Directing Near-Perfect (001) Orientation in Wafer-Scale BiOI Thin Films via Halide-Mediated van der Waals Stacking

Controlling crystallographic orientation and facet exposure in 2D bismuth oxyhalide materials is critical for numerous applications, including efficient (photo)catalysis and enhanced charge transport. Oriented bismuth oxyhalide thin films are of strategic importance not only for their intrinsic properties but also as a gateway to many other valuable materials via their ability to undergo facile topotactic reactions. To progress the numerous applications linked to this family of materials this work identifies, for the first time, the mechanism regulating crystal orientation in bismuth oxyhalide (BiOI) thin films. Detailed experiments, backed by density functional theory (DFT) calculations, reveal the pivotal role of iodine (halide) anions to modify the van der Waals stacking energies required to direct orientation. Using our methodology, we demonstrate (001) BiOI thin films on commercially relevant wafer scales with unprecedented (001) crystallographic alignment-addressing the key challenge of scalability. To probe structure-function relations, we fabricate BiOI photodetectors. Compared to unoriented devices, our (001)-oriented films demonstrate vastly increased operational stability, fewer trap states, ON/OFF signal switching up to 10⁵, low (pA) noise levels, and millisecond response times. Our work can be used as a general methodology to control orientations in BiOX and related chalcohalide systems.