High Pressure Study Of Mn-Doped Lead Halide Perovskite Nanocrystals

In the past decades,lead halide perovskites have emerged as prominent materials in various research fields owing to their low cost,simple synthesis,tunable bandgap,and long carrier diffusion length.CsPbBr₃,a typical three-dimensional（3D）lead halide perovskite,exhibits excellent charge transport characteristics and high luminescence efficiency.However,the stability issues associated with its own structure significantly limit its practical application in devices.The introduction of long-chain organic layer PEA⁺（PEA⁺=C₆H₅C₂H₄NH₃⁺）to reduce the material dimension can effectively enhance the stability of the material.However,this reduction in dimension also leads to new issues,such as a large bandgap and a small emission range.Reasonable and effective chemical element doping is an important strategy to prepare more stable and higher luminescence efficiency lead halide perovskite materials.Doping with transition metal manganese（Mn）into lead halide perovskite nanocrystals（NCs）not only enhances the stability of the material but also introduces new luminescent centers and expands the emission range.Moreover,due to the difference in radius between Mn²⁺and Pb²⁺,the octahedral skeleton undergoes local distortion,leading to carrier localization.Nevertheless,further studies are necessary to determine the influence of Mn²⁺doping on the photoluminescence（PL）properties,electronic structure,and crystal structure of lead halide perovskite NCs.Investigating the structure-optical property relationship of Mn-doped lead halide perovskite NCs has crucial guiding implications for designing high-stability and high-luminescence efficiency optoelectronic materials in the future.High pressure is an important thermodynamic parameter that can decrease atomic distances and facilitate effective regulation of electronic structure and crystal structure.Therefore,it is considered one of the best experimental methods to explore the relationship between material structure and optical properties.This paper systematically explores the structure-property relationship of Mn-doped traditional 3D all inorganic perovskite CsPbBr₃ NCs,doped three-dimensional（2D）organic-inorganic hybrid perovskite（PEA）₂PbBr₄ NCs,and doped（PEA）₂PbCl₄ NCs using high-pressure experimental techniques.（1）Under ambient conditions,the PL of Mn-doped CsPbBr₃NCs exhibited dual emission peaks,including intrinsic emission（439 nm）and Mn-related emission（613nm）.When the pressure reached to～2.1 GPa,the intrinsic PL disappeared completely,and the Mn-related emission rapidly enhancement,accompanied by the emergence of new emission.Further compression（2.1 GPa to 14.5 GPa）,the intensity of the new emission peak continuously increased,accompanied by a slight redshift in its peak position,while the Mn-related emission continuously shifted to longer wavelengths and weakens.High-quality single-component white emission（CIE color scale（0.330,0.325））of Mn-doped CsPbBr₃ NCs was achieved at a pressure of 7.6 GPa.Further combined with in situ high-pressure ultraviolet-visible（UV-Vis）absorption spectrum,a gradual blue-shift was observed after 1.8 GPa.The opposite trend between the fluorescence peak position and the absorption edge under high pressure indicates that the new emission does not belong to the free exciton emission.In addition,under the same pressure,the intensity of the new emission showed linear growth with the increase of excitation power density.Therefore,it can be concluding that the new emission originates from radiative recombination of self-trapped excitons（STEs）.Furthermore,combined with in situ high-pressure angle dispersive X-ray diffraction（ADXRD）analysis,it was found that the Mn-doped CsPbBr₃ NCs exhibited an isostructural phase transition at a pressure of around 2.2 GPa,that is,an electronic structure transition.The isostructural phase transition is the fundamental reason for the emergence of the new emission.The quantitative analysis of the distorted structure under high pressure also shows that with the increase of pressure,there are differences in the pressure response of the[MnBr₆]and[PbBr₆]octahedra,which leads to an increase in the distortion of the octahedral skeleton.The distortion of the octahedra promotes the enhancement of electron-phonon coupling,which further leads to the formation of self-trapped states due to the distortion of the excited state,and the binding energy of the exciton increases to form stable STEs emission.In this work,the synergistic effect of high pressure and Mn doping has been utilized to achieve radiative transitions of STEs in traditional 3D all-inorganic metal halide perovskites,clarifying the quantitative relationship between STEs radiative recombination and halide octahedral distortion.（2）For this part,we presented a comprehensive in situ high-pressure study of Mn-doped 2D（PEA）₂PbBr₄ NCs.The introduction of long-chain organic layers reduces the dimensionality of the perovskite structure,forming a 2D layered organic-inorganic hybrid perovskites and addressing the stability issue of traditional3D perovskites.Under ambient pressure,the PL of Mn-doped（PEA）₂PbBr₄ NCs exhibited dual emission peaks of intrinsic emission and Mn-related emission.The quantum confinement effect of the 2D structure and the carrier localization caused by Mn doping provide a prerequisite for the radiative recombination of STEs.Thus,the synergistic effect of pressure engineering and Mn doping leads to the appearance and enhancement of STEs emission through strong exciton-phonon coupling induced by octahedral distortion of the lattice.Meanwhile,by applying pressure,successive bandgap narrowing（reduced by 480 me V）and marked piezochromism（changed from orange to bluish violet）of the Mn-doped 2D（PEA）₂PbBr₄ NCs were achieved.The sustainable range of the intrinsic emission and the narrowed bandgap can reach up to9.2 GPa,which is larger than that of Mn-doped CsPbBr₃ NCs.Furthermore,combined with a quantitative analysis of the compression behavior of layered structures under pressure,it is found that in the low-pressure range,the asymmetric organic cations are significantly compressed,leading to a decrease in bandgap due to the increased van der Waals interactions.Moreover,the unique layered structure of the organic layer PEA⁺provides a certain“spring”protection to the inorganic octahedral layer under pressure,slowing down octahedral distortion and thus leading to a larger sustainable range of intrinsic emission under pressure.This chapter elucidate the structure-property relationship of Mn-doped layered（PEA）₂PbBr₄ NCs,thus providing a basis for their applications in opto-pressure sensing.（3）This chapter presented a systematic study of the high-pressure optical properties of Mn-doped（PEA）₂PbCl₄ NCs,aiming to compare with Mn-doped（PEA）₂PbBr₄ NCs and reveal the in situ high-pressure regulation rules of Mn-doped2D organic-inorganic hybrid perovskites.Under pressure,the intensity of PL emission increased by about 3.5 times at lower pressures（0.4 GPa）.As the pressure increased to 1.2 GPa,a new emission appeard near the exciton absorption peak,with a narrow FWHM due to exciton radiation recombination.The intensity of the new emission gradually increased and then decreased until the exciton emission and Mn-related emission disappeared at 12.7 GPa,leaving only STEs emission.In situ high-pressure absorption spectra showed that before 12.7 GPa,the exciton absorption peak was red-shifted,and the bandgap decreased by 390 meV.Combined with in situ high-pressure ADXRD,the isostructural phase transition at 1.4 GPa is the root reason for the appear of excitons emission.Notably,the irreversibility of the NCs structure before and after high-pressure treatment leads to irreversible changes in PL and absorption.Compared with Mn-doped（PEA）₂PbBr₄ NCs,the optical properties and structural changes of Mn-doped（PEA）₂PbCl₄ NCs under high pressure are complex,and the same organic and different halides may have different PL mechanisms.Differences in initial crystal phases,structural responses to pressure,and halogen element electronegativity may all cause differences in optical property responses under high pressure.The research in this chapter provides an experimental basis for the“capture”of high pressure-phase,and lays a scientific foundation for the design of new pressure induced emission materials.