A Mechanistic Study of Phase Transformation in Perovskite Nanocrystals Driven by Ligand Passivation

Abstract

Active control over the shape, composition, and crystalline habit of nanocrystals has long been a goal. Various methods have been shown to enable postsynthesis modification of nanoparticles, including the use of the Kirkendall effect, galvanic replacement, and cation or anion exchange, all taking advantage of enhanced solid-state diffusion on the nanoscale. In all these processes, however, alteration of the nanoparticles requires introduction of new precursor materials. Here we show that for cesium lead halide perovskite nanoparticles, a reversible structural and compositional change can be induced at room temperature solely by modification of the ligand shell composition in solution. The reversible transformation of cubic CsPbX3 nanocrystals to rhombohedral Cs4PbX6 nanocrystals is achieved by controlling the ratio of oleylamine to oleic acid capping molecules. High-resolution transmission electron microscopy investigation of Cs4PbX6 reveals the growth habit of the rhombohedral crystal structure is composed of a zero-dimensional layered network of isolated PbX6 octahedra separated by Cs cation planes. The reversible transformation between the two phases involves an exfoliation and recrystalliztion process. This scheme enables fabrication of high-purity monodispersed Cs4PbX6 nanoparticles with controlled sizes. Also, depending on the final size of the Cs4PbX6 nanoparticles as tuned by the reaction time, the back reaction yields CsPbX3 nanoplatelets with a controlled thickness. In addition, detailed surface analysis provides insight into the impact of the ligand composition on surface stabilization that, consecutively, acts as the driving force in phase and shape transformations in cesium lead halide perovskites.

Design Principles for Trap-Free CsPbX3 Nanocrystals: Enumerating and Eliminating Surface Halide Vacancies with Softer Lewis Bases

“…species do not bind to the surface. [40][41][42] Surface cesium can be replaced by the oleylammonium head group in a substitution mechanism. 41 Alternatively, oleylammonium can bind surface halides through an addition mechanism.…”

Zero-Dimensional Cesium Lead Halides: History, Properties, and Challenges

“…These Cs 4 PbX 6 NCs are spherical or hexagonally shaped for small NCs (<20 nm) and often rhombohedral for larger NCs, which strongly differs from the highly cubic CsPbX 3 NCs, and strongly reflects their hexagonal crystal structure. 13,33,37 Interestingly, these NCs are often synthesized under similar conditions to those of the highly luminescent CsPbBr 3 NCs, but they use a higher amount of alkylamine. Here, the nucleation of CsPbBr 3 can be completely suppressed because oleylamine strongly binds to Pb 2+ .…”

“…13,27 This can be done via the insertion of PbBr 2 , the extraction of CsBr with water or with Prussian Blue, or the extraction of Pb 2+ with amines. 13,27,28,33,37 Several groups have reported that the size and shape of the starting Cs 4 PbBr 6 NCs can be used to tune the size of the final CsPbBr 3 NCs (Figure 5b). 13,27,28,33,37 The transformed CsPbBr 3 NCs exhibit a strong green PL, similar to the PL of directly synthesized CsPbBr 3 NCs, and they show no excitonic absorption in the UV from their parent Cs 4 PbBr 6 NCs (Figure 5c).…”

“…(b) TEM image showing the preservation of NC size from Cs 4 PbBr 6 NCs to CsPbBr 3 NCs, adapted from ref (13). (c) First excitonic absorption peak and (d) XRD pattern of NCs after several reversible exchanges from CsPbBr 3 to Cs 4 PbBr 6 and back, adapted from ref (37). …”

Automated microfluidic screening of ligand interactions during the synthesis of cesium lead bromide nanocrystals

“…This again suggest that the ligand branching, affecting surface packing and ligand diffusion in solution and through the ligand monolayer, is a critical factor. Further, as it is known that surface trap states in LHP nanocrystals, 57 and the crystal phase, 66 both strongly affect photoluminescence brightness, it seems that the changing base : acid ratio in our study is having a significant effect on surface structure, passivation and/or crystal structure, but a lesser effect on the overall size of the nanocrystals.…”