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Dielectrophoretic Assembly of Graphene Nanostructures

Written by: 
Jeff Morse, Ph.D.
Burg, et. al., demonstrate a controllable and reliable method for the scalable, high-yield directed assembly of ultrathin graphene sheets.

Reviewed by Jeff Morse, Ph.D., National Nanomanufacturing Network

Burg Figure 1
Chip schematic.
The availability of two-dimensional graphene sheets has received considerable attention in recent years due to the enhanced electronic properties of this unique material, most notably an electron mobility in excess of 200,000 cm2V-1s-1, for potential applications in nanoscale electronics, energy, sensors, and nanocomposites. At present, graphene handling and manipulation remains dominated by mechanical exfoliation techniques that suffice for laboratory research and analysis, but does not facilitate large-scale integration. In order to advance the potential of graphene materials for these application, a nanomanufacturing process technology is needed to enable a parallel, controllable assembly method.

Recently, Burg, et. al., report a significant step toward this goal, demonstrating the scalable, high-yield integration and assembly of soluble two-dimensional ultrathin graphene sheets. Utilizing dielectrophoresis—a  method for the directed movement of nanoscale objects in nonuniform electric fields—the  authors studied the assembly of functionalized graphene sheets dispersed in aqueous solvents. The dielectrophoretic force effectively attracts particles having polarizability greater than the suspension medium to regions of high electric field, while repelling others, and is often used as a technique for separations within microfluidic structures and devices. Using photolithographic techniques, the authors patterned electrode arrays having 1-2 µm gaps onto an oxidized silicon substrate. Voltage applied between the electrodes provided the necessary electric field required for dielectrophoretic assembly. Chemically treated, purified, and exfoliated graphene oxide sheets were dispersed in solvent suspensions. A 5 µl droplet of the suspension was placed over the electrode array, after which an AC voltage signal (5 V @ 5 MHz) was applied between the electrodes. Graphene oxide sheets a few monolayers thick (3-12 nm) assembled over the electrode gap. As the initial graphene oxide sheets were insulating, thermal annealing was conducted on the assembled structures to reduce the graphene oxide to metallic graphene. A 450°C, 1-hour rapid thermal anneal step reduced the sheet thickness to 2-10 nm and  drove off any interstitial water and functional groups in the material.

Subsequent electrical testing of the graphene sheets between the electrodes demonstrated that the resistance dropped from >10 GΩ to approximately 40 KΩ after the thermal anneal step, confirming the reduction process in forming metallic graphene. The average conductivity of the sheets was measured to be 15-36 S/cm when accounting for the thickness, corroborating  previously reported values.  The authors report in addition that by increasing the voltage signal applied during assembly, the thickness of the layer increases, therefore some control over the properties of the graphene layers can be demonstrated.

This process represents an inherently controllable and reliable method for the directed assembly of ultrathin graphene sheets. These results represent a key step towards site selective deposition of two-dimensional nanostructures, including water soluble or functionalized graphene. Although issues for device and process integration must be investigated, such directed assembly techniques advance the state of the technology towards feasible device and system studies employing these unique materials systems.

Image reproduced with permission from Burg BR, Lutolf F, Schneider J, Schirmer NC, Schwamb T, and Poulikakos D.2009. High-yield Dielectrophoretic Assembly of Two-dimensional GrapheneNanostructures. Appl Phys Lett 94(053110). Copyright 2009, American Institute of Physics.

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