**2. Generally growth mechanism of CVD-based graphene**

CVD growth of graphene is a chemical process for the formation of SLG or FLG on an arbitrary substrate by exposing the substrate to the gas-phase precursors at controlled reaction conditions [69]. Owing to the versatile nature of CVD, intricately mixed homogeneous gas-phase and heterogeneous surface reactions are involved [70]. In general, as the partial pressure and/ or temperature in the reaction substances are increased, homogeneous gas-phase reactions and the resulting homogeneous nucleation become significant [80]. To grow a high-quality graphene layer, this homogeneous nucleation needs be minimized [70]. A general mechanism for CVD-based graphene growth on catalytic metal substrates, for the growth of uniform and highly crystalline graphene layer on the surface, includes eight steps as follow: (1) mass transport of the reactant, (2) reaction of the film precursor, (3) diffusion of gas molecules, (4) adsorption of the precursor, (5) diffusion of the precursor into substrate, (6) surface reaction, (7) desorption of the product, and (8) removal of the by-product (**Figure 2**) [71].

Typically, CVD growth of 2D materials (e.g. graphene) involves catalytic activation of chemical reactions of precursors at the growth substrate surface/interface in a properly designed

**Figure 2.** Diagram of generally growth mechanism of CVD-based graphene: Transport and reaction processes. Reproduced with permission from [71]. Copyright 2011, Freund Publishing.

environment. Generally speaking, the roles of precursors, conditions (e.g. fast growth rates, large domain size, or very high crystalline quality), atmosphere, substrates and catalysts are the key factors affecting the final quality of the grown 2D materials. So far, significant efforts have been made to prepare highly crystalline 2D materials (e.g. graphene), but many challenges are still ahead. For example, due to the rough feature of catalytic metal surface, growth of uniform and high quality graphene is considerably difficult. The 2D material research community is also interested in new precursors (e.g. solid precursor only, gas precursor or solid precursor mixed with certain solvents) that could induce the formation of high-quality uniform graphene with minimal defect density. Another question is the effect of growth rate on the catalytic metal surface on the quality of graphene. Currently, it is difficult to give an exact answer, as investigations are progressing at an exponential rate.

devices, such as sensors [63–65], black silicon solar cells [66], cambered micro-optics [67], 3D microelectromechanical system (MEMS) [68], or CMOS technology-based applications [2]. Here, we present an overview of various recently reported strategies for direct graphene growth on flexible substrates. In addition, a wide-range of applications as well as the per-

CVD growth of graphene is a chemical process for the formation of SLG or FLG on an arbitrary substrate by exposing the substrate to the gas-phase precursors at controlled reaction conditions [69]. Owing to the versatile nature of CVD, intricately mixed homogeneous gas-phase and heterogeneous surface reactions are involved [70]. In general, as the partial pressure and/ or temperature in the reaction substances are increased, homogeneous gas-phase reactions and the resulting homogeneous nucleation become significant [80]. To grow a high-quality graphene layer, this homogeneous nucleation needs be minimized [70]. A general mechanism for CVD-based graphene growth on catalytic metal substrates, for the growth of uniform and highly crystalline graphene layer on the surface, includes eight steps as follow: (1) mass transport of the reactant, (2) reaction of the film precursor, (3) diffusion of gas molecules, (4) adsorption of the precursor, (5) diffusion of the precursor into substrate, (6) surface reaction,

**2. Generally growth mechanism of CVD-based graphene**

(7) desorption of the product, and (8) removal of the by-product (**Figure 2**) [71].

Typically, CVD growth of 2D materials (e.g. graphene) involves catalytic activation of chemical reactions of precursors at the growth substrate surface/interface in a properly designed

**Figure 2.** Diagram of generally growth mechanism of CVD-based graphene: Transport and reaction processes. Repro-

duced with permission from [71]. Copyright 2011, Freund Publishing.

spectives and challenges are also addressed.

74 Flexible Electronics

However, non-catalytic direct-growth of graphene on semiconducting and dielectric rigid and flexible substrates follows different mechanisms according to our best insights. To date, the understanding of the concept of the general mechanism of the direct growth of graphene is still not yet adequate, neither experimentally nor theoretically, with many proposed possible growth mechanisms, e.g. vapor-solid-solid [72], or vapor-solid [73], or solid-liquidsolid [74]. There has been arguments on the direct-growth mechanism of graphene domains on dielectric rigid and flexible substrates or non-catalytic substrates in previous reports [59, 60, 75], but the mechanism for the entire process of the carbon precursor transformation to the crystalline graphene structures has not yet been fully understood. Thus, understanding the graphene growth mechanism and the effect of various growth conditions will be of significant interest to the 2D material research community to obtain large-scale, high-quality graphene.
