  Single layered molybdenum disulfide with a direct bandgap is a promising
two-dimensional material that goes beyond graphene for next generation
nanoelectronics. Here, we report the controlled vapor phase synthesis of
molybdenum disulfide atomic layers and elucidate a fundamental mechanism for
the nucleation, growth, and grain boundary formation in its crystalline
monolayers. Furthermore, a nucleation-controlled strategy is established to
systematically promote the formation of large-area single- and few-layered
films. The atomic structure and morphology of the grains and their boundaries
in the polycrystalline molybdenum disulfide atomic layers are examined and
first-principles calculations are applied to investigate their energy
landscape. The electrical properties of the atomic layers are examined and the
role of grain boundaries is evaluated. The uniformity in thickness, large grain
sizes, and excellent electrical performance of these materials signify the high
quality and scalable synthesis of the molybdenum disulfide atomic layers.
