  The emergence of transition metal dichalcogenides (TMD) as crystalline
atomically thin semiconductors has created a tremendous amount of scientific
and technological interest. Many novel device concepts have been proposed and
realized (1-3). Nonetheless, progress in k-space investigations of
ground/excited state electronic structures has been slow due to the challenge
to create large scale, laterally homogeneous samples. Taking advantage of
recent advancements in chemical vapor deposition, here we create a wafer-size
MoS2 monolayer with well-aligned lateral orientation for advanced electron
spectroscopy studies (4-6). Low energy electron diffraction and scanning
tunneling microscopy (STM) demonstrate atomically clean surfaces with in-plane
crystalline orientation. The ground state and excited state electronic
structures are probed using scanning tunneling spectroscopy (STS),
angle-resolved photoemission (ARPES) and time-resolved (tr-)ARPES. In addition
to mapping out the momentum-space quasiparticle band structure in the valence
and conduction bands, we unveil ultrafast excited state dynamics, including
inter- and intra-valley carrier scattering and a rapid downward energy shift by
~ 0.2eV lower than the initial free carrier state at Sigma point.
