The highly unusual properties of graphene has triggered a flurry of research activity in 2D carbon worldwide over the past four years. The isolation of graphene (high quality single layer of carbon) or few layers graphene (FLG) by mechanical or chemical exfoliation of highly oriented pyrolytic graphite (HOPG) bulk crystals turns out to be the most popular and successful method for fabricating graphene based devices, but the main downside of the method is the resulting random dispersion of the graphene sheets on the substrate. The lack of site specificity in the deposition of graphene prevents any possibility of high throughput fabrication of graphene-based devices.
Site-specific transfer-printing has recently been successfully demonstrated, making use of stamps with protrusions coated with ‘glue’ to exfoliate graphene from bulk HOPG crystals and ‘fixing agents’ to deposit the graphene onto the substrates. A team from the Center for Emergent Materials at the Ohio State University exposed a physical hypothesis based on theory and ab initio modeling of a conceptually simpler technique for site specific stamping of patterned FLG on oxidised Si substrates with no use of interfacial layers such as glue or fixing agents [Li et al., Adv. Mat. (2008) 20, 1].
The authors consider three different scenarios of a semi-infinite graphite single crystal as it is pulled away from an amorphous SiO2 substrate, in vacuum: 1) full separation between the crystal and the substrate, 2) cleavage of the graphite at a distance from the graphite/SiO2 interface and 3) cleavage in the vicinity of the interface leaving a single sheet of graphene to adhere to the substrate. They express theoretically the in vacuo Dupre work of adhesion parameter in each case. Values of this parameter found in the literature and confirmed by density functional theory (DFT) total energy calculations reinforce the notion that the third scenario, ie the graphene- or FLG-stamping situation, is expected to be energetically the most favourable.
Li et al. then go on to demonstrate experimentally the feasibility of the site-specific stamping technique of patterned FLG and the high reproducibility of the method for a variety of patterns. If the stamping of even fewer layers of graphene over a large area still needs better engineering, the stamping of large-area complex patterns of FLG at predetermined sites is a distinct possibility. The site-specific nature of the patterned stamps could pave the way for possible high-throughput fabrication of graphene-based integrated devices in the future.