Structure Control And Growth Mechanism Of Graphene On Copper Substrates By Chemical Vapor Deposition

Graphene is a hexagonal honeycomb two-dimensional material composed of sp²hybrid carbon atoms.Since graphene was found in 2004,many novel physical and chemical properties of graphene have been revealed experimentally.Andre Geim and Konstantin Novoselov who discovered graphene using mechanical exfoliation method for the first time were awarded the Nobel Prize in 2010,which immediately caused a tremendous upsurge of scientific interests in graphene.Due to its high specific surface area,conductivity,stability and carrier mobility,graphene has broad application prospects in the fields of energy storage,catalysis and semiconductor.Therefore,it's necessary to control the structure of graphene,which makes graphene better used in specific fields.Polycrystalline graphene has low cost and is more suitable for transparent conducting electrodes.Single-crystal graphene has high carrier mobility and is more suitable for semiconductor devices.On the other hand,the properties of graphene are related to layer number.Monolayer graphene and ABA-stacked trilayer graphene are semimetal without band gap.Bilayer graphene and ABC-stacked trilayer graphene are semiconductor with a controllable band gap.Chemical vapor deposition(CVD)method is recognized as the most promising industrialization method for graphene.Although CVD graphene has been developed greatly in recent years,some issues in structure control and growth mechanism were still to be resolved,such as,structure control of polycrystalline graphene,layer number control of single-crystalline graphene and their growth mechanism.This paper focuses on these issues and studies layer-number control and growth mechanism of graphene film on dielectric substrate and single-crystal graphene.The main research contents and results are as follows.The transfer process before graphene device fabrication could degrade the performance of graphene device greatly.A high-quality graphene film with controlled thickness was grown on dielectric substrate,using the by-product carbon in metal film deposition process as the carbon source of graphene.When deposition temperature was350 ^oC,pressure was 5000 Pa and growth time was 60 min,the copper films were epitaxially grown on Al₂O₃(0001)substrates.After annealing and etching process,the copper layers were etched and high-quality graphene films with I_D/I_G?0.86 were obtained on Al₂O₃(0001)substrates.By adjusting the thickness of carbon layers in films,3,5,7 and 10 layers graphene films with transmittance of up to 93.5%and square resistance of 0.8 k?·sq^-1were obtained and a high-performance electrochromic device was fabricated using 3 layers graphene films as electrodes.The rolling lines of cold-rolling copper foils could increase graphene nucleation greatly,so we developed high-quality electroplated copper films instead of cold-rolling copper foils.Single-crystal monolayer graphene with the size of around 400 mm was prepared on the high-quality electroplated copper film by pre-oxidation method.Roughness and thickness of the Cu film were positively correlated with the electrodeposition temperature and current density,respectively.With increasing electrodeposition temperature of Cu film from 20 to 65°C,the nucleation density of graphene increased from 124 to 448 mm^-2.The nucleation density of graphene increased from 124 to 192 mm^-2 with increasing the current density from 0.03 to 0.07A/cm^-2.Finally,we obtained single-crystal monolayer graphene about 400 mm in size using optimized condition and the field effect transistor(FET)device shown a good electrical performance with a carrier transport performance of?_h=4040 cm²V^-1s^-1 and?_e=2580 cm²V^-1s^-1 at room temperature.Because of the self-limited mechanism,it's difficult to control the layer number of graphene.Uniform single-crystal trilayer graphene with the size of 80 mm was obtained on premelting copper.When deposition temperature was near the melting temperature(1084 ^oC)of copper,a premelting copper layer with a thickness of about0.9 to 1.4 nm(4?6 copper atomic layers)formed on copper surface.The premelting copper may separate the graphene from the underlying solid copper and intensify the diffusion of carbon atoms,finally,a large-size single-crystal trilayer graphene was obtained.Single-crystal umbrella-like graphene(ULG)with the size of 600 mm was prepared on melting copper.ULG consisted of two parts,the panel and the rib.The thickness of penal was the thinnest,about 10 layers,while that of rib gradiently changed from 10 to 30 layers.The growth of ULG followed immersion growth theory.Since ULG partially immersed into liquid copper during the nucleation stage,carbon atoms could reach each layer of ULG,leading to the continual growth of ULG graphene.Moreover,the growth of umbrella-like graphene and monolayer graphene were competitive process.The larger hydrogen flow was beneficial to the growth of ULG.