▎ 摘　　要

Research efforts in large area graphene synthesis have been focused on increasing grain size. Here, it is shown that, beyond 1 mu m grain size, grain boundary engineering determines the electronic properties of the monolayer. It is established by chemical vapor deposition experiments and first-principle calculations that there is a thermodynamic correlation between the vapor phase chemistry and carbon potential at grain boundaries and triple junctions. As a result, boundary formation can be controlled, and well-formed boundaries can be intentionally made defective, reversibly. In 100 mu m long channels this aspect is demonstrated by reversibly changing room temperature electronic mobilities from 1000 to 20,000 cm(2) V-1 s(-1). Water permeation experiments show that changes are localized to grain boundaries. Electron microscopy is further used to correlate the global vapor phase conditions and the boundary defect types. Such thermodynamic control is essential to enable consistent growth and control of two-dimensional layer properties over large areas.