Size Control of CH3NH3PbBr3 perovskite Quantum Dots Via Ostwald Anti-Ripening
Authors: Umemoto Kazuki , Tezuka Yuki , Inose Tomoko , Uji-i Hiroshi , Yoshida Tsukasa , Masuhara Akito 
Published: February 2020

「
Introduction
」

Organic-inorganic perovskite crystals have been aggressively applied to perovskite solar cells due to their attractive properties such as long carrier diffusion, am-bipolar conductivity, broad color-tunability, and their small exciton binding energy1. These perovskite crystals have been also applied to various applications like light-emitting diodes (LED), photodetectors, field effect transistors, and lasers due to perovskite crystals can be simply prepared from low-cost precursors with simple solution processes1. These tremendous potential for various applications with high performance are based on perovskite electro-optical features and they are expected to be used as an alternative semiconductor material to silicon and compound materials.
Recently, bright luminescence properties from methylammonium lead tri-bromide (MAPbBr3) perovskite quantum dots (PeQDs) have been reported owing to the development of methods for preparing these PeQDs. Zhang et al. developed a ligand-assisted reprecipitation (LARP) inspired by the reprecipitation method for preparing organic and nano / micro crystals2. LARP can simply obtain MAPbX3(X = I, Br, and Cl) PeQDs through mixing a solution dissolving precursors and poor solvent. Although synthesis methods representative for LARP with narrow size-distributions are of particular interest for the successful implementation of PeQDs into LED with narrow emission, size-controlled PeQDs by nanometers have not been developed.
In this work, we report on Ostwald ripening as a size-tunable technique for MAPbBr3PeQDs using LARP.

「
Experimental
」

Size-controlled synthesis of MAPbBr3PeQDs, and a simple strategy is as follows; a precursor solution was dropped into poor solvent followed by centrifuged to remove large-sized MAPbBr3crystals. Herein, MAPbBr3PeQD dispersions were aged at the 50℃using water bath to accelerate Ostwald ripening in MAPbBr3PeQD dispersions. Finally, we obtained size-controlled MAPbBr3PeQDs through centrifugation to remove grown MAPbBr3crystals.

「
Results and Discussion
」

MAPbBr3PeQD dispersions can be clearly observed that as aging time increases, PL peaks are blue shifted from 517 nm to 456 nm through aging for 4 hours. The emission colors irradiated UV lamp at 254 nm of MAPbBr3PeQD dispersions also changed from green to cyan and deep blue , while these dispersions close to be color less. When MAPbBr3PeQD dispersion is aged 1 hour, compared with an initial MAPbBr3PeQD dispersion, maximum PL peak was not clearly blue shift (ΔE = 42 meV) because of mean QD sizes were not drastically changed and also these square shape is maintained. In contrast, MAPbBr3PeQD dispersion is aged for 2 hours, a maximum PL peak drastically blue shifted from 508 nm to 489 nm. Because morphologies of samples such as sizes and shapes were completely changed from square to spherical with 7.2 nm size , result in PL energy extremely change (ΔE = 95 meV). Moreover, as further aging times increases, their mean QD sizes down to approach 5.3 nm by aging at the 50℃for 4 hours, in which clearly appear increasing band gap of MAPbBr3PeQDs due to quantum confinement effect.

「
References
」

1) B. R. Sutherland, et al.,
Nature, 2016, 10, 295-302.
2) F. Zhang, et al.,
ACS Nano, 2015, 9, 4533-4542.