Davydov Splitting and Evolution of Molecular Excitons in Two-Dimensional PTCDA Crystals

Two-dimensional (2D) van der Waals materials have recently gained much research interest because of their potential in various applications. Unlike 2D atomic crystals represented by graphene, MoS₂, and hexagonal BN, however, 2D molecular crystals have rarely been studied. In this work, single and few-layer PTCDA (perylene-3,4,9,10-tetracarboxylic dianhydride) crystals were grown on hexagonal BN by self-limited vapor assembly. Polarized photoluminescence (PL) spectra revealed high and low-energy excitonic emissions with a strong dependence on thickness. The former Frenkel exciton exhibited significant vibronic progressions with pronounced Davydov splitting for 0-0 peak, indicating the presence of multiple basis molecules in unit cells, which was also confirmed in absorption spectra. The temperature-dependence of PL signals suggested thermal equilibration between the two Davydov states. By determining transition dipole moments using polarized PL analysis, we directly show the charge-transfer character in the latter is enhanced for thicker layers with increased π-π interactions.