@article{Frenzel2021, 
author = {Florian Frenzel and Christian Würth and Oleksii Dukhno and Frédéric Przybilla and Lisa M. Wiesholler and Verena Muhr and Thomas Hirsch and Yves Mély and Ute Resch-Genger},
title = {Multiband emission from single β-NaYF4(Yb, Er) nanoparticles at high excitation power densities and comparison to ensemble studies},
year = {2021},
journal = {Nano Research},
volume = {14},
number = {11},
pages = {4107-4115},
keywords = {upconversion luminescence, single nanoparticle study, high power excitation, emission color, luminescence decay kinetics},
url = {https://www.sciopen.com/article/10.1007/s12274-021-3350-y},
doi = {10.1007/s12274-021-3350-y},
abstract = {Ensemble and single particle studies of the excitation power density (P)-dependent upconversion luminescence (UCL) of core and core–shell β-NaYF4: Yb, Er upconversion nanoparticles (UCNPs) doped with 20% Yb3+ and 1% or 3% Er3+ performed over a P regime of 6 orders of magnitude reveal an increasing contribution of the emission from high energy Er3+ levels at P > 1 kW/cm2. This changes the overall emission color from initially green over yellow to white. While initially the green and with increasing P the red emission dominate in ensemble measurements at P < 1 kW/cm2, the increasing population of higher Er3+ energy levels by multiphotonic processes at higher P in single particle studies results in a multitude of emission bands in the ultraviolet/visible/near infrared (UV/vis/NIR) accompanied by a decreased contribution of the red luminescence. Based upon a thorough analysis of the P-dependence of UCL, the emission bands activated at high P were grouped and assigned to 2–3, 3–4, and 4 photonic processes involving energy transfer (ET), excited-state absorption (ESA), cross-relaxation (CR), back energy transfer (BET), and non-radiative relaxation processes (nRP). This underlines the P-tunability of UCNP brightness and color and highlights the potential of P-dependent measurements for mechanistic studies required to manifest the population pathways of the different Er3+ levels.}
}