Studies On Formation Mechanism Of Single-crystal Graphene

Single crystal graphene has excellent physical and chemical properties, thedeveloping chemical vapor deposition (CVD), using catalytic copper substrate, wassupposed to be the most promising method for the production of large-sized andhigh-quality graphene. However, neither quality nor size of graphene synthesizedcurrently could meet the needs of practical application, so deeper insights into itsmicroscopic growth mechanism and then improve the process parameters reasonablyhave the vital significance to improve the quality of single-crystal graphene.Using first-principles calculations within density function theory, the adsorption andformation behaviors of transitional CxHy clusters during graphene nucleation and growthon a Cu (111) surface were investigated in this thesis. The adsorption energy, theformation energy, and the average distance between Cu (111) surface and carbon atom ofCxHy are employed to characterize the eventual calculation results. So we draw theconclusion that:(1) The study on the formation of CxHy small clusters on Cu (111) surface based on Cand H atoms show that: with the increase of the value of hydrogen saturation and the numberof hydrogen atoms, the formation energy per atom of CxHy clusters decreases, the averagedistance between carbon atom and the copper surface increases, and the average length of C-Cbond also shows a trend of increase. Nevertheless, when the generated CxHy clusters withspatial symmetrical structure, the above rules are broken to some extent. Amazingly, theformed C3H4and C3H5symmetric clusters possess graphene-like spatial structure, these kindof structure may be plays an important role in the synthesis of graphene.(2) The study on the formation of C3H4and C3H5clusters on Cu (111) surface basedon C and CHx show that: When the initial particles do not contain CH3radicals, C3H4and C3H5clusters generated on Cu (111) surface. And when the initial particlescontaining CH3radical, there is no hydrocarbon clusters generated with correspondingnumber of carbon atoms. But the interactions between CHx radicals accelerate the decomposition of CH3, then the final state structure formed with CH, CH2, C2H4and othersmaller CxHy clusters adsorbed on copper surface independently. In addition, C4H5and C4H6clusters produced respectively by the combination of CH and symmetrical C3H4and C3H5clusters were also studied in this thesis, computing results indicate that, compared with C4H5,The C4H6cluster have a lower formation energy, a higher symmetry and a more stablestructure.On the basis of theoretical research, a preliminary experimental exploration on graphenegrowth was conducted. Using plasma chemical vapor deposition method under the conditionof low pressure, we intend to synthesize single-crystal graphene on copper, with methane ascarbon source. Pretreatment can effectively improve the surface roughness of the coppersubstrate, the specific process is as follows: firstly a copper foil was onicated in10%dilutedhydrochloric acid using an ultrasonic cleaner, and then was Electrochemical-Polished underthe applied voltage of10V for3min, and finally was High-pressure annealed under ahydrogen atmosphere. Atomic force microscopy images show that, the maximum bulge ofsuch pretreated copper foil was lower than10.0nm. The thesis designs a complete set ofschemes for grephene growth by microwave plasma chemical vapor deposition method. In thefollow-up work, we will conduct the single-crystal graphene growth research, with abovepretreated copper foil as substrates.