Two-dimensional Lead Halide Perovskite:Synthesis And Photoluminescence Properties

In recent years,two-dimensional(2D)Ruddlesden-Popper(R-P)-type perovskites have been considered as an excellent optoelectronic material with high exciton binding energy,quantum confinement effect and better water stability.They are expressed with a general formula of(A')₂(A)_n-1B_nX_3n+1,in which the different cation ligands(A'),metal ions(B),halogen(X)and the number(n)of layers of inorganic units is capable to tailor their crystal structure and optical properties,respectively.In the early researh stage,there have been many studies on quasi-2D perovskite thin films,but limited reports on the nanomaterials of quasi-2D perovskites.And with the fact that the photoluminescence quantum efficiency(PLQY)of 2D perovskites is low.We therefore designed a series of experiments to prepare 2D and quasi-2D perovskites by adjusting the colloidal synthesis process.Their crystal structure and optical properties were further analyzed in depth,and PLQY was improved by metal ion doping.It mainly includes the following aspects:(1)Hot injection method can control the size and morphology of materials,and has been widely used in the preparation of traditional chalcogenide and full-inorganic halide perovskite quantum dots(QDs).However,it is rarely employed to prepare the quasi-2D perovskite nanosheets.For the first time,the quasi-2D(OAm)₂CsPb₂Br₇(n=2)perovskites was prepared by the surface engineering involving metal ions,where OAm⁺(C₁₈H₃₅NH₃⁺)and Cs⁺occupy the A site together.Beyond the synthesis system of conventional 3D CsPbBr₃ QDs,we introduced Mg²⁺with a high ratio to Pb²⁺,which promoted the adsorption of ligands and changed the arrangement of ligands.This in return contributed to a lamellar soft template,serving the plane-confined growth of[PbBr₆]^4-components,resultant of anisotropiclayered structure.And,the deep blue PL at 440 nm with PLQY of 35%was detected in the quasi-2D perovskites nanosheets.This doping strategy is extended sucessfully to other metal cations,including the Ca²⁺,Co²⁺,Sr²⁺and Mn²⁺.Different from the preparation method of controlling the feed ratio of the reactants according to the general formula of the quasi-2D perovskite,we controlled the synthesis of the quasi-2D perovskites through surface engineering,and proposed a new idea toward the synthesis of low-dimensional perovskites.(2)On the basis of the previous research,we have prepared Mn²⁺-doped quasi-2D(OAm)₂Cs(Pb_1-xMn_x)₂Br₇(n=2)nanosheets.By changing the molar ratio of Mn²⁺to Pb²⁺solely,the conversion of 3D perovskite QDs to quasi-2D perovskite nanosheets was first realized,concurrently the Mn²⁺doped quasi-2D perovskite was prepared.The efficient energy transfer from free exciton in quasi-2D perovskite to Mn²⁺dopant caused the d-d transition of Mn²⁺,thus realizing the intrinsic emission of Mn²⁺at 614 nm.The full-spectral emission from 400 nm to 700 nm was achieved by adjusting the feed amount of Mn²⁺.Finally,Mn²⁺doped quasi-2D perovskite based LEDs with orange and white emission on was prepared,respectivley.(3)The strong quantum confinement effect of 2D perovskite(n=1)results in a wide band gap,making it difficult to achieve emission at longer wavelength other than blue and green light.On the one hand,its large exciton binding energy accelerates the exciton thermal quenching,resulting in a very low PLQY.On the other hand,tightly bound excitons are more conducive to the energy transfer from the host to the dopant.In this scenario,we performed a simple,efficient and low-energy crystallization method at room temperature to prepare a series of Mn²⁺-doped 2D(PEA)₂PbBr₄ nanosheets.The as-prepared Mn²⁺-doped(PEA)₂PbBr₄ has two PL emission peaks centered at 410 nm and 600 nm,corresponding to the free exciton transition of the host perovskite and the d-d transition of Mn²⁺,respectively.After adjusting the optimal Mn²⁺doping amount,the PLQY of the material is as high as 54% thanks to the efficient exciton energy transfer.Temperature-dependent and time-resolved PL spectroscopy confirmed such effective energy transfer process,from the host to Mn²⁺.Finally,we prepared a series of white LEDs based on Mn²⁺-doped2D(PEA2)PbBr₄ with different color temperatures.In short,we have perfected the collodial preparation process and studied the crystal structure and optical properties of(quasi)-2D perovskite nanosheets,laying the foundation for its application in optoelectronic devices.