Study On Preparation And Luminescence Mechanism Of 5s~2 Lone-Pair Metal Halides

In recent years,lead-free organic-inorganic metal halide perovskite have attracted extensive attention due to their excellent photoelectric properties,long-term stability and environmental friendliness.At molecular level,metal halide perovskite can be divided into three-dimensional(3D),two-dimensional(2D),one-dimensional(1D)and zero-dimensional(0D)structures.Zero-dimensional metal halide has strong quantum-limited effect,large exciton binding energy and strong electron-phonon coupling due to its unique crystal structure and isolated polyhedral unit.Generally showing broad full width at half-maximum(FWHM),large Stokes shift and efficient broadband emission,it has gradually become a research hotspot in the field of luminescence.Among them,low-dimensional halide perovskites based on 5s2 configuration ions(such as Sb3+and Te4+)are widely used in various high-efficiency optoelectronic devices due to their special chemical properties of 5s2 lone-pair electrons,such as wide-band emission and temperature-dependent lifetime.However,the relationship between the luminescent properties and the crystal structure as well as the corresponding luminescence mechanism is still unclear,and the optical properties of these materials cannot be accurately regulated from the molecular level or the structural level,so it needs to be further explored.Based on this,this research focuses on the development of metallic perovskite materials containing 5s2 lone-pair electrons with excellent luminescence properties and stable structure,and further exploration of their luminescence mechanism.Specific research contents are as follows:(1)The Effect of Host Lattice Environment on the Expression of 5s2 Lone-Pair Electrons in a 0D Bismuth-Based Metal Halide.ns2-metal halide perovskites have attracted wide attention due to their fascinating photophysical properties.However,achieving high photoluminescence(PL)properties is still an enormous challenge,and the relationship between the lattice environments and ns2-electron expression is still elusive.Herein,an organic-inorganic Bi3+-based halide(C5H14N2)2BiCl6·Cl·2H2O(C5H14N22+=doubly protonated 1-methylpiperazine)with six-coordinated structure has been successfully prepared,which however exhibits inferior PL properties due to the chemically inert expression of Bi3+-6s2 lone-pair electrons.After reasonably embedding Sb3+with 5s2 electrons into the lattice of(C5H14N2)2BiCl6·Cl·2H2O,the host lattice environment induces Sb-Cl moiety changing from original five-coordinated to six-coordinated structure,thereby resulting in a broadband yellow emission with the PL efficiency up to 50.75%.By utilizing the host lattice of(C5H14N2)2BiCl6·Cl·2H2O,the expression of Sb3+-5s2 lone-pair electrons is improved and thus promotes the radiative recombination from Sb3+-3P1 state,resulting in the enhanced PL efficiency.This work will provide a deeper insight into the effect of local structure on the expression of Sb3+-5s2 lone-pair electrons.(2)Synthesis and Luminescence Mechanism of(C8H20N)2TeCl6 with Yellow Emission.Low-dimensional metal halides with ns2 lone-pair electrons have been considered as a new generation of emitters for various optoelectronic applications.In the previous work,we discussed zero-dimensional metal halides with 5s2 active lone-pair electrons(Sb3+).However,tellurium-based halides,which also have 5s2 lone-pair electrons,have not received much attention despite their attractive photoluminescence(PL)properties.Therefore,in this work we develop a novel organic-inorganic hybrid tellurium-based halide single crystal(C8H20N)2TeCl6,in which the inorganic unit[TeCl6]2-octahedron is separated by the organic cation tetraethylammonium to form a typical zero-dimensional(0D)structure.Spectroscopic analysis characterizations show that(C8H20N)2TeCl6 exhibits bright broadband yellow emission in the range of 450-800 nm with a photoluminescence quantum yield of 17.25%.Steady-state fluorescence spectroscopy,temperature-variable fluorescence spectroscopy,time decay curves and other characterization methods combined with theoretical calculations demonstrate that the broadband yellow emission with large Stokes shift and long lifetime within(C8H20N)2TeCl6 originates from triplet self-trapped excitons(STEs)emission localized in[TeCl6]2-octahedra.This study provides a guidance for the preparation of low-dimensional tellurium-based halide materials with excellent luminescence properties and also provides a deeper understanding of the PL behavior of tellurium-based halides.(3)Study on the Luminescence Properties and Stability of High-Performance 0D Te4+-Based Metal Halide.In our previous work,we synthesized a novel zero-dimensional tellurium-based halide with 5s2 lone-pair electrons,but the optical properties of this material were not very satisfactory.Therefore,the development of tellurium-based halide with excellent luminescence and stable structure is the focus of our next research.In this work,we have prepared a Te4+-based metal halide(C26H56N)2TeCl6 with excellent optical properties by a simple room temperature crystallization method.The material exhibits bright orange-red broadband emission in the range of 450-850 nm with FWHM of 172 nm,stokes shift of 225 nm and PLQY of 65.82%,which is the highest quantum yield of Te4+-based metal halides that has been reported so far.Steady-state and transient spectral characterizations combined with theoretical calculations indicate that self-trapped excitons(STEs)localized in[TeCl6]2-octahedra are mainly responsible for the broadband red emission.In particular,in addition to the common 3P1→1S0 process,3P2→1S0 process was also observed at low temperatures due to the broken symmetry of the(C26H56N)2TeCl6 crystal.In addition,(C26H56N)2TeCl6 has excellent water stability and can be stable in extreme water environments for about 60 days,and the long-chain hydrophobic group structure of(C26H56N)+organic ligands may be responsible for the excellent water stability.We also discuss the relationship between structural distortion of[TeX6]2-octahedra and luminescence properties in Te4+-based metal halides,which provides an important idea to explain the structure-property relationship in Te4+-based metal halides.This work provides a key guidance for the synthesis of tellurium-based metal halides with high luminescence properties and excellent water stability.