Controllable Synthesis Of Graphene Films Tuned By Oxygen

Graphene has received extensive attention in the last decade due to its excellent physical and chemical properties together with potential applications.Up to now,large-area graphene films are mainly prepared by chemical vapor deposition（CVD）based on catalytic Cu substrate,which has made great progress.However,the controllability of defects within the crystalline domains and layer uniformity still needs to be improved,and the relevant mechanisms still lack understanding.The presence of oxygen is unavoidable in the fabrication of graphene.an increasing number of studies have shown that even trace amounts of oxygen can have a dramatic effect on the growth of graphene.To investigate and solve this issue,this dissertation aims at the effect of oxygen on graphene growth,provides a deep understanding of its mechanism,and then regulates graphene defects and layer number by tuning of oxygen.The research content and main conclusion are as follows:（1）The effect of oxygen on the quality of graphene at different growth temperatures was designed to investigate intracrystalline defects.It was found that a small amount of oxygen at 950-1050°C would have no effect on graphene,and multilayer strips would be formed at the folds at 750-850°C.With the further decrease of temperature,etching of graphene began at 650°C,the strongest etching occurred at 450°C,and then etching ability gradually weakened.The mechanism was revealed by thermodynamic calculations of the gas-phase reaction.The reaction between oxygen and reducing gases,such as hydrogen and methane,decreased with temperature lowdown,resulting in an increase in residual oxygen concentration,which in turn lead to the promotion of graphene additional layer growth and etching.The weakening of etching at lower temperatures（<450°C）could be attributed by the inability to provide sufficient kinetic driving force.Based on this finding,it was proposed to enhance the quality of graphene films by higher purity gases,rapid and low-pressure cooling.（2）Because of the difficulty of completely eliminating adlayers in large-area monolayer graphene and the limited number of layers in the preparation of multilayer graphene on Cu substrates,the factors of multilayer graphene growth were investigated.It was found that the Cu substrates configuration has significant influence on the growth of graphene adlayers,and the mechanism can be attributed to the asymmetric adlayers growth mechanism based on back diffusion.Further,it is proposed that the growth of adlayers is effectively suppressed by placing the Cu substrate in suspension to obtain symmetrical growth of graphene on both sides of the Cu foil,and large-area full monolayer graphene films are prepared.Multilayer graphene films were prepared by suppressing the growth of graphene on one side of the Cu foil with oxygen to enable the continued growth of multilayer graphene on the other side.（3）By studying the effect of the O/C ratio on the growth of graphene,it was found that as the O/C ratio increased,there was a trend of"adlayer etching-adlayer growth-all layer etching",and the same trend occurred with increasing time at a constant O/C ratio.Oxygen-assisted penetration etching and growth of graphene additional layers were proved to be underneath the continuous graphene film by carbon isotope labeling.Furthermore,the graphene adlayers were etched or promoted to grow by tuning the O/C ratio,so that large-area full monolayer and large-area basic bilayer（bilayer coverage>90%）graphene films could be prepared by oxygen post-treatment,respectively.（4）By studying the effect of the quartz tube use time in low-pressure CVD system on the repeatability of graphene film preparation,it was found that the Cu deposited on the wall of the quartz tube will be gradually oxidized to Cu O and Cu₂O in the repeated exposure to air process.On the one hand,the rough surface of deposited Cu oxide adsorbed water which was the oxygen sources during the graphene preparation process.,On the other hand,both hydrogen and methane may reduce the Cu oxide and consequently brought water into the reaction atmosphere.Therefore,it was necessary to continuously revise the graphene preparation parameters as Cu deposition continues to increase.The research results of this dissertation not only deepen the understanding of the growth mechanism of graphene,but also further promote the development of controllable preparation technology of graphene.