Seeing graphene-based sheets

Graphene is two-dimensional nanomaterial consisting of a single layer of sp2 network of carbon atoms (Fig. 1a)1. While the thickness of a graphene sheet is on the order of a single atomic unit, its lateral dimension can approach up to tens of microns. Graphene and its derivatives such as graphene oxide (GO) (Fig. 1b), and reduced graphene oxide (r-GO, a.k.a. chemically modified graphene) (Fig. 1c) have attracted great interests in both fundamental science and technology due to their intriguing structures and excellent electronic, mechanical and thermal properties2-4. Graphene-based sheets (GBS) have been shown to be very promising for high-performance nanoelectronics1,5-7, transparent conductor8-12, polymer composite13,14 and electron microscopy support15-17, etc. Initially graphene was discovered and prepared at the individual sheet level by mechanical exfoliation from highly ordered pyrolytic graphite (HOPG) crystals – the so-called Scotch tape trick1. To scale up the production, various synthetic methods are being developed. At a slightly larger “single substrate” scale, high quality graphene thin films have been prepared epitaxially on SiC surface18, and most recently by chemical vapor deposition (CVD) on catalytic metal surfaces5,19-21. At an even larger scale, bulk production typically yields GBS in forms of solvent dispersion, which can then be used by solution processing techniques in various forms. Some recent success includes chemical exfoliation through the formation of derivatized graphene sheets such as GO22-25, r-GO12,26 or halogenated graphene27, solvent assisted ultrasonic exfoliation28 and solvothermal reduction of organic solvent using alkali metals29. GBS are essentially the world’s thinnest materials: they are single atomic layers with lateral dimension extends from nanometers well into tens of microns. The first characterization step is typically microscopy imaging to determine whether single layers are present, their positions on the substrate and sizes. It is an indispensable quality control tool for manufacturing GBS materials since it can provide immediate feedback to improve synthetic and processing strategies. Graphene-based sheets such as graphene, graphene oxide and reduced graphene oxide have stimulated great interest due to their promising electronic, mechanical and thermal properties. Microscopy imaging is indispensable for characterizing these single atomic layers, and oftentimes is the first measure of sample quality. This review provides an overview of current imaging techniques for graphene-based sheets and highlights a recently developed fluorescence quenching microscopy technique that allows high-throughput, high-contrast imaging of graphene-based sheets on arbitrary substrate and even in solution.