In Situ Ion Passivation For Preparation Of Highly Luminescent And Stable CH₃NH₃PbBr₃ Perovskite Nanocrystals

The ABX₃-type lead halide perovskite nanocrystals（PNCs）have attracted more and more attention by virtue of their unique merits of narrow band emission,good color-tunability,high photoluminescence quantum yield（PLQY）in combination with easy synthesis and low fabrication cost.Despite success in boosting the optoelectronics properties,the intrinsic instability of PNCs seriously impedes their large-scale commercial and industrial applications.In general,the low formation energy of lead halide perovskite materials,highly ionic properties and large surface-to-volume（S/V）ratio of nanocrystals（NCs）lead to the poor stability of PNCs.To address the issue,many efforts are paid to improve the luminescence performance and stability of PNCs.Therefore,in this paper,the ligand-assisted reprecipitation（LARP）method has been used to prepare CH₃NH₃PbBr₃ nanocrystals.During the process of LARP,the thiocyanate anions and guanidine cations are introduced to realize the in-situ passivation of PNCs,as well as control of the optical properties and stability of PNCs.Firstly,an in-situ thiocyanate inclusion method for the fabrication of highly luminescent and stable CH₃NH₃PbBr₃ perovskite nanocrystals（NCs）is developed,employing Pb（SCN）₂ as the lead precursor to partially or totally replace PbBr₂ in the ligand-assisted reprecipitation（LARP）process.The in-situ approach not only avoids the introduction of impurity elements,but more interestingly,incorporation of thiocyanate can control the crystallinity,particle size,luminescence and stability of CH₃NH₃PbBr₃ NCs in a simple and effective manner.By adjusting the thiocyanate concentration,the photoluminescence（PL）of the synthesized CH₃NH₃PbBr₃ NCs can be tuned in the range of 473-526 nm,as characterized by narrow emission line widths of 21-28 nm and outstanding photoluminescence quantum yields（PLQYs）of 73%to96%.Meanwhile,the stability of CH₃NH₃PbBr₃ NCs can be greatly improved as the amount of thiocyanate increases.The improvement in optical performance and stability of CH₃NH₃PbBr₃ NCs is mainly due to the contribution of higher crystallinity and,more stable and defect-free surface passivation induced by the presence of thiocyanate.Secondly,guanidine hydrobromate,together with OA and OLA,is used as ligands to improve the stability and regulate the luminescence performance of CH₃NH₃PbBr₃ nanocrystals.The in-situ introduction of guanidine cations can control the crystallinity,particle size,luminescence and stability of CH₃NH₃PbBr₃ NCs.The results show that the CH₃NH₃PbBr₃ nanocrystals exhibit the best stability and the highest fluorescence quantum yield when the molar ratio of guanidine hydrobromate to CH₃NH₃PbBr₃ is 0.15:1.Our work paves a novel way for preparing highly luminescent and stable CH₃NH₃PbBr₃ NCs.