Luminescence properties of lanthanide-doped nanocrystal labels

Abstract

The aim of this thesis is to incorporate luminescent labels in nanoparticles and study their optical properties under various conditions. This helps in the development of new luminescent labels for nanoparticles that can be used to trace the distribution of nanoparticles in the environment and organisms. Lanthanide ions incorporated into a nanocrystalline host are promising as luminescent label since the narrow emission lines at specific energies can be used to create a variety of unique luminescent labels by changing and combining the nature of the lanthanide ion incorporated into the nanocrystalline host lattice. In addition, the stability of the lanthanide doped crystals is high which makes the labels robust. For this reason, lanthanide ions doped into a variety of nanocrystalline host lattices with different sizes are synthesized and their optical properties are investigated under various conditions to obtain a better insight in the optical properties of lanthanide ions in nanocrystals. First, a method is described to grow silica around LaPO4 nanocrystals. The LaPO4 nanocrystals have sizes ranging from 4 to 8 nm and are doped with europium or cerium and terbium. After silica growth, monodisperse silica spheres are obtained with a LaPO4 nanocrystal in the center. The size of the silica spheres can be tuned between 25 and 55 nm by addition of small volumes of methanol during the silica growth reaction. Next, the influence of the size of the host material on the optical properties of lanthanide doped crystalline materials is investigated. The influence of disorder in nanocrystals on the optical properties of lanthanide dopants is investigated by studying the inhomogeneous linewidth and luminescence decay curves for LaPO4:Eu3+ samples of different sizes (4 nm to bulk) and core-shell configurations (core, core-isocrystalline shell and core-silica shell). The emission linewidths increase strongly for nanocrystals. Luminescence decay curves reveal an increased non-radiative decay rate for Eu3+ in nanocrystals. The effects are strongest in core and core-silica shell nanocrystals and can be reduced by growth of an isocrystalline LaPO4 shell. Moreover, the role of phonon confinement in lanthanide doped nanocrystals is investigated. High resolution emission spectra at temperatures down to 2.2 K are reported for various Ln3+ ions (Er3+, Yb3+, Eu3+) doped into monodisperse 10 nm NaYF4 nanocrystals and compared with spectra for microcrystalline material. No evidence for phonon bottleneck effects is found in the emission spectra. Emission from closely spaced higher Stark levels is observed only at elevated excitation powers and explained by laser heating. Finally, the robustness of various types of nanoparticles that can be used in correlative microscopy is investigated. The luminescence of 130 nm dye-labelled silica, 15 nm quantum dots and 230 nm Y2O3:Eu3+ are measured for (single) nanoparticles after various cycles of electron beam exposure. The dye-labelled silica nanoparticles and quantum dots are quenched after exposure to low doses, while Y2O3:Eu3+ NPs are robust and continue to show luminescence after five times the low doses. In addition, it is shown that the intensity loss does not depend on the electron flux, but on the total electron dose.