Study On The Diffusion Mechanism Of Graphene Prepared By Chemical Vapor Deposition On Copper Substrates

Since graphene was produced by mechanically exfoliating graphite in 2004 and got the Nobel Prize in Physics in 2010,graphene undoubtedly has become a hot star material,and set off a variety of research interests.Despite the Nobel aura,graphene does have a lot of eye-catching properties,such as high strength,high transmittance,high carrier mobility,high thermal conductivity,chemical stability and so on.Materials nature determines purpose,so graphene has a lot of potential applications on the fields such as touch screens,electronic devices,energy storage devices and biomedical engineering waiting to be explored.The premise for the realization of these wonderful applications,are to prepare high-quality graphene.During various preparation methods of graphene explored by scientists,graphene prepared on copper substrates by chemical vapor deposition(CVD)method stands out and is considered the most promising synthesis method because of high preparation quality,controllable growth,simple preparation and could be transferred to any substrates.Graphene synthesis based on this CVD method has been newly discovered.The size of monolayer graphene domain from the initial microns has been developed to millimeters,or even centimeters,and the size of orderly stackded bilayer graphene also has reached hundred microns.On the growth mechanism of graphene on copper substrates,Ruoff group proved the surface self-limitted growth mechanism by isotope labelling experiment.This mechanism explains well the reason why most of graphene produced on the copper surface is monolayer.However,there are many reports about bilayer graphene produced on copper substrates.These phenomena can not help but make people doubt,what is.the mechanism forming bilayer graphene on copper substrates?Is the self-limitted growth mechanism wrong or imperfect?About the etching study,is hydrogen or oxygen played an etching role during the process of graphene preparation?Do there exist any other factors causing etching?Is it possible to achieve simple and controllable preparation of AB-stacked bilayer graphene with openable bandgap?We hope to solve these puzzles in this thesis.The main research work is summarized as follows:1.We used copper pocket to create different environment between the inner side and the outer side of a same piece of copper foil for graphene growth.The correlation of the graphene domains grown at the inner side and the outer side of the copper pocket disclosed a new graphene growth mechanism based on carbon atom fast diffusion through the 25 micrometer-thick copper foil.Such carbon diffusion based growth process,with the assistance of the surface mediated graphene growth process found five years ago,yielded bilayer graphene with coverage about 90%at the outer side of the copper pocket.2.We reported a special etching phenomenon on the outer surface of a copper(Cu)pocket,while large-size graphene domains grow slowly on the inner surface.A systematic study was conducted to investigate this etching process through isotope-tracing.Combined with our previous work,we conclude that the etching phenomenon is analogous to a counter diffusion process that.keeps a stable monolayer of graphene on both sides of the Cu foil.Low C solubility and poor C saturation of Cu appear to drive this counter diffusion and help keep the stable state.Furthermore,we used fast-growing to break this stable state and realized 85%coverage rates of bilayer graphene growth on the outer surface of Cu pocket.This work sheds light on the graphene growth mechanism on Cu substrates.3.We designed a sandwiched structure simply by embedding one piece of Cu sheet into a Cu pocket to establish an environment,which suppresses Cu evaporation and ensures both surfaces of Cu sheet are smooth to grow large-size bilayer graphene(BLG)and multilayer graphene(MLG).Single-diffusion and double-diffusion mechanisms help explain graphene growth on both the Cu pocket and the Cu sheet,respectively.On the basis of the double-diffusion mechanism,we prepared AB-stacked sub-millimeter BLG and MLG with diameters up to 603 ?m and 793 ?m respectively.Our work regarding the improvement of the quality and single-crystal size of graphene domains helps broaden the potential applications in materials chemistry and microelectronic devices.