Organic Metal Chalcogenide Films With Giant Birefringence

Birefringent films emerge as a compelling candidate for integrated photonic systems, gracefully balancing superior processability with seamless substrate compatibility. However, birefringent films typically retain only ∼20% of bulk crystal performance, which consequently limits film-based birefringence to values rarely exceeding 0.1. To address this limitation, we introduce a two-dimensional van der Waals (2D-vDW) organic metal chalcogenide (OMC) PbHBT (HBT = 4-mercaptophenol), featuring "molecular-scale superlattices" of alternating inorganic PbS and organic Ph-OH ligands. PbHBT crystal exhibits a birefringence of 0.39@546 nm. A nanoscale thickness-controllable, highly oriented PbHBT film was prepared through an economical in situ layer-by-layer (LBL) spin-coating liquid-phase epitaxy (LPE) approach. The birefringence of the PbHBT film is 0.3 at 546 nm, maintaining 77% of the birefringence of its bulk crystal. Importantly, the PbHBT film achieves a birefringence of 0.65 in the visible spectrum and 0.43 in the near-infrared, which are the highest notes ever achieved by a birefringent film. This excellent birefringence performance stems from the anisotropy of the electron cloud density of different crystal planes of the PbHBT and the high consistency of grain orientation after being fabricated into a film. This work opens avenues for thin-film birefringent materials with large intrinsic birefringence and high retention, bridging the bulk-film performance gap.