Measuring the internal quantum yield of upconversion luminescence for ytterbium-sensitized upconversion phosphors using the ytterbium(iii) emission as an internal standard

Abstract

A method is presented for estimating the internal quantum yield (IQY) of NIR-to-NIR and NIR-to-visible upconversion (UC) luminescence for Yb³⁺-sensitized energy-transfer upconversion (ETU) phosphors. The method does not require an integrating sphere or a secondary standard, but rather uses the 1 μm emission of the Yb³⁺ sensitizer as an internal standard. The method requires the acquisition of the 1 μm emission decay curve of the UC phosphor using low pulse-energy density, an estimation of the radiative decay constant of the 1 μm emission, and emission spectra corrected for instrument response. This method is valid for UC emission spectra acquired via pulsed or continuous wave (cw) excitation. The method is demonstrated for cw excitation to obtain IQY for UC and downshifted luminescence for β-phase NaYF₄: 0.5%Tm, 25%Yb and NaYF₄: 2%Er, 18%Yb nanocrystals (with and without a passivating NaYF₄ shell) over a range of excitation irradiance. The corresponding results are consistent with those obtained using integrating spheres and numerical simulations, respectively. For pulsed excitation, an additional alternative method is described which requires acquisition of the 1 μm emission decay curve at each excitation pulse-energy density for which the IQY is to be determined. The proposed methods should be particularly useful for samples having very low absorbance at the excitation wavelength, for which direct determination methods are impractical.