Alloy Effect On The Underlying Self-catalysis Mechanism Of Carbon Atoms Segregation From The Sub-surface Of Ni(111)

Known as the firstly discovered two dimensional material,graphene has attracted a great deal of attention since it was discovered in 2004.Correspondingly,various methods for the synthesis of graphene have been established.Among these approaches,chemical vapor deposition(CVD)growth on transition-metal surfaces(Cu,Ni,and their alloys)is considered to be the most promising route for synthesizing large-size graphene.There have been many papers about the mechanism of graphene growth on these catalyst surfaces,however,to the best of our knowledge,the underlying kinetic mechanism of the initial stage of the C atoms segregating from the Ni(111)substrate onto the surface is still open.Here,using first-principles calculations,we find that there is a self-catalysis during the segregation of C atoms in the Ni(111)substrate.Specifically,the smaller the separation of the C-C atoms,the lower the diffusion barrier.We further investigated the alloy effect on modulating such a self-catalysis.Interestingly,we find that the alloy effect is strongly influenced by the local structures of the dopants.In chapter 1,we firstly introduced the background about graphene,including its various unique superior properties,practical,and potential applications.To meet graphene's remarkable potential applications,a cost-efficient,reliable,and high-throughput synthesis of a high-quality single-crystal monolayer graphene is required.Then as for the most promising route for synthesizing large-size graphene,we put forward our main research content.In chapter 2,we briefly described the methods: first-principles calculations based on the density of functional theory(DFT).In addition,the simulation code and software used in this paper are also introduced.In chapter 3,we present systematic first-principles calculations on the dynamic behaviors of the C atoms segregating from the sub-surface of Ni(111).We reveal that,energetically,carbon atoms may be separately soluted in the Ni(111)substrate;however,kinetically,carbon atoms segregation prefers a synergistic behavior(rather than one atom by one atom)from the Ni(111)substrate.Specifically,in the segregation process,the diffusion barrier of a C atom will be significantly lowered by another neighboring C atom.Intriguingly,the C atoms prefer to diffuse from the substrate onto the surface through a square-like window when existing neighboring C atoms,which effectively reduce the local strain as compared with the triangular-window observed in the case of segregation without such synergistic behavior.We further investigate the hybridization of C atom with its adjacent surface Ni atoms during its segregation,and we find that C atom own the same value of hybridization in the saddle point all the time.Then through the analysis of the hybridization of C atom with its adjacent suface Ni atoms in the initial state,we give an explanation for the formation of different diffusion admittance.In chapter 4,based on the above mentioned underlying mechanism,taking Ni-Co and Ni-Cu alloys as prototypical examples,we demonstrate that the alloy effect can further significantly modulate such a synergistic process in the framework of the d-band theory.Chapter 5: conclusion and outlook.