title: 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 (ΦUC) of popular β-NaYF4:20% Yb3+,2% Er3+UCNPs of different surface chemistries in organic solvents, D2O, 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 (Ig/r) in water and organic solvents and P-dependent population pathways of the different emissive energy levels of Er3+. 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.
authors.affiliation: BAM Federal Institute for Materials Research and Testing, Division 1.10 Biophotonics, Richard-Willstaetter-Str. 11, 12489 Berlin, Germany. ute.resch@bam.de.
authors.forename: C
authors.identifier: http://orcid.org/0000-0002-0204-9727
authors.initials: C
authors.lastname: Würth
authors.affiliation: BAM Federal Institute for Materials Research and Testing, Division 1.10 Biophotonics, Richard-Willstaetter-Str. 11, 12489 Berlin, Germany. ute.resch@bam.de.
authors.forename: M
authors.initials: M
authors.lastname: Kaiser
authors.affiliation: Institute of Analytical Chemistry, Chemo- and Biosensors, University of Regensburg, 93040 Regensburg, Germany.
authors.forename: S
authors.initials: S
authors.lastname: Wilhelm
authors.affiliation: BAM Federal Institute for Materials Research and Testing, Division 1.10 Biophotonics, Richard-Willstaetter-Str. 11, 12489 Berlin, Germany. ute.resch@bam.de.
authors.forename: B
authors.initials: B
authors.lastname: Grauel
authors.affiliation: Institute of Analytical Chemistry, Chemo- and Biosensors, University of Regensburg, 93040 Regensburg, Germany.
authors.forename: T
authors.identifier: http://orcid.org/0000-0002-4224-4762
authors.initials: T
authors.lastname: Hirsch
authors.affiliation: BAM Federal Institute for Materials Research and Testing, Division 1.10 Biophotonics, Richard-Willstaetter-Str. 11, 12489 Berlin, Germany. ute.resch@bam.de.
authors.forename: U
authors.initials: U
authors.lastname: Resch-Genger