Excitation power dependent population pathways and absolute quantum yields of upconversion nanoparticles in different solvents

Abstract

The rational design of brighter upconversion nanoparticles (UCNPs) requires a better understanding of the radiationless deactivation pathways in these materials. Here, we demonstrate the potential of excitation power density (P)-dependent studies of upconversion (UC) luminescence intensities, slope factors, and absolute quantum yields (Φ) of popular β-NaYF:20% Yb,2% Er UCNPs of different surface chemistries in organic solvents, DO, and water as a tool to gain deeper insight into the UC mechanism including population and deactivation pathways particularly of the red emission. Our measurements, covering a P regime of three orders of magnitude, reveal a strong difference of the P-dependence of the ratio of the green and red luminescence bands (I) in water and organic solvents and P-dependent population pathways of the different emissive energy levels of Er. In summary, we provide experimental evidence for three photon processes in UCNPs, particularly for the red emission. Moreover, we demonstrate changes in the excited population dynamics via bi- and triphotonic processes dependent on the environment, surface chemistry, and P, and validate our findings theoretically.

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

“…The solid lines pointing upwards represent energy absorption, the dashed lines represent energy‐transfer processes, the wavy lines represent nonradiative relaxation processes, the blue dotted lines represent cross‐relaxation (CR) processes between excited Er 3+ species, and the solid lines pointing downwards represent visible and NIR emissions. The excitation energy transfer from Yb 3+ ions to Er 3+ ions occurs by long‐range energy migrations from the Yb 3+ absorption site through the Yb sublattice …”

“…The excitation energy transferf rom Yb 3 + ions to Er 3 + ions occursbyl ong-range energy migrationsf rom the Yb 3 + absorption site through the Yb sublattice. [67] (UCNP core @NaYF 4 )w ere also evaluated. Typically,t he vibrational coupling interaction between the solvent( or ligand)a nd the lanthanide ions resultsi nad ecrease in the UC emission.T his coupling is much more effective for the 2 F 5/2 level of the Yb 3 + ions or the 4 I 11/2 and 4 I 13/2 levels of the Er 3 + ions, which are crucial for the population mechanism of the state responsible for the UC emission ( 4 F 9/2 , 2 H 11/2 ,o r 4 S 3/2 ).…”

NaYF4:Yb,Er/NaYF4 Core/Shell Nanocrystals with High Upconversion Luminescence Quantum Yield

“…A systematic variation with the particles size is not only observed for the quantum yield and the luminescence lifetimes but also for the red‐to‐green ratio in the emission spectrum. It is well established that the red‐to‐green ratio strongly increases with increasing P and this dependence is observed for all our samples (Figure ) . Figure shows that the cross‐over point where the red and the green emission band have the same intensity, shifts systematically to lower P with increasing particle size and, hence, increasing quantum yield.…”

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

“…Corresponding values for red (800 nm) Tm 3 + ion upconverted emission were equal to approximately % 57 and % 66 %. [57] Thus, for the purpose of rough estimationofthe influence of the performed functionalization process on the spectroscopicp arameters characterizingt he studied luminescent labels, we expect the QY value of NaYF 4 :Tm 3 + , Yb 3 + NPs used in this work as al uminescent cargo for encapsulationw ith % 8nmi ns ize and ac ubic structure, to be in the range of approximately 0.005 %-0.001 %( referred to as QY NPs ). 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.…”

“…[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. [57] Thus, for the purpose of rough estimationofthe influence of the performed functionalization process on the spectroscopicp arameters characterizingt he studied luminescent labels, we expect the QY value of NaYF 4 :Tm 3 + , Yb 3 + NPs used in this work as al uminescent cargo for encapsulationw ith % 8nmi ns ize and ac ubic structure, to be in the range of approximately 0.005 %-0.001 %( referred to as QY NPs ). [52,53] Those values can be used as ar eference for calculation of the corresponding parameter characterizing the loaded As already mentioned, the co-encapsulation of NaYF 4 :Tm 3 + , Yb 3 + NPs and VP molecules gives the possibilityt ot rigger the anticancer treatment with NIR light within the so called "biological window"o fc ellsa nd tissues.F igure 4a shows the upconversion emission spectra of the NC water suspensions coloaded with NaYF 4 :Tm 3 + ,Y b 3 + NPs and increasing amount of VP (concentration range:0mm up to 1335 mm).…”