Self-Passivation of 2D Ruddlesden-Popper Perovskite by Polytypic Surface PbI₂ Encapsulation

Two-dimensional Ruddlesden-Popper (2D RP) halide perovskites, C₂MA_n-1Pb_nI_3n+1 (C = bulky ammonium cation; MA = methylammonium) with low n-members (n < 5), have been garnering sensational attention for photovoltaic and optoelectronic applications because of the long carrier diffusion lengths, long-term stability, and tunable bandgap. Yet, the surface modification of 2D RP under kinetic particle irradiation, such as light or electron irradiation, is ambiguous, even though it is imperative to elucidate long-stabilized conversion efficiency. Herein, we present molecular-scale observations of dynamic surface reconstruction of BA₂MA₂Pb₃I₁₀ (n = 3) 2D RP induced by the electron beam. The surface dynamics reveal lateral growth of polytypic PbI₂ with 3R, 4H, and 2H structures at the edge and surface of the 2D perovskite, accompanied by simultaneous annihilation at the other edges. Local radiolysis occurs dominantly by the internal energy increase of electron momentum transfer, which triggers a sequential layer-by-layer degradation into PbI₂. In situ observation of the polytypic PbI₂ growth at the whole surface and edges of 2D RP under electron irradiation elucidates how the outer PbI₂ self-passivation can protect inner 2D RP, causing longer operando stability.