Synthesis parameter optimization for uniform and stable perovskite quantum dots

. Colloidal lead halide perovskites nanocrystals, also known as quantum dots, have been intensively studied as promising optoelectronic materials and have attracted widespread attention due to their unique optical versatility, high photoluminescence quantum yield and convenient synthesis. Among them, the potential of all-inorganic halide caesium lead perovskite (CsPbX 3 , X = Cl, Br, I) is particularly prominent. In this work, by adjusting the experimental parameters, including precursor preservation condition, reaction temperature and the isolation/purification process, CsPbBr 3 quantum dots with uniform size, neat arrangement, narrow full width at half maxima and excellent luminescent properties have been successfully prepared, which lays a good foundation for its potential application in practical optoelectronic devices.

Since Møller [5] reported CsPbX 3 (X = Cl, Br or I) perovskite in 1958, the research on lead halide perovskite nanocrystals has made rapid progress in the past few decades. Protesescu [6] et al. prepared high luminescent perovskite-based colloidal QDs and demonstrated a new way to prepare halide perovskites. They used cheaper materials and simple methods to synthesize monodisperse colloidal nanocubes of all-inorganic caesium halide lead perovskite with cubic shape and cubic perovskite crystal structure, which opened a new door for the preparation of lead halide perovskite nanocrystals.
However, the performance of CsPbBr 3 QDs needs to be improved for practical applications. Therefore, meticulous preparation of QDs with multiple controllable parameters has become a top priority [7][8][9][10] . Herein, we successfully synthesized uniform and stable CsPbBr 3 QDs by thermal injection method. More importantly, by optimizing experimental parameters, including precursor preservation condition, reaction temperature and the isolation/purification process, the size distribution, colour purity, and storage stability of CsPbBr 3 QDs have been significantly improved, which lays a foundation for its future application [11][12][13] .

Preparation of Cs−Oleate Solution
Cs 2 CO 3 (407 mg), 20 mL of ODE and 1.25 mL of OA were added to a 50 mL three-necked flask and maintained under vacuum at 120 o C for 60 min. The mixture was stirred under N 2 to 150 o C until all Cs 2 CO 3 was dissolved. The solution was put aside for later use with the heating temperature being kept by various methods.

Synthesis of CsPbBr3 QDs
PbBr 2 (414 mg) and 30 mL of ODE were placed in a 100 mL three-necked flask and maintained under vacuum at 120 o C for 60 min. Then the reaction system was switched to nitrogen protection, 3 mL of OA and 3 mL of OAm were injected into the solution. Then the temperature was raised to 150 o C,160 o C and 180 o C, respectively, and the Cs-oleate was swiftly injected. After 5 s, the reaction solution was put in an ice water bath and cooled to room temperature.

The influence of precursor preservation condition on the morphology of CsPbBr3 QDs
As mentioned in the synthetic method, the synthesis of CsPbBr 3 QDs consisted of two steps, namely the precursor solution preparation followed by nucleation of the nanocrystals. The precursor solution played an important role in the whole experimental process, and its quality directly affected the results of the experiment.
In the preparation of precursor solution, CsCO 3 was dissolved in organic solvent under high temperature. The obtained precursor solution was preserved in a vacuum oven at 120 o C, such method has been normally adopted in literature. However, after a period of time, the dissolved CsCO 3 partially precipitated and caused the precursor solution to become turbid (Fig.1a). Therefore, we modified the precursor preservation condition by keeping it in an oil bath with inert atmosphere. As shown in Fig.1b, a clear and transparent precursor solution can be obtained. The different precursor preservation conditions had great influence on the resultant morphologies of obtained CsPbBr 3 QDs. TEM tests were carried out on the QDs prepared by the two thermal preservation methods. As shown in Fig.2 (a-c), QDs prepared by the precursor preserved in the vacuum oven was large in size and varied greatly in dimension. In comparison, as shown in Fig.2 (df), the QDs prepared by the precursor preserved in the oil bath are very uniform in size and neatly arranged. The average size of these QDs was about 8 nm as measured from TEM images. XRD measurement was carried out on the prepared QDs shown in Fig. 2(d-f) to examine their crystal structure in comparison with the standard PDF card. It can be obtained from Fig.3 that the prepared QDs had a cubic crystal structure, and all diffraction peaks corresponded perfectly to the standard card (PDF#54-0752) of CsPbBr 3 QDs.

The influence of reaction temperature on the size of CsPbBr3 QDs
The reaction temperature had a very crucial effect on the size of CsPbBr 3 QDs, The average size of QDs have been successfully adjusted by adjusting the reaction temperature. Fig.4 (a-c) shows the TEM images of CsPbBr 3 QDs prepared at different temperatures of 150 o C, 160 o C and 180 o C. The average particle sizes of QDs prepared at 150 o C, 160 o C and 180 o C were measured to be 7.35 nm, 7.44 nm and 8.15 nm respectively, with the size distributions shown in Fig. 4(d-f), indicating that in a certain range, the higher the preparation temperature, the larger the average particle size.

The influence of isolation/purification on optical properties of CsPbBr3 QDs
Isolation/purification of prepared QDs is a very important step that determines the dispersity and optical properties of QDs. During the isolation/purification of QDs, the excessive reagent as well as surface ligand were removed. Thus, the choice of solvents and washing times is the key to the long-term storage of QDs. Improper reagents will directly cause the agglomeration of QDs, and even destroy its structure, resulting in photoluminescence (PL) quenching [14] . So it is necessary to select appropriate reagents from a large number of attempts. We divided the optimization of isolation/purification into three steps, namely choice of washing reagent, composition of washing reagent, and washing times. Four reagents were selected to purify QDs, namely TL, mixed solution of MAC and Hex, mixed solution of EAC and Hex, and mixed solution of TBA and Hex. PL test was carried out on the samples obtained by these four methods (Fig.5). The full width at half maxima (FWHM) of the four samples were measured and listed in Table 1. It can be seen that the FWHM of the QDs washed with TL was the largest, up to 23.5 nm. The FWHM of the QDs washed by MAC/Hex and EAC/Hex were smaller, being 21.5 nm and 20 nm, respectively. And the FWHM of the QDs washed by mixed solution of TBA and TL was the smallest, being only 16 nm. However, TBA would lead to easy agglomeration of QDs, thus reducing the optical performance and storage stability of QDs. In the previous attempt, the FWHM of the QDs washed by MAC/Hex and EAC/Hex were small, so we further adjusted the proportions of different reagents in the mixed solution. The PL spectra of QDs washed by the optimum reagent ratio of 3:1 are shown in Fig.6. It can be seen from Table 2 that the FWHM of the QDs washed with EAC: Hex=3:1 is smallest, being 19 nm, while those of the QDs washed with MAC: Hex=3:1 and Hex were larger, both of which being 21 nm. Finally, the influence of washing times on QDs was also analysed. By fixing the washing reagent of EAC: Hex=3:1, QDs were washed by different times and then analysed by PL test (Fig.7). As shown in Table 3, the FWHM of the QDs obtained after twice washing was 18.5 nm, which is significantly smaller than those of QDs washed once and three times. The smaller the FWHM, the higher the luminous purity of QDs. Therefore, the optimized washing parameters of QDs was determined to be washed twice with EAC: Hex=3:1. CsPbBr 3 QDs can be greatly affected by the polarity of organic solvents. Solvent with strong polarity would have a significant negative impact on the optical property and structure intergrity of QDs. Therefore, Hex with less polarity was used as the solvent of CsPbBr 3 QDs.
It can be seen from Fig.8, after adjusting the experimental parameters, the synthesized CsPbBr 3 QDs was a clear and transparent light green solution, which emits green fluorescence under the irradiation of ultraviolet (UV) lamp (365nm).

Research on storage stability of CsPbBr3 QDs
CsPbBr 3 QDs are very fragile and may lose their structure integrity and optical properties by a variety of external stimuli, including light, heat, and oxygen, which is a major limiting factor for its practical application [15,16] . The prepared CsPbBr 3 QDs were stored in a fridge frozen chamber, and taken out for PL and UV tests every other week. It can be seen from Fig.9 that after nearly a month of storage, the PL and UV curves of QDs were basically unchanged, being highly coincident with the beginning curve, showing good storage stability of CsPbBr 3 QDs prepared by optimized method.

Conclusion
In conclusion, we demonstrate a systematic analysis on the synthesis parameters of CsPbBr 3 QDs by adjusting precursor preservation condition, reaction temperature and isolation/purification process. Compared with the previously reported CsPbBr 3 QDs, the QDs synthesized in this work show a high degree of monodispersity and narrow FWHM. More importantly, satisfactory storage stability has been demonstrated, which is of great significance for the potential practical application of perovskite QDs.