Evolution Of Copper-graphene Interface Steps And The Effect Mechanism Of External Assistance On The Graphene Growth

In 2004,Geim et al.obtained the two-dimensional graphene material through mechanical exfoliation.The exceptional electrical,mechanical,thermal,and optical properties of graphene have attracted widespread interest from both the scientific and industrial communities.This has led to an unprecedented development in graphene preparation methods.Among them,chemical vapor deposition(CVD)is considered the most effective method for producing high-quality graphene in large quantities,and thus holds great potential for industrial production.This is because the graphene grown by CVD is higher in quality,larger in size and easier to transfer.By adjusting substrate treatment methods and growth process parameters,high-quality singlecrystal graphene or graphene films can be obtained through CVD.However,despite its advantages,the complex reaction process and various influencing factors of CVD still pose many unsolved fundamental scientific problems,such as the problem of Cu step beams during the cooling process and the problem of insufficient driving force for graphene growth,which presenting numerous technical challenges for the batch preparation of high-quality graphene.Therefore,this dissertation delves into an in-depth study of the growth evolution of step beams at the copper-graphene interface,the impact of external assistance on graphene growth on copper substrates,and the mechanical properties of the resulting graphene.The primary research contents are as follows:1.Evolution of copper-graphene interface step beamsThis part systematically investigates the correlation factors of evolution of coppergraphene interface step beams induced by thermal stress between graphene and copper foil.The mechanisms of Cu crystal orientation,graphene layer number,and copper foil surface undulation on the morphology of copper step beams are elucidated.The results show that step beams tend to occur more readily on loosely packed surfaces,and that step beams on highindex surfaces are significantly taller than those on low-index surfaces.In addition,the atomic surface on the uppermost surface of the Cu foil tends to have a more stable arrangement(closepacked surface)when forming step beams to resist the greater deformation caused by stress.Step beams formed beneath bilayer and trilayer graphene are deeper and wider than those formed beneath monolayer graphene,with an increase in surface roughness of over 50%.This is because the ability of bilayer and trilayer graphene to wrinkle is weakened,and excess stress can only be released through the formation of step beams.The surface undulation of copper also forms nanoscale step beams,which can take on a terrace morphology on certain crystal facets.Finally,Raman spectroscopy confirms that the formation of interface step beams releases the thermal stress between graphene and copper foil,with deeper and steeper step beams releasing more stress.These findings provide important insights into the effective utilization and avoidance of step beams.2.Effect mechanism of oxygen element on graphene growthTo produce high-quality single-crystal graphene,this part investigates the affect mechanism of oxygen element on graphene growth.A stable Cu2O intermediate layer was prepared through pre-oxidation and gradient annealing and the reaction mechanism of copper and oxygen in this process was explored.A multistage growth scheme was designed to prepare millimeter-scale single-crystal graphene with the assistance of Cu2O intermediate layer.Combined with theoretical calculations,the effects of CuO nanoparticles and Cu2O intermediate layer on graphene growth mechanism were investigated.CuO can induce graphene growth by forming nanocrystals,and as it has two types of easily adsorbed sites(step and surface),graphene grown by CuO-seeded growth tends to form bilayer graphene.Conversely,the Cu2O intermediate layer provides the optimal adsorption site for graphene at the surface,altering the growth mode of graphene from edge-attachment-limited to diffusion-limited.This leads to a decrease in the nucleation density of graphene and promotes the growth of large-size monolayer graphene.Additionally,the effects of oxygen partial pressure,temperature,and oxidation time on the oxidation products of copper foil were analyzed and clarified,providing data for the industrial preparation of copper oxide foil for graphene.3.Effect mechanism of copper vapor on graphene growthIn this part,a method for preparing high-quality graphene films on Cu(100)substrates using copper vapor-assisted chemical vapor deposition is presented.Copper vapor is introduced in the form of copper foam,which provides a continuous driving force for the decomposition of the carbon source.The abundant copper vapor is conducive to the growth of high-quality graphene without leaving elemental copper deposits.The copper foam also acts as an excellent auxiliary catalyst for graphene growth by bringing additional carbon active species,which can prevent the cracks that appear during the growth process and reduce graphene defect density.The Raman spectroscopy analysis indicates that the ID/IG ratio of the graphene grown with copper foam assistance can decrease by an order of magnitude.By constructing a limited-space device,the copper foam is used to produce copper vapor and the growth of adlayer graphene on the substrate is regulated.After adjusting the growth process parameters,a uniform graphene film with a single-layer rate of 99.8%is successfully prepared.This method offers a promising approach for the bulk growth of low-defect monolayer graphene films on Cu(100)substrates.4.Characterization of mechanical properties of graphene prepared by CVDIn order to investigate the mechanical properties of CVD graphene,the Young’s modulus of graphene was comprehensively characterized based on the atomic force microscope nanoindentation method,and the fitting methods of Young’s modulus were discussed.By controlling the testing parameters,the influence of loading value and loading cycles on the elastic modulus of suspended graphene was studied.Molecular dynamics simulation was used to explore the fracture process of perfect graphene under point loading,and further analysis was carried out on the mechanical property differences between perfect graphene and graphene prepared by CVD method.This is expected to expand the application of nanoindentation in the mechanical characterization of two-dimensional materials.