Giant Exciton Binding Energy in a Three-Dimensional Organic-Inorganic Hybrid Semiconductor

In a conventional three-dimensional (3D) semiconductor, exciton binding is significantly weaker than that of a low-dimensional system due to the absence of geometry confinement and strong dielectric screening. β-ZnTe(en)_0.5, a 3D II-VI organic-inorganic hybrid semiconductor, with subnanometer inorganic sheets, is predicted to possess an exceptionally large exciton binding energy, orders-of-magnitude higher than that of bulk ZnTe. However, experimental validation of this prediction is elusive. This study provides the first experimental confirmation of a large E_b in β-ZnTe(en)_0.5 using one-photon photoluminescence to detect the exciton ground state (1s) and two-photon photoluminescence excitation spectroscopy to probe the 2p-like exciton excited state. The 1s-2p separation can be taken as the lower-bound value of E_b and further used for estimating E_b by applying a hydrogenic model. The results reveal a 1s-2p separation of 280 meV at 300 K, which yields E_b exceeding 315 meV based on a hydrogenic model, the largest among 3D structures. The large E_b in β-ZnTe(en)_0.5 is derived from its 2D-like electronic properties. This work experimentally confirms the giant E_b in β-ZnTe(en)_0.5 together with the information on the exciton excited states, thereby setting a critical benchmark for more rigorous theoretical studies and establishing a novel spectroscopic approach for investigating a large group of hybrid semiconductors.