The Effect Of Catalyst Substrate To The Shape And Orientation Of Graphene’s Chemical Vapor Deposition Growth

Due to the success in getting high quantity and large area of production uncomplicatedly and in low cost, chemical vapor deposition(CVD) has been the main method of fabricating graphene in experiment. Practically many experimental conditions like the concentration of carbon feedstock, temperature, partial pressure of hydrogen, carrier gas and metal catalyst surface etc. could affect the growth of graphene, and among them the metal catalyst substrate plays an important role. However the growth mechanism of graphene on metal substrate has not been understood totally despite of many great success have been achieved in experiment. For example, why the graphene growing on Ge(110) surface can be a single crystal in large scale and what determine the shapes of single crystal in CVD growth, especially how a triangular graphene islands form on some metal substrate. Basing on the result observed in experiment and puzzles mentioned above, here we studied the orientation and shape of graphene island on two typical catalyst substrates (Ge(110) surface is semiconducting and Ni(111) is metallic) by using first principle theory calculation. The calculation results we got matches the experiment data well and explains sorts of experimental phenomena.Our calculation reveals that on Ge(110) surface passivated by hydrogen, the vdw between the graphene and substrate are not able to anchor the graphene in specific direction. And through analyzing the phase diagram of combination between the graphene and terrace(step) on Ge(110) under the condition of T～1100-1200K, PH2～50-100Torr, we proposed that the graphene interact with substrate differed obviously when it is on terrace and step. The graphene islands on terrace have their edges passivated by H atoms, therefore there is only week vdw interaction between them. Whereas near the steps, the graphene will form strong chemical bonds with step atoms. Due to the graphene lattice can matches the step’s well in a specific orientation, the graphene would bound with the steps in a primary direction which means when the AC direction of graphene allies with [-110] direction of Ge(110) substrate, the interaction between graphene edge atoms and step become strongest. Therefore, graphene domains grown on Ge(110) surface with relative large number of steps usually have the same orientation, while it is hard to keep graphene domains in same direction on flat terrace. The theory we proposed can explain the experimental observation of union alliance of graphene domains on Ge(110) substrate essentially, also confirmed by the cooperative experiment team.We investigated the shape of graphene on Ni(111) substrate thermodynamically and kinetically by calculating the thermodynamic stability and kinetic growth barrier of four different edges(ZZ, ZZ-klein, AC, AC-klein) on Ni(111) substrate. Combining with thermodynamic and kinetic Wulff Construction theory, we find that under thermodynamics equilibrium the most graphene on Ni(111) surface have hexagonal shape. However under some suitable circumstances, triangular shape graphene islands could form if the differences of kinetic growth of edges are taken into consideration. And our conclusion could explain the phenomenon observed before.