Mn²⁺-doped Zero-dimensional Cs₄PbCl₆ Perovskite Nanocrystals And Their Optical Properties

Zero-dimensional(0D)perovskite derivative Cs₄PbX₆(X=Cl,Br,and I)nanocrystals(NCs)have attracted increasing attention as a new class of optoelectronic materials for applications in light-emitting diodes,lasers and photodetectors,owing to their excellent optical properties.However,because of the insulator nature,Cs₄PbX₆ perovskites inherently do not luminesce in the visible region,which limits their application as light-emitting materials in a wide spectral range.Manganese ion doping is an effective way for fine-tuning the electronic,optical and magnetic properties of perovskite NCs.A fundamental research on the optical spectroscopy of Mn²⁺-doped Cs₄PbX₆ perovskite NCs is of vital importance for tailoring the optical properties of 0D perovskites and expanding their applications.In this thesis,we have systematically investigated the controlled synthesis and optical properties of Mn²⁺-doped 0D Cs₄PbCl₆ perovskite NCs.By means of temperature-dependent steady-state/transient photoluminescence(PL)and electron paramagnetic resonance(EPR)spectroscopies,the excited-state dynamics of Mn²⁺in Cs₄PbCl₆NCs was unveiled.The detailed research contents are as follows:1.We have developed a unique strategy for the synthesis of monodispersed Mn²⁺-doped0D Cs₄PbCl₆ perovskite NCs with controllable morphology and particle size through a hot-injection method.Powder XRD measurements showed that the as-synthesized NCs had a high crystallinity and maintained pure-phase rhombohedral Cs₄PbCl₆for Mn²⁺doping concentrations up to 30 mol%.TEM images showed that the NCs were monodispersed and uniform in size(?25 nm)and morphology,and Mn²⁺doping had no significant influence on the size and morphology of the NCs.Energy dispersive X-ray spectroscopy,X-ray photoelectron spectroscopy,and inductively coupled plasma-atomic emission spectroscopy confirmed pure phase and high crystallinity of the resulting NCs,and also revealed the substitution of Pb²⁺by Mn²⁺in isolated[PbCl₆]^4-octahedra.2.We have comprehensively surveyed the optical properties of Cs₄PbCl₆:Mn²⁺NCs with different Mn²⁺doping concentrations through PL spectroscopy.Optical absorption spectra revealed that Mn²⁺doping has little influence on the electronic structure of Cs₄PbCl₆host,and the NCs displayed an intense and narrow absorption band at 284 nm,corresponding to a bandgap of 4.37 e V.Upon excitation at 289 nm,the NCs exhibited two broad excitation bands with central wavelengths at?358 and?617 nm,which were assigned to the ³P_0,1?¹S₀transition of Pb²⁺and the ⁴T_1g?⁶A_1gtransition of Mn²⁺,respectively.The PL intensity of Mn²⁺increased gradually at the expense of that of Pb²⁺with increasing the Mn²⁺concentration,indicative of an efficient energy transfer from Pb²⁺to Mn²⁺in Cs₄PbCl₆:Mn²⁺NCs.The highest efficiency of energy transfer Pb²⁺to Mn²⁺was determined to be 46.0%.Besides,the overall PL intensity of the NCs was significantly enhanced upon Mn²⁺doping,as a merit of efficient energy transfer from Pb²⁺to Mn²⁺which alleviates the energy migration through Pb²⁺sublattice to the intrinsic or surface defects of the NCs.Owing to the the spatially confined 0D structure of Cs₄PbCl₆ perovskite,Mn²⁺ions in these NCs exhibited an unusually long PL lifetime of?26.2 ms and a superior PL quantum yield of 25.8%in solid form,which is much higher than those(?0.1%)of Cs Pb X₃counterparts previously reported.3.We have unraveled the excited-state dynamics of Mn²⁺in 0D Cs₄PbCl₆ perovskite NCs through temperature-dependent steady-state and transient PL spectroscopy.By fitting the the temperature dependence of the integrated PL intensities and full widths at half maximum of Pb²⁺and Mn²⁺emissions,the activation(or binding)energies of Pb²⁺and Mn²⁺were determined to be 251 and 145 me V,and their electron–phonon coupling strengths were determined to be 243 and 75 me V,respectively.This demonstrated the large exciton binding energies and high electron–phonon coupling strengths of Cs₄PbCl₆:Mn²⁺NCs.Furthermore,an anomalous temperature evolution of Mn²⁺-PL decay from a dominant slow decay(in tens of ms scale)at 300 K to a fast decay(in 1 ms scale)at 10 K was observed,as a result of strong Mn-Mn dipole-dipole coupling interactions induced by lattice distortion at low temperatures.In summary,we have systematically investigated the optical properties and excited-stated dynamics of Mn²⁺in pure-phase 0D Cs₄PbCl₆ perovskite NCs.The substitution of Pb²⁺by Mn²⁺in isolated[PbCl₆]^4-octahedra resulted in intense orange-red Mn²⁺emission at?617 nm through efficient Pb²⁺-to-Mn²⁺energy transfer.Owing to the spatially confined 0D structure of Cs₄PbCl₆ perovskite,Mn²⁺ions in these NCs exhibited a large binding energy and strong electron–phonon coupling strength,which led to a remarkably long PL lifetime(26.2 ms)of Mn²⁺and a superior PL quantum yield(25.8%)for NC solids to their 3D Cs Pb Cl₃analogues.Specifically,an anomalous temperature evolution of Mn²⁺-PL decay from a dominant slow decay to a fast decay with the temperature decrease was observed.These findings provide fundamental insights into the excited-state dynamics of Mn²⁺in 0D Cs₄PbCl₆NCs,thus laying a foundation for future design of 0D perovskite NCs through metal ion doping towards versatile applications.