Protection from Below: Stabilizing Hydrogenated Graphene Using Graphene Underlayers

Graphene Transfer: A Physical Perspective

Graphene, synthesized either epitaxially on silicon carbide or via chemical vapor deposition (CVD) on a transition metal, is gathering an increasing amount of interest from industrial and commercial ventures due to its remarkable electronic, mechanical, and thermal properties, as well as the ease with which it can be incorporated into devices. To exploit these superlative properties, it is generally necessary to transfer graphene from its conductive growth substrate to a more appropriate target substrate. In this review, we analyze the literature describing graphene transfer methods developed over the last decade. We present a simple physical model of the adhesion of graphene to its substrate, and we use this model to organize the various graphene transfer techniques by how they tackle the problem of modulating the adhesion energy between graphene and its substrate. We consider the challenges inherent in both delamination of graphene from its original substrate as well as relamination of graphene onto its target substrate, and we show how our simple model can rationalize various transfer strategies to mitigate these challenges and overcome the introduction of impurities and defects into the graphene. Our analysis of graphene transfer strategies concludes with a suggestion of possible future directions for the field.

Graphene-enabled block copolymer lithography transfer to arbitrary substrates

We describe a method for phase separating and transferring block copolymer (BCP) nanoscale patterns to arbitrary substrates for surface-independent nanolithography. The enabling technology is a hydrogenated or oxidized graphene thin film that only weakly adheres to its substrate. BCPs are applied to these graphene-based materials and solvent annealed to effect nanoscale phase separation. Then, taking advantage of the weak interaction of the graphene film and its substrate, the BCP/graphene stack is delaminated easily in water. A target substrate is then used to retrieve the stack, which can then serve as a lithographic mask. The use of water as a lift-off agent allows for chemically mild retrieval of the phase-separated BCP, extending the BCP lithography technique to essentially arbitrary substrates. We demonstrate this graphene-enabled BCP lithography on silicon nitride and polyethylene. We also show that using reduced graphene oxide (RGO) as a thin film enables the transfer of wafer-scale BCP films and lithography on SiOx and Si. We use an RGO support to produce phase-separated BCP solvent-annealed patterns on polystyrene, a result which is not possible using standard BCP solvent annealing and which shows the utility of this technique. Finally, we demonstrate the ability to create nanopatterns of higher complexity by stacking multiple BCP masks, a capability that is not possible using conventional BCP lithography. This technique may have applications in fabricating nanoporous membranes and photonically active coatings.

Transfer of printed electronic structures using graphene oxide and gelatin enables reversible and biocompatible interface with living cells

We present a low-cost, easy-to-implement platform for printing materials and interfacing them with eukaryotic cells. We show that thermal or chemical reduction of a graphene oxide thin film allows water-assisted delamination of the film from glass or plastic. The chemical and physical properties and permeability of the resulting film are dependent on the method of reduction and deposition of the graphene oxide, with thermal reduction removing more oxidized carbon functionality than chemical reduction. We also developed a method to attach the films onto cell surfaces using a thin layer of gelatin as an adhesive. In general, the films are highly impermeable to nutrients and we observed a significant amount of cell death when gelatin was not used; gelatin enables diffusion of nutrients for sustained cell viability. The combination of nanoscale membranes with a low melting point biopolymer allows us to reversibly interface cells with cargo transferred by graphene oxide while maintaining cell viability. To demonstrate delivery of electronic structures, we modified a commercial off-the-shelf printer to print a silver-based ink directly onto the reduced graphene oxide films which we then transferred to the surface of the cells.

Copper (0) Mediated Single Electron Transfer-Living Radical Polymerization of Methyl Methacrylate: Functionalized Graphene as a Convenient Tool for Radical Initiator

Polymer nanocomposites have been synthesized by the covalent addition of bromide-functionalized graphene (Graphene-Br) through the single electron transfer-living radical polymerization technique (SET-LRP). Graphite functionalized with bromide for the first time via an efficient route using mild reagents has been designed to develop a graphene based radical initiator. The efficiency of sacrificial initiator (ethyl α-bromoisobutyrate) has also been compared with a graphene based initiator towards monitoring their Cu(0) mediated controlled molecular weight and morphological structures through mass spectroscopy (MOLDI-TOF) and field emission scanning electron microscopy (FE-SEM) analysis, respectively. The enhancement in thermal stability is observed for graphene-grafted-poly(methyl methacrylate) (G-g-PMMA) at 392 °C, which may be due to the influence ofthe covalent addition of graphene, whereas the sacrificial initiator used to synthesize G-graft-PMMA (S) has low thermal stability as analyzed by TGA. A significant difference is noticed on their glass transition and melting temperatures by DSC. The controlled formation and structural features of the polymer-functionalized-graphene is characterized by Raman, FT-IR, UV-Vis spectroscopy, NMR, and zeta potential measurements. The wettability measurements of the novel G-graft-PMMA on leather surface were found to be better in hydrophobic nature with a water contact angle of 109 ± 1°.

Liquids relax and unify strain in graphene

Solid substrates often induce non-uniform strain and doping in graphene monolayer, therefore altering the intrinsic properties of graphene, reducing its charge carrier mobilities and, consequently, the overall electrical performance. Here, we exploit confocal Raman spectroscopy to study graphene directly free-floating on the surface of water, and show that liquid supports relief the preexisting strain, have negligible doping effect and restore the uniformity of the properties throughout the graphene sheet. Such an effect originates from the structural adaptability and flexibility, lesser contamination and weaker intermolecular bonding of liquids compared to solid supports, independently of the chemical nature of the liquid. Moreover, we demonstrate that water provides a platform to study and distinguish chemical defects from substrate-induced defects, in the particular case of hydrogenated graphene. Liquid supports, thus, are advantageous over solid supports for a range of applications, particularly for monitoring changes in the graphene structure upon chemical modification.

Here, the authors report water as a superior platform to suspend graphene compared to solid substrates that induce non-uniformity and do not provide structural flexibility. They utilize confocal Raman spectroscopy to study graphene floating freely on the surface of water to show that a liquid support relieves the pre-existing strain.