Layer-by-layer epitaxial growth of perovskite heterostructures with tunable band offsets

Halide perovskites exhibit superior optoelectronic properties but lack precise thickness and band offset control in heterojunctions, which is critical for modular multilayer architectures such as multiple quantum wells. We demonstrate vapor-phase, layer-by-layer heteroepitaxial growth exemplified by CsPbBr₃ deposition on single crystals of PEA₂PbBr₄ (PEA: 2-phenylethylammonium). Angstrom-level thickness control and subangstrom smooth layers enable quantum-confined photoluminescence of CsPbBr₃ from monolayer, bilayer, and through to bulk. The interfacial structure controls the electronic structure from a Cs‒PEA-terminated interface (type II heterojunction) to a PEA‒PEA-terminated interface (type I heterojunction), with a layer-tunable band offset shift exceeding 0.5 electron volts. Electron transfer from CsPbBr₃ to PEA₂PbBr₄ for a type II Cs‒PEA heterojunction results in delayed electron-hole recombination beyond 10 microseconds. Precise quantum confinement control and large band offset tunability unlock perovskite heterojunctions as platforms for scalable, superlattice-based optoelectronic applications.