Photoinduced Anion Exchange in Cesium Lead Halide Perovskite Nanocrystals.

Cesium lead halide (CsPbX) perovskite nanocrystals (NCs) possess the unique capability of post-synthesis anion exchange providing facile tunability of the optical properties, which is usually achieved by mixing NCs with reactive anion precursors. In this work, we show that the controllable anion exchange can be achieved in a dihalomethane solution of CsPbX NC in the absence of any spontaneously reacting anion source using photoexcitation of CsPbX NCs as the triggering mechanism for the halide ion exchange. The reaction begins with the photoinduced electron transfer from CsPbX NCs to dihalomethane solvent molecules producing halide ions via reductive dissociation, which is followed by anion exchange. The reaction proceeds only in the presence of excitation light and the rate and extent of reaction can be controlled by varying the light intensity. Furthermore, the asymptotic extent of reaction under continuous excitation can be controlled by varying the wavelength of light that self-limits the reaction when light becomes off-resonance with the absorption of NCs. The light-controlled anion exchange demonstrated here can be utilized to pattern the post-synthesis chemical transformation of CsPbX NCs, not readily achievable using typical methods of anion exchange.