Phase-conversion synthesis of LaF3:Yb/RE (RE = Ho, Er) nanocrystals with Ln2(OH)4SO4·2H2O type layered compound as a new template, phase/morphology evolution, and upconversion luminescence

Layered hydroxide sulfate (Ln2(OH)4SO4·2H2O) was employed as a new template for (La0.97Yb0.02RE0.01)F3 nanocrystal synthesis via fluorination (RE=Ho, Er), with the course of phase/morphology evolution being studied in detail via systematically varying the reaction temperature (up to 180 °C) and reaction duration (up to 24 h). It was shown that phase conversion can be completed by reaction at room temperature for 24 h or at 180 °C for ∼2 h and the hexagonal structure of LaF3:Yb/RE preferentially develops with ab instead of (00l) basal planes. The upconversion (UC) performance under 978 nm laser excitation was examined with the nanocrystals synthesized via 24 h of reaction at 180 °C (average crystallite size ∼31 nm), and it was found that LaF3:Yb/Ho exhibits luminescence at ∼540 nm (5S2 → 5I8 transition, medium strong), 642 nm (5F5 → 5I8, strong) and 749 nm (5F4 → 5I7, weak), with chromaticity coordinates of about (0.47, 0.52), and LaF3:Yb/Er shows strong green (∼520/540 nm, 2H11/2,4S3/2 → 4I15/2) and even stronger red (∼656/668 nm, 4F9/2 → 4I15/2) luminescence, with color coordinates of around (0.45, 0.54). Analyzing the UC intensity against excitation power (0.6-1.5 W) via logarithmic data transformation found approximately two laser photons are needed to populate the emitting state, implying a two-photon mechanism for the observed UC luminescence of the two types of fluoride nanocrystals. The 642 and 656 nm main emissions of Ho3+ and Er3+ were found to decay in a single exponential manner and have fluorescence lifetimes of ∼742 and 1453 μs, respectively.