Controlling Luminescence Properties Of Mn²⁺:CsPbCl₃ Nanocrystals Via Incorporation Of Alkali Metal Ions

The general formula of metal halide perovskite is ABX₃,where A is a monovalent cation（A=CH₃NH₃（MA）,CH₃（NH₂）₂（FA）,or Cs⁺）,and B is a divalent metal cation(B=Pb²⁺,Sn²⁺,Ni²⁺,or Cd²⁺),X is a halogen anion（X=Cl^-,Br^-,I^-）.In the past few years,the synthesis of metal halide perovskites and their optoelectronic application have made tremendous progress.The study on enhancement of photoluminescence（PL）performance of perovskite nanocrystals has attracted widespread attention.The photoelectric properties of perovskite nanocrystals（such as absorption band gap,PL and quantum yield（PL QYs））and stability are effectively controlled by metal ion doping.Recently,some scientific researchers have reported some metal cations doping at A and B sites to enhance the PL QYs of perovskite nanocrystals and greatly improve the photoelectric properties of perovskite nanocrystals.Therefore,metal ion doping can effectively improve the optical and structural properties of perovskite nanocrystals.These Mn²⁺-doped cesium lead halide perovskite nanocrystals have potential applications in white LEDs.However,it has been found that the surface ligands of Mn²⁺-doped cesium lead halide perovskite nanocrystals are easy to fall off,resulting in formation of defects,which affect the optoelectronic properties of perovskite nanocrystals and the corresponding devices.Therefore,it is necessary to improve the stability of Mn²⁺-doped nanocrystals.In this paper,A-site（Rb⁺,K⁺）ion doped Mn:CsPbCl₃nanocrystals were synthesized,and their doping effects onthe luminescence properties and stability of Mn:CsPbCl₃nanocrystals are studied by using steady-state and time-resolved PL spectroscopy.Firstly,the luminescence properties of Mn:CsPbCl₃ nanocrystals with Rb⁺ions doping were studied.The Mn:Cs_1-xRb_xPb Cl₃ NCs（x=0,0.1,0.2,0.3 and 0.4）with various ratios of Mn/Pb and Rb/Cs were successfully synthesized by thermal injection methodand the effects of Rb ion doping on the luminescence properties of Mn:CsPbCl₃ nanocrystals were studied in detail.It is found that Rb⁺doping improves the Mn²⁺emission quantum yield of Mn:CsPbCl₃ nanocrystals.As the content of Rb⁺increases,the doping efficiency of Mn²⁺decreases.It was observed that the Mn doped CsPbCl₃ nanocrystals with Rb⁺content of 0.1 exhibited the highest Mn²⁺emission quantum yield up to 63.18%.The X-ray diffraction（XRD）and transmission electron microscopy（TEM）experimental results showed that all Mn:Cs_1-xRb_xPb Cl₃nanocrystals are in cubic phase,and the average size of NCs decreases as the Rb⁺doping content increases.The temperature-dependent PL spectra demonstrated the enhanced Mn²⁺emission of Mn doped CsPbCl₃ nanocrystals at low temperature of 80K,and as the temperature rises to 300 K,the PL intensity of Mn²⁺remains almost unchanged.The experimental results show that adding Rb⁺ to Mn:CsPbCl₃ nanocrystals can increase its crystallinity and reduce defects,thereby improving the thermal stability of Mn:CsPbCl₃ nanocrystals.Secondly,the luminescence properties of Mn:CsPbCl₃ nanocrystals with K⁺ions doping were studied.The Mn:Cs_1-xK_xPb Cl₃ nanocrystals with different ratio of Mn/Pb and K/Cs synthesized by thermal injection method were used to study the effect of K ion doping on the luminescence properties of Mn:CsPbCl₃ nanocrystals.It is noted that K⁺doping has a clear influence on optical properties of Mn:CsPbCl₃ nanocrystals.The doping efficiency of Mn²⁺decreases with the increase of K⁺content,which is consistent with the results of Mn:CsPbCl₃ nanocrystals doped with Rb⁺ions.The Mn doped CsPbCl₃ nanocrystals with K⁺content of 0.1 exhibited the highest Mn²⁺emission quantum yield up to 77.62%.In order to further study the luminescence properties of Mn:Cs_1-xK_xPb Cl₃ nanocrystals,the nanocrystals were dissolved in n-hexane,and the emission wavelength of excitons was adjusted with addition ofPbBr₂ solution.The samples were characterized by UV absorption and PL spectra.It was found that the exciton absorption and PL bandsshift to the red with the increase of Br content,while the PL peak of Mn²⁺hardly moved.The PL emission of Mn²⁺was still very strong when the emission peak of exciton was shifted to 450 nm.