Applications Of Imidazolium Cations In The Surface Modifications And Controllable Synthesis Of Halide Perovskites

All-inorganic lead halide perovskite quantum dots(CsPb X₃QDs,where X=Cl,Br,I)have received an unparalleled attention as luminescent materials due to their high quantum yields,narrow emission peaks,and facilely tunable emission wavelength.However,their weak stability against light,heat and moister remain as a major challenge towards practical application in the field of displays and lighting.The conventional ligands are extremely easy to desorb from the surface or undergo chemical reactions under environmental stimulation,which is in urgent needs to be solved.Given the above problems,this thesis intends to modify the QDs with aromatic heterocyclic ionic ligands from the perspective of surface ligand engineering.At the same time,the effects of such ligands on the luminescence properties and crystal growth are studied.We used a simple two-step ligand exchange method to put a series of di-alkylimidazole ions onto the surface of QDs prepared by hot-injection(HI)in replacement of the conventional ligands oleic acid(OA)and oleylamine(OAm).The phases and shapes were found to be well preserved after the ligands exchange.Fournier transfer Infrared(FT IR)spectra and X-ray photoelectron spectroscopy(XPS)confirmed the successful replacement of surface ligands.The conformation and conjugated heads of imidazole ions possess different characteristics from traditional amine ligands.The frontier molecular orbital(FMOs)energy levels and the surface structure of the QDs were simulated theoretically,where a strong interaction between the imidazole ring and the QDs was found.The imidazolium modification not only improves the stability of the quantum dots,but also regulates the luminescent properties of the QDs.We then adopted the CsPb Br₃/Cs₄Pb Br₆ microscale endodaxial system a facile antisolvent precipitation method by the assist of imidazolium cations.Due to the differences in the solubility of metal ions and total energy of the two phases,the precursor would trigger the formation of CsPb Br₃ at first before Cs₄Pb Br₆ could grow using the former CsPb Br₃ NCs as seeds since the lattice of Cs₄Pb Br₆ matches well with that of CsPb Br₃in both crystal structure and electronic structure.Compared with conventional QDs,this composite powder has better resistance against heat,and thus a better performance in the application as a phosphor.