Showing papers by "Rodney S. Ruoff published in 2009"
TL;DR: It is shown that graphene grows in a self-limiting way on copper films as large-area sheets (one square centimeter) from methane through a chemical vapor deposition process, and graphene film transfer processes to arbitrary substrates showed electron mobilities as high as 4050 square centimeters per volt per second at room temperature.
Abstract: Graphene has been attracting great interest because of its distinctive band structure and physical properties. Today, graphene is limited to small sizes because it is produced mostly by exfoliating graphite. We grew large-area graphene films of the order of centimeters on copper substrates by chemical vapor deposition using methane. The films are predominantly single-layer graphene, with a small percentage (less than 5%) of the area having few layers, and are continuous across copper surface steps and grain boundaries. The low solubility of carbon in copper appears to help make this growth process self-limiting. We also developed graphene film transfer processes to arbitrary substrates, and dual-gated field-effect transistors fabricated on silicon/silicon dioxide substrates showed electron mobilities as high as 4050 square centimeters per volt per second at room temperature.
10,663 citations
TL;DR: The use of colloidal suspensions to produce new materials composed of graphene and chemically modified graphene is reviewed, which is both versatile and scalable, and is adaptable to a wide variety of applications.
Abstract: Interest in graphene centres on its excellent mechanical, electrical, thermal and optical properties, its very high specific surface area, and our ability to influence these properties through chemical functionalization. There are a number of methods for generating graphene and chemically modified graphene from graphite and derivatives of graphite, each with different advantages and disadvantages. Here we review the use of colloidal suspensions to produce new materials composed of graphene and chemically modified graphene. This approach is both versatile and scalable, and is adaptable to a wide variety of applications.
6,178 citations
TL;DR: An improved transfer process of large-area graphene grown on Cu foils by chemical vapor deposition is reported on, finding that the transferred graphene films have high electrical conductivity and high optical transmittance that make them suitable for transparent conductive electrode applications.
Abstract: Graphene, a two-dimensional monolayer of sp2-bonded carbon atoms, has been attracting great interest due to its unique transport properties. One of the promising applications of graphene is as a transparent conductive electrode owing to its high optical transmittance and conductivity. In this paper, we report on an improved transfer process of large-area graphene grown on Cu foils by chemical vapor deposition. The transferred graphene films have high electrical conductivity and high optical transmittance that make them suitable for transparent conductive electrode applications. The improved transfer processes will also be of great value for the fabrication of electronic devices such as field effect transistor and bilayer pseudospin field effect transistor devices.
3,017 citations
TL;DR: In this paper, several nanometer-thick graphene oxide films were exposed to nine different heat treatments (three in Argon, three in Argon and Hydrogen, and three in ultra-high vacuum), and also a film was held at 70°C while being exposed to a vapor from hydrazine monohydrate.
2,990 citations
TL;DR: It is reported that homogeneous colloidal suspensions of chemically modified graphene sheets were readily produced in a wide variety of organic solvent systems and "paperlike" materials generated by very simple filtration of the reduced graphene oxide sheets had electrical conductivity values as high as 16,000 S/m.
Abstract: We report that homogeneous colloidal suspensions of chemically modified graphene sheets were readily produced in a wide variety of organic solvent systems. Two different sets of solubility parameters are used to rationalize when stable colloidal suspensions of graphene oxide sheets and, separately, of reduced graphene oxide sheets in a given solvent type are possible and when they are not. As an example of the utility of such colloidal suspensions, “paperlike” materials generated by very simple filtration of the reduced graphene oxide sheets had electrical conductivity values as high as 16 000 S/m.
1,541 citations
TL;DR: This work used carbon isotope labeling in conjunction with Raman spectroscopic mapping to track carbon during the growth process and shows that at high temperatures sequentially introduced isotopic carbon diffuses into the Ni first, mixes, and then segregates and precipitates at the surface of Ni forming graphene and/or graphite.
Abstract: Large-area graphene growth is required for the development and production of electronic devices. Recently, chemical vapor deposition (CVD) of hydrocarbons has shown some promise in growing large-area graphene or few-layer graphene films on metal substrates such as Ni and Cu. It has been proposed that CVD growth of graphene on Ni occurs by a C segregation or precipitation process whereas graphene on Cu grows by a surface adsorption process. Here we used carbon isotope labeling in conjunction with Raman spectroscopic mapping to track carbon during the growth process. The data clearly show that at high temperatures sequentially introduced isotopic carbon diffuses into the Ni first, mixes, and then segregates and precipitates at the surface of Ni forming graphene and/or graphite with a uniform mixture of 12C and 13C as determined by the peak position of the Raman G-band peak. On the other hand, graphene growth on Cu is clearly by surface adsorption where the spatial distribution of 12C and 13C follows the pre...
1,494 citations
TL;DR: In this paper, the authors used a carbon isotope labeling technique to elucidate the growth kinetics and unambiguously demonstrate that graphene growth on Cu is by surface adsorption whereas on Ni is by segregation-precipitation.
Abstract: Large-area graphene is a new material with properties that make it desirable for advanced scaled electronic devices1. Recently, chemical vapor deposition (CVD) of graphene and few-layer graphene using hydrocarbons on metal substrates such as Ni and Cu has shown to be a promising technique2-5. It has been proposed in recent publications that graphene growth on Ni occurs by C segregation2 or precipitation3, while that on Cu is by surface adsorption5. In this letter, we used a carbon isotope labeling technique to elucidate the growth kinetics and unambiguously demonstrate that graphene growth on Cu is by surface adsorption whereas on Ni is by segregation-precipitation. An understanding of the evolution of graphene growth and thus growth mechanism(s) is desired to obtain uniform graphene films. The results presented in this letter clearly demonstrate that surface adsorption is preferred over precipitation to grow graphene because it is a self-limiting process and thus manufacturable.
1,108 citations
TL;DR: In this paper, a homogeneous aqueous colloidal suspension of chemically cross-linked graphene oxide sheets was generated by addition of polyallylamine to an aqous suspension of graphene oxide, followed by sonication of the mixture.
Abstract: We report that a homogeneous aqueous colloidal suspension of chemically cross-linked graphene oxide sheets was generated by addition of polyallylamine to an aqueous suspension of graphene oxide sheets followed by sonication of the mixture This is the first example for producing a homogeneous colloidal suspension of cross-linked graphene sheets As a demonstration of the utility of such a colloidal suspension, “paper” material samples made by simple filtration from such a suspension of cross-linked graphene oxide sheets showed excellent mechanical stiffness and strength
503 citations
TL;DR: In this paper, carbon atoms decomposed from methane in a metal substrate at high temperatures were precipitated on metal surfaces upon cooling, and large area uniform few-layer graphene (FLG)/graphite films were transferred to glass slides after dissolving the metal substrate in an aqueous solution of Fe(NO3)3.
Abstract: By dissolving carbon atoms decomposed from methane in a metal substrate at high temperatures, large area uniform few-layer graphene (FLG)/graphite films were precipitated on metal surfaces upon cooling. The thickness could be controlled by varying the amount of carbon atoms in the metal. Such films were transferred to glass slides after dissolving the metal substrate in an aqueous solution of Fe(NO3)3. Sheet resistances as low as 200 Ω/◻ with a transmittance of 85% were obtained from FLG films. The resistance and transmittance can be changed over one order of magnitude, making such films potentially useful for transparent thin conducting electrodes.
362 citations
TL;DR: The thermal stability and reduction kinetics of graphene oxide were studied by measuring the electrical resistivity of single-layer graphene films at various stages of reduction in high vacuum and by performing temperature programmed desorption (TPD) measurements of multilayer films in ultrahigh vacuum.
Abstract: The thermal stability and reduction kinetics of graphene oxide were studied by measuring the electrical resistivity of single-layer graphene films at various stages of reduction in high vacuum and by performing temperature programmed desorption (TPD) measurements of multilayer films in ultrahigh vacuum. The graphene oxide was exfoliated from the graphite oxide source material by slow-stirring in aqueous solution, which produces single-layer platelets with an average lateral size of ∼10 μm. From the TPD measurements, it was determined that the primary desorption products of the graphene oxide films for temperatures up to 300 °C are H2O, CO2, and CO, with only trace amounts of O2 being detected. Resistivity measurements on individual single-layer graphene oxide platelets resulted in an activation energy of 37 ± 1 kcal/mol. The TPD measurements of multilayer films of graphene oxide platelets give an activation energy of 32 ± 4 kcal/mol.
220 citations
TL;DR: Transparent conducting films have been fabricated in one step, combining self-assembly and chemical reduction of graphene oxide platelets dispersed in water, and their thickness can be controlled by the concentration of the graphene oxide suspension as discussed by the authors.
Abstract: Transparent conducting films have been fabricated in one step, combining self-assembly and chemical reduction of graphene oxide platelets dispersed in water. The films are of centimeter scale and their thickness can be controlled by the concentration of the graphene oxide suspension. The optical transmittance values at a wavelength of 550 nm were 87% and 96% for the films made from 1.5 and 0.5 mg/ml suspensions, respectively, and have sheet resistances of 11.3 and 31.7 kΩ/◻. Scanning and transmission electron microscopy, atomic force microscopy, and x-ray photoelectron spectroscopy were used to characterize the films.
TL;DR: Calculations suggest that the epoxide reduction by hydrazine may predominantly follow a direct Eley-Rideal mechanism rather than a Langmuir-Hinshelwood mechanism.
Abstract: Mechanisms for epoxide reduction with hydrazine on a single-layer graphene sheet are examined using quantum mechanical calculations within the framework of gradient-corrected spin-polarized density-functional theory. We find that the reduction reaction is mainly governed by epoxide ring opening which is initiated by H transfer from hydrazine or its derivatives. In addition, our calculations suggest that the epoxide reduction by hydrazine may predominantly follow a direct Eley–Rideal mechanism rather than a Langmuir–Hinshelwood mechanism. We also discuss the generation of various hydrazine derivatives during the reduction of graphene oxide with hydrazine and their potential contribution to lowering the barrier height of epoxide ring opening.
TL;DR: In this article, the authors used an electrostatic force-directed assembly technique at room temperature for dry decoration of graphene oxide sheets with aerosol Ag nanocrystals synthesized from an arc plasma source.
Abstract: Facile dry decoration of graphene oxide sheets with aerosol Ag nanocrystals synthesized from an arc plasma source has been demonstrated using an electrostatic force directed assembly technique at room temperature. The Ag nanocrystal-graphene oxide hybrid structure was characterized by transmission electron microscopy (TEM) and selected area diffraction. The ripening of Ag nanocrystals on a graphene oxide sheet was studied by consecutive TEM imaging of the same region on a sample after heating in Ar at elevated temperatures of 100 °C, 200 °C, and 300 °C. The average size of Ag nanocrystals increased and the number density decreased after the annealing process. In particular, migration and coalescence of Ag nanocrystals were observed at a temperature as low as 100 °C, suggesting a van der Waals interaction between the Ag nanocrystal and the graphene oxide sheet. The availability of affordable graphene-nanocrystal structures and their fundamental properties will open up new opportunities for nanoscience and nanotechnology and accelerate their applications.
TL;DR: In this paper, thin layers of graphite oxide sheets were dispersed in dimethylformamide and dielectrophoretically assembled onto predefined and opposing metal electrodes, resulting in the deposition of multiple layers.
TL;DR: Raman signals due to storage lipids (specifically triglycerides) were clearly identified in the nitrogen‐starved C. sorokiniana and N. oleoabundans, but not in their healthy counterparts, and the time evolution of fluorescence background was observed while acquiring Raman signals from the algae.
Abstract: Preliminary feasibility studies were performed using Stokes Raman scattering for compositional analysis of algae. Two algal species, Chlorella sorokiniana (UTEX #1230) and Neochloris oleoabundans (UTEX #1185), were chosen for this study. Both species were considered to be candidates for biofuel production. Raman signals due to storage lipids (specifically triglycerides) were clearly identified in the nitrogen-starved C. sorokiniana and N. oleoabundans, but not in their healthy counterparts. On the other hand, signals resulting from the carotenoids were found to be present in all of the samples. Composition mapping was conducted in which Raman spectra were acquired from a dense sequence of locations over a small region of interest. The spectra obtained for the mapping images were filtered for the wavelengths of characteristic peaks that correspond to components of interest (i.e., triglyceride or carotenoid). The locations of the components of interest could be identified by the high intensity areas in the composition maps. Finally, the time evolution of fluorescence background was observed while acquiring Raman signals from the algae. The time dependence of fluorescence background is characterized by a general power law decay interrupted by sudden high intensity fluorescence events. The decreasing trend is likely a result of photo-bleaching of cell pigments due to prolonged intense laser exposure, while the sudden high intensity fluorescence events are not understood.
TL;DR: Electrogenerated chemiluminescence can be obtained from an electrochemically oxidized highly oriented pyrolytic graphite electrode or a graphene oxide suspension in an aqueous solution containing 0.1 M NaClO(4), phosphate buffer (PBS) and 13 mM tri-n-propylamine.
Abstract: Electrogenerated chemiluminescence (ECL) can be obtained from an electrochemically oxidized highly oriented pyrolytic graphite electrode or a graphene oxide suspension in an aqueous solution containing 0.1 M NaClO(4), phosphate buffer (PBS) (pH = 7.0) and 13 mM tri-n-propylamine. Single ECL events from individual graphene oxide nanosheets can be observed. The observation of ECL in such systems may lead to the development of a new class of carbon-based nanomaterials for potentially highly sensitive analytical applications.
TL;DR: In this article, the characteristics of thin-film transistors with pentacene active layers and source/drain contact layers consisting of either Au, Au coated with highly reduced graphene oxide (HRG), or plain HRG, are compared.
Abstract: The characteristics of thin-film transistors (TFTs) with pentacene active layers and source/drain contact layers consisting of either Au, Au coated with highly reduced graphene oxide (HRG), or plain HRG, are compared It is shown that the incorporation of HRG as an interfacial material between gold source/drain contacts and pentacene in TFT devices results in improved electrical characteristics The effect of the HRG layer is to improve the gold/pentacene interface leading to better charge injection, lower losses at the interface, and, consequently, higher effective carrier mobility
Patent•
27 Apr 2009TL;DR: In this paper, an electrochemical device comprising a chemically modified graphene material is disclosed, along with a method of making an ultracapacitor, the method comprising forming two electrodes, wherein at least one of the two electrodes comprises a graphene material, and positioning each of the electrodes such that each is in contact with an opposing side of a separator and a current collector.
Abstract: An electrochemical device comprising a chemically modified graphene material is disclosed. An ultracapacitor comprising a chemically modified graphene material is disclosed, along either with a method of making an ultracapacitor, the method comprising forming two electrodes, wherein at least one of the two electrodes comprises a graphene material, and positioning each of the two electrodes such that each is in contact with an opposing side of a separator and a current collector
TL;DR: In this article, the synthesis of isotopically-labeled graphite films on nickel substrates by using cold-wall chemical vapor deposition (CVD) was reported. But the results from transport measurements indicate that 13C-labeling does not significantly affect the electrical transport properties of graphene.
Abstract: We report the synthesis of isotopically-labeled graphite films on nickel substrates by using cold-wall chemical vapor deposition (CVD). During the synthesis, carbon from 12C- and 13C-methane was deposited on, and dissolved in, a nickel foil at high temperature, and a uniform graphite film was segregated from the nickel surface by cooling the sample to room temperature. Scanning and transmission electron microscopy, micro-Raman spectroscopy, and X-ray diffraction prove the presence of a graphite film. Monolayer graphene films obtained from such isotopically-labeled graphite films by mechanical methods have electron mobility values greater than 5000 cm2·V−1·s−1 at low temperatures. Furthermore, such films exhibit the half-integer quantum Hall effect over a wide temperature range from 2 K to 200 K, implying that the graphite grown by this cold-wall CVD approach has a quality as high as highly oriented pyrolytic graphite (HOPG). The results from transport measurements indicate that 13C-labeling does not significantly affect the electrical transport properties of graphene.
15 May 2009
TL;DR: In this article, a method to grow large-area graphene films on Cu foils and graphene transfer methods to other substrates was reported, and the transferred graphene films were characterized by optical, scanning and transmission electron microscopy, Raman and UV-VIS spectroscopy, and four point probe electrical measurements.
Abstract: Graphene and few-layer graphene films exhibit unique properties and show promise as electronic devices as well as for passive applications. A significant challenge however is the synthesis of large area graphene and/or multilayer films. A method that is showing promise is the growth of graphene on metal substrates by chemical vapor deposition. In this paper we report on a method to grow large-area graphene films on Cu foils and graphene transfer methods to other substrates. The transferred graphene films were characterized by optical, scanning and transmission electron microscopy, Raman and UV-VIS spectroscopy, and four-point probe electrical measurements. The data shows that graphene can be grown directly on Cu substrates by chemical vapor deposition. The graphene films transferred to SiO2/Si substrates show low defects as determined by the near absence of the “D” band at 1350 cm -1 in the Raman spectrum. In addition, the films were found to have high optical transmittance and electrical conductivity.
TL;DR: In this paper, a microscale, thermally actuated, uniaxial testing stage for nanofiber materials has been designed and fabricated, which allows two-point electrical measurements simultaneously with in situ mechanical testing.
Abstract: A microscale, thermally actuated, uniaxial testing stage for nanofiber materials has been designed and fabricated. Electrical separation of portions of the stage allows two-point electrical measurements simultaneously with in situ mechanical testing. Using this stage, a nanofiber consisting of a carbon nanotube (CNT) surrounded by amorphous carbon was subjected to mechanical loading and simultaneous electrical impedance characterization, which provides a means to derive fiber resistance measurements when a fiber is mechanically coupled using highly resistive contacts. Stress applied to the nanofiber was estimated using measurements of the stage displacement and the input power supplied to the thermal actuator.
TL;DR: In this paper, the growth of boron carbide nanowires is achieved by pyrolysis of diborane and methane at 650-750'°C and around 500 mTorr in a quartz tube furnace.
Abstract: Catalyst-free growth of boron carbide nanowires is achieved by pyrolysis of diborane and methane at 650–750 °C and around 500 mTorr in a quartz tube furnace. Electron-diffraction analysis using a novel diffraction-scanning transmission electron microscopy (D-STEM) technique indicates that the crystalline nanowires are single-crystal orthorhombic boron carbide. TEM images show that the nanowires are covered by a 1–3 nm thick amorphous layer of carbon. Elemental analysis by electron energy loss spectroscopy (EELS) shows only boron and carbon while energy-dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS) show the presence of oxygen as well as boron and carbon.
TL;DR: The structure of graphite oxide has been studied by neutron diffraction and is found to be nano-crystalline, the diffraction pattern being a mixture of Bragg and diffuse scattering.
01 Jan 2009
TL;DR: In this paper, an improved device for whole-cell patch clamp recordings using nanoscale pipettes which more readily form seals with the cell membrane and a movable metal electrode to decrease the access resistance, increase the signal bandwidth, and clear debris from the pipette tip.
Abstract: The gold standard for studying the properties of ion channels is the patch clamp, a technique that measures pA currents passing through ion channels in an isolated patch of cell membrane. The authors sought to design an improved device for whole-cell patch clamp recordings using nanoscale pipettes which more readily form seals with the cell membrane and a movable metal electrode to decrease the access resistance, increase the signal bandwidth, and clear debris from the pipette tip. Electrical characterization of the instrument by impedance spectroscopy showed that advancing the metal electrode towards the pipette tip decreased the impedance distinctly in different frequency bands, the greatest decrease occurring between 1 and 100 kHz, and the smallest at 100 kHz. Finite element simulations of the electrode, incorporating the electrode/electrolyte interface, suggest that optimal signal transmission can be achieved with athick-walled pipette, nearly filled with alarge Ag/AgCl electrode, whose impedance can ...
TL;DR: In this article, a method of metal deposition was developed to improve the contacts of β-SiC nanowires supported at both ends of the wires. But this method consists of multiple depositions at different angles in order to avoid the shadow effect and reduce the compressive residual stress.
Abstract: In this paper, we report the measurement of the deflection of β-SiC nanowires supported at both ends. Such wires hold promise as active elements in NEMS/MEMS devices. To ensure the stable mechanical clamping and electrical contacts between electrodes and nanowires, we have developed a method of metal deposition to improve the contacts. This method consists of multiple depositions at different angles in order to avoid the shadow effect and reduce the compressive residual stress. The improvement of the contacts was verified via SEM observation and electrical transport measurements. To suspend the nanowire, a dielectric layer underneath was removed, followed by critical point drying. The change of electrical resistance was measured when the suspended nanowires were deflected by either capillary forces arising from the surface tension or electrostatic forces.
01 Jan 2009
Journal Article•
01 Jan 2009
TL;DR: In this article, an inorganic catalyst based glucose Biofuel cell using nanoporous silica coating as a functional membrane is presented, which is capable of replacing the conventional polymer based membranes with improved performance.
Abstract: In this paper, we report the design, fabrication and characterization of an inorganic catalyst based glucose Biofuel cell using nanoporous silica coating as a functional membrane. Blood vessel implantable biofuel cells are subjected to higher glucose concentrations and blood flow rates. However, reduction in the implant thickness is critical for the intra-vascular implantable Biofuel cells. Platinum thin-film (thickness: 15 nm) deposited on Silicon (500 µm) served as the anode while Graphene (65 µm) was used as the cathode. Control experiments involved the use of polypropylene based porous membrane (50 µm) and activated Carbon (198 µm) electrodes. We report that nanoporous silica thin film (270 nm) is capable of replacing the conventional polymer based membranes with improved performance.