Thesis (Ph.D.)--University of Colorado at Boulder, 2020
Includes bibliographical references
This thesis studies the photoluminescence (PL) enhancement of lanthanide-doped upconversion nanoparticles (UCNPs). In order to improve the upconversion luminescence from UCNPs, we first exploit localized surface plasmon resonance (LSPR) that were provided by noble metal nanoparticles. The experimental study on the PL performance of Au-UCNP nanoclusters in colloidal solution shows the molar concentration ratio between Au and UCNP plays an essential role in PL results. The absorption enhancement as well as the emission enhancement due to the amplification of local electric field near the metal particles' surface are also quantified in simulation and qualitatively explain the observations in experiments. The rapid temperature increase in the solution environment upon near-infrared (NIR) excitation demonstrates the possibility of the nanocluster sample being used as a photothermal agent. In addition, the thesis studies the luminescence performance of regiospecific nano-assemblies of TiO2 capped gold nanorod (AuNR) and UCNP at single particle level. This method is proven not only to alleviate the ensemble average effect that was inevitable in colloidal solution, but also takes the most advantage of the highly enhanced local field of LSPR. Upconversion luminescence is observed to increase twofold. The polarization dependent PL illustrates the important role of incident polarization direction on affecting LSPR. The latter part of this thesis explores the luminescence enhancement effect by incorporating UCNPs into a two-dimensional (2D) photonic crystal (PhC) structure with a square lattice of air hole arrays. A self-assembled method enables UCNPs being selectively filled in the holes. By tuning the band edge frequency to the absorption wavelength of sensitizer ions, the electric field is highly concentrated inside the voids. The absorption enhancement leads to more than two orders of magnitude PL enhancement. The quenching effect was proved to be much less than plasmonic structure that is made of metal. The optimization of the 2D PhC structure for upconversion enhancement was explored in the final portion by performing calculation on finite-difference time-domain (FDTD) based simulation software. We observe spectral shift of the resonances and change of field concentration while the structural parameters, such as hole diameter, periodicity and slab thickness are varied. This study enables further improvement of the enhancement performance
Electronic reproduction. Ann Arbor, Mich. : ProQuest, 2020
