  Focused ion beam (FIB) techniques have been frequently used to section
metal-halide perovskites for microstructural investigations. However, the ion
beams directly irradiated to the sample surface may alter the properties far
different from pristine, potentially leading to modified deterioration
mechanisms under aging stressors. Here, we combine complementary approaches to
measure the subsurface characteristics of polished perovskite and identify the
chemical species responsible for the measured properties. Analysis of the
experimental results in conjunction with Monte Carlo simulations indicates that
atomic displacements and local heating occur in the subsurface of
methylammonium lead iodide (MAPbI3) by glazing Ar+ beam irradiation (15 nm by 4
kV at 3 degree). The lead-rich, iodine-deficient surface promotes rapid phase
segregation under thermal aging conditions. On the other hand, despite the
subsurface modification, our experiments confirm the rest of the MAPbI3 bulk
retains the material integrity. Our observation supports that polished
perovskites could serve in studying the properties of bulk or buried junctions
far away from the altered subsurface with care.
