Real-time atomic force microscopy visualization of humidity-induced surface reconstruction on MAPbBr₃ hybrid perovskites

Humidity can rapidly alter hybrid-perovskite surfaces, yet the earliest topographic processes linking water adsorption to chemical degradation remain poorly resolved. Here, we tracked the same air-cleaved methylammonium lead bromide (MAPbBr₃) single-crystal surface using consecutive non-contact atomic force microscopy measurements during a controlled increase in relative humidity (RH). The initial surface contained atomically flat terraces, single-unit-cell steps, and pre-existing PbBr_x clusters approximately 5-6 nm high. Within the 74.9-78.2% RH range, surface intermediate islands with an apparent height of approximately 0.35 nm became observable and subsequently expanded laterally while maintaining a nearly constant apparent height. Their growth was correlated with decreasing and disappearing PbBr_x clusters, spillover across atomic steps, and pronounced shifts of step boundaries. The characteristic height agrees with the structural offset previously identified between MABr-terminated flat and PbBr₂-terminated vacant surface structures, supporting assignment of the islands as a Pb-Br-rich, MA-deficient intermediate. Photoluminescence from comparably prepared crystals decreased substantially after exposure to approximately 85% RH for 2 h while retaining the dominant MAPbBr₃ band-edge emission, whereas liquid-water immersion produced nearly complete quenching. These results reveal a humidity-assisted surface-redistribution pathway involving Pb-Br-containing material, pre-existing clusters, and atomic steps.