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 th