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The synthesis of wafer-scale single crystal graphene remains a challenge toward the utilization of its intrinsic properties in electronics. Until now, the large-area chemical vapor deposition of graphene has yielded a polycrystalline material, where grain boundaries are detrimental to its electrical properties. Here, we study the physicochemical mechanisms underlying the nucleation and growth kinetics of graphene on copper, providing new insights necessary for the engineering synthesis of wafer-scale single crystals. Graphene arises from the crystallization of a supersaturated fraction of carbon-adatom species, and its nucleation density is the result of competition between the mobility of the carbon-adatom species and their desorption rate. As the energetics of these phenomena varies with temperature, the nucleation activation energies can span over a wide range (1-3 eV) leading to a rational prediction of the individual nuclei size and density distribution. The growth-limiting step was found to be the attachment of carbon-adatom species to the graphene edges, which was independent of the Cu crystalline orientation.

Transport properties of progressively reduced graphene oxide (GO) are described.Evolution of the electronic properties reveals that as-synthesized GO undergoes insulator-semiconductor-semi-metal transitions with reduction. The apparent transport gap ranges from 10 ~ 50 meV and approaches zero with extensive reduction. Measurements at varying degrees of reduction reveal that transport in reduced GO occurs via variable-range hopping and further reduction leads to increased number of available hopping sites.

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 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 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 uncharged graphene in solution. Degradation of the molecules intercalated in GICs can be triggered by high temperature treatment or microwave irradiation, creating a gas pressure surge in graphite and exfoliation. Electrochemical exfoliation by applying a voltage in an electrolyte to a graphite electrode can be tuned by varying precursors, electrolytes and potential. Graphite electrodes can be either negatively or positively intercalated to obtain GICs that are subsequently exfoliated. We also discuss the materials that can be amenable to exfoliation, by ...

Toward the large-area deposition of MoS 2 layers, we employ metal−organic precursors of Mo and S for a facile and reproducible van der Waals epitaxy on c-plane sapphire. Exposing c-sapphire substrates to alkali metal halide salts such as KI or NaCl together with the Mo precursor prior to the start of the growth process results in increasing the lateral dimensions of single crystalline domains by more than 2 orders of magnitude. The MoS 2 grown this way exhibits high crystallinity and optoelectronic quality comparable to singlecrystal MoS 2 produced by conventional chemical vapor deposition methods. The presence of alkali metal halides suppresses the nucleation and enhances enlargement of domains while resulting in chemically pure MoS 2 after transfer. Field-effect measurements in polymer electrolyte-gated devices result in promising electron mobility values close to 100 cm 2 V −1 s −1 at cryogenic temperatures. KEYWORDS: Chemical vapor deposition, two-dimensional transition metal dichalcogenides, nucleation and growth, microstructure engineering, FET devices T he chemical vapor deposition of two-dimensional materials is a highly promising method to produce atomically thin layers at a large scale for harnessing their attractive properties. Monolayer MoS 2 is a model 2D semiconductor that can be used to realize field-effect transistors with high current on/off ratios. 1 It is a naturally occurring material with a good chemical stability that exhibits a wide range of attractive properties such as a spin−orbit couplinginduced band splitting, 2,3 a mechanically tunable bandgap, 4−8 and a low temperature superconductivity. 9−13 Toward the large-scale synthesis of MoS 2 thin films, a conventional chemical vapor deposition method of producing MoS 2 monolayers typically involves low vapor pressure solid powder precursors such as MoO 3 and sulfur. It has been investigated for centimeter-scale deposition of polycrystalline monolayer MoS 2 with grain sizes of nanometer to micrometer and with controllable coverage. 14,15 However, low vapor pressures of the solid precursors require them to be loaded inside a heated zone of the reactor chamber leading to a limited control over the vapor phase composition and deposition rate. Thus, this synthesis approach heavily undermines the ability to control the nucleation density, thickness, and coverage.Here, we aim to address this issue by employing well-known metal−organic precursors of molybdenum, Mo(CO) 6 , which is a high vapor pressure solid, and of sulfur, H 2 S in gas phase. 16−19 This metal-organic chemical vapor deposition (MOCVD) approach allows reliably setting the concentration of precursor gases within the gaseous mixture that is transported to the substrate by controlling the evaporation rates of the solid precursor and mass flow rates. An extensive vapor phase thermodynamics study performed by Kumar et al. 19 has shown that growth temperatures above 850°C at atmospheric pressure lead to layer-by-layer growth of MoS 2 without extraneous deposition of carbon ...