Quantum yield measurements of Yb,Ho co-doped upconverting nanomaterials: The impact of methods, reference materials and concentration

Quantitative comparison of emission intensity between lanthanide doped upconverting (UC) nanoparticles is critical to evaluate the impact of host matrix and nanocrystal size, active- and passive- dopants, core-shell chemical architecture, surface biofunctionalization and others, which are typically a subjects to optimization. While relative luminescence intensity has been commonly used for that purpose, quantum yield (QY) is another accurate measure, because here, number of UC emission photons is normalized to the number of photons being absorbed. However, this type of measurements have been hindered by limited availibility of commercial instrumentation. Here we show a novel, simple and affordable UCQY setup design and versatilely characterize its performance using Yb3+ and Ho3+ co-doped colloidal NaYF4 nanoparticles dispersed in chloroform. The UCQY showed a liner dependence on excitation intensity, and reached up to 0.41% and 0.25% for the 2% Ho3+, 20% Yb3+ and 0.1% Ho3+, 20% Yb3+ materials, respectively. Interestingly, the UCQY values depend on the selection of reference material and concentration of NPs. In particular, the un-doped reference NPs of the same diameter as the sample NPs show higher UCQY values than the ones obtained for the pure solvent. Moreover, UCQY depend on the concentration of the NPs. These two effects were related to the scattering of photoexcitation beam in the sample and reference materials. Additionally, no saturation of power dependent UC was seen up to ~270 W/cm2 excitation intensities, which we relate to the photoexcitation methodology. In opposite to the Continuous Wave photoexcitation, we used Pulse Width Modulation of photoexcitation intensity, and confirm such behavior with a simple, time - resolved differential rate equations based model. These findings and reliable UCQY instruments are important to quantitatively compare and further optimize the performance of UCNPs.