm of NH3, 1 ppm of NO2, and 1 ppm O3 were measured, which shows that our graphene based gas sensor is highly sensitive.
5:30 PM - L2.8
New Insights into the Structure and Reduction of Graphite Oxide.
Wei Gao 1 , Lawrence Alemany 1 , Lijie Ci 1 , Pulickel Ajayan 1 Show Abstract
1 , Rice University, Houston, Texas, United States
Graphite oxide (GO) is one of the main precursors of graphene-based materials, highly promising for various technological applications because of their unusual electronic properties. But although epoxy and hydroxyl groups are widely accepted as its main functionalities, its complete structure has remained elusive. Interpreting spectroscopic data in the context of the major functional groups believed to be present in GO, we now show evidence for the presence of 5- and 6-membered-ring lactols. Based on this chemical composition, we devised a complete reduction process through chemical conversion by sodium borohydride and sulfuric acid treatment, followed by thermal annealing. Only small amounts of impurities are present in the final product (less than 0.5 wt% of sulphur and nitrogen, compared with about 3 wt% with other chemical reductions). This method is particularly effective in the restoration of the π conjugated structure, and leads to highly soluble and conductive graphene materials.
5:45 PM - L2.9
Oligo (Polyethylene Glycol) Functionalized Reduced Graphene Oxide and Its Water Solubility.
Shifeng Hou 1 , Robert Cuellari 1 , Najeeb Hoshang Hakimi 1 , Krutika Patel 1 , Pratik Shah 1 , Matthew Gorring 2 , Stefanie Brachfeld 2 Show Abstract
1 Chemistry & Biochemistry, Montclair State University, Montclair , New Jersey, United States, 2 Department of Earth & Environmental Studies, Montclair State University, Montclair , New Jersey, United States
Since first discovered in 2004, graphene has shown various unique properties, including superior mechanical strength and low density and high heat conductance. Many potential applications are based its unique mechanical and electrical properties: graphene–polymer composites, electronic devices, drug delivery and biosensors. Graphene oxide is water soluble with low conductivity and the reduced graphene oxide is good conductivity with poor solubility in water. Several techniques have been developed to enhance the solubility of reduced graphene oxide and most of these are based on physically absorbed functional molecules onto graphene sheets. In this report, a chemical modification process was developed to functionalize reduced graphene oxide with specific groups. Graphene oxide was obtained from graphite powder through a modified Hummers method and then reduced by NH2-NH2, LiAlH4 or NaBH4. The reduced graphene oxide had been functionalized with amine and halogen successfully firstly and these graphene sheets can be used as precursors for further functionalization. Oligo (Polyethylene glycol) molecules were linked to single-layer graphene sheets through covalent bond. The FT-IR, X-Ray diffraction, TEM, SEM and Uv-spectroscopy techniques were used to characterize polyethylene glycol modified reduced graphene oxide (PEG-RGO). PEG-RGO could be dispersed in water, tetrahydrofuran, and ethylene glycol, with individual, single-layer graphene sheets automatically. The dispersion behavior of PEG-RGO in aqueous solvent has been investigated. It was demonstrated that the addition of PEG groups to the surface reduced graphene can enhance the solubility of RGO in water. PEG-RGO was well-dispersed as individual sheets in water. A series of solutions of PEF-RGO with concentrations of 0.001% to 1.5% were prepared and the PEG-RGO dispersions exhibited long-term stability, similar to the case of graphene oxide dispersions in water. In addition, a PEG-RGO film with layered structure and high conductivity has been successfully prepared by filtration, spin-coating or evaporation technique. We have further characterized the compatibility of PEG-RGO with a series of polymer matrixes. Our results demonstrate the feasibility of combining conventional organic synthesis techniques with techniques for the modification of reduced graphene oxide. The technique developed here can be extended to synthesize various functionalized graphene dispersions by using other functional groups and provides a general route for preparing solutions and conducting films based on functional graphene and seeks different applications of graphene-based materials.1.K. S. Novoselov, A. K.Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva, A. A. Firsov, Science, 2004, 306, 666-669.2.S. Park, R. S. Ruoff, Nature Nanotechnology, 2009, 4, 217 - 224.3.Y. Xu, H. Bai, G. Lu, C. Li and G. Shi, J. Am. Chem. Soc., 2008, 130, 5856–5857.
L3: Poster Session: Chemically Derived Graphenes III
Monday PM, November 30, 2009
Exhibit Hall D (Hynes)
9:00 PM - L3.1
Efficient Reduction of Graphite Oxide by Sodium Borohydride and its Effect on Electrical Conductance.
Hyeon-Jin Shin 1 2 , Ki Kang Kim 2 , Anass Benayad 1 , Seon-Mi Yoon 1 , Hyeon Ki Park 2 , In-Sun Jung 1 , Mei Hua Jin 2 , Hae-Kyung Jeong 2 , Jong Min Kim 1 , Jae-Young Choi 1 , Young Hee Lee 2 Show Abstract
1 Samsung Advanced Institute of Technology, Samsung Electronics Co., LTD., Youngin-si Korea (the Republic of), 2 Sungkyunkwan Advanced Institute of Nanotechnology, Sungkyunkwan University, Suwon-si Korea (the Republic of)
We systematically modulated the conductivity of graphite oxide film by using reducing agents. We found that the sheet resistance of graphite oxide film reduced by sodium borohydride (NaBH4) was much lower than that reduced by hydrazine (N2H4). This was attributed to the formation of C-N groups in the case of N2H4 that may act as a donor state and as a consequence, compensate the hole carriers in the reduced graphite oxide. In the case of reduction by NaBH4, the interlayer distance was slightly expanded first by the formation of intermediate boron oxide complex and then contracted by the gradual removal of carbonyl and hydroxyl groups together with boron oxide complex. The fabricated transparent conducting film with the reduced graphite oxide by NaBH4 revealed the sheet resistance comparable to that of the dispersed graphene.
9:00 PM - L3.10
Graphene Growth Based on Dissolution and Segregation of C in Ni.
Ageeth Bol 1 Show Abstract
1 TJ Watson Research Center, IBM, Yorktown Heigths, New York, United States
We developed a scalable process to synthesize large area graphene films based on dissolution/segregation of Carbon in Nickel. A thin film of Ni (typically 300 nm) is exposed to ethylene at 1000°C at low pressure (500 mTorr) and subsequently cooled down in inert atmosphere. In this way we obtain a continuous film consisting of overlapping graphene flakes. HRTEM, LEED, XPS and Raman spectroscopy show that the flakes consist of 1-10 layers of graphene. The graphene films are rather robust and can easily be lifted off from the Ni films and placed on any desirable substrate. Hall mobility measurements show very uniform electrical characteristics. Over a piece of 0.5 cm2 the Hall mobility varies from 940 to 990 cm2/Vs with a stable carrier density of 8.5x10.12 cm-2 (channel width 200 um, channel length 460 um). With in-situ LEEM we demonstrate that our growth technique is based on dissolution and segregation of Carbon in Nickel rather than chemical vapor deposition. At high temperatures ethylene breaks down on the Ni surface and C dissolves into the Ni and saturates the film. In-situ LEEM shows that by cooling the C-saturated Ni film in inert atmosphere, the Ni becomes over-saturated with C and the C crystallizes out on the surface of the Ni in the form of graphene.With in-situ XRD we show that controlling the cooling down rate is important in this process. The cooling down rate has a large influence on how the Ni film orders and therefore how the graphene grows.
9:00 PM - L3.11
A Magneto-Catalytic Technique for Writing Complex Patterns in Graphene.
Lutfiye Bulut 1 , Robert Hurt 1 Show Abstract
1 , Brown University, Providence, Rhode Island, United States
There is tremendous interest in the patterning of graphene and other carbon films for a variety of applications. In addition to lithographic methods, graphene patterning may be possible through selective etching by catalytic gasification. Several studies have created single-particle channels in graphite or graphene using gasification catalyzed by iron, nickel, cobalt, or silver particles. It has been reported that channeling catalyst particles follow preferred crystallographic directions, which if fully understood could lead to precise patterning through rule-based etching protocols. Nevertheless, this class of technique would be much more powerful and flexible if one had a method to explicitly control the channeling directions, or to change direction dynamically to produce complex and preselected channel patterns. Here we explore the use of the dioxygen reaction (C + O2 -> CO/CO2) to reduce reaction temperature below the Curie point of magneto-catalytic particles allowing them to be steered in situ by externally applied magnetic fields. We demonstrate that graphene can be etched into arbitrarily complex and preset patterns using cobalt-based catalyst particles for dioxygen gasification at temperatures near 550 oC using a close-proximity Sm2Co17 magnet. We demonstrate vertical pitting, horizontal unidirectional channeling, bidirectional channeling, and tridirectional channeling under identical reaction conditions by varying only the position and motion of the external magnetic. Etch rates as high as 2 um/min are achieved and the etch tracks and XRD studies suggest the gradual decrease in magneto-catalytic activity over the course of the reaction through cobalt oxide formation. The results suffice to demonstrate active steering for particles/channels down to about 200 nm. This is to our knowledge the first demonstration of a magneto-catalytic writing technique for graphene, and it may find application in the massively parallel production of uniform flow channels, pores, ribbons, devices, or circuits.
9:00 PM - L3.12
Electrochemical Synthesis of CdSe Quantum Dot Array on Graphene Basal Plane using Mesoporous Silica Thin Film Templates.
Yong Tae Kim 1 , Jung Hee Han 2 , Byung Hee Hong 1 2 , Young-Uk Kwon 1 2 Show Abstract
1 Chemistry, BK-21 School of Chemical Materials Science, Sungkyunkwan University, Suwon Korea (the Republic of), 2 Chemistry, SKKU Advanced Institute of Nanotechnology, Sungkyunkwan University, Suwon Korea (the Republic of)
Since the discovery of microscale single-layer graphene, graphene and related materials have received intensive attention as promising materials for nanoelectronics due to their fascinating electrical, mechanical, and chemical properties. In addition, the recent large-scale synthesis of high-quality graphene films suggests their applications to bendable and/or stretchable transparent electrodes for solar cells, sensors and displays. Surface grafting on the graphene with functional materials will be an indispensable technique towards these directions of progress. This, however, requires the problems arising from the lack of reactivity of the graphene basal plane to be resolved. A perfect graphene does not have dangling bonds on the basal plane for chemical bond formation. The chemical potential of the basal plane is lower than the edges or defects. Therefore, deposition directly on graphene results in grafting on the edges and defects only. In case of using vacuum deposition techniques, this problem has been solved by modifying the graphene surface i.e., ozone treatment, forming metal underlayer, and attaching organic molecules with functional groups. On the contrary, there has been no report on grafting on the basal plane by electrochemistry. Here, we demonstrate that the use of a nanoporous mask can be a viable means to form a uniform nanostructured film on the graphene basal plane. We applied a mesoporous silica thin film whose pore structure is composed of about 8 nm sized vertical channels in a hexagonal symmetry on the graphene surface as a nanoporous mask. The nanochannels exert resistance against the diffusion of electrolytes and, thus, function as a potential-equalizer to suppress the preference for the edge and defect sites. By depositing CdSe, we formed CdSe quantum dots into a hexagonal array structure.
9:00 PM - L3.13
Formation of Graphene Nanostructures on Vicinal SiC(000-1) Surfaces.
Chenda Srey 1 , Seigi Mizuno 2 , Satoru Tanaka 1 Show Abstract
1 Applied Quantum Physics, Kyushu Univ., Fukuoka Japan, 2 Molecular and Material Sciences, Kyushu Univ., Fukuoka Japan
Graphene, a single sheet of sp2-bonded carbon arranged in a honeycomb structure has attracted a lot research interests due to its superior electronic properties . Graphene can be epitaxially grown either on Si or C face of SiC surface by heating at high temperatures in vacuum .The self-organized nanofacet formation was found by high temperature H2 gas etching on vicinal SiC surfaces consisting of step/terraces or facets and that is the one of the promising ways to be applicable to nanostructure formation. The high temperature etching on 4° (vicinal toward [11-20]) C-terminated 6H-SiC(000-1) at 1360°C result in a periodic surface structure . In our experiments, we focus on the growth of graphene layers, in vacuum, with SiC nanofacet surface, which can be a good template for the nanostructure formation. First, the growth of graphene has been carried out on Si face of SiC (0001) surfaces with nanofacet structure. The surfaces exhibited the (√3×√3)R30° due to silicate adlayer . After annealing to higher temperature than 1100°C, the surface structure was changed to (6√3×6√3)R30°  before the monolayer or few layers of graphene were grown . The nanofacet structure was thermally destroyed at the temperature higher than 1050°C confirmed by atomic force microscope (AFM). Thus, the nanostructure was conclusively difficult to form on Si face of SiC substrate.On the other hand, the C face of SiC(000-1) surface initially exhibited (√3×√3)R30° by reflection high electron energy diffraction (RHEED) due to silicate adlayer  after H2 etching. Then, the surface was changed to (3×3) when annealed to 1000°C, and the mixure of (3×3) and (2×2) phases were appeared after annealing at 1050°C. Further anneal to 1100°C for 5 minutes, a faint streak of graphene was only observed outside of the (+2/3) streak (invisible at the (-2/3) streak), viewed with [1-100] azimuth parallel to the facet/step edges. This asymmetric pattern indicates site-selective formation of graphene on (11-2n) nanofacets. This was confirmed by the detailed RHEED analysis, showing a ~8° inclined graphene streak from SiC rods. Importantly, the surface morphology checked by AFM indicated the periodic structure similar to the SiC nanofacet surface. We can here infer that the nucleation of graphene takes place at the facet region, resulting in the formation of the nano-structure of graphene at (11-2n) nanofacets. The further results of characterizations using LEED, STM, AFM, and micro-Raman will be given and the formation mechanisms of graphene nanowires will be discussed.References K. S. Novoselov et al., Nature 438, 197 (2005) I. Forbeaux et al.,Surf. Sci. 442, 9 (1999) H. Nakagawa et al., Phys. Rev. Lett. 91, 226107 (2003) J. Bernhardt et al., Appl. Phys. Lett. 74, 1084 (1996) L.I Johansson et al., Phys. Rev. B. 53, 13793 (1996)
9:00 PM - L3.14
Morphology of Graphene Surfaces on Hydrophobic/Hydrophilic Domain Nanopatterns.
Takahiro Tsukamoto 1 , Toshio Ogino 1 Show Abstract
1 , Yokohama National University, Yokohama Japan
Graphene attracts much interest as a new material for the next generation of electronic devices owing to its remarkable electrical and mechanical properties. In many studies, graphene samples have been prepared using mechanical exfoliation of graphite and its deposition onto a SiO2 substrate. Substrate-induced structures or mesoscopic corrugations on the deposited graphene are important for their transport. Since a well-defined graphene layer on a solid substrate is necessary for device applications, a well-defined surface is also required to tightly attach the graphene to the substrate. Therefore, the selection of the substrate is one of the important issues. Previously, we have reported on a graphene sheet on a sapphire surface with a terrace/step structure . Graphene is closely adhered on the sapphire surface and the height of a single layer graphene is estimated to be approximately 0.36 nm when using atomic force microscopy (AFM) in air. In the present study, we report on a graphene sheet on the sapphire surface on which hydrophilic and hydrophobic domain nanopatterns coexist. Sapphire substrates with step bunches accompanied with crossing steps were treated by a H2SO4 and H2O2 mixed solution (H2SO4:H2O2=3:1) for 10 min followed by an ultrasonic treatment in water for 5 min to clean the surface. Graphene was deposited on the sapphire surface by mechanical exfoliation of graphite. The surface morphology of the graphene was observed using the contact mode in AFM in air. Previously, we found that a graphene sheet tightly adheres to a sapphire surface with regularly ordered terrace/step structure and the buried step structure on the sapphire surface clearly appeared on the graphene surface. In the case of a sapphire surface with elliptical hydrophobic domains surrounded with hydrophilic areas, graphene adhered on the hydrophobic domains and did not closely attach to the surrounding hydrophilic areas. The weaker adhesion on the hydrophilic areas is believed to be owing to water or gas molecules confined at the interface. The graphene is hydrophobic. When the graphene attaches to the hydrophobic domains, the confined water or gas molecules are transferred from the hydrophobic domains to the hydrophilic areas. The water or gas molecules are concentrated in the hydrophilic areas. Our experimental results indicate that, for a good contact of graphene with a solid surface, hydrophobicity is important in addition to the flatness.  T. Tsukamoto et al., Appl. Phys. Express, 2 (2009) 075502.
9:00 PM - L3.15
Electric Transport in Epitaxial Graphene on Vicinal SiC Substrate with Periodic Atomic-scale Facets.
Shunsuke Odaka 1 2 3 , Hisao Miyazaki 1 3 , Akinobu Kanda 3 4 , Kouhei Morita 5 , Satoru Tanaka 5 , Yasumitsu Miyata 6 , Hiromichi Kataura 6 , Kazuhito Tsukagoshi 1 3 6 , Yoshinobu Aoyagi 3 7 Show Abstract
1 , MANA, NIMS, Tsukuba Japan, 2 , Tokyo Institute of Technology, Yokohama Japan, 3 , JST-CREST, Kawaguchi Japan, 4 , Institute of Physics and TIMS, University of Tsukuba, Tsukuba Japan, 5 , Kyushu University, Fukuoka Japan, 6 , AIST, Tsukuba Japan, 7 , Ritsumeikan University, Kusatsu Japan
High mobility in single or multiple carbon atomic sheets of graphene have been revealed in theoretical and experimental studies. For a development of the field-effect transistors (FETs) fabricated in graphene sheet, large-area sheet of an epitaxial graphenes on SiC substrate were grown at high temperatures on a vicinal SiC substrate. However, the vicinal SiC surface was corrugated with atomic-scale facet arrays. The atomic-scale facets caused an anisotropic transport in the graphene sheets, depending on current flow direction. A series of graphene FETs were fabricated on vicinal SiC substrates with the channels parallel (parallel-channel FET) or perpendicular to the facet (perpendicular-channel FET). The graphene with a few layers were grown on the Si face of vicinal 4H-SiC substrates by thermal decomposition at 1600 °C in vacuum. Cross-sectional transmission electron microscopy revealed that the epitaxial graphene layers continuously covered the facets. The graphene was composed of two regions: flat regions with atomic flatness and slop regions on facets. The typical period of the flat and slop cycle was 30 nm. The graphene layers were patterned by dry etching with O2 plasma. Source and drain electrodes, and a top gate structure were formed on the graphene layers. The sheet resistivity as a function of the gate voltage was measured at room temperature in parallel or perpendicular configuration. Ambipolar resistance modulation with respect to the gate voltage change was observed in the both FETs. In comparison of the two FETs, the parallel-channel FET reproducibly exhibited lower sheet resistivity. Based on the observed result, we could clarify the resistivity and gate-voltage response of the graphene in facet regions and atomic flat regions. As the most probable case, the resistivity of terrace region could be smaller than the resistivity of the facet region. This is because the Si atom was decomposed from the facet region, resulting in introducing the defects in the graphene. Furthermore, since graphitization must be started from the facets, there would be disconnection at the facets. More details of the graphene transport with atomic-scale corrugation will be discussed.
9:00 PM - L3.16
Graphene-on-insulator Substrates by In-place Bonding of Graphene Grown on Si/SiO2/Metal Templates.
Katherine Saenger 1 , James Tsang 1 , Jack Chu 1 , Ageeth Bol 1 , Conal Murray 1 , Alfred Grill 1 Show Abstract
1 , IBM T.J. Watson Research Center, Yorktown Heights, New York, United States
Graphene synthesis by chemical vapor deposition on metallic template layers is a potentially promising route to large-area electronic-quality graphene, providing that workable methods can be found for post-growth removal of the metal and transfer of the graphene sheets to an insulating substrate. The in-place bonding approach presented here, in whic