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Integration of semiconductor on 2D atomic layers has offered flexible optoelectronic devices. Especially, GaN grown on chemically vapor-deposited (CVD) graphene has the potential to be utilized for the practical application, but high-angle grain boundaries originated from CVD graphene would degrade the luminescence performance in GaN thin films by acting as a non-radiative recombination centers. We calculated the local electronic structure of grain boundaries to understand the atomistic role of them on physical properties of GaN films.