Controllable synthesis and performance research of CsPbBr<sub>3</sub>@CsPb<sub>2</sub>Br<sub>5</sub> nanocrystals with high stability and quantum yield

Abstract

In the past decade, all-inorganic lead halide perovskite nanocrystals have emerged as promising active materials for optoelectronics, such as photovoltaics, lasers, and photodetectors, owing to their size-tunable optical bandgaps, attractive light absorption, and extraordinary charge-transport properties. Besides these photoelectric applications, nanostructured lead halide perovskites have shown good wavelength tunability, high photoluminescence quantum yields (PLQYs), and high color purity with narrow full width at half maximum (FWHM), rendering them potential candidates for high-performance light-emitting diodes (LEDs). Unfortunately, despite the remarkable advancements of all-inorganic lead halide perovskite, the problem of their unsolved stability upon exposure to thermal and humid environments has greatly hampered their further practical and commercial applications. Thus, many researchers have been bent their mind to improve the water stability of perovskites from moist air or solutions, such as moisture-impermeable polymer embedding, water proof organic ligand coating, inorganic encapsulation, and superhydrophobic framework structures. CsPb2Br5, which has an indirect band gap of 3.87 eV and a two-dimensional structure with [Pb2Br5]– layers spaced by Cs+ cations, has been reported to modify the stability of CsPbBr3. However, the arrangement of CsPbBr3 and CsPb2Br5 phases, which is significant for both the improvement of the stability and the investigation of the mechanism, is still a bottleneck. In order to improve the stability of the all-inorganic lead halide perovskite, an innovative method—Pseudo-peritectic method, that is a peritectic reaction in the solution, was proposed to achieve the same element surface modification of the CsPbBr3 perovskite by reasonably controlling the ratio of reactants. In our experiment, the CsPbBr3 nanocrystals were synthesized by solvothermal method first, and then injected into the PbBr2 solution. The peritectic reaction between CsPbBr3 and PbBr2 occurred in the solution. The synthesized CsPb2Br5 phase is excepted to disperse on the surface of the as-prepared CsPbBr3 phase. The study of the crystal structure, morphology, composition and photoluminescence of the prepared composite perovskite CsPbBr3@CsPb2Br5 were carried out. The X-ray diffraction (XRD) pattern and morphologies of the samples synthesized by the pseudo-peritectic method indicated that the composite perovskite CsPbBr3@CsPb2Br5 with uniformly distributed CsPb2Br5 phase on the surface can be prepared by controlling the ratio of reactants. With the ratio of CsPbBr3: PbBr2 changing from 1:1 to 2.5:1, the CsPb2Br5 phase increased. When the ratio of CsPbBr3:PbBr2 was 2.5:1, the CsPb2Br5 phase well dispersed on the surface of the as-prepared CsPbBr3 phase. However, when the ratio increased to 3:1, even 5:1, the synthesized CsPb2Br5 phase decreased. The photoluminescence spectroscopy and fluorescence quantum yield of the composite material showed that the presence of the same element phase CsPb2Br5 greatly improved the stability and the photoluminescence (PL) properties. The uniformly distributed CsPb2Br5 phase on the surface of the composite material reduced the surface defect concentration of CsPbBr3, and protected the CsPbBr3 phase to a certain extent, that improved the luminescence and stability of the CsPbBr3 perovskite. The method for synthesizing composites provides a new pathway to ensure a controllable surface modification structure, which would dramatically enhance the stability of CsPbBr3 nanocrystals in highly polar solvents such as water. The water-stable enhanced nanocrystals make it possible for long-term stable electronic applications in the atmosphere.