Multiband emission from single β-NaYF4(Yb,Er) nanoparticles at high excitation power densities and comparison to ensemble studies
Florian Frenzel, Christian Würth, Oleksii Dukhno, Frédéric Przybilla, Lisa M. Wiesholler, Verena Muhr, Thomas Hirsch, Yves Mély, Ute Resch‐Genger
Abstract
Abstract Ensemble and single particle studies of the excitation power density ( P )-dependent upconversion luminescence (UCL) of core and core-shell β-NaYF 4 :Yb,Er upconversion nanoparticles (UCNPs) doped with 20% Yb 3+ and 1% or 3% Er 3+ performed over a P regime of 6 orders of magnitude reveal an increasing contribution of the emission from high energy Er 3+ levels at P > 1 kW/cm 2 . 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/cm 2 , the increasing population of higher Er 3+ 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 Er 3+ levels.