Lead-free Cs₃MnCl₅ and CsMnCl₃ crystals: rapid on-chip crystallization, phase transition and fluorescence sensing...

Manganese-based perovskites have attracted tremendous interest due to their structural diversity, tunable emissions and low toxicity. In this work, manganese perovskite crystals, Cs₃MnCl₅ and CsMnCl₃, are successfully fabricated through a proposed on-chip rapid crystallization strategy. Fabrication of Cs₃MnCl₅ and CsMnCl₃ crystals can be finely controlled by adjusting the crystallization rate of precursor solution. The formation mechanisms of CsMnCl₃ and Cs₃MnCl₅ crystals are clarified based on the classical crystal nucleation and growth theory. The morphology and phase structure of the obtained Cs₃MnCl₅ and CsMnCl₃ crystals are determined through scanning electron microscopy (SEM) and X-ray diffraction (XRD) characterization studies. Luminescence properties of Cs₃MnCl₅ and CsMnCl₃ crystals are analyzed using UV-Vis absorption spectroscopy, steady-state photoluminescence spectroscopy and time-resolved photoluminescence spectroscopy. The observed green and red emissions for Cs₃MnCl₅ and CsMnCl₃ crystals are well explained according to the varied coordination surroundings of Mn²⁺ cations. Besides, phase transformation from Cs₃MnCl₅ to CsMnCl₃ was accompanied by a fluorescence color change from green to red. Furthermore, red luminescence of CsMnCl₃ would be quenched under high relative humidity and fully recovered after heating, which makes it possible for application in humidity detection. After 15 cycles of the quenching-recovery test, the maximum fluorescence intensity of CsMnCl₃ remained at 75% of its initial value, showing high operational stability and potential for application in fluorescence-based sensors.