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.
Highlights
Lanthanide-based upconversion nanoparticles (UCNPs) with their photoluminescence (PL) in the ultraviolet (UV), visible, and near infrared (NIR) following the absorption of two or more low energy photons have been increasingly studied in the last years [1,2,3,4,5,6]
As revealed by many photophysical studies of UCNPs of different size [15, 16], doping ion concentration, and particle architecture in apolar solvents and aqueous environments [17, 18], the intensity and relative spectral distribution of upconversion luminescence (UCL) are controlled by the interplay of energy transfer (ET) processes between the lanthanide doping ions involved in thepopulation of the radiative energy levels of the activator ions
For our mechanistic studies we chose a set of four core only and core–shell UCNPs (5 nm-thick NaYF4 shell) (Fig. 1(a)) of similar size doped with 1% and 3% Er3+
Summary
Due to their excellent photostability, UCNPs have gained interest as reporter for fluorescence microscopy using high P in the range of kW/cm to MW/cm. Emerging applications include single particle (SP) tracking and super-resolution microscopy [14, 20]. This initiated an increasing number of spectroscopic studies of these nanomaterials on the SP level [21,22,23], assessing UCL features with the overall goal to identify optimum particle architectures for these high P regimes. With the aim to correlate UCL ensemble spectroscopic measurements and high P microscopic studies and gain a deeper mechanistic insight into the (de)population processes at P > 1,000 W/cm, we assessed the optical properties of a set of 28 nm-sized core and core–shell UCNPs doped with 20% Yb3+ and 1% or 3% Er3+ at low, medium as well as high P. Based upon an in-depth analysis of the P-dependence of UCL, we derived a classification of the different high P-activated emission bands and their assignment to 2–3, 3–4, and 4 photonic processes
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