Stimuli-responsive Photoluminescence Of Two-dimensional Lead Halide Perovskites

Two-dimensional（2D）lead halide perovskites（LHPs）have been widely used in the field of solar cells,light-emitting diodes,photodetectors,and micro-nano lasers due to their easy structural tailoring,tunable bandgaps,high environmental stability,high photoluminescence（PL）quantum yield,and high spectral purity.Despite significant progress have been made in the optical properties of 2D LHPs,their stimuli-responsive photoluminescence property,which would not only provide in-depth understanding to achieve highly efficient PL,but also extend their application in the field of stimuli-responsive smart materials,are ignored and urgent to be explored.In this thesis,taking several 2D LHPs as examples,the effects of chemical composition,lattice distortion,electron-phonon coupling on their PL properties were investigated by the combination of temperature-responsive PL,pressure-responsive PL,first principle calculations and molecular dynamics simulations,and their applications in pressure sensing,piezoelectric energy harvesting are explored.The main discoveries of this thesis are listed as below:（1）The effects of the length of the interlayer organic amine on the pressure-reponsive PL and pressure-induced phase transition in a series 2D LHPs-[C_nH_2n+1NH₃]₂Pb I₄（n=4,8,12）are investigated.The results show that the critical pressures for driving structural transitions and amorphization,and PL quenching are proportional to the length of alkylamine cations,which arise from their scaling counterbalance to compression energy via conformational rearrangements.In addition,it is demonstrated that their PL emission alters from free excitons to bound excitons,then to self-trapped excitons under successive pressure manipulation.Furthermore,the high-pressure treatment is discovered as a new method can transform the sample from bulk micrometer powder to thin nanosheets.This work not only reveals the key role of interlayer organic spacers on the structural evolution and PL properties of 2D LHPs,but also paves new avenue for regulating the stimuli-responsive behavior of 2D LHPs by chemical method.（2）Temperature-and pressure-responsive PL properties of novel chiral 2D LHPs-R-[BPEA]₂Pb I₄ and S-[BPEA]₂Pb I₄ are investigated.The variable temperature PL experiment show that there is a strong electron-phonon coupling effect in S-[BPEA]2Pb I4,and its deformation potential is 1.7×10⁹ e V/cm.High-pressure PL experiments show that the PL intensity of S-[BPEA]₂Pb I₄ is approximately doubled at～0.6 GPa,which is originated from the pressure-induced suppression of defect states;meanwhile,S-[BPEA]₂Pb I₄ exhibits obvious piezochromism effect,that is,the color of the crystal can change from orange to red to black during under compression.In addition,the circularly polarized luminescence（CPL）and piezoelectric energy harvesting properties of R-[BPEA]₂Pb I₄ and S-[BPEA]₂Pb I₄are also explore.The degree of polarization of S-[BPEA]₂Pb I₄ can reach 11.3%at room temperature and an output voltage of 0.6 V and an output current of 1.5μA can be generated under a periodic pressure of 2 N based on chiral perovskite-containing composite films.This work not only reveals the effect of interlayer chiral molecules on the stimuli-responsive fluorescence properties of 2D LHPs,but also expands the application of chiral 2D LHPs for efficient CPL and piezoelectric energy harvesting.（3）Temperature-and pressure-responsive PL properties of novel broadband emissive R-[4Me OPEA]₂Pb Br₄ and S-[4Me OPEA]₂Pb Br₄ are investigated.Crystal structure analysis reveals the broadband emission is originated from the highly distorted Pb Br₆ octahedra induced formation of self-trapped excitons.The variable temperature PL experiments show that the broadband emissive behavior can be maintained down to 93 K,with negligible emission energy and width change during cooling.The high-pressure PL experiments reveal that the broadband emission transform to narrow emission under compression,originating from the self-trapped excitons transfer to free excitons.The combined experimental and simulated structural evolution under pressure unveils that the distortion level of Pb Br₆octahedra undergoes a giant reduction at high pressure,along with the reduced atomic displacements,would inevitably reduce the electron-phonon coupling and self-trapped energy barrier,and consequently release the self-trapped excitons to free excitons.This work not only provides direct evidence of structural origin of broadband emission in 2D LHPs,but also paves a new way for studying their stimulus-responsive behaviors.