Comparison of Quantum Yield of Upconversion Nanocrystals Determined by Absolute and Relative Methods

Photoluminescence quantum yield (phi) is a key parameter of any luminescent material. There are two main ways to determine this value: 1) absolute, which requires calculation of the number of emitted and absorbed photons; and 2) relative, which utilizes the emission of a reference sample with known phi. Both methods become more complicated in case of upconversion (UC) photoluminescence, due to its nonlinear nature. The main obstacle to employing the relative method is the lack of a suitable reference with known UC quantum yield (UCQY, phi(UC)). Herein, a new UCQY reference material is presented, based on SrF2:1%Yb3+,1%Er3+ single crystal, for the relative measurement of phi(UC) for near-infrared (976 nm)-to-visible UC. When utilizing this reference material, the phi(UC) is determined to be 2.5% (at 100 W cm(-2)) for alpha-NaYF4:18%Yb3+,2%Er3+@CaF2 nanocrystals (NCs). This result coincides very well with the value for the same NCs determined using the absolute method of phi(UC) = 2.4% (at 100 W cm(-2)). The intensity dependence of UCQY yield for the NCs determined using the SrF2:1%Yb3+,1%Er3+ reference exhibits good agreement with the results acquired with the absolute method. In addition, various effects that can have an impact on the measured UCQY using absolute and relative methods are discussed.