Optical bandgap anomaly with tuning dimensionality in germanium perovskites: Interplay between quantum confinement and lone pair expression

The optical bandgaps of 2D halide perovskites (HPs) generally decrease with increasing inorganic layer thickness because of the diminishing quantum confinement (QC) effect. Here, we report a reverse bandgap trend in a class of Ge-based HPs, achieved through a judicious selection of organic cations, which exhibits an enhanced stereochemical lone pair expression (LPE) on the Ge2+ cation with increasing inorganic layer thickness. We find the enhanced LPE arises from cooperative interlayer compression and intralayer expansion on the inorganic lattice induced by a small spacer cation and a large cage A-cation, respectively. Experimental and theoretical studies show that such enhanced LPE can shift up the bandgap, which is significant enough to offset the QC effect, leading to the bandgap anomaly with tuning dimensionality. Our results reveal a previously underexplored avenue to tune the optical and electronic properties of 2D HPs by rationally controlling the LPE through interlayer and intralayer engineering.