Structure and Optical Anisotropy of Monolayer Tetracene Crystals

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.