Power-dependent upconversion quantum yield of NaYF₄:Yb³⁺,Er³⁺ nano- and micrometer-sized particles – measurements and simulations

Abstract

Photophysical studies of nonlinear lanthanide-doped photon upconverting nanoparticles (UCNPs) increasingly used in biophotonics and photovoltaics require absolute measurements of the excitation power density (P)-dependent upconversion luminescence (UCL) and luminescence quantum yields (Φ) for quantifying the material performance, UCL deactivation pathways, and possible enhancement factors. We present here the P-dependence of the UCL spectra, Φ, and slope factors of the different emission bands of representative 25 nm-sized oleate-capped β-NaYF:17% Yb, 3% Er UCNPs dispersed in toluene and as powder as well as Φ of 3 μm-sized upconversion particles (UCμP), all measured with a newly designed integrating sphere setup, enabling controlled variation of P over four orders of magnitude. This includes quantifying the influence of the beam shape on the measured Φ and comparison of experimental Φ with simulations utilizing the balancing power density model of the Andersson-Engels group and the simulated Φ of UCμP from the Berry group, underpinned by closely matching decay kinetics of our UC material. We obtained a maximum Φ of 10.5% for UCμP and a Φ of 0.6% and 2.1% for solid and dispersed UCNPs, respectively. Our results suggest an overestimation of the contribution of the purple and an underestimation of that of the red emission of β-NaYF:Yb,Er: microparticles by the simulations of the Berry group. Moreover, our measurements can be used as a guideline to the absolute determination of UCL and Φ.

Rapid Synthesis of Sub‐10 nm Hexagonal NaYF4‐Based Upconverting Nanoparticles using Therminol® 66

“…[4][5][6][7][8][9][10][11][12][13] To date, the mostf requently studied UCNPs consist of hexagonal closed packed (hcp) b-NaYF 4 crystal host lattice co-dopedw ith as ensitizer-activator combination of Ln 3 + ions (primarily Yb 3 + combined with Er 3 + ,T m 3 + ,o rH o 3 + as activator ions). [14][15][16][17][18][19][20][21][22][23][24] Despite the fact that progress has been made in the synthesis of lanthanide-doped UCNPs, scalable size-controlled production of phase-pure b-NaYF 4 nanocrystals still remains ag reat challenge.I np articular,r eproducible synthetic routes that can provide accesst os ub-10 nm nanocrystals with strictly controlled morphology,h igh crystal phasep urity,a nd exceptional upconversion (UC) luminescence are highly desirable. Reported methodsf or the synthesis of hcp b-NaYF 4 particlesw ith diameters below 10 nm mostly rely on the use of additives such as Gd 3 + and Eu 3 + ion in high concentrations.…”

Förster Resonance Energy Transfer‐Activated Processes in Smart Nanotheranostics Fabricated in a Sustainable Manner

“…[49].T he calculated h Q values for encapsulated NaYF 4 : Tm 3 + ,Y b 3 + NPs are much lower than the ones for ligand-free or silanized NaYF 4 :Tm 3 + ,Y b 3 + NPs. [50] Arppe et al [48] ;however,they allowed us to confirm the positive shielding effect provided by the selected emulsification/solvent evaporation methodf ollowed by the LbL encapsulationp rocess. Another quantitative parameter used for the characterization of luminescent probes is the quantum yield (QY), defined as the ratio between the number of emitted photonst ot he absorbed ones.…”

“…[49].T he calculated h Q values for encapsulated NaYF 4 : Tm 3 + ,Y b 3 + NPs are much lower than the ones for ligand-free or silanized NaYF 4 :Tm 3 + ,Y b 3 + NPs. [50][51][52] There are several papers availablei nt he literature reporting the QY values for different types of up-converting nanomaterials (Table S2, SupportingI nformation) to be in the range of 3a nd 0.0022 %f or core-onlyc olloidal NaYF 4 NPs with sizes over 100 nm and below 10 nm, respectively. Corresponding values for red (800 nm) Tm 3 + ion upconverted emission were equal to approximately % 57 and % 66 %.…”

“…[50] Arppe et al [48] investigated the quenching of up-conversion luminescenceb yw ater in % 30 nm sphere-shaped NPs ando bserved quenching efficiencies of blue emission (470 nm) at the level of % 61 %f or ligand-free and % 66 %f or silica-functionalized NaYF 4 :Tm 3 + , Yb 3 + NPs. [28,50,51,[53][54][55][56] In the case of the nanocarrierss tudied by us, the QY of the loadedN Cs (QY NCs )w ill mainly dependo nt he corresponding value for as-synthesized NaYF 4 :Tm 3 + ,Y b 3 + NPs, with additional quenching introduced by the encapsulation process and water environment. These values cannotb ed irectly compared with those calculated for encapsulatedNaYF 4 :Tm 3 + ,Yb 3 + NPs investigated within this work, due to the large difference in NPs size (30 vs. 8nm);however,they allowed us to confirm the positive shielding effect provided by the selected emulsification/solvent evaporation methodf ollowed by the LbL encapsulationp rocess.…”

“…However,t he exact determination of the QY value for up-converting materials is complex, mostly due to the lack of standard reference fluorophores and experimental setups, and requires careful validation of all intermediate steps, such as, among many others:c onstruction of ad edicated measurementss etup, the choice of appropriate excitation power range, selection of reference material ands ample concentration. [50][51][52] There are several papers availablei nt he literature reporting the QY values for different types of up-converting nanomaterials (Table S2, SupportingI nformation) to be in the range of 3a nd 0.0022 %f or core-onlyc olloidal NaYF 4 NPs with sizes over 100 nm and below 10 nm, respectively. [28,50,51,[53][54][55][56] In the case of the nanocarrierss tudied by us, the QY of the loadedN Cs (QY NCs )w ill mainly dependo nt he corresponding value for as-synthesized NaYF 4 :Tm 3 + ,Y b 3 + NPs, with additional quenching introduced by the encapsulation process and water environment.…”