All-inorganic CsPbX? perovskite quantum dots (QDs) are ideal materials for high-quality single-photon sources in quantum information applications, as the performance of single-photon sources is closely related to the size of perovskite QDs. However, the lack of effective methods to reduce the size of CsPbX₃ QDs remains one of the major obstacles to achieving single-photon emission. In this study, we employ an efficient and low-cost doping synthesis strategy, directly introducing ammonium bromide (NH₄Br) into the lead precursor via the hot-injection method, successfully preparing CsPbBr₃ perovskite QDs. By utilizing NH₄Br to regulate crystal growth kinetics and passivate surface defects, we effectively suppress the Ostwald ripening process, significantly reducing the average size from the original 10.07 nm to 6.87 nm while improving size uniformity. The size reduction enhances the quantum confinement effect, leading to a blue shift in the photoluminescence (PL) emission peak from 520 nm (undoped) to 505 nm. Additionally, autocorrelation tests reveal that the g2(0) value of the doped QDs decreases from 0.45 to 0.22, which indicates a significant improvement in single-photon purity. We present an innovative and straightforward synthesis strategy, successfully producing CsPbBr₃ perovskite QDs with a narrow size distribution. The incorporation of NH?Br enhances the single-photon purity of the QDs, which provides an ideal material system for single-photon emission applications and lays an important foundation for their commercialization.

In our study, CsPbBr? QDs, and NH?Br-doped CsPbBr? QDs are synthesized using the hot-injection method to achieve size reduction and improved size uniformity. The synthesis process consists of two main steps. Firstly, the cesium precursor is prepared by heating a mixture of cesium carbonate, oleic acid, and 1-octadecene in an inert atmosphere at 120 ℃ for 2 h, followed by increasing the temperature to 160 ℃. The resulting solution is then cooled to room temperature and stored under sealed conditions. Secondly, the synthesis of CsPbBr? QDs and NH?Br-doped CsPbBr? QDs is carried out by heating a mixture of lead bromide, ammonium bromide, oleic acid, oleylamine, and 1-octadecene in an inert atmosphere for 2 h, with the temperature raised to 150 ℃. Subsequently, 0.4 mL of cesium oleate precursor is rapidly injected. After 5 s of reaction, the mixture is immediately cooled using an ice-water bath. Well-dispersed QD solutions are obtained by high-speed centrifugation. Throughout the synthesis process, QDs with different doping concentrations are prepared by controlling the amount of ammonium bromide added.

The prepared NH₄Br-CsPbBr₃ QDs, with the Br/Pb molar ratio less than 7, exhibit a distinct decreasing trend in particle size as the Br/Pb molar ratio increases. The average particle size of CsPbBr? QDs significantly decreases from 10.07 to 6.87 nm, which results in a blue shift of the PL emission peak from 520 nm (undoped) to 505 nm (Br/Pb molar ratio is 7). Statistical analysis of the same number of grains further shows that the particle size distribution narrows from 4?19 nm to 5.5?9 nm, which confirms that ammonium bromide doping effectively improves the morphological uniformity of the QDs. However, when the Br/Pb molar ratio exceeds 7, excess ammonium bromide disrupts the controllability of the QD morphology, which leads to distortion of the cubic structure. Therefore, the effective doping range of ammonium bromide is limited to a Br/Pb molar ratio of less than or equal to 7. Furthermore, the doping of ammonium bromide strengthens the covalent bonding of Pb-Br, which enhances the stability of the QD crystal structure and increases the quantum yield of the QDs from 31.22% to 61.65%, while the fluorescence lifetime extends from 1.15 ns to 2.80 ns. The reduction in QD size induced by NH?Br doping enhances the quantum confinement effect. This enhanced quantum confinement leads to a more discrete energy level structure in the QDs, which significantly reduces the probability of multiphoton emission, thereby improving the purity of single-photon emission. After doping, the second-order autocorrelation function g2(0) of the QDs decreases from the original value of 0.45 to 0.22, which indicates that ammonium bromide doping effectively improves the single-photon emission purity of the QDs.

Our study systematically reveals the effect of NH?Br doping on the size control of CsPbBr? perovskite QDs, its intrinsic mechanisms, and the effect on single-photon properties. Experimental results show that within the doping concentration range where the Br/Pb molar ratio is less than or equal to 7, the QD size decreases in a regular pattern as the doping concentration increases, with the average particle size significantly reducing from 10.07 nm (undoped) to 6.87 nm (at a Br/Pb molar ratio of 7). More importantly, the uniformity of the doped QDs’ size is improved, with the particle size distribution narrowing from 4?19 nm to 5.5?9 nm. This improvement primarily stems from the bromine-rich environment provided by ammonium bromide, which not only effectively fills the bromine vacancies on the QD surface [as confirmed by X-ray photoelectron spectroscopy (XPS) quantitative analysis, showing an increase in the Br/Pb molar ratio from 3.85 to 4.21] but also narrows the size distribution by suppressing Ostwald ripening and through the synergistic coordination of NH${}_{\mathrm{4}}^{+}$ ions with the [PbBr₆]^4- octahedral structure. We find that after doping, the g2(0) value of the QDs decreases from 0.45 to 0.22, which indicates that NH₄Br doping helps improve the single-photon purity of the QDs. The innovative findings of this study open new avenues for the application of perovskite crystal QDs in single-photon sources, which is of great significance for advancing the development of single-photon technology in cutting-edge fields, such as quantum communication, quantum computing, and quantum information processing.