Multiband emission from single β-NaYF4(Yb,Er) nanoparticles at high excitation power densities and comparison to ensemble studies

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.

Nanosecond kinetics of multiphoton upconversion in an optically trapped single microcrystal

“…A diagram (Fig. S2, ESI †) relying on Judd-Ofelt analyses [45][46][47][48][49] and previous studies 27,37 were used for clarifying the complex population processes of the Yb 3+ -Er 3+ system, which involves energy transfer (ET), Er 3+ -Yb 3+ BET, and nRPs.…”

“…F. Frenzel et al have investigated the power density-dependent multiband emission of NaYF 4 :Yb/Er, 37 which demonstrated the existence of four-photon UC processes. Indeed, the kinetic multiband UCL population and emission processes are complex, making them difficult to identify and predict.…”

The impact of Yb3+ concentration on multiband upconversion in a single NaYF4:Yb/Er microcrystal determined via nanosecond time-resolved spectroscopy

“…We subsequently focused on the main UCL peaks, which can be clearly distinguished from each other. According to the previous studies 12,24,[33][34][35][36][37] , transient spectra of multiband UCL are determined based on the emission wavelengths and their corresponding transitions, as shown in Error! Reference source not found.d.…”

“…) and pathway B ( 4 I 15/2 → 4 I 11/2 → 4 I 13/2 → 4 F 9/2 → 2 H 9/2 → 2 D 5/2 ). 24,33,38,39 The UCL at 542 nm ( 4 S 3/2 → 4 I 15/2 ), 522 nm owing to the high dopant of Yb 3+ . [19][20][21][22] In the timescale from 50 Please do not adjust margins Please do not adjust margins ns to 80 ns after excitation, the emergence of 402 nm ( 2 P 3/2 → 4 I 13/2 ) and 767 nm ( 2 P 3/2 → 4 S 3/2 ) emissions demonstrate the occurrence of the four-photon process.…”

Quantification of the Activator and Sensitizer Ion Distributions in NaYF4:Yb3+, Er3+ Upconverting Nanoparticles Via Depth‐Profiling with Tender X‐Ray Photoemission

“…[44] While Hudry et al provided evidence for intermixing in individual UCNPs, our depth-profiling analysis with HAXPES gives complementary evidence for intermixing averaged over an ensemble of UCNPs. Since the optical properties are typically measured over an ensemble of UCNPs, although single particle studies do exist, [38,[72][73][74][75] averaged depth-profiles can be more readily correlated with optical measurements. In addition, core-level HAXPES is a more direct method to quantitatively determine chemical speciation, for example the small amount of Yb 2+ observed in the UCNP particle core of sample A.…”