Interface And Optical Properties Of Perovskite Lead Halide And Layered Oxide Composite

All-inorganic perovskite nanocrystals of CsPbX₃(X=Cl,Br,I)have drawn considerable attention for their excellent electronic and optical properties,such as high photoluminescence(PL)quantum yield,narrow emission peaks,tunable emission colors,and so on.These distinct advantages make CsPbX₃ as promising materials for many optoelectronic applications,including but not limited to light-emitting diodes,solar cells,lasers,or photodetectors.Thin films of lead halide perovskite are commonly used in most new-type nano-micro optoelectronic devices.Therefore,the controllable growth of lead halide perovskite thin films is very important,and the research of the interface/interaction between lead halide perovskite thin films and substrates is of great significance.At present,the growth methods of CsPbX₃ thin films reported in relevant literatures are limited to vapor phase epitaxy growth,evaporation,spin coating and so forth.However,it is a great challenge to grow high-quality thin films matching well with the substrates by solution processing.Layered two-dimensional oxide nanosheets can be utilized as substrates to grow/composite lead halide perovskite heterostructure,due to their similar structures and simple chemical preparation process.This dissertation aims to explore a new method and mechanism for the growth of CsPbX₃ on layered oxides,and discuss the potential applications.This thesis is divided into seven chapters.In chapter 1,the research progresses of all-inorganic perovskite CsPbX₃,layered oxide materials and CsPbX₃-based heterostructures are briefly reviewed.Based on this,the purpose,significance and research subject of this thesis are proposed.In chapter 2,the syntheses and characterizations of the samples are presented.For chapter 3-6,the transformation behavior and mechanism of Cs₄PbBr₆ to CsPbBr₃ triggered by niobate nanosheets(KNO NSs),PL properties of CsPbBr₃/KNO composites prepared by solvent evaporation,interfacial interaction of CsPbBr₃/CTO nanocomposites obtained by in situ synthesis,as well as the application of CCTO:Yb,Er@CsMX₃ composites are systematically studied.The final chapter summarizes the whole work and proposes a prospect.The specific achievements are presented as follows:(1)The composite with heterostructure is prepared by the phase transition of Cs₄PbBr₆precursor triggered by KNO NSs nanosheets.After the exfoliation of K₄Nb₆O₁₇ by OAm,KNO NSs nanosheets are obtained.Interestingly,when KNO NSs are added,the colorless toluene solution dispersing Cs₄PbBr₆ nanocrystals exhibits green emission immediately under UV light.But after a while,this mixed solution with green emission transformed to colorless again.After analyzing the test data,we found that KNO NSs could trigger the transformation of Cs₄PbBr₆to CsPbBr₃ and finally result in the decomposition of CsPbBr₃.DFT calculation results show that the adsorption of Cs⁺or Pb²⁺ions will reduce the surface energy and KNO NSs tend to preferentially adsorb Cs⁺ions.Based on the above data and analysis,we propose a mechanism of the transformation from Cs₄PbBr₆ to CsPbBr₃ triggered by KNO NSs.There is an equilibrium of crystal dissolution and crystallization in toluene solution when dispersing Cs₄PbBr₆ or CsPbBr₃ nanocrystals.However,the addition of negatively-charged KNO NSs will break the crystal dissolution recrystallization equilibrium process,which leads to the above phenomena.The research in this chapter makes a preliminary technical exploration and knowledge reserve for further optimizing the preparation method of negatively charged layered oxide and lead halide perovskite heterostructure.(2)CsPbBr₃/KNO heterostructure is in situ synthesized in DMSO solution of Cs⁺and Pb²⁺ions by solvent evaporation.It is found that KNO NSs nanosheets preferentially adsorb Cs⁺ions,which simultaneously results in disorderly restacked nanosheets.What's more,Cs⁺ions are mainly distributed between the interlayers of restacked KNO NSs nanosheets,while Pb²⁺ions tend to be on the surface of the restacked layered materials.SEM and TEM results show that CsPbBr₃ is generally distributed in the edge of interlayers for the restacked KNO NSs.Due to the asymmetry in the faces of KNO NSs,these nanosheets will spontaneous scrolling to form nanotubes.It is also found that some CsPbBr₃ are encapsulated inside the niobate nanotubes for the CsPbBr₃/KNO composites.(3)Bulk perovskite-like layered material CsCa₂Ta₃O₁₀ is exfoliated into two-dimensional negatively charged Ca₂Ta₃O^-₁₀(CTO)nanosheets as seeds to in situ synthesize and composite CsPbBr₃.It is found that the(112)planes of orthorhombic CsPbBr₃ match the(004)lattice planes of CTO referring to the lattice distance,which is beneficial to epitaxial growth.The as-synthesized CsPbBr₃/CTO nanocomposites possess improved green emission with apparently prolonged decay time with reference to bare CsPbBr₃ QDs.The decay time can retrieve to a normal state when the nanocomposites are treated with some water.It is found that the CTO acts as a defect to trap the bound exciton of the loaded CsPbBr₃.Protons from water can preferably replace Cs⁺at the interface of the nanocomposites,resulting in the separation of the nanosheets and CsPbBr₃ and retrieving the decay time.X-ray photoelectron spectroscopy results also indicate the strong interaction between CsPbBr₃ and CTO with reference to the physical mixing sample of bare CsPbBr₃ QDs and CTO nanosheets.The decoration of ultrathin2D charge-bearing oxide nanosheets on the nanocrystals benefits significant improvements in humidity resistance and photostability performance in light-emitting diode devices.(4)Perovskite-like oxide and perovskite halide composites with core-shell-like structure are designed and prepared.The composites are prepared by a facile hot-injection method,which includes adhesive growth of Cs(Pb_xMn_1-x)(Cl_yBr_1-y)₃ on the surface of CsCa₂Ta₃O₁₀:Yb,Er(abbreviated as CCTO:Yb,Er@CsMX₃)via sharing Cs⁺at the interface.The unique structure makes it possess sensitivity to some additive decrypted inks,offering one more intriguing anti-counterfeiting dimension beyond the common dual-mode fluorescence and time-gating technique.The resultant CCTO:Yb,Er@CsMX₃ composites are further fabricated into different luminescent inks for producing 5D anticounterfeiting barcodes via luminescence printing,by utilizing the fluorescent colors,luminescence lifetimes and additive decrypted inks.