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Measuring Activation and Luminescence Time Scales of Upconverting NaYF₄:Yb,Er Nanocrystals C

Laurence, Ted A., Liu, Yang, Zhang, Ming, Owen, Matthew J., Han, Jinkyu, Sun, Lingdong, Yan, Chunhua, Liu, Gang-yu
Journal of physical chemistry 2018 v.122 no.41 pp. 23780-23789
differential equation, energy transfer, fluorescence, ions, models, nanocrystals, nanoparticles, physical chemistry
Accurate determination of upconversion and luminescence lifetimes requires kinetic modeling of the complete time-resolved response for upconversion luminescence of NaYF₄:Yb,Er nanocrystals. Prior investigations typically perform exponential fitting of the tail in the time profile or employ complex systems of differential equations to extract lifetimes. To simplify analysis while fitting the entire time-resolved response, this work introduces a set of simplified models that model the response as a convolution of upconversion and fluorescence processes. Models for two- and three-photon upconversion processes are developed and tested for NaYF₄:Yb,Er nanocrystals excited by a 980 nm laser. The results are presented for the transitions ²H₁₁/₂, ⁴S₃/₂ → ⁴I₁₅/₂ (530 nm, green emission), ⁴F₉/₂ → ⁴I₁₅/₂ (650 nm, red emission), and ²H₉/₂ → ⁴I₁₅/₂ (400 nm, blue emission). Even with the same number of fitting parameters, the two- and three-photon models resulted in better fitting than the simpler models of single and two convolved exponentials for 30 nm, 400 nm, and silica-coated 30 nm nanoparticles. We provide evidence that the longest time scales (0.5–4 ms) are due to the luminescence of the final state, that the energy transfer waiting times leading to upconversion are shorter (<0.1 ms), and that more than two waiting times are required to explain the entire time response. The long-time response deviates from single-exponential behavior more strongly in uncoated and silica-coated 30 nm particles than for 400 nm particles. This deviation may result from heterogeneity of the local environment of ions within the 30 nm particles.