Production and processing of graphene and related materials

We present an overview of the main techniques for production and processing of graphene and related materials (GRMs), as well as the key characterization procedures. We adopt a 'hands-on' approach, providing practical details and procedures as derived from literature as well as from the authors' experience, in order to enable the reader to reproduce the results. Section I is devoted to 'bottom up' approaches, whereby individual constituents are pieced together into more complex structures. We consider graphene nanoribbons (GNRs) produced either by solution processing or by on-surface synthesis in ultra high vacuum (UHV), as well carbon nanomembranes (CNM). Production of a variety of GNRs with tailored band gaps and edge shapes is now possible. CNMs can be tuned in terms of porosity, crystallinity and electronic behaviour. Section II covers 'top down' techniques. These rely on breaking down of a layered precursor, in the graphene case usually natural crystals like graphite or artificially synthesized materials, such as highly oriented pyrolythic graphite, monolayers or few layers (FL) flakes. The main focus of this section is on various exfoliation techniques in a liquid media, either intercalation or liquid phase exfoliation (LPE). The choice of precursor, exfoliation method, medium as well as the control of parameters such as time or temperature are crucial. A definite choice of parameters and conditions yields a particular material with specific properties that makes it more suitable for a targeted application. We cover protocols for the graphitic precursors to graphene oxide (GO). This is an important material for a range of applications in biomedicine, energy storage, nanocomposites, etc. Hummers' and modified Hummers' methods are used to make GO that subsequently can be reduced to obtain reduced graphene oxide (RGO) with a variety of strategies. GO flakes are also employed to prepare three-dimensional (3d) low density structures, such as sponges, foams, hydro- or aerogels. The assembly of flakes into 3d structures can provide improved mechanical properties. Aerogels with a highly open structure, with interconnected hierarchical pores, can enhance the accessibility to the whole surface area, as relevant for a number of applications, such as energy storage. The main recipes to yield graphite intercalation compounds (GICs) are also discussed. GICs are suitable precursors for covalent functionalization of graphene, but can also be used for the synthesis of unchargedgraphene in solution. Degradation of the molecules intercalated in GICs can be triggered by hightemperature treatment or microwave irradiation, creating a gas pressure surge in graphite andexfoliation. Electrochemical exfoliation by applying a voltage in an electrolyte to a graphite electrodecan be tuned by varying precursors, electrolytes and potential. Graphite electrodes can be eithernegatively or positively intercalated to obtain GICs that are subsequently exfoliated. We also discussthe materials that can be amenable to exfoliation, by employing a theoretical data-mining approach.The exfoliation of LMs usually results in a heterogeneous dispersion of flakes with different lateralsize and thickness. This is a critical bottleneck for applications, and hinders the full exploitationof GRMs produced by solution processing. The establishment of procedures to control themorphological properties of exfoliated GRMs, which also need to be industrially scalable, is oneof the key needs. Section III deals with the processing of flakes. (Ultra)centrifugation techniqueshave thus far been the most investigated to sort GRMs following ultrasonication, shear mixing,ball milling, microfluidization, and wet-jet milling. It allows sorting by size and thickness. Inksformulated from GRM dispersions can be printed using a number of processes, from inkjet to screenprinting. Each technique has specific rheological requirements, as well as geometrical constraints.The solvent choice is critical, not only for the GRM stability, but also in terms of optimizing printingon different substrates, such as glass, Si, plastic, paper, etc, all with different surface energies.Chemical modifications of such substrates is also a key step.Sections IV-VII are devoted to the growth of GRMs on various substrates and their processing aftergrowth to place them on the surface of choice for specific applications. The substrate for graphenegrowth is a key determinant of the nature and quality of the resultant film. The lattice mismatch betweengraphene and substrate influences the resulting crystallinity. Growth on insulators, such as SiO2, typicallyresults in films with small crystallites, whereas growth on the close-packed surfaces of metals yieldshighly crystalline films. Section IV outlines the growth of graphene on SiC substrates. This satisfies therequirements for electronic applications, with well-defined graphene-substrate interface, low trappedimpurities and no need for transfer. It also allows graphene structures and devices to be measured directlyon the growth substrate. The flatness of the substrate results in graphene with minimal strain and rippleson large areas, allowing spectroscopies and surface science to be performed. We also discuss the surface engineering by intercalation of the resulting graphene, its integration with Si-wafers and the production of nanostructures with the desired shape, with no need for patterning. Section V deals with chemical vapour deposition (CVD) onto various transition metals and on insulators. Growth on Ni results in graphitized polycrystalline films. While the thickness of these films can be optimized by controlling the deposition parameters, such as the type of hydrocarbon