The Synthesis And Characterization Of Large-Scale Graphene Single Crystals

Graphene,as a new two-dimensional thin film materials,has been attracting enormous research interest for its excellent electrical,mechanical and thermal properties.However there is still a long way to go until its large-scale industrial applications,due to the application property of graphene can be influenced by its synthesis process easily.Among all method to grow graphene,chemical vapor deposition(CVD)on copper(Cu)substrate is a popular method due to its low cost and simple operation,and it’s hoped to grow single-crystal graphene for industry in future.While how to prepare large-area single-crystal graphene remains a serious problem for us.Before large area single-crystal graphene growth,we propose a new method to identify graphene multilayer structure in a large scale under optical microscopy directly,and this method improves the next experimental progress of graphene multilayer growth greatly;Ultraviolet and heating treatment were employed to graphene/Cu to produce copper oxide film with various thickness under different graphene layer,resulting in a different color contrast under optical microscopy which can realize graphene multilayer visualization under optical microscopy.A single nucleation method was adopted firstly to grow millimeter-size single-crystal graphene.We discussed the influence of hydrogen partial pressure to graphene shape,size,nucleation numbers and multilayer,and found that a lower hydrogen partial pressure can make a faster graphene growth,flower-like shape and more graphene multilayer under the first graphene layer,while higher hydrogen partial pressure can make a slower graphene growth,hexagon shape and only one nucleation in the center of graphene.So we put forward a two-step method by combining the two graphene growth model.That is,grow graphene in lower hydrogen partial pressure firstly and transformed to higher hydrogen partial pressure.And compared to traditional CVD method,a 100%increase for the first graphene layer and 50%decrease for multilayer which exists only one nucleation is achieved finally.Meanwhile,according to the influence of hydrogen to graphene growth,in optimized conditions,that is,200 standard-state cubic centimeter per minute(sccm)hydrogen and 0.3 sccm methane,continuous poly-crystal graphene membranes can be acquired with millimeter-size single-crystal graphene grains by prolonging growth time,which decreases the number of grain boundary in poly-crystal graphene membranes derived from CVD a lot.Then a multi nucleation method was employed in this paper to grow centimeter-size single-crystal graphene.We performed a simple pre-heating process on Cu that can be acquired easily by common purchasing channels,so that copper oxide can form on Cu surface that decreases the melting point of Cu during heating at high temperature,and a lower energy Cu(111)crystal can be reconstructed.Then we can achieve a less graphene nucleation number and same-orientated graphene on reconstructed Cu(111)grain crystal.There is no grain boundary between two same-orientated graphene grain crystal when they merge together,so large-area single-crystal graphene can be achieved when all graphene grain crystal merge together.This method realizes large-area single-crystal graphene growth on a low-cost common Cu by a simple experimental process,which provides a good way for future industrial applications.