New Insights into Mn-Mn Coupling Interaction-Directed Photoluminescence Quenching Mechanism in Mn~(2+)-Doped Semiconductors

摘要

Strong Mn-Mn coupling interactions (dipole-dipole and spin-exchange), predominantly determined by statistically and apparently short Mn-Mn distances in traditional heavily Mn~(2+)-doped semiconductors, can promote energy transfer within randomly positioned and close-knit Mn~(2+) pairs. However, the intrinsic mechanism on controlling Mn~(2+) emission efficiency is still elusive due to the lack of precise structure information on local tetrahedrally coordinated Mn~(2+) ions. Herein, a group of Mn~(2+)-containing metal-chalcogenide open frameworks (MCOFs), built from [Mn_4In_(16)S_(35)] nanoclusters (denoted T4-MnInS) with a precise [Mn4S] configuration and length-variable linkers, were prepared and selected as unique models to address the above-mentioned issues. MCOF-5 and MCOF-6 that contained a symmetrical [Mn_4S] core with a D_(2d) point group and relatively long Mn...Mn distance (～3.9645 A) exhibited obvious red emission, while no room-temperature PL emission was observed in MCOF-7 that contained an asymmetric [Mn_4S] configuration with a C_1 point group and relatively short Mn-Mn distance (～3.9204 A). The differences of Mn-Mn dipole-dipole and spin-exchange interactions were verified through transient photoluminescent spectroscopy, electron spin resonance (ESR), and magnetic measurements. Compared to MCOF-5 and MCOF-6 showing a narrower/stronger ESR signal and longer decay lifetime of microseconds, MCOF-7 displayed a much broader/weaker ESR signal and shorter decay lifetime of nanoseconds. The results demonstrated the dominant role of distance-directed Mn-Mn dipole-dipole interactions over symmetry-directed spin-exchange interactions in modulating PL quenching behavior of Mn~(2+) emission. More importantly, the reported work offers a new pathway to elucidate Mn~(2+)-site-dependent photoluminescence regulation mechanism from the perspective of atomically precise nanoclusters.