Upconverting lanthanide doped fluoride NaLuF4:Yb3+-Er3+-Ho3+ - optical sensor for multi-range fluorescence intensity ratio (FIR) thermometry in visible and NIR regions

Luminescent materials co-doped with Er3+-Ho3+ activator ions can work as multi-range Vis-NIR optical sensors of temperature. The upconverting NaLuF4:16%Yb3+-2% Er3+-0.5%Ho3+ hexagonal microcrystals, were successfully synthesized under hydrothermal conditions. The material obtained showed bright yellow up-conversion luminescence, under 975 nm CW laser irradiation. The emission spectra were measured in a broad spectral range, i.e. 450-1650 nm, revealing the presence of intense emission bands both in the visible and NIR range. The intensities of the Vis-NIR emission bands characteristic of Er3+ and Ho3+ changed with temperature, and the evolution of their intensity ratios were correlated with temperature and calibrated. The high sensitivity found for the intensity ratio coming from the Ho3+: I-5(5)-> I-5(8) (887 nm) and Er3+: I-4(9/2)-> I-4(15/2) (817 nm) transitions, located in the first biological optical window, is useful for optical thermometry applications.