How Graphene Islands Are Unidirectionally Aligned on the Ge(110) Surface

The unidirectional alignment of graphene islands is essential to the synthesis of wafer-scale single-crystal graphene on Ge(110) surface, but the underlying mechanism is not well-understood. Here we report that the necessary coalignment of the nucleating graphene islands on Ge(110) surface is caused by the presence of step pattern; we show that on the preannealed Ge(110) textureless surface the graphene islands appear nonpreferentially orientated, while on the Ge(110) surfaces with natural step pattern, all graphene islands emerge coaligned. First-principles calculations and theoretical analysis reveal this different alignment behaviors originate from the strong chemical binding formed between the graphene island edges and the atomic steps on the Ge(110) surface, and the lattice matching at edge-step interface dictates the alignment of graphene islands with the armchair direction of graphene along the [-110] direction of the Ge(110) substrate.