Study On Mn²⁺ Doped Luminescent Materials At High Pressure And Low Temperature By Time-Resolved Photoluminescence Spectroscopy

Mn2+ ion-doped luminescent materials have unique and excellent properties,and have been extensively studied for a long time.However,there are still many unknowns about the mechanisms of their excitation,energy transfer and luminescence,which need further theoretical and experimental research.Pressure and temperature are important factors affecting the luminescence of materials,which can effectively adjust the luminescence properties of materials in situ;time is an important dimension of luminescence properties and contains detailed information on the luminescence dynamics of materials.It is foreseeable that combining the adjustment of pressure and temperature with time resolution can reveal much more detailed information on the luminescent properties of materials.This doctoral dissertation selects MnS/ZnS core-shell quantum dots and Cs2NaBiCl6:Mn2+ Crystals,which are complicative representative of Mn2+ ion-dopedⅡ-Ⅵ semiconductor quantum dots and Mn2+ ion-doped perovskite materials,respectively.They are mainly studied and analyzed by high pressure and low temperature experiments combined with time-resolved spectroscopy.The following is a brief introduction to each chapter.Chapter 1 is an introduction,which introduces some basic theories of luminescent m aterials;quantum dot materials,perovskite materials and their research progress;Mn2+ ion luminescence characteristics and research progress;the development history,research progress and related experimental techniques of high pressure science;the principle of time-resolved spectroscopy and the experimental method of time-resolved spectroscopy combined with high pressure and low temperature techniques.Finally,the research objectives,main contents and technical routes of this paper are elaborated.Chapter 2 is about the structure and steady-state spectra of MnS/ZnS core-shell quantum dots under high pressure.Through direct excitation at 514 nm and indirect excitation at 325 nm,the different properties of different components of Mn2+ ion luminescence under high pressure were studied.The high-pressure fluorescence spectrum shows that at about 7.5 GPa,the luminescence of Mn2+ions is significantly attenuated and fluorescence quenching occurs.Two possible reasons were proposed:(1)the phase transition of the crystal structure of the MnS core;(2)the intersection of the 2T2(2I)and 4T1(4G)energy levels.At higher pressures,the luminescence of Mn2+ ions exhibited obvious differences under direct excitation and indirect excitation.This is attributed to the change in the relative proportions between the luminescence of coupled and isolated Mn2+ ions with different pressure responses.Chapter 3 is the time-resolved spectroscopic study of MnS/ZnS core-shell quantum dots under high pressure and low temperature.Four main luminescence components were observed and discussed:(1)a very fast broad peak in the visible range,attributed to defect luminescence,such as Zn vacancy defects;(2)a very fast narrow peak in the ultraviolet range,attributed to exciton-related luminescence;(3)faster broad peaks in the shorter wavelength range,attributed to impurity luminescence;(4)the slow Mn2+ ion 4T1(4G)luminescence in the longer wavelength range.The Mn2+ ion 4T1(4G)luminescence can be further divided into coupled Mn2+ ions with longer wavelength and faster decay,and isolated Mn2+ ions with shorter wavelengths and slower decay.The normalized time-resolved spectra show that the average emission rate of Mn2+ ion 4T1(4G)emission remains nearly unchanged until 10 μs.The defect luminescence showed no obvious shift with increasing pressure,while Mn2+ ion 4T1(4G)luminescence showed a typical crystal field enhancement-induced red shift.These strongly confirm that the historically controversial nanosecond fluorescence lifetime component of ZnS:Mn QDs at wavelength of 590 nm does not originate from Mn2+ ion 4T1(4G)luminescence,but from the faster defect state luminescence.Coupled Mn2+ ions have different pressure coefficients than isolated Mn2+ ions.This is attributed to the different local environments of them,such as the different local compressibility,resulting in different changes in the crystal field splitting with pressure.The results of lowtemperature time-resolved spectroscopy show that the defect emission and the excitonrelated emission weaken with the increase of temperature,while the emission of Mn2+ions increases a little,indicating that there may be energy transfer from excitons and defect states to Mn ions,which partially enhances with increasing temperature.Chapter 4 is the time-resolved spectroscopic study of Cs2NaBiCl6:Mn2+ crystals at low temperature.Two kinds of Cs2NaBiCl6:Mn2+ crystals with different Mn2+ ion concentrations were synthesized by hydrothermal method.Among them,the crystal obtained from sample No.1 is small,and the electron paramagnetic resonance(EPR)spectrum results show that it has obvious Cl-vacancy defects.The crystal of sample No.2 is larger,and there is no obvious signal of Cl-vacancy defect.Under the excitation of different wavelengths,the Mn2+ ion emission of sample No.1 has obvious tail at 700 nm,while sample No.2 has almost no tail.This tail may be related to the Cl-vacancy defect.This tail at 700 nm was found by time-resolved spectroscopy to correspond to the luminescence peak at 750 nm,which decays faster than the Mn2+ ion 4T1g(4G)luminescence.450 nm luminescence and 650 nm luminescence are also found,which are possibly exciton-related luminescence.A weak,distinctly red-shifted,and slowly decaying Mn2+ ion luminescence peak is found at low temperature and long Delay.Compared to the previous stronger and faster Mn2+ ion emission,this may originate from Mn2+ions on different doping site(Bi3+ion or Na+ ion site).As the temperature increases,the luminescence intensity of Mn2+ ions increases at first,and then decreases sharply after about 240 K.Other luminescent components have also undergone obvious changes.The complex changes of different luminescent components at different temperatures sh ow that there are complex energy transfer processes.Low temperature Raman spectroscopy shows no obvious structural phase transition of the sample.Therefore,the complex luminescence changes may be the result of the combined effects of energy transfer and non-radiative relaxation among Mn2+ ions,excitons and some unclear states,which need further study.Chapter 5 is the summary and prospect of this thesis.