GRAPHENE GROWTH ON CRYSTALLINE AND POLYCRYSTALLINE NICKEL SURFACES: INSIGHT FROM NUMERICAL SIMULATIONS

In this thesis, we start analysing epitaxial graphene on single crystal (111) and (100) Nickel surfaces as fundamental starting points to model the case of cheaper polycrystalline Nickel substrates. Through ab-initio computational techniques based on Density Functional Theory (DFT) we characterize the ground state structure and the electronic properties of the system, while its dynamical evolution is studied through a homemade Kinetic Monte Carlo (KMC) code. Both simulation methods are thoroughly explained in the first part of the thesis. The results are continuously compared with the experimental findings, giving both a critical feedback of the quality of our studies and helping the understanding and the interpretation of the experimental observations. Graphene can be easily grown by Chemical Vapor Deposition (CVD) on polycrystalline nickel substrate, adapting itself to crystal surface modulations without any lattice break or discontinuity. The configuration of graphene depend on the mismatch and the misorientation angle between the hexagonal graphene lattice and the one of the underlying Nickel surface (Ni(111), Ni(100), Ni(010) etc.). A part from the case of graphene on Ni(111) surface, in general the epitaxial graphene is not flat and the modulation of its structure can originate local peculiar environments for nanoconfined catalysis. First, we characterized by Scanning Tunneling Microscopy (STM) simulations and experimental measurements the intrinsic vacancy defective structures of epitaxial graphene on (111) Nickel surface (Gr/Ni(111)). Trapping of Nickel adatoms is generally favored on these structures, whereas empty vacancies are not and the stability and the bonding configuration of the observed Ni-doping defects are discussed in light of the calculated charge distribution. Concerning the stable structures of graphene on (100) Nickel surface (Gr/Ni(100)) a systematic study has been done in function of the misorientation angle between the two lattices. DFT simulations shed light on spatial corrugation and interfacial interactions: depending on the misorientation angle, graphene is either alternately physi- and chemisorbed or uniformly chemisorbed, the interaction being modulated by (sub)nanometer-sized moiré superstructures. The electronic properties were investigated combining Scanning Tunneling Microscopy and Scanning Tunneling Spectroscopy simulations, highlighting the peculiarities of the different moiré regions, in excellent agreement with the experimental findings. Ni(100) micrograins appear to be a promising substrate to finely tailor the electronic properties of graphene at the nanoscale, with relevant perspective applications in electronics and catalysis. Furthermore, we afforded the problem of the moiré superlattices stability. KMC and DFT simulations have been used to study the evolution of the moiré patterns induced by carbide segregation at the Gr/Ni(100) interface observed by cooling down the sample. The last part of this thesis has been experimentally observed by STM, and always the graphen with a simple model of polycrystalline Nickel, combining together (100)-(111)-(010) surfaces. Evidence of surface steps bunch opening in Gr/Ni(100) system has been experimentally observed by STM, and always the graphene foil follows the surface modification without breaking. A systematic study for different stepped surfaces, with single- or multilayer steps with (111) or (110) facets, has been addressed in order to understand how one single layer of graphene can adapt to a stepped surface.