Upconversion quantum yield and luminescence of beta-NaYF4:Yb3+,Er3+ nanoparticles

Lanthanide-based upconversion (UC) nanometer (nm)-sized particles (UCNPs) exhibit the unique ability to emit one higher-energy photon after the sequential absorption of two or more near-infrared (NIR) photons. This UC luminescence (UCL) makes UCNPs very attractive for the use as optical probes in the biomedical area. In this respect, the main advantages of UCNPs are the suppression of environment-based background fluorescence as well as their excitability with NIR-light allowing imaging applications in deep tissue layers of several centimeters (cm). The future progress in the development of brightly-luminescent UCNPs requires reliable quantification methods of their optical properties to ensure comparability on an international level. For this purpose, in the first study of this thesis an integrating sphere setup (ISS), as well as corresponding relevant guidelines for the measurement conditions, has been developed enabling the absolute determination of the key parameter upconversion quantum yield &Phi UC with minimum uncertainty (QYUC := ratio of emitted high-energy photons to absorbed low-energy photons). The main features of the developed ISS setup present the wide tunability of P of over four orders of magnitude and a high linear detection range of over 10 orders of magnitude allowing the comprehensive characterization of the nonlinear luminescence behavior of UC materials. As the main subject of the investigations of this thesis served a UC system based on a transparent host lattice beta-NaYF4, which was doped with trivalent ytterbium ions (Yb3+), acting as absorber/antenna, and trivalent erbium ions (Er3+), acting as emitter. The suppression of radiationless deactivation processes via the particle surface presents a major challenge in the development of brightly-luminescent UCNPs. In order to quantify the luminescence quenching of UCNPs, commercial µm-sized beta-NaYF4:Yb3+(21%),Er3+(2%) particles (UCµP), with particle diameters large enough to neglect surface effects, were comprehensively optically characterized. In this respect, a maximal absolute QYUC of 10.5% at P = 30 Wcm-2, considering the spectral region from 360 - 900 nm, was measured for these UCµP. New detailed insights of the UC processes of beta-NaYF4:Yb3+,Er3+ UCNPs were gained in the second and third study by the investigation of high-quality UCNPs sample series with systematical varied parameters. These UCNPs sample series were optically characterized regarding their P-dependent QYUC and UCL of the different Er3+ emission bands as well as of the decay behavior of the intensity of the Er3+ and Yb3+ emission bands. The computer-assisted simulation of these experimental data by utilizing a coupled rate equation system turned out to be a powerful tool to underpin the photophysical interpretations. A special highlight of this advanced analysis represents the clarification of the conditions for the controversially debated bi- and triphotonic population pathways of the red-emitting Er3+ 4F9/2 energy level. In the following, the main results of these studies are briefly presented. The second study deals with 23 nm-sized UCNPs with different surrounding environments, its interaction with the particle surface, and the accompanied influence on the UC processes. A main conclusion of this study points out that for UCNPs dispersed in water (H2O), at low P, the Er3+ 4F9/2 energy level is populated biphotonically by a nonresonant Yb3+-Er3+ energy transfer (ET) from the low energetic Er3+ 4I13/2 energy level. Thereby, the high population density of the low Er3+ energy levels is favored due to the coupling of excited Er3+ ions near the UCNPs particle surface with the O-H vibrational modes of the H2O molecules. However, the use of very high P of ca. 1kWcm-2 compensates H2O-induced increase of nonradiative relaxation rates, and consequently, leads to moderately high QYUC of about 0.5% with a mainly triphotonic activation of the Er3+ 4F9/2 energy level for the UCNPs dispersed in H2O. In the third study, the optical properties of 33 nm-sized UCNPs dispersed in toluene as function of the ion-ion distances were investigated. These distances can be directly tuned by variation of the Yb3+ and the Er3+ dopant concentrations. In this respect, it was demonstrated that the increase of the Yb3+ dopant concentration causes an enhancement of the triphotonic activation of the red-emitting Er3+ 4F9/2 energy level. This effect is induced by an increased back energy transfer from Er3+ to Yb3+ due to the reduction of the Er3+-Yb3+-distance. Although the QYUC is reduced by a faster energy migration to the UCNP surface, the overall UCL intensity was overcompensated due to the higher number of absorbing Yb3+ ions. However, the increase of the Er3+ concentration resulted in different trends of the relative Er3+ red emission intensity. The intensity increase, observed at low P, was attributed to increased biphotonic activation by enhanced nonradiative relaxation of the green-emitting Er3+ 2H11/2/4S3/2. Contrary to that, at high P, the Er3+ red emission intensity diminishes when the Er3+ concentration increases. Consequently, a rate equation analysis revealed that this UCL behavior is a sign of a yet unknown depopulation rate.

Author dc:creator

• Kaiser, Martin

Advisors dc:contributor.advisor

• Hoffmann, Axel
• Resch-Genger, Ute

Licence dc:rights.uri

• http://rightsstatements.org/vocab/InC/1.0/

Language dc:language.iso

en

Identifier URI

http://dx.doi.org/10.14279/depositonce-11457

OAI identifier oai:identifier

oai:depositonce.tu-berlin.de:11303/12647