## 3 Results and Discussion  
As schematically shown in Figure 1a, the synthetic process was performed in two steps: the formation of CsPbBr\({}_{3}\) nanocrystals and size controlling by adding APTES. Details of the process have been described in the Experimental section. The sol sample showed bright green under the ultraviolet light (395 nm) (Figure 1c), and CsPbBr\({}_{3}\) nanocrystals/polyacrylic composites can be prepared via an ultraviaolet polymerization method (Figure 1b,c). Figure 2a shows the absorption and fluorescence spectra of CsPbBr\({}_{3}\) nanocrystals prepared in IBOMA. An emission peak at 510 nm was observed, and the corresponding absorption peak located at 485 nm. According to the formula proposed by [PERSON] and [PERSON], the calculated bandgap of as-prepared CsPbBr\({}_{3}\) nanocrystals is 2.25 eV [39; 40]. However, red shift can be observed in the emission spectra of CsPbBr\({}_{3}\) prepared in other three acrylic monomers, indicating the particle size increasing [41] (Figure S1 in Supplementary Materials). It was noted that the PLQY of CsPbBr\({}_{3}\) nanocrystals prepared in IBOMA reached 87.5%, much higher than the samples prepared in other acrylic monomers (Table S1 in Supplementary Materials). In order to study the crystal structure of the samples, the XRD patterns of CsPbBr\({}_{3}\) powder samples are shown in Figure 2b and Figure S1, and CsPbBr\({}_{3}\) nanocrystals possessing a cubic structure (PDF # 54-0752) were synthesized in IBOMA. However, a small impurity peak (Figure S1) identified as PbBr\({}_{2}\) existing in the pattern of CsPbBr\({}_{3}\) nanocrystals synthesized in IBOA. The XRD results demonstrated that cubic CsPbBr\({}_{3}\) nanocrystals can be synthesized at room temperature by this method. Figure 1: (a) In-situ one-step synthesis process of CsPbBr\({}_{3}\) nanocrystals sol; (b) preparation of solid CsPbBr\({}_{3}\) nanocrystals composites; (c) nanocrystals sol, blocks and films. TEM images of the as-synthesized CsPbBr\({}_{3}\) nanocrystals are shown in Figure 3. Compared to other three samples (Figure S2 in Supplementary Materials), CsPbBr\({}_{3}\) synthesized in IBOMA (Figure 3a) has a smaller particle size about \(8.4\pm 1.3\) nm (Figure 3b). The HRTEM image (Figure 3c) reveals high crystallinity with a lattice spacing of 0.295 nm, which fits well with the (200) plane of cubic CsPbBr\({}_{3}\). According to the TEM results and the PLQY performance, CsPbBr\({}_{3}\)-IBOMA sol has been used for the subsequent studies and tests. The CsPbBr\({}_{3}\)-IBOMA nanocrystals sol exhibited excellent stability. After being placed in air without any protection for 2 weeks, the particle size remained nearly unchanged (Figure 4a), which was attributed to the capping of hydrolyzed APTES ligand [32]. APTES hydrolyzes on the surface of nanocrystals, forming a network structure to prevent the desorption of ligands and to subsequently prohibit size increases, which enhanced the stability of nanocrystals [37; 42]. To further prove the effect of APTES, CsPbBr\({}_{3}\)-IBOMA nanocrystals without APTES were synthesized, and their morphology and size were determined by TEM (Figure 4b). The size (\(45.7\pm 7.7\) nm) was obviously larger than the sample using APTES (\(8.4\pm 1.3\) nm). The reaction with and without APTES revealed a large difference (Video S1). Because of unrestricted increasing of particle size, the APTES-free sample rapidly turned to yellow after mixing two precursors, and finally the sample subsided [43]. Meanwhile, the sample with APTES remained unchanged and no aggregation was observed, suggesting that the addition of APTES effectively prevents the size increasing of CsPbBr\({}_{3}\) nanocrystals. FTIR spectrum was used to demonstrate the capping of APTES, as shown in Figure 4c, in which it exists a N-H vibration absorption peak at 3448 cm\({}^{-1}\), originating from the -NH\({}_{2}\) group of APTES. The peak at 948 cm\({}^{-1}\) was ascribed to the vibration absorption of the -Si-OH group of hydrolyzed APTES.