| Graphene is a two-dimensional material with excellent electrical conductivity,thermal conductivity,strength and infrared wave permeability.It is used as a coating on sapphire??-Al2O3?substrate,which has a potential application in some fields as infrared window,sensors,semiconductor devices,transparent electrodes.In this paper,Based on the first-principles calculation method,the growth process of graphene film on ?-Al2O3?0001?surface is simulated.Those models for the decomposition of methane?CH4?on ?-Al2O3?0001?surface and the formation of graphene are built.The paper is focused on studying the structure and electronic morphology of the atoms on the ?-Al2O3?0001?surface,the adsorption process and the bonding characteristics of the products from CH4 on the surface of the ?-Al2O3?0001?,the mechanism of substrate surface is etched,the graphene`s nucleation precursor and graphene nucleation process,the influence of H2 on the graphene`s growing process during CVD respectively.The main conclusions are as follows:?1?By optimizing the structure of ?-Al2O3 substrate,it is found that the atomic layers of Al2O3?0001?are relaxed to the interior,and the relaxation value of the outermost O atom layer is the most obvious,up to 0.752 ?.After the relaxation of the substrate structure microstructure,the simulation value of the atomic layer spacing is a good agreement with experimental value.And after the surface atoms relaxing,the distribution of electron density also changes,and the surface electrons will have a greater chance of locating on the O atoms around.The groups and the atoms from the CH4 decomposition have interacted with the substrate,and the surface microstructure of the substrate is calculated.It is found that the surface roughness of ?-Al2O3 substrate decreased due to C atom dissociation from CH4.?2?The first-principles calculation of the decomposition process of CH4 on ?-Al2O3?0001?surface shows that ?-Al2O3 only has physical adsorption on CH4,but decomposition products as CH3,CH2,CH group and C,H atoms have strong chemical adsorption behvour.The CH3 group and the C and H atoms are preferentially adsorbed at the top of the O atom.Those CH2,CH groups preferentially adsorbed between the O and Al atoms.The decomposition of CH4 on the surface of ?-Al2O3?0001?is an endothermic process and the ?-Al2O3 substrate has no catalytic effect on the decomposition of CH4.?3?In four-step decomposition process,namely CH4 ? CH3,CH3 ? CH2,CH2 ? CH,CH ? C on the surface of ?-Al2O3?0001?,the reaction energy of the simulation system increased in turn.The complete decomposition of CH4 requires 9.037 eV of energy.At the same time,we calculate the coupling process between the CH groups?CH + CH ? C2H2,CH2 + CH ? C3H3?on the surface of the substrate.It was found that,on the above process,the total energy of the system is declining.After further molecular dynamics simulation,we find that the precursor of the graphene nucleation on ?-Al2O3 is CH group.The coupling of CH groups on the surface of ?-Al2O3?0001?is the basis of the nucleus of the graphene.The CH groups are first coupled to form a larger?CH?x microstructure,then the?CH?x structure remove H atoms and form graphene.?4?The relationship between H2 contents and the growth of graphene was established by the thermodynamic analysis on the sapphire substrate by CVD.It is found that the higher the K value of CH4 /H2 mixed gas and the higher the vacuum of the reaction furnace,the more favorable the growth of graphene. |
PDF Full Text Request