High Pressure Study Of Zero-Dimensional Cesium Lead Bromine Perovskite Nanocrystals

The recent success of organometallic halide perovskites in the field of solar cell,illumination and displays has triggered research activities on three-dimensional?3D?cesium lead halide perovskite CsPbX₃?X=Cl,Br,I?.The breakthrough development of CsPbX₃ in photoelectric devices indicates the one of the most inviting vista in commercialization due to their good stability,ultrahigh photoluminescence quantum yield?PLQY?,and composition dependent luminescence with a wide color gamut.Reducing the dimensionality of perovskites demonstrated a remarkable increase in PLQY due to the enhancement of its confinement effect and free exciton binding energy.Cs₄PbBr₆,as a typical zero-dimensional?0D?inorganic perovskite,have attracted considerable interests in the whole scientific community.However,in the research process,the comprehension associated with the optical properties of Cs₄PbBr₆ triggered a enormous academic debate.Cs₄PbBr₆ do not exhibit any emission in the visible region because of seldom display of excited wide bandgap and the non-emissive Cs₄PbBr₆ nanocrystals?NCs?have also been synthesized by many research groups.However,some recent works have demonstrated that Cs₄PbBr₆possesses a higher PLQY at ambient condition both in its single crystals and nanocrystalline forms.In a few cases,the emission intensity is found to be much higher than that of CsPbBr₃.Accordingly,whether or not emissive Cs₄PbBr₆ at ambient condition occur sparked a sudden great discussion.In addition,more low-dimensional perovskite materials exhibited the radiative recombination of self-trapped excition?STE?,a kind of broad emission,and lowering the dimensionality to 0D makes exciton self-trapping easier.Unfortunately,the 0D perovskite Cs₄PbBr₆ do not show the exciting emission property related to STEs,although Cs₄PbBr₆ possesses the lowest electronic dimension.?Note that the emission from emissive Cs₄PbBr₆ possesses a good monochromaticity,inconsistent with the characteristization of the radiative recombination of STEs.?The main reason for the above debate is mainly attributed to the different understanding about the relationship between the structure and optical properties of Cs₄PbBr₆.Pressure,as a unique thermodynamic variable,provides a powerful means to studying the structural and emissive behaviors of perovskite materials.Therefore,we focus on whether using the high-pressure technology can resolve the emissive debate of Cs₄PbBr₆ and deepen insight into its emissive properties.The Cs₄PbBr₆ NCs have been selected as our research system,and conducted a systematical study on its high-pressure structure and optical properties.?1?We conducted a systematic high-pressure study of the non-emissive Cs₄PbBr₆ NCs.In situ high-pressure photoluminescence?PL?experiment exhibited that the Cs₄PbBr₆ NCs exhibit an unexpected pressure-induced broad emission at room temperature when the intrinsically non emitting nanomaterials are compressed to 3.01 GPa.The new broad emission is attributed to the radiative recombination of STEs.In situ high-pressure angle-dispersive synchrotron X-ray diffraction?ADXRD?,Raman and optical absorption measurements indicated that the Cs₄PbBr₆ NCs undergo a structural phase transition from rhombohedral to monoclinic structure beginning at 3.01 GPa.During this process,the PbBr₆ regular octahedra experience a structural distortion.First principles energetic calculations show that the monoclinic structure becomes energetically more favorable than the rhombohedral phase with increasing pressure.In addition,first principles calculations also indicate that the distorted PbBr₆ octahedra within high-pressure phase enhance the optical activity and improve the binding energy of STEs.Both the enhanced optical activity and binding energy result in the pressure-induced broad emission in 0D Cs₄PbBr₆ NCs.Our results suggest that the STE with octahedral distortion is also responsible for the observed broadband emission,though it is the lowest-dimensional structure.?2?We have also conducted a systematical investigation on the relationship between the structure and optical properties of the emissive Cs₄PbBr₆ NCs through density functional theoretical?DFT?and high-pressure experimental methods.The ADXRD was carried out to obtain the detail structures under different pressure,which were used to theoretical calculation.Therefore,the theoretic simulation will be more compatible with experimental processes.The dependent charge-transition level of Cs₄PbBr₆ with Br vacancy on the external pressure was obtained via the first-principles calculations,which exhibited a persistent decrease in the separation of the defect states and conduction band minimum?CBM?with increasing pressure.We deduced from this calculated result that the PL from Br vacancy should undergo a persistent redshift in wavelength with increasing pressure.However,a high pressure PL peak shift was detected upon compression as follows:a gradual redshift initially occurred below 1.41 GPa,followed by a persistent blueshift with further increasing pressure.Upon further compression,a distinct broad emission appeared at 3.02 GPa,consistent with the PL behavior of non-emissive Cs₄PbBr₆ NCs.The disagreement between the first-principles calculation and high pressure experiment excludes the possibility of Br vacancy.The high-pressure PL measurements is completely consistent with the pressure-dependent emission of CsPbBr₃ NCs.Meanwhile,the absorption spectrum from a mixture of the CsPbBr₃ and non-emissive Cs₄PbBr₆ NCs revealed that the abnormal absorption tail of emissive Cs₄PbBr₆ NCs originates from the superposition of the absorption band from non-emissive Cs₄PbBr₆ and the impurity CsPbBr₃.This result confirms that the green emission of Cs₄PbBr₆ NCs mainly derived from the impurity of CsPbBr₃ NCs embedded in Cs₄PbBr₆ matrix rather than its intrinsic emission from a halogen vacancy.The sudden appearance of a broad emission under more higher pressure derived from pure Cs₄PbBr₆ NCs.Our findings offer a new evidence?pressure evidence?for the emissive causes of emissive Cs₄PbBr₆ NCs and facilitate the understanding and designing of a high-performance perovskite materials in the future.