Two-photon Nonlinear Optical Response Of CsPbBr₃ Nanocrystal Superlattices

Halide perovskite materials have important potential application value in the field of new optoelectronic devices due to their advantages of low cost,easy of fabrication and excellent photoelectric response.Among them,perovskites have the characteristics of strong defect tolerance,high photon capture efficiency and long carrier lifetime and diffusion length,which have been widely used in solar cells,semiconductor lasers,photodetectors,light-emitting diodes,filed-effect transistors and photocatalysis,etc.Due to their strong quantum confinement,remarkable exciton effect,and structural diversity,halide perovskites also exhibit important applications in nonlinear optics,such as two-/third-harmonic emission,two-photon pump amplified spontaneous emission,two-photon absorption and two-photon fluorescence.In recent years,CsPbBr₃ nanocrystal superlattice synthesized by self-assembly method has attracted extensive attention.Its structure has the characteristics of close ordered arrangement,high phase purity and crystal quality,strong particle coherence,high exciton state density and low band width,etc.Based on this,there are photoelectric responses such as super-fluorescence emission,ultra-thin quantum barrier and can overcome the inherent light coupling efficiency limitation in perovskite quantum dot led.Considering the enhancement of two-photon nonlinear optical response by high phase purity,crystal quality and tight ordered arrangement structure,so the CsPbBr₃ nanocrystal superlattice is expected to be used to enhance two-photon nonlinear optical response.In this paper,highly stable CsPbBr₃ colloidal nanocrystals（NCs）were synthesized by hot injection method,and a well-defined three-dimensional cubic superlattice was fabricated by a self-assembly method of the superlattice induced by solvent evaporation.Each superlattice is composed by millions of nanocrystals arranged in regular order through high resolution scanning electron microscopy.To explore the nonlinear collective response of the superlattice composed of individual nanocrystals,two-photon emission and absorption studies were performed on individual superlattices using microscopic light paths.The experimental results show that the superlattice has strong two-photon emission and absorption properties.The main research contents of the paper are as follows:（1）The colloidal CsPbBr₃ nanocrystals were prepared by hot injection method.And the colloidal CsPbBr₃ nanocrystals with high size uniformity could be obtained by controlling the reaction temperature and centrifugation conditions,which provided the basis for the subsequent self-assembly preparation.（2）Based on the self-assembly method of superlattice induced by slow evaporation of solvent,discretely distributed CsPbBr₃ nanocrystal superlattice structures were fabricated on silicon or glass substrates.The close ordered arrangement of the nanocrystals can be observed by high-resolution SEM,and the two Bragg reflection peaks at 2θ≈15°and 30.5°and the splitting peak at 2θ≈15°in the X-ray diffraction pattern further prove that the structure is superlattice.The thickness of the superlattice was calibrated by AFM,and its thickness was1.5～2.1μm,which was much higher than the thickness（213～458 nm）of randomly oriented films with different solvent evaporation conditions.The normalized absorption and photoluminescence（PL）spectra of the superlattice showed no relative shift in the characteristic peaks compared to the purified nanocrystals.And the average radiation lifetime of the superlattice is 3.015ns,which is 2.35 times shorter than that of the randomly oriented film.On the basis of the above characterization results,the conditions for the preparation of high-quality superlattices by the self-assembly method were further optimized by changing the size uniformity,solution concentration,dropping amount,reaction temperature and solvent evaporation time of the nanocrystals during the self-assembly process.（3）The results of two-photon nonlinear optical properties of CsPbBr₃ nanocrystal superlattice structure show that the superlattice has a strong two-photon emission intensity under 800nm excitation,which is 16 times stronger than that of randomly oriented films.The wavelength of the pump light source was changed in the range of 750-980 nm,and the experimental results showed that the superlattice exhibited inter-band photoluminescence under the excitation of photons below the band gap energy,and the luminescence intensity decreased with the increase of the excitation wavelength.Moreover,the two-photon fluorescence intensity of the superlattice changes periodically at 90°with the change of the polarization angle of the pump light,which is similar to the single crystal structure.In addition,the superlattices have the effect of enhancing two-photon absorption.Its two-photon absorption coefficient is 5 times that of the randomly oriented thin films,which is in the same order of magnitude as the traditional semiconductor materials（such as WS₂ and Zn O）,which shows the potential to replace the traditional semiconductor materials as the design of new nonlinear optical devices.