The Study On Structure And Optical Properties Of Antimony(Bismuth)-Based Two-Dimensional Layered Halide Perovskite Nanocrystals Under High Pressure

Lead-based halide perovskite nanocrystals have attracted extensive attention because of their superior optoelectronic properties,such as low defect density and high absorption coefficient.However,their high toxicity and low stability of these materials severely limit their commercialization potential.As a response to the national"dual carbon"strategy,researchers are now focused on developing green and environmentally friendly halide perovskites.The highly toxic problem was partially solved by replacing highly toxic lead with less toxic elements,such as bismuth（Bi）and antimony（Sb）.However,the stability problem of perovskite remains unsolved after replacement of elements,which is attributed to the distortion of octahedral structure in the classical three-dimensional perovskite nanocrystals.Reducing the dimension of Sb（Bi）-based perovskite material and thus reducing the probability of octahedral configuration distortion is a feasible method to improve the structural stability of Sb（Bi）-based perovskite nanocrystals.Unfortunately,reducing the dimensionality has also created new issues such as wider band gaps and lower luminescence efficiency.In addition,the optical properties of Sb（Bi）-based two-dimensional perovskite nanocrystals can also be altered by substituting elements or changing the stoichiometry,which exhibits the complex diversity of their optical properties.Therefore,it is necessary to further explore the relationship between their structure and optical properties,so as to improve the photoelectric properties of two-dimensional Sb（Bi）perovskite nanocrystals,laying a foundation for the rational design and preparation of non-lead low-dimensional halide perovskite.Pressure,as another important thermodynamic parameter independent of temperature and composition,provides an important dimension for studying the regulation of structure and functions of materials.The optical properties of materials can be further regulated by changing the crystal structure,electronic structure and molecular interaction of materials.In order to understand the relationship between the electronic structure and optical properties of perovskite materials,high pressure dimension is introduced to regulate the distortion behavior of halide octahedron and the connection of the octahedron,so as to achieve effective regulation of the band gap and optical properties of this kind of materials.Therefore,Cs₃A₂X₉（A=Sb,Bi;X=Cl,B,I）two-dimensional layered perovskite nanocrystals were studied under high pressure to explore the relationship between their structure and optical properties,revealing the effects of different halogens on the structural stability of two-dimensional layered perovskite nanocrystals.（1）In this paper,the study of high pressure was carried out on the two-dimensional layered perovskite Cs₃Sb₂I₉nanocrystals.Under atmospheric pressure,the band gap of Cs₃Sb₂I₉nanocrystals was found to be 2.05 e V.With the pressure increasing to 20.0 GPa,the band gap continuously decreased to 1.36 e V continuously,with a band gap compression rate of 33.7%.The band gap of Cs₃Sb₂I₉nanocrystals after pressure relief is slightly smaller than that under the initial condition,resulting from the incomplete recrystallization of Cs₃Sb₂I₉nanocrystals after pressure relief.In situ high pressure ADXRD and in situ high pressure Raman spectra of Cs₃Sb₂I₉nanocrystals indicated that there was no phase transition under high pressure.Meanwhile,when the pressure increased to about 14.0 GPa,the structural became amorphous.After releasing pressure,the structural changes were reversible.Moreover,First-principles calculations further demonstrated that the compressed Sb-I bond length and I-Sb-I bond angle under pressure enhance the overlap of electron clouds between Sb and I orbitals,leading to the narrowing band gap.Meanwhile,Cs₃Sb₂Br₉nanocrystals were further synthesized by halogen substitution and studied under high pressure.The band gap of Cs₃Sb₂Br₉nanocrystals and Cs₃Sb₂I₉nanocrystals both decreased gradually under high pressure.However,Cs₃Sb₂Br₉nanocrystals underwent a structural phase transition from trigonal to monoclinic at about 9.0 GPa,indicating that the structural stability of two-dimensional layered perovskite materials formed by different halogen elements varied under pressure.（2）The effect of halogens on the structural stability of two-dimensional perovskite nanocrystals was further investigated under pressure.High-pressure studies were conducted on the Bi-based layered perovskite Cs₃Bi₂Br₉nanocrystals.The band gap of the Cs₃Bi₂Br₉nanocrystals underwent a continuously reduction from 2.66 e V to 1.97 e V upon compression,resulting from the compressed Bi-Br bond and Br-Bi-Br bond angle within the[Bi Br₆]^3-octahedra.In situ high pressure ADXRD and in situ high pressure Raman spectra indicated that the Cs₃Bi₂Br₉nanocrystals changed from trigonal structure to monoclinic structure as the pressure increased to 6.0 GPa.With continued pressure,Cs₃Bi₂Br₉nanocrystals began to amorphous,and this structural change were reversible once pressure relief.The change rate of pressure-dependent band gap increased at 6.0 GPa,because of the structural phase transition.It was further confirmed by the first-principles calculation that the pressure-induced distortion of the[Bi₂Br₉]^3-octahedral frame effectively changed the overlap of Bi and Br electron orbitals,leading to the continuous reduction of the band gap.In addition,the above results indicated that two-dimensional layered perovskite materials with halogen Br are more prone to undergo structural phase transitions under pressure.（3）Since Cs₃Bi₂I₉lacks two-dimensional layered structure,Cs₃Bi₂Cl₉nanocrystals with two-dimensional layered structure were selected as the research object and systematically studied under high pressure.In this study,the pressure-induced emission（PIE）accompanied by a remarkable pressure-enhanced emission intensity was achieved without phase transition in Cs₃Bi₂Cl₉nanocrystals.Note that the initial Cs₃Bi₂Cl₉nanocrystals possessed strong electron-phonon coupling,leading to the easy annihilation of trapped excitons by the phonon-assisted pathway.Upon compression,pressure could effectively suppress phonon-assisted non-radiative decay and give rise to an intriguing emission from no emission“0”to emission“1”.Furthermore,the increased oscillator strength from 1 atm to 0.8 GPa indicated an enhanced optical activity,promoting to an observed PIE in Cs₃Bi₂Cl₉nanocrystals under high pressure.As the pressure continued to increase,the enhanced self-trapping exicition states（STEs）enhanced,and so was the binding energy and detrapped activation energy.As a result,STEs were effectively prevented from converting into bound excitons to increase the concentration of STEs,thus promoting the possibility of radiation recombination.Likewise,the contraction of halide octahedra of Cs₃Bi₂Cl₉nanocrystals decreased the electron-phonon coupling strength and inhibited the formation of highly distorted excited states.Therefore,more STEs occupied in high-energy excited states with slight distortion degree,leading to the enhanced high-energy emission with a relative gaussian profile.As pressure exceeded6.0 GPa,the electron-phonon coupling was further suppressed and more distorted excited states were restricted.The STEs at high energy was instable,thus facilitating the formation of bound excitons to dominate the decrease in PL intensity.In situ high pressure synchrotron radiation ADXRD and in situ high pressure Raman spectra demonstrated that the Cs₃Bi₂Cl₉nanocrystals did not undergo structural phase transition under pressure.In this work,the luminescent properties of Cs₃Bi₂Cl₉nanocrystals were improved by adjusting the distortion degree of octahedra under pressure,and the breakthrough from no emission"0"to emission"1"was realized.The results of the above series of experiments show that the two-dimensional layered Sb（Bi）-based perovskite with Br halogen are more likely to undergo structural phase transition under high pressure.