Gaseous Oxidants-controlled Growth Of Graphene By Chemical Vapor Deposition

Two-dimensional materials have attracted significant attention due to their super properties and great potentials for various applications.Among them,there is no doubt that graphene is the most outstanding material on which tremendous and valuable pioneering research works have been conducted.It is critical to achieve precise synthesis controls over its parameters and related physical and chemical properties.Especially,chemical vapor deposition(CVD)has emerged as an approach that combines high-quality and low cost for graphene synthesis.On the other hand,this method offers a versatile platform for both synthesis and fundamental research of two-dimensional crystals.It provides a plenty space for adjusting the final product and further process of the material with substrates for certain purpose.As a result,CVD growth of graphene is not only the material synthesis technology,but also has the potential of subqequent engineering process for further applications.However,understanding the intrinsic growth mechanism to control the material preparation forms the prerequisite and fundamental for material application.In this paper,we focus on two key issues in CVD growth of graphene:controllable synthesis of large-size single crystals and grain boundary formation mechanism.The main contents are listed as follows:(1)Grains and grain boundaries in graphene are vital for the control of its properties.Engineering or controlling grain boundaries of graphene by growth is highly desirable.Firstly,we use the combined graphene etching,mild air oxidation and transmission electron microscopy techniques to probe grains and grain boundaries in chemical vapor deposited polygonal graphene flakes.We correlate grains and grain boundaries,various polygonal shapes,graphene etching shapes and growth process,and demonstrate that the landscape of grain boundaries in polygonal graphene flakes can be predicted from their geometric shapes or etched patterns.Grain boundary is determined to be a continuous and straight line that bisects the angle between polygonal edges,leading to symmetric tilt grain boundary in polygonal graphene flakes.We also show that grain boundary impedes electrical transport of graphene.This work presents a significant step forward in engineering grain boundaries in graphene.(2)Controlling the growth behavior of graphene during the CVD process is a major issue in high-quality graphene synthesis.One of the most critical obstacles encountered in large-area single crystalline graphene growth is the suppression of nucleation density.In the present study,we demonstrate that oxidative etchants are highly responsible for graphene etching,which significantly govern the nucleation behavior and growth rate.Single crystalline hexagonal graphene domain of 1 cm size can be obtained by precisely controlling the amount of O2.(3)Rapid growth of large-area graphene single crystals is essential to realize the application of high-quality graphene in various applications.Fast growth rate and low nucleation density are two critical points to obtain large graphene single crystals.However,they can not be realized by simply ajusting growth paremeters at the same time,which remains an obstacle so far.In this paper,we develop a one-step approach that compromises the merits of both fast growth rate and low nucleation density.It is proposed that trace amounts of oxidizing impurities can act as promoters for CH4 dissociation and active C adatoms absorption,leading to formation of more graphene nuclei.By diminishing oxidizing impurities as much as possible,graphene nucleation density can be dramatically suppressed without decreasing growth rate.Large-size grpahene single crystals can be syntheiszed with maxium growth rate up to 190 ?m min-1.