Mechanisms of rare earth ion distribution in fluorosilicateglass containing KMnF3 nanocrystal

Luminescent materials with an efficient single-(pure) color up-conversion luminescence (UCL) are expectedto be applied to three-dimensional (3D) display, lighting, biological imaging, promoting plant growth andimproving the photoelectric conversion efficiency of solar cells. In this work, perovskite-type KMnF3 fluoridenanocrystals (NCs) are grown in situ in a fluorosilicate glass co-doped with rare earth (RE) ions Yb3+/Er3+ by acontrolled thermal treatment. Compared with precursor glass (PG), the nano-glass composites (also referred toas glass ceramics, or GC in short) thus obtained exhibit a significantly enhanced (by 6 times) red UCLemission. Although a weak green UCL emission can be also observed in the GC, the intensity ratio of the redUCL emission to green UCL emission is as high as 30, implying a good color purity. It is suggested that thedramatic enhancement of UCL emissions in the GCs is due to the doping of RE ions into the KMnF3 NCs witha much lower phonon energy (330 cm-1) than that of the silica glass matrix about 1100 cm-1. However, thedoping mechanisms of RE ions into KMnF3 nano-glass composites are not yet conclusive, mainly because of thecharge and ionic radius mismatch between RE ion dopants and cations of KMnF3. This work combines thehigh-resolution transmission electron microscopy (HR-TEM) analysis technology and the first principlescalculation, to unravel the doping mechanism of RE ions in KMnF3 nano-glass composites. First, the HR-TEMstudy provides straightforward evidence that RE ions are preferentially accumulated in KMnF3 NCs embeddedin the glass matrix. Then, through the first-principles calculation considering the charge balance, it is foundthat the formation energy of RE ions substituting for K+ is lower than for Mn2+ lattice sites in KMnF3, which ismost likely related to the fact that the ionic radius of the eight-fold coordinated K+ is larger than that of thesix-fold coordinated Mn2+ and thus is more conductive to accommodating the large size RE ions. The electronicdensities of states at the top of the valence band and the bottom of the conduction band of KMnF3 increaseafter doping the RE ions. It is inferred from the profile of partial density of state that RE ions have a strongbonding tendency with F- in the crystal. Benefiting from the efficient energy transfer between RE ions andMn2+ in KMnF3, the green UCL emission is dramatically quenched, and consequently, the GC is endowed witha highly pure red UCL emission. The present study is expected to deepen the understanding of RE ions dopingmechanisms in NCs and facilitate the design of highly efficient UCL materials based on nano-glass composites