Structure and Optical Anisotropy of Two-Dimensional Tetracene Crystals

Quantum size effects have long been observed in various semiconducting and metallic materials except for organic molecular solids. Because of weak intermolecular binding, well-defined and stable low-dimensional molecular crystals have been limited to large molecular building blocks or in-vacuo cryogenic environments that are technically incompatible with sophisticated laser spectromicroscopy methods. Here, we report the formation, structure and optical anisotropy of two-dimensional (2D) tetracene (Tc) crystals. Single and few-layer Tc crystals are sandwiched between two inorganic 2D crystals of graphene or hexagonal BN for ambient and photo-stability. The excitonic absorption and emission of 2D Tc consist of attenuated vibronic side bands and enlarged Davydov splitting because of reduced dielectric screening. The crystallographic domains and orientations mapped with polarized wide-field imaging and second-harmonic generation show distinctive registry between the 2D organic and inorganic crystals. The dimensional effects observed in 2D Tc will be universal for other 2D molecular crystals and useful in modifying their properties.