Dynamic Evolution of 2D Layers Within Perovskite Nanocrystals via Salt Pair Extraction and Reinsertion

Metal halide perovskite (MHP) semiconductors exhibit unprecedented optoelectronic properties coupled with low formation energies that enable scalable, cost-efficient solution processing. The low formation energies additionally facilitate dynamic transformation of the chemical composition and crystal structure of the MHP material. In this work, we show that CsBr salt is selectively extracted from CsPbBr₃ nanocrystals (NCs) to yield PbBr₂ NCs. The PbBr₂ NCs are then exposed to different glacial acetic acid ABr salt solutions to generate a variety of emissive compounds with the generic structure A′₂A_n-1Pb_nBr_3n-1X′₂, where A = cesium (Cs⁺), methylammonium (MA⁺), formamidinium (FA⁺); A′ = A or H⁺; X′ = Br^- or acetate (CH₃COO^-); and n is the number of lead halide layers, where n = 1, 2, 3,..∞. We systematically vary the ratios of PbBr₂/ABr/CH₃COOH and show that certain ratios result in isolable single-phase APbBr₃ NCs - an effective A-site cation exchange from the parent CsPbBr₃ NCs. Importantly, time-resolved photoluminescence (PL) spectroscopy shows the dynamic evolution of many additional species as evidenced by blue-shifted emission peaks from 2.85-2.49 eV for MA⁺-based structures. We assign these species to n = 1, 2, 3, 4, and 5 quasi-two-dimensional network (2DN) sheets, in which CH₃COO^- anions and Br^- anions compete for the c-axis X′ sites separating haloplumbate(II) layers within the A′₂A_n-1Pb_nBr_3n-1X′₂ NCs. Finally, we demonstrate the degree of CH₃COO^- incorporation, and thus the 2DN layer thickness and PL energy, is controlled in the early reaction times by kinetic factors. After a longer time (3 h), thermodynamic forces dictated by Le Chatelier's principle tune the structure in A′₂A_n-1Pb_nBr_3n-1X′₂ NCs from exclusively n = 1 to n = ∞ depending on the PbBr₂/ABr/CH₃COOH ratio.