Preparation,Structure And Properties Of Two-Dimensional Perovskite （OA）₂（MA）_n-1Pb_nI_3n+1

In the past several years,two-dimensional（2D）perovskites have drawn increasing attention and shown great potential in plenty of fields such as solar cells,light-emitting devices,photodetectors and lasers,in virtue of their distinct properties including tunable bandgap,high absorption coefficient,large exciton binding energy,and excellent stability,etc.However,due to the particularity of its intrinsic structure,it is extremely difficult to synthesize 2D perovskite with large number of layers and with pure phase at the macro scale.Coexistence of various phases is such a common phenomenon in the prepared 2D perovskite materials that it makes traditional macroscopic measurements helpless in identifying and characterizing the structure and property of each phase,hindering not only their large-scale application but our thorough comprehension of their structure-property relationships.For instance,the previous reports of Ruddlesden-Popper perovskite whose intrinsic structure has been clearly characterized mainly focus on those with number of layers no more than 5,leaving spacious room for exploring 2D perovskites with more layers.Moreover,the strategy of using post-processing methods to adjust or optimize the crystal structure has been widely applied in the preparation of 3D perovskites,which has not received enough attention and not been taken full advantages of in the 2D perovskites,however.Besides,the optical behaviors observed in the 2D perovskites basically originate from their intrinsic structure at present.To adjust the optical properties of 2D perovskites with the help of morphology and structure engineering is facing severe challenges but full of opportunities.Therefore,by means of the fluorescence microscopy with high spatial resolution,we investigated the structure and optical properties of（OA）₂（MA）_n-1Pb_nI_3n+1（OA=CH₃（CH₂）₇NH₃,MA=CH₃NH₃）with pure phase at the micron level.We determined the band gap structure of（OA）₂（MA）_n-1Pb_nI_3n+1with larger number of layers,quantitatively characterized the excitonic feature in the 2D perovskites,deeply understood the interaction force in the（OA）₂（MA）_n-1Pb_nI_3n+1and developed a post-treatment method to induce their structure rearrangement and to optimize their crystal structure.On the other hand,we investigated the linearly polarized photoluminescence that depended on the microscopic geometric structure of（OA）₂（MA）_n-1Pb_nI_3n+1.The main contents can be classified into the following three parts:（1）Characterization of the intrinsic structure of（OA）₂（MA）_n-1Pb_nI_3n+1with pure phase and the effect of the intrinsic structure on the luminescent behavior.We at first identified the pure phases of the 2D perovskites with various number of layers in the selected micro-areas by using the fluorescence microscopy and for the first time,we experimentally characterized the featured emission spectra of the 2D perovskite（OA）₂（MA）_n-1Pb_nI_3n+1 with layers up to 10.Combined with theoretical calculation,we found the exponential relationship between band gap energy and number of layers,instead of the pure relationship from quantum confinement effect,which suggests that the band gap of the 2D perovskite is determined not only by quantum confinement effect,but also by other factors including chemical components and dielectric effect.（2）Adjustment of the crystal structure of 2D perovskite.Annealing and solvent engineering have been considered as practical post-treatment methods for adjusting the crystal structure and are widely used in 3D perovskites.In this paper,we tried to blow ethanol vapor onto the（OA）₂（MA）_n-1Pb_nI_3n+1films and monitored the variations of their luminescent behaviors in real time.Through emission spectra and XRD patterns,we found that the ethanol molecules can insert into（OA）₂（MA）Pb₂I₇ crystal lattice and interact with MA ions through hydrogen bonding to change the MA/OA ratio and thus to induce the reorganization of the crystal structure of 2D perovskite materials.On the other hand,the ethanol molecules can adsorb onto the crystal surface to induce the photoluminescence（PL）enhancement by passivation of the surface defects.（3）Impact of the microscopic geometric structure on the luminescent behavior of（OA）₂Pb I₄.Polarized luminescent materials have special applications in various fields,so in this paper we synthesized（OA）₂Pb I₄ film with crumpled morphology and detected the linearly polarized photoluminescence from crumples,whose polarization degree was up to～0.65 at room temperature.Based on the characterization of emission spectra,luminescent intensity,X-ray diffraction and temperature-dependent behaviors,we thoroughly explored the origin of the linear emission polarization in the（OA）₂Pb I₄ films from several respects including light transmission,re-absorption effect,out-of-plane exciton,defect states and thermodynamic behavior,and at last confirmed that the selectivity of their geometric structure to the polarization orientation of light was the major cause.When the photoluminescence of the 2D perovskite continued to transmit in the film,the total internal reflection at the surface played a significant role.In this process,polarized component parallel to the film’s surface possessed higher reflection efficiency,for which it was easier to reach the condition to escape and emit from the2D perovskite film when the light passed through the crumples,and consequently the photoluminescence from the crumples exhibited linear polarization.This discovery verifies that the microscopic geometric structure can significantly impact the luminescent behavior of 2D perovskite,and morphology engineering will show great significance for the exploitation of 2D perovskites’new optical properties and applications.