Dimensional control of the band-gap crossover in layered lead iodide
Before assessing the suitability of a semiconductor for specific applications, the first question to ask is whether it possesses a direct or indirect band gap. This distinction is fundamental, as the operation of devices such as light-emitting diodes, solar cells, and photodetectors is closely tied to the band-gap nature. Semiconductors that exhibit a band-gap crossover, from indirect to direct or vice versa, offer enhanced versatility for optoelectronic applications. Prominent examples include transition metal dichalcogenides and the subject of this study, PbI2. The nature of the band gap, and its crossover, can only be directly determined in reciprocal space by tracking the valence-band maximum and conduction-band minimum. Here, we directly visualize the thickness-dependent crossover of PbI2 from an indirect to a direct band gap using angle-resolved photoemission spectroscopy. Our measurements reveal a shift of the valence-band maximum toward the Brillouin-zone center as the film thickness exceeds a monolayer. Supported by density functional theory calculations, our results show that this crossover is driven by interlayer interactions and the hybridization of iodine pz orbitals. These findings demonstrate the tunable electronic structure of PbI2 and its potential for optoelectronic applications.