Study On Pressure-Induced Emission Enhancement Of Ruddlesden-Popper Type 2D Monolayer Perovskite Crystals

Organic-inorganic hybrid perovskites as a new type of optoelectronic materials have drawn tremendous attention in the fields of photovoltaic,light-emitting diode,detector and laser due to its excellent optical and electrical properties.In the past decade,three-dimensional（3D）perovskite materials have been extensively researched.More requirements have been put forward for the stability and excellent optical properties of perovskite materials,which makes perovskite materials develop towards low-dimensional-networked.Due to reducing the dimension of organic-inorganic hybrid perovskite materials,not only the stability of the material can be effectively improved,but also the quantum confinement effect can be produced in the structure.Ruddlesden-Popper type two-dimensional（2D）single-layer perovskite crystal materials are the most typical one in 2D perovskite system.The monolayer inorganic skeleton is separated by two layers of large organic cations with hydrophobic substituents.The structure of the monolayer inorganic skeleton has strong quantum confining characteristics,high stability and greater controllability,which has attracted extensive attention of researchers.Interval cation of this kind of material,however,will be formed in a 2D perovskite insulating layer,hinder the carrier transmission,low quantity of fluorescent rate is a critical problem has restricted the two-dimensional perovskite photoelectric device efficiency,thus realize the exciton behavior of structure,research the relationship between the crystal structure and luminescence behavior and improve fluorescence performance has become the Ruddlesden-Popper 2D single perovskite crystal material direction important scientific problems needed to resolve.As an independent thermodynamic parameter,pressure provides a new dimension for material control.It can effectively regulate the crystal structure,electronic structure and physical properties of materials without changing the chemical composition.It plays a vital role in discovering new phenomena,explaining new mechanisms,and preparing new materials.Therefore,in this paper,we choose have different halogen atoms under atmospheric conditions with free exciton luminescence（ETA）₂Pb I₄,with self-trapping exciton luminescent（2mept H₂）Pb Cl₄ and nonluminous environment-friendly double perovskite（BA）₄Ag Bi Br₈are three typical Ruddlesden-Popper type 2D single-layer perovskite crystal materials.The relationship between optical properties and structure is studied from different luminescent mechanisms to deepen the understanding of Ruddlesden Popper layered 2D perovskite materials.The optical properties and structural changes of Ruddlesden-Popper type 2D single-layer perovskite crystal materials upon compression were systematically studied by using various in-situ high-pressure testing methods and first principles calculation methods,hoping to deeply understand the structure property relationship of 2D halide perovskite materials,and provide experimental basis for future design of new materials and improvement of device performance.（1）First of all,we studied the free exciton luminescence（ETA）₂Pb I₄ by combining experimental and computational methods.In the low-pressure range of less than 1.5GPa,the photoluminescence spectrum shows obvious enhancement,accompanied by the increase of the symmetry of the photoluminescence peak,which is related to the unique 2D perovskite quantum well structure.At atmospheric pressure,the large volume of organic cations will seriously distort the inorganic lattice,and the strong exciton phonon coupling will form too many bound excitons.The radiative and nonradiative transitions of bound excitons will lead to the weakening of free exciton luminescence and the emergence of band tail from the low energy region.In the low-pressure region,the lattice shrinks regularly under the protection of organic cations caused harden the phonons,which reduces the exciton phonon interaction,inhibits the generation of exciton defects,greatly reduces the nonradiative recombination pathway,and improves the photoluminescence efficiency in the low-pressure region.In addition,the decrease of Pb-I bond length and the slow decrease of Pb-I₁-Pb bond angle caused the increase of atomic orbital coupling of metal halides,which leads to the narrowing of the band gap in the pressure range of 10 GPa and the obvious piezochromism of the crystal.Our results provide new information for people to understand the pressure response of 2D halide perovskites.The obvious piezochromism of photoluminescence upon compression also indicates that（ETA）₂Pb I₄ crystal may be used in the field of pressure optical fiber sensor and photovoltaic.（2）The（2mept H₂）Pb Cl₄ crystal with extrinsic self-trapping exciton emission has also been systematically studied upon compression.Under mild pressure,the weak photoluminescence of the sample suddenly increased under the pressure at 2.1 GPa,and then continuously increased until 9.9 GPa.It is easy to form localized self-trapped excitons in layered materials with natural quantum well structure.When the pressure is2.1 GPa,the volume of the sample collapses along the organic layer direction accompanied with the distortion of the crystal structure enhanced,which leads to the enhancement of the self-trapped state photoluminescence,resulting in a sudden photoluminescence enhancement.We used series high-pressure in situ measurement technology,combined with theoretical calculation,to calculate the relationship between the structure,exciton radiation,nonradiative recombination behavior and photoluminescence efficiency.Upon compression,the photoluminescence intensity of the sample is increased by 190 times.At the same time,the pressure can inhibit the nonradiative loss and promote the radiation recombination rate in the range of 2.1GPa-9.9 GPa.Therefore,by adjusting the binding energy and molecular confinement of self-trapped excitons,pressure can produce highly localized excitons,reduce the scattering of defects and phonons,and effectively promote the photoluminescence of self-trapped excitons.And,the fluorescence color of the sample is controlled by the pressure,so that it always emits"warm"white light.This work not only found that pressure can effectively regulate the behavior of self-trapped excitons in 2D metal halides,improve their luminescence performance and regulate the color temperature of white light,but also provides a quantitative analysis of the pressures-induced luminescence enhancement of self-trapped excitons and describes the microscopic mechanism.（3）Finally,we conducted a systematic high-pressure study of the non-emissive environment-friendly（BA）₄Ag Bi Br₈.The pressure-induced emission（PIE）phenomenon was successfully extended to 2D double perovskite system.The wide band emission with large Stokes shift proves that the emission is attributed to the radiative recombination of STEs.The results of in situ high pressure synchrotron radiation X-ray diffraction（ADXRD）and in situ high pressure infrared absorption spectra show that（BA）₄Ag Bi Br₈ crystal undergoes a structural transition from C2/m to P2₁/C in monoclinic phase at 2.5 GPa.During the phase transition,the rotation distortion between the adjacent[Ag Br₆]^5-and[Bi Br₆]^3-inorganic octahedrons makes the new crystal structure with higher optical activity and stronger self-trapping exciton binding energy,which promotes the emergence of new photoluminescence peaks.With the further compression,the serious distortion of the structure of the inorganic octahedron increases the activation energy of self-trapping excitons,reduces the lattice relaxation energy,and leads to the blue shift of photoluminescence enhancement peak.At the same time,the continuous decrease of the bandgap is mainly due to the distortion of the Ag-Br₁-Bi bond angle between the inorganic octahedrons of the phaseⅠ,and the rapid decrease of the band gap of phase II caused the quickly decrease of the Bi-Br₁and Ag-Br₁ bond length.Our results not only prove the relationship between crystal structure and optical properties under pressure,but also provide important reference for the improvement of optical properties of related environmentally friendly two-dimensional double perovskite materials.