Lead halide perovskites is a fast-advancing solar technology with the potential of achieving high efficiency and low production costs. They therefore appear as pivotal building blocks in current energy as well as optical related application. In order to realize the full potential of lead halide perovskites, an in-depth understanding of the structural details is crucial.
In view of this, a high pressure technique implementing diamond anvil cells in conjuction with in-situ X-ray scattering methods as well as characterization tools present a comprehensive approach for the investigation of the structure property relationship of materials. For instance, it has been reported that electrical resistance as well as photocurrent of methylammonium lead bromide perovskites are highly sensitive to the crystal structure change under applied pressure. It was later found that the band gap alignment of the methylammonium lead bromide perovskites is limited to bulk perovskites without taking into account the perovskite crystal sizes, assemblages, and shapes.
Researchers led by Professor Ou Chen at Brown University studied, for the first time, the characteristics of self-assembled lead bromide perovskite nanocube superlattices under high applied pressure. They derived in-depth correlations of the band gaps as well as atomic mesoscale structures of the perovskite as functions of pressure. Their research work is published in Advanced Materials.
The research team prepared the lead bromide perovskite nanocubes by modifying an existing method. The electronic absorption peak as well as sharp emission peak indicated the monodispersity of the specimens. Through the transmission electron microscopy, the authors were able to confirm that the nanocubes were sufficiently uniform and with an average edge length of 10.2±0.6 nm.
The conventional square pattern of the atomic fringes with an approximate d-spacing of 5.8 Å was indicative of high crystallinity of the nanocubes. The authors formed the self-assembled nanocubes superlattices by solvent evaporation of the lead bromide nanocubes suspension. The superlattices had simple cubic superstructure with a 12.5 nm lattice constant pattern, which confirmed the simple cubic superstructure of the superlattices.
In general, the authors probed the pressure induced structural dynamics as well as property enhancements of lead bromide nanocube-assembled superlattices under a 17.5 GPa compression. The lead bromide nanocubes experienced structural transitions from a combination of cubic and orthorhombic phases, to a single orthorhombic phase, which was followed by a quasi-amorphous phase.
In the course of these processes, the superlattices changed from a simple cubic to a lamellar structure. Concerning the structural changes, the photoluminescence of the superlattices indicated a small pressure induced blue-shift and a six-fold improvement in intensity at 0.1 GPa pressure, which was followed by a red-shift with a decrease in intensity until it was undetectable at about 1.3 GPa.
2-D lateral nanoplatelets with perfect cube phases were formed via a pressure-gradient-induced inter-nanocube fusion process. The resulting nanoplatelets had improved crystallinity and uniform thickness, and displayed a 1.6 time improvement of photoluminescence intensity with a prolonged ensemble lifetime as compared to the initial lead bromide nanocubes.
The outcomes of their study present a new method for combining nanocrystal assembly with pressure processing in order to come up with functional materials with improved crystallinity, improved properties, and structural stability for various applications in light emitting diodes as well as solar devices.
Yasutaka Nagaoka, Katie Hills-Kimball, Rui Tan, Ruipeng Li, Zhongwu Wang, and Ou Chen. Nanocube Superlattices of Cesium Lead Bromide Perovskites and Pressure-Induced Phase Transformations at Atomic and Mesoscale Levels. Advanced Material 2017, 29, 1606666.Go To Advanced Material