NIR-to-NIR and NIR-to-blue light upconversion in stoichiometric NaYb(MO4)2 (M = Mo, W)

Abstract

Polycrystalline NaYb(MoO₄)₂ and NaYb(WO₄)₂ exhibited NIR-to-NIR and NIR-to-blue light upconversion under 973 nm excitation. The emission spectra were dominated by a strong NIR (∼795 nm) band. Blue (∼475 nm), green (∼525 and 545 nm), and red (∼650 nm) bands were also observed. The origin of these bands was investigated using a combination of steady-state and time-dependent spectrofluorometry, elemental analysis, and Rietveld analysis of synchrotron X-ray diffraction data. The strong NIR emission at ∼795 nm was assigned to two-photon upconversion from Yb³⁺-sensitized Tm³⁺, which was found to be present at trace levels (∼1 ppm) in both NaYb(MoO₄)₂ and NaYb(WO₄)₂. Due to the high efficiency of the energy-transfer from Yb³⁺ to Tm³⁺, the intensity of the NIR emission exhibited a linear dependence on the excitation power. Green and red bands were assigned to two-photon upconversion from Yb³⁺-sensitized Er³⁺, which was also found to be present at trace levels in both hosts (∼1 ppm). In the case of the blue emission, power-dependence and time-dependent spectrofluorometric studies favored cooperative luminescence of Yb³⁺–Yb³⁺ dimers, rather than three-photon upconversion from Yb³⁺-sensitized Tm³⁺. Local clustering of Yb³⁺ ions yielding Yb³⁺–Yb³⁺ dimers that interact cooperatively under NIR excitation was feasible considering (i) the intrinsic disorder of Na⁺ and Yb³⁺ over the same crystallographic site, and (ii) the shortest distance between two adjacent Yb³⁺ ions (∼3.81 Å). The significance of probing energy-transfer processes relevant to light absorption and emission in fully concentrated metalate hosts is highlighted.