  Quantum size effects in molecular crystals have been beyond the reach of
systematic investigation because of lack or instability of their well-defined
low-dimensional forms. Here, we report the formation, structure and optical
anisotropy of two-dimensional (2D) tetracene (Tc) crystals template-grown and
photo-stabilized with inorganic 2D crystals of graphene or hexagonal BN.
Molecular orientation and long-range order mapped with polarized wide-field
photoluminescence imaging revealed high crystallinity of 2D Tc and its
distinctive molecular registry with 2D inorganic crystals, which were verified
with first-principles calculations. The reduced dielectric screening in 2D
space was manifested by enlarged Davydov splitting and attenuated vibronic
sidebands in the excitonic absorption and emission of monolayer Tc crystals.
Photostable 2D molecular crystals and their size effects can be observed for
various molecular systems and will lead to novel photophysical principles and
photonic applications.
