The Theoretical Study Of Metal Catalytic Mechanism Of Graphene Growth And Cutting

Since found in 2004,graphene has attracted many researchers' attention due to its fantastic physical and chemical properties and great application potentials in semiconductor electronics and microelectronics.Therefore,synthesizing large-scale and high quality graphene becomes experimental researchers' dream.From the initial mechanical exfoliation to the present chemical vapor deposition,people continue to propose new experiments to synthesis large-scale graphene which can be used in applications.But over ten years past,how to synthesis large-scale and high quality graphene is still a great challenge needed to be solved.Beside experimental researchers,researchers in theoretical calculations use the divided and conquer strategy to study the whole graphene growth process.They have conducted so many calculations in each stages like the hydrocarbon's dissociation,nucleation and the lateral growth of the nucleation island and so on.The other question that graphene can be used in semiconductor electronics is how to change it from gapless semimetal to semiconductor.The common band-gap engineering approaches include altering graphene's strain,chemical modification and so on.Synthesizing graphene nanoribbon is another method to open a gap and give more interesting electrical properties.Graphene nanoribbon with smooth edge and suitable width can be produced by transition metal nanoparticle cutting graphene.Not only experiments researchers,but also theoretical calculation researchers try to reveal graphene growth and cutting mechanism.Graphene growth and cutting are both surface heterogeneous catalysis reactions.In theoretical calculations,the general research routine on these kind of catalytic reactions is that using the flat and perfect metal surface model to represent the metal surface and nanoparticle used in experiments,then searching the reactant and product's stable adsorption structures on the surface,determining the transition states of these reactant and product pairs,obtaining the reaction barriers and inentifying the dominate reaction paths at last.However,the temperature in these catalytic reactions generally are over 1000 K in experiments.The surface and nanoparticle are almost pre-melting or melting at such high temperature and far away from the perfect flat surface at low temperature.The effect of temperature and the role of entropy may be remarkable at high temperature.So it also may be not sufficient to study the chemical kinetic just from potential energy surface.The better idea is to calculate free energy in the studies of high-temperature catalytic reactions.Although the model is reasonable,there are a great spatial and temporal scale gap between molecular simulations and experiments.It's difficult to directly simulate the processes at macroscale level.To realize this goal,multiscale simulation uses a serial of methods ranging from first-principles calculation and molecular force field at atomic level,Monte Carlo at mesoscopic scale and even continuum method at marcoscale to get some information which can be compared to experiment data.In this thesis,multiscale simulation and free energy calculation are used to study the mechanism of graphene growth in chemical vapor deposition and nickel nanoparticle cutting of graphene.(1)We have calculated the free energy of methane dissociation on Cu(111)surface by first-principles based molecular dynamics,analysized the dissociation barriers of different small hydrocarbon radicals and guessed the possible dominate growth species.Also we try to propose some general rules for these kind of reactions.(2)Multiscale simulation is used to simulate nickel nanoparticle cutting of graphene and reveal the underlying cutting mechanism.We firstly used reactive molecular dynamics to simulate the process of the nickel nanoparticle cutting of graphene directly.From the trajectories,we speculated that different graphene edges had different etching rates and found the detail process that how nickel atoms break edge C-C bonds.The results of the free energy calculation of different edge C-C bond breaking confirmed our speculation.The energy barriers of Ni atoms breaking graphene C-C bonds from the more accurate density functional theory calculations were consistent with our conclusion drawn from classical force field based simulation and removed the doubt of the force field's transferability which strengthened our conclusion.With these results,we inferred that the experiment observation that the methane production rate changes linearly with the square of the nanoparticle's radius resulting from the difference etching rates between different edges.The following kinetic Monte Carlo simulation showed the same trend with the experiment as we expect.So we revealled a brand-new cutting mechanism different from that inferred from experiment data,deepened our understanding on the cutting process and provided some guide for experiment researchers.