Red And Near-infrared Luminescence Of Zero-dimensional Perovskite

All-inorganic perovskite has garnered much attention in optoelectronic community due to their compelling optical properties including enhanced photo-and thermo-stability,nearly-unity photoluminescence quantum yield(PL QY),narrow emission bands,and tunable bandgap.In the Chapter 1 of this article,the structures of zero-dimensional(0D)perovskites and three-dimensional(3D)perovskites are compared.For the 3D perovskite CsPbBr₃,it features a framework of corner-sharing[Pb Br₆]^4-octahedra that extend in all three dimensions.In stark contrast,0D perovskite consists of an array of isolated[Pb X₆]^4-octahedra that are separated by large cations.Such a molecular array governs reflects the intrinsic properties of single[Pb X₆]^4-unit where quantum confinement effect was highly pronounced,and therefore arises considerable attention in last few years.Due to the effect of quantum confinement,pristine0D perovskites usually have a large bandgap up to 4?5 e V,making them a typical insulator material.However,it can emit bright green light after being excited.At present,there are two main conjectures about the luminescence mechanism of zero-dimensional perovskite:one is that the 3D phase impurities contained in the 0D perovskite emit light.The other is defects that are easily formed in the soft lattice of 0D perovskite are believed to act as emission centers.It should be noted that there are currently fewer types of zero-dimensional lead halide perovskites in the luminescent phase.Only the Br-based zero-dimensional perovskite Cs₄Pb Br₆is found to emit light.Other halide perovskites such as Cs₄Pb Cl₆and Cs₄Pb I₆have never been reported as an emissive phase to date.The Pb element contained in the zero-dimensional lead halide perovskite is harmful to human health.In order to solve this problem,researchers chose to substitute Pb²⁺with other less toxic elements,such as Ag⁺,Na⁺and In³⁺.The compound obtained in this way is called lead-free double perovskite,which belongs to 0D perovskite structure.At present,most of the relevant reports are the research on lead-free double perovskite nanocrystals,and there are few reports on lead-free double perovskite single crystals.And most of the research on the luminescence of inorganic halide perovskite is mainly focused on the emission of visible light,and there are few reports on near-infrared luminescence.In view of the above-mentioned problems of zero-dimensional perovskite,the main body of the thesis was briefed as follows:1.In the Chapter 2,red-emitting Cs₄Pb I₆zero-dimensional perovskite nanocrystals were successfully synthesized by co-precipitation at room temperature,which are regular hexagons with a size of about 380 nm.The Cs₄Pb I₆nanocrystals show both a sharp emission at 680 nm and a sharp excitation peak at 450 nm which is usually ignored in daily PL screening.The composition of the Cs₄Pb I₆crystal was analyzed by energy dispersive spectrometer(EDS).the results suggesting an I-rich nature of luminescent Cs₄Pb I₆nanocrystals.The X-ray diffraction(XRD)pattern of the Cs₄Pb I₆nanocrystal shows that it does not contain its three-dimensional phase impurities.The power-dependent PL proves that a certain defect in the Cs₄Pb I₆nanocrystal serves as the luminescence center.Importantly,the Cs₄Pb I₆perovskite nanocrystals exhibit highly enhanced stability in both anion-exchange and photobleaching experiment when compared to its 3D counterpart.Based on the above,this work breaks the uniqueness of Br-based phosphors,increases the types of zero-dimensional perovskites in the luminescent phase,and broadens the field of zero-dimensional perovskites.2.In Chapter 3,Cs₂Na_0.2Ag_0.8In Cl₆single crystal with a centimeter size was grown by hydrothermal method.It emits bright yellow light under the irradiation of 365-nm light,and the PL spectrum shows a broad peak around 565 nm.In this work,Cs₂Na_0.2Ag_0.8In Cl₆:Yb³⁺single crystal were also synthesized by hydrothermal method,which achieved dual-emission in the visible light band(640 nm)and near-infrared band(996 nm).The structure diagram shows that Yb³⁺Ions replaced part of In³⁺ions.Time-resolved PL measurement reveals the energy transfer between the self-trapped state and Yb³⁺energy level in this crystal.Surprisingly,the irradiated Cs₂Na_0.2Ag_0.8In Cl₆:Yb³⁺single crystal showed afterglow characteristics in the near-infrared(NIR)region after the ultraviolet(UV)light source was turned off.The power decay curve shows that the afterglow time is much more than two hours.Cs₂Na_0.2Ag_0.8In Cl₆:Yb³⁺crystal has obvious photochromic phenomenon,which is related to the traps in its crystal lattice.Combined with the thermoluminescence test,a possible afterglow generation mechanism was proposed.In the Chapter 4,a brief summary of the above work is given,combined with the current research status of zero-dimensional perovskite,and several prospects for other existing problems are put forward.