Effect of Gas-Phase Reaction on the CVD Growth of Graphene

Chemical vapor deposition (CVD) is considered as the most promising method for the mass production of high-quality graphene films owing to its fine controllability, uniformity, and scalability. In the past decade, significant efforts have been devoted to exploring new strategies for growing graphene with improved quality. During the high-temperature CVD growth process of graphene, besides the surface reactions, gas-phase reactions play an important role in the growth of graphene, especially for the decomposition of hydrocarbons. However, the effect of gas-phase reactions on the CVD growth of graphene has not been analyzed previously. To fill this gap, it is essential to systematically analyze the relationship between gas-phase reactions and the growth of graphene films. In this review article, we aim to provide comprehensive knowledge of the gas-phase reactions occurring in the CVD system during graphene growth and to summarize the typical strategies for improving the quality of graphene by modulating gas-phase reactions. After briefly introducing the elementary steps and basic concept of graphene growth, we focus on the gas-phase dynamics and reactions in the CVD system, which influence the decomposition of hydrocarbons, nucleation of graphene, and lateral growth of graphene nuclei, as well as the merging of adjacent graphene domains. Then, a systematic description of the mass transport process in gas phase is provided, including confirmation of the states of gas flow under different CVD conditions and introduction to the boundary layer, which is crucial for graphene growth. Furthermore, we discuss the possible reaction paths of carbon sources in the gas phase and the corresponding active carbon species existing in the boundary layer, based on which the main impact factors of gas-phase reactions are discussed. Representative strategies for obtaining graphene films with improved quality by modulating gas-phase reactions are summarized. Gas-phase reactions affect the crystallinity, cleanness, domain size, layer number, and growth rate of graphene grown on both metal and non-metal substrates. Therefore, we will separately review the detailed strategies, corresponding mechanisms, key parameters, and latest status regarding the quality improvement of graphene. Finally, a brief summary and proposals for future research are provided. This review can be divided into two parts: (1) gas-phase reactions occurring in the high-temperature CVD system, including the mass transport process and the reaction paths of hydrocarbons; and (2) the synthesis of high-quality graphene film via modulation of the gas-phase reaction, in order to improve the crystallinity, cleanness, domain size, layer number, and growth rate of graphene.