Structural And Optical Properties Of Inorganic Double Perovskite Nanocrystals Under High Pressure

In the last decade,metal lead halide perovskites have been widely used in solar cells,light emitting diodes,lasers and photodetectors for their excellent optoelectronic properties,such as easy processing,high absorption coefficients,high photoluminescence quantum yields and long charge carrier diffusion lengths.However,these materials are highly susceptible to degradation of element Pb when they are exposed to light,moisture and ultraviolet radiation over long periods of time,resulting in a significant reduction in their stability.In addition,the toxicity of Pb has been contrary to the purpose of building an environmental and friendly society,which limits its commercial application.In view of these limitations,the urgent need for a new stable and non-toxic lead-free halide perovskites material is one of the key challenges to be solved in the field of halide optoelectronics today.Recent theoretical calculations demonstrate that a halide double perovskite structure,A₂B（I）B（III）X₆,which could be formed through a replacement of two toxic Pb²⁺in the crystal lattice with a pair of nontoxic heteroallene（i.e.,mono-and trivalent）metal cations.These halide double perovskites not only have the same three-dimensional structure as Pb-based perovskites,but also are highly stable and non-toxic.Substantial studies show that these double halide perovskites can achieve carrier diffusion lengths,bandgap and tunable properties comparable to Pb-based perovskite,leading to high-performance,high-stability and sustainable optoelectronic devices,so they have become one of the candidates in the field of optoelectronics.However,the wide bandgap and low luminescence of lead-free double perovskites limit their application in commerce.Therefore,it is a subject worthy of further research that how to tune the structural and optical properties of lead-free double perovskites,thus extending the range of applications.Pressure,as a powerful tool,is the effective tool to investigate the intrinsic connection between structure and properties,and can effectively alter the physicochemical properties of perovskite materials and generate novel phenomena.The systematic high-pressure studies of all-inorganic lead-free double halide perovskites nanocrystals（NCs）can not only explain the relationship between structural and optical properties of double halide perovskite systems,but also provide new ideas and methods for the preparation of novel double halide perovskites by using diamond anvil cell and high-pressure test techniques.Therefore,we choose three typical all-inorganic lead-free halide perovskite NCs as the objects to investigate their structural and optical properties systematically under high pressure.Firstly,we performed a systematic high-pressure study of the inorganic double halide perovskite Cs₂AgBiBr₆ NCs with an indirect band-gap.Through in situ high-pressure synchrotron X-ray diffraction（ADXRD）and Raman spectra,we found that the Cs₂AgBiBr₆ NCs underwent a phase transition from cubic（Fm-3m）to tetragonal phase（I4/m）at 2.3 GPa.In situ high pressure absorption spectra show that the absorption peak shows a red shift in the low-pressure region and then has a continuous blue shift with the increase of pressure.High-pressure leads to first decreasing and then increasingin the band gap.Combined with first-principles calculations,we found that the evolution of band gaps was related to the orbital interactionof the[Ag Br₆]^5-,[Bi Br₆]^3-octahedra tilting and distortion in the cubic and tetragonal phase.When the pressure is completely released,part of tetragonal phase is trapped to ambient pressure and the dimensional inhomogeneity of the sample after the pressure releasing,lead collectively to a blue shift of the absorption peak compared with the initial.Considering the limitation of the indirect band gap of Cs₂AgBiBr₆ NCs,limits their application in the field of luminescence.Researchers have used In³⁺replaces Bi³⁺to form luminescent double perovskite Cs₂AgInCl₆ NCs with a direct band gap.However,the luminescence mechanism of Cs₂AgInCl₆ NCs is still controversial.The debate on the luminescence mechanism of blue light revolves around defective state radiative complex luminescence and a parity-forbidden transition.This is mainly due to the different understanding of the relationship between the structurale and optical properties of Cs₂AgInCl₆ NCs.Therefore,we hope to systematically investigated the relationship between the structural and optical properties of Cs₂AgInCl₆ NCs by introducing high-pressure and combining theoretical calculations to delve into the luminescence mechanism.In situ high pressure photoluminescence experiments showed that the fluorescence intensity of Cs₂AgInCl₆ nanocrystals abruptly decreased in the low-pressure interval of 1.0 GPa.As the pressure increases,the PL intensity gradually decreases,while the fluorescence peak position is red-shifted.After 10.0 GPa,the PL peak is blue-shifted and gradually decreases until it disappears completely.If the PL of Cs₂AgInCl₆ NCs originates from a small parity-forbidden transition between the conduction and valence bands,then such an abrupt change in luminescence would not have occurred prior to the structural phase transition.It is further shown that the PL mechanism of Cs₂AgInCl₆ NCs is related to the leap radiation between the conduction band minimum and the defect state.The high pressure enhances the interaction between the NCs and the surface organic ligands,blunting the surface defects and reducing the chance of radiation complexation between the conduction band bottom and the defect state,resulting in a significant reduction in fluorescence.When the pressure is fully released,we find that the PL intensity does not fully return to its initial intensity,which is mainly attributed to the presence of tangential stresses between the highly aggregated nanocrystals that can disrupt the structure of the chalcogenide crystal after decompression,and this tangential force is the main driving force behind the fluorescence weakening.Our study provides a new evidence,pressure ecudence,for the emission of Cs₂AgInCl₆ NCs,also achieves microscopic modulation of the band gap of Cs₂AgInCl₆ NCs under high pressure,providing a theoretical basis and new ideas for improving the optical properties of Cs₂AgInCl₆ NCs.Although we successfully prepared Cs₂AgInCl₆ NCs with PL,their weak PL still greatly limits their practical applications.Despite the fact that the PL efficiency of Cs₂AgInCl₆ NCs can be effectively improved by Mn²⁺doping,the energy transfer and conformational relationship between the PL of Mn²⁺and Cs₂AgInCl₆ NCs under extreme compression needs to be further investigated.Here,we have chosen Mn²⁺-doped Cs₂AgInCl₆ NCs as the subject of our study and conducted a systematic in situ high-pressure experimental study.In situ high-pressure PL experiment exhibited that the Mn²⁺doped Cs₂AgInCl₆ NCs exhibits a pressure-induced emission enhancement at 1.6 GPa and14.8 GPa.The UV-Vis absorption spectra showed that the absorption edge shifted red with the increase of pressure,and followed by a blue shift when the pressure exceeded9.9 GPa.Combining ADXRD and the analysis of cell parameters and cell volume,it is indicated that the Mn²⁺doped Cs₂AgInCl₆ NCs underwent an isostructural phase transition.The pressure-induced emission enhancement of Mn²⁺doped Cs₂AgInCl₆ NCs was attributed to the inhibition of the electronic leap between CBM to defect state with increasing pressure,while transferring energy to the Mn²⁺d electrons,facilitating the leap from ⁴T₁ to ⁶A₁ of the Mn²⁺d electrons.As the pressure increase,the Mn-Cl-Mn exchange enhances thereby decreasing the PL intensity of Mn²⁺.After the phase transition,the energy of CBM increased,thus suppressing the energy transfer between the CBM to Mn²⁺,which eventually leads to enhanced PL of Cs₂AgInCl₆ NCs.This phenomenon is opposite to the variation of pure Cs₂AgInCl₆ NCs.And the PL intensity of Cs₂AgInCl₆ NCs reached its maximum when the Mn²⁺-related peak disappeard.Compared with Cs₂AgInCl₆ NCs,the PL intensity of Mn²⁺doped Cs₂AgInCl₆ NCs was increased 4.5times,while pressure-induced PL enhancement of Cs₂AgInCl₆ NCs occurred after phase transition.Color modulation was also chieved during high pressure.Our finding provides a theoretical basis for the preparation of reasonable double halide and a new direction for its application in optoelectronic devices.