Vincent C. Tung

High-throughput solution processing of large-scale graphene

The electronic properties of graphene, such as high charge carrier concentrations and mobilities, make it a promising candidate for next-generation nanoelectronic devices. In particular, electrons and holes can undergo ballistic transport on the sub-micrometre scale in graphene and do not suffer from the scale limitations of current MOSFET technologies. However, it is still difficult to produce single-layer samples of graphene and bulk processing has not yet been achieved, despite strenuous efforts to develop a scalable production method. Here, we report a versatile solution-based process for the large-scale production of single-layer chemically converted graphene over the entire area of a silicon/SiO(2) wafer. By dispersing graphite oxide paper in pure hydrazine we were able to remove oxygen functionalities and restore the planar geometry of the single sheets. The chemically converted graphene sheets that were produced have the largest area reported to date (up to 20 x 40 microm), making them far easier to process. Field-effect devices have been fabricated by conventional photolithography, displaying currents that are three orders of magnitude higher than previously reported for chemically produced graphene. The size of these sheets enables a wide range of characterization techniques, including optical microscopy, scanning electron microscopy and atomic force microscopy, to be performed on the same specimen.

Practical Chemical Sensors from Chemically Derived Graphene

We report the development of useful chemical sensors from chemically converted graphene dispersions using spin coating to create single-layer films on interdigitated electrode arrays. Dispersions of graphene in anhydrous hydrazine are formed from graphite oxide. Preliminary results are presented on the detection of NO(2), NH(3), and 2,4-dinitrotoluene using this simple and scalable fabrication method for practical devices. Current versus voltage curves are linear and ohmic in all cases, studied independent of metal electrode or presence of analytes. The sensor response is consistent with a charge transfer mechanism between the analyte and graphene with a limited role of the electrical contacts. A micro hot plate sensor substrate is also used to monitor the temperature dependence of the response to nitrogen dioxide. The results are discussed in light of recent literature on carbon nanotube and graphene sensors.

Low-Temperature Solution Processing of Graphene−Carbon Nanotube Hybrid Materials for High-Performance Transparent Conductors

We report the formation of a nanocomposite comprised of chemically converted graphene and carbon nanotubes. Our solution-based method does not require surfactants, thus preserving the intrinsic electronic and mechanical properties of both components, delivering 240 ohms/square at 86% transmittance. This low-temperature process is completely compatible with flexible substrates and does not require a sophisticated transfer process. We believe that this technology is inexpensive, is massively scalable, and does not suffer from several shortcomings of indium tin oxide. A proof-of-concept application in a polymer solar cell with power conversion efficiency of 0.85% is demonstrated. Preliminary experiments in chemical doping are presented and show that optimization of this material is not limited to improvements in layer morphology.

A One-Step, Solvothermal Reduction Method for Producing Reduced Graphene Oxide Dispersions in Organic Solvents

Refluxing graphene oxide (GO) in N-methyl-2-pyrrolidinone (NMP) results in deoxygenation and reduction to yield a stable colloidal dispersion. The solvothermal reduction is accompanied by a color change from light brown to black. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) images of the product confirm the presence of single sheets of the solvothermally reduced graphene oxide (SRGO). X-ray photoelectron spectroscopy (XPS) of SRGO indicates a significant increase in intensity of the C=C bond character, while the oxygen content decreases markedly after the reduction is complete. X-ray diffraction analysis of SRGO shows a single broad peak at 26.24 degrees 2theta (3.4 A), confirming the presence of graphitic stacking of reduced sheets. SRGO sheets are redispersible in a variety of organic solvents, which may hold promise as an acceptor material for bulk heterojunction photovoltaic cells, or electromagnetic interference shielding applications.

The effects of thionyl chloride on the properties of graphene and graphene–carbon nanotube composites

Anionic dopants have been used to reduce the overall sheet resistance of carbon nanotube and graphene films for transparent conductor applications. These enhanced electronic properties are attributed to an increased number of p-type charge carriers. While there have been many reports of its use, there is little reported insight into the chemical interactions of a commonly used dopant, thionyl chloride (SOCl2), with pristine graphene and its chemically converted derivatives. Here, we explore the effects of thionyl chloride on the physical and chemical properties of graphene and hybrid graphene–carbon nanotube films, focusing on how the changes in conductivity correlate to the morphology of chemically converted graphene and carbon nanotube composites.

Versatile solution for growing thin films of conducting polymers.

The method employed for depositing nanostructures of conducting polymers dictates potential uses in a variety of applications such as organic solar cells, light-emitting diodes, electrochromics, and sensors. A simple and scalable film fabrication technique that allows reproducible control of thickness, and morphological homogeneity at the nanoscale, is an attractive option for industrial applications. Here we demonstrate that under the proper conditions of volume, doping, and polymer concentration, films consisting of monolayers of conducting polymer nanofibers such as polyaniline, polythiophene, and poly(3-hexylthiophene) can be produced in a matter of seconds. A thermodynamically driven solution-based process leads to the growth of transparent thin films of interfacially adsorbed nanofibers. High quality transparent thin films are deposited at ambient conditions on virtually any substrate. This inexpensive process uses solutions that are recyclable and affords a new technique in the field of conducting polymers for coating large substrate areas.

Temperature dependent Raman spectroscopy of chemically derived graphene

Reduced graphite oxide (GO) has shown promise as a scalable alternative to mechanically exfoliated specimens. Although many measurements show that reduced GO has properties approaching those of pristine graphene, it has been difficult to quantify the extent to which the graphitic network is restored upon reduction. Raman spectroscopy is widely used for the characterization of mechanically exfoliated graphene, but has not been fully explored for reduced GO. In this work, hydrazine suspensions of reduced GO are deposited on micro-hot-plates and examined over a range of temperatures by Raman spectroscopy. The work highlights the benefits of solution processing.

Stenciling Graphene, Carbon Nanotubes, and Fullerenes Using Elastomeric Lift-Off Membranes

Graphene, a 2D crystal comprised of single-atom-thick sheets of hexagonal sp2 carbon atoms, has garnered much attention recently owing to potential applications in field-effect transistors (FETs), sensors, composite reinforcement, supercapacitors, and emissive displays.[1–5] While other low-dimensional allotropes of carbon have been extensively studied since their discoveries over 20 years ago,[6,7] graphene research is still in its infancy. Although many methods to synthesize these nanomaterials have been demonstrated, there are relatively few approaches to systematically organize these materials that meet the high throughput requirements for practical device applications. In this Communication, we demonstrate that thin elastomeric membranes comprised of poly(dimethylsiloxane) (PDMS) can be utilized as physical stencils for patterning chemically converted graphene (CCG), carbon nanotubes (CNTs), and fullerenes, all of which can be dispersed in hydrazine. Furthermore, this method represents a simple and versatile process to selectively register these carbon nanomaterials into configurations suitable for nanoelectronic devices.