Jonathan K. Wassei

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.

Graphene, a promising transparent conductor

New electronic devices such as touch screens, flexible displays, printable electronics, solid-state lighting and thin film photovoltaics have led to a rapidly growing market for flexible transparent conductors. Standard indium tin oxide films are unlikely to satisfy future needs due to losses in conductivity on bending and the escalating cost of indium which is in limited supply. Recent advances in the synthesis and characterization of graphene indicate that it may be suitable for many electronic applications including as a transparent conductor. Graphene hybrids with, for example, carbon nanotubes, may prove to be especially interesting.

Graphene flash memory.

Graphenes single atomic layer of sp(2) carbon has recently garnered much attention for its potential use in electronic applications. Here, we report a memory application for graphene, which we call graphene flash memory (GFM). GFM has the potential to exceed the performance of current flash memory technology by utilizing the intrinsic properties of graphene, such as high density of states, high work function, and low dimensionality. To this end, we have grown large-area graphene sheets by chemical vapor deposition and integrated them into a floating gate structure. GFM displays a wide memory window of ∼6 V at significantly low program/erase voltages of ±7 V. GFM also shows a long retention time of more than 10 years at room temperature. Additionally, simulations suggest that GFM suffers very little from cell-to-cell interference, potentially enabling scaling down far beyond current state-of-the-art flash memory devices.

Continuity of Graphene on Polycrystalline Copper

The atomic structure of graphene on polycrystalline copper substrates has been studied using scanning tunneling microscopy. The graphene overlayer maintains a continuous pristine atomic structure over atomically flat planes, monatomic steps, edges, and vertices of the copper surface. We find that facets of different identities are overgrown with graphenes perfect carbon honeycomb lattice. Our observations suggest that growth models including a stagnant catalytic surface do not apply to graphene growth on copper. Contrary to current expectations, these results reveal that the growth of macroscopic pristine graphene is not limited by the underlying copper structure.

Chemical Vapor Deposition of Graphene on Copper from Methane, Ethane and Propane: Evidence for Bilayer Selectivity

To study the effects of hydrocarbon precursor gases, graphene is grown by chemical vapor deposition from methane, ethane, and propane on copper foils. The larger molecules are found to more readily produce bilayer and multilayer graphene, due to a higher carbon concentration and different decomposition processes. Single- and bilayer graphene can be grown with good selectivity in a simple, single-precursor process by varying the pressure of ethane from 250 to 1000 mTorr. The bilayer graphene is AB-stacked as shown by selected area electron diffraction analysis. Additionally propane is found to only produce a combination of single- to few-layer and turbostratic graphene. The percent coverage is investgated using Raman spectroscopy and optical, scanning electron, and transmission electron microscopies. The data are used to discuss a possible mechanism for the second-layer growth of graphene involving the different cracking pathways of the hydrocarbons.

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.

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.