Regulation Of Structure And Optical Properties In Low-Dimensional Lead Bromine Metal Halides Under High Pressure

Low-dimensional metal halides,as star materials in the field of optical functional materials and optoelectronic devices,have great application value in the research fields of light-emitting diodes,photodetectors,and lasers.These materials exhibit rich physical and chemical properties due to their diverse composition and adjustable structure.The optical properties of low-dimensional metal halides are usually closely related to inorganic structural units.Therefore,it is of great scientific significance to explore the microstructure of low-dimensional halide inorganic units and effectively regulate their luminescence properties.In addition to chemical synthesis means,physical means can also be used to control the structure and optical properties of such materials.As a safe and effective physical means,pressure can continuously and accurately modulate the structure and optical properties of low-dimensional metal halides.In this paper,three low-dimensional organic metal halides with different polyhedral configurations and luminescence properties are selected as research objects.They are one-dimensional organic metal halide(C₆H₁₃N₄)₃Pb₂Br₇ with inorganic unit dimers,which are self-trapped excitons recombination luminescence under ambient conditions and show the white light;zero-dimensional organic metal halide(C₁₃H₁₉N₄)₂Pb Br₄ with tetrahedral inorganic units,which are localized excitons luminescence under ambient conditions and exhibit the blue light;zero-dimensional organic metal halide(C₉NH₂₀)₆Pb₃Br₁₂ with inorganic unit trimers,which are defect luminescence under ambient conditions and show the green light.Using a variety of in situ high-pressure spectral measurement techniques（photoluminescence spectroscopy,ultraviolet-visible absorption spectroscopy,time-resolved fluorescence spectroscopy,synchrotron radiation X-ray diffraction,Raman scattering spectroscopy）based on diamond anvil cells combined with theoretical computational simulation,the evolution of the structural and optical properties of three kinds of low-dimensional organic metal halides under high pressure were studied.Utilize the unique advantage of pressure to fine control their microstructure,reveal the structure-property relationship of low-dimensional organic metal halide materials under high pressure,clarify the mechanism of pressure to control their energy level structure,luminescence characteristics and exciton behavior,and explore the possibility of using high pressure to optimize and obtain low-dimensional organic metal halide materials with excellent luminescence properties.（1）Firstly,the one-dimensional organic metal halide(C₆H₁₃N₄)₃Pb₂Br₇ with self-trapped excitons luminescence was systematically studied by using high-pressure experiments and theoretical calculations.The results show that(C₆H₁₃N₄)₃Pb₂Br₇achieves white light emission enhancement in a mild pressure range below 0.6 GPa,accompanied by an increase in fluorescence lifetime.This is due to the pressure-induced lattice contraction of the inorganic unit[Pb₂Br₉]^5-dimer,which effectively reduces the electron-phonon coupling strength,thereby reducing the non-radiative transition and achieving luminescence enhancement in the low-pressure range At the same time,with the increase of pressure,the decrease of Pb-Br bond length and Br-Pb-Br bond angle increases the orbital coupling between Pb atoms and Br atoms,resulting in band gap narrowing during compression.During the entire pressurization process,the evolution of the structure of(C₆H₁₃N₄)₃Pb₂Br₇ is consistent with the change in its optical properties,and the structure and optical properties are reversible after decompression.The results show that(C₆H₁₃N₄)₃Pb₂Br₇ crystal has good pressure response characteristics,and also shows that pressure can be used as an effective strategy to adjust the emission of white-light materials.（2）Secondly,the zero-dimensional organic metal halide(C₁₃H₁₉N₄)₂Pb Br₄ with local excitons luminescence was comprehensively studied by means of high-pressure experiments and theoretical calculations.In the pressure range of less than 0.8 GPa,the blue emission intensity of(C₁₃H₁₉N₄)₂Pb Br₄ decreases with the increase of pressure;as the pressure further increases to 5.1 GPa,the blue emission gradually increases;continue to increase the pressure,the fluorescence intensity is weakened.The change in fluorescence intensity is accompanied by the change of fluorescence color,from the initial dark blue to light blue and finally to green.By analyzing the fluorescence spectra with different excitation wavelengths,time-resolved fluorescence spectra,synchrotron radiation X-ray diffraction,and Raman experiment under high pressure,the high-pressure luminescence behavior of(C₁₃H₁₉N₄)₂Pb Br₄ can be attributed to the ultrafast exciton trapping of(C₁₃H₁₉N₄)₂Pb Br₄ after photoexcitation at ambient conditions,forming two localized states with different depths,denoted as energy levels S₁ and S₂,respectively.Since the self-trapped excitons tend to occupy lower energy levels,more excitons are gathered in the energy level S₂.The emission is dominated by the S₂ energy level.With the increase of pressure,the inorganic unit Pb Br₄^2-tetrahedron lattice is distorted,the energy level S₂gradually rises and is higher than the energy level S₁,which leads to the aggregation of more self-trapped excitons in the energy level S₁,the emission is dominated by the S₁ energy level.The results show that pressure can be used as a means to adjust the emission transition between different energy levels to achieve better luminescence performance.（3）Finally,the zero-dimensional organic metal halide(C₉NH₂₀)₆Pb₃Br₁₂ with defect luminescence was thoroughly studied by high-pressure experiments combined with theoretical calculations.In the pressure range of less than 0.8 GPa,the emission intensity of(C₉NH₂₀)₆Pb₃Br₁₂ decreases with the increase of pressure;and then presents a large emission enhancement in the pressure range of 0.8-9.3 GPa,and the fluorescence intensity increases by more than 120 times,from weak green light to bright white light;with the further application of pressure,the fluorescence intensity decreases.It can be attributed to the structural distortion of the inorganic unit[Pb₃Br₁₂]^6-trimer under high pressure,which effectively regulates the transition of self-trapped excitons from non-radiative transition to radiative transition,thereby achieving bright broadband white light emission.The luminescence color after pressure release is green,and the luminescence intensity is 6 times that at atmospheric pressure,and the synchrotron radiation X-ray diffraction spectrum and Raman spectrum do not return to the initial state after pressure release.It shows that the efficient green light emission can be successfully“intercepted”by pressure treatment,and the irreversible pressure-induced structural change is the main reason for the efficient emission after decompression.The research results show that the defect luminescence can be controlled by pressure,and the optical properties of high-efficient emission can be retained after pressure relief,which provides a new perspective for the luminescence behavior of zero-dimensional metal halides and enriches the photophysical mechanism of metal halides under high pressure.