  Achieving large-size two-dimensional (2D) crystals is key to fully exploiting
their remarkable functionalities and application potentials. Chemical vapor
deposition (CVD) growth of 2D semiconductors such as monolayer MoS2 has been
reported to be activated by halide salts, yet clear identification of the
underlying mechanism remains elusive. Here we provide unambiguous experimental
evidence showing that the MoS2 growth dynamics are halogen-dependent through
the Br{\o}nsted-Evans-Polanyi relation, based on which we build a growth model
by considering MoS2 edge passivation by halogens, and theoretically reproduces
the trend of our experimental observations. These mechanistic understandings
enable us to further optimize the fast growth of MoS2 and reach record-large
domain sizes that should facilitate practical applications.
