Graphene Synthesis Essay

2. GRAPHENE FABRICATION TECHNIQUES

Fabrication of graphene can be divided into two different approaches. They are bottom-up and top-down approaches. Bottom-up methods involve synthesizing graphene from alternative carbon containing sources, whereas top-down methods involve breaking apart the stacked layers of graphite to yield graphene. Figure shows the schematic representation of the bottom-up and top-down graphene synthesis. Figure 10: A schematic of ‘bottom-up’ and ‘top-down’ graphene synthesis. 2.1 BOTTOM-UP APPROACHES

For bottom-up methods high levels of graphitization must be promoted to produce good quality material, so these methods generally require high temperatures. The processes involved are usually simple, although the material

Graphene growth occurs due to the precipitation of the graphite from carbon species within the metal. Yu et al. grew few-layer graphene sheets on polycrystalline Ni foils. The foils were first annealed in hydrogen and then exposed to a CH4-Ar-H2 environment at atmospheric pressure for 20 mins at a temperature of 10000C. The foils were then cooled at different rates between 200C/s and 0.10C/s. The thickness of the graphene layers was found to be dependent on the cooling rate, with few layer graphene (typically 3-4 layers) being produced with a cooling rate of 100C/s. Faster cooling rates result in thicker graphite layers, whereas slower cooling prevents carbon from segregating to the surface of the Ni foil [21]. To transfer the graphene layers to an insulating substrate, the Ni foil with graphene was first coated in silicon rubber and covered with a glass slide, then the Ni was etched in HNO3. Li et al. used a similar process to produce large-scale monolayer graphene on copper foils. 25 μm thick copper foils were first heated to 10000C in a flow of 2 sccm (standard cubic centimeters per minute) hydrogen at low pressure and then exposed to methane flow of 35 sccm and pressure of

This unzipping results in graphene ‘nanoribbons’, with ribbon widths dictated by the diameter of the tubes. Graphene nanoribbons are considered as quasi one dimensional material, and have different properties depending on their width and edge type (armchair/zigzag). Carbon nanotube unzipping occurs via C–C bond fission which is often initiated at defect sites, leading to irregular cutting. Recently the synthesis of well regulated nanoribbons has been demonstrated via unzipping of flattened carbon nanotubes, where attack occurs preferentially along the bent edges (Figure2.5). Producing graphene from carbon nanotubes is set to raise interesting questions as graphene is often seen as a replacement or alternative to