Plasmon-enhanced upconversion emissions from lanthanide-doped nanocrystals hybridized with metal nanoparticles

PolyU Library Call No.: [THS] LG51 .H577M AP 2016 Heviii, 119 pages :color illustrationsLanthanide-doped upconversion (UC) nanocrystals (NCs), which can absorb near-infrared light and emit visible light via an anti-Stokes process, have attracted great interest due to potential applications ranging from biomedical imaging to photovoltaics. However, UCNCs often face low emission efficiency and strong luminescence quenching in aqueous solvents, and these limitations significantly hinder their practical utilization. Many methods have thus been developed to overcome these weaknesses: One of the most effective approaches is to couple UCNCs with localized surface plasmon resonances (LSPRs), which are collective oscillations of free electrons in metal nanoparticles. In this thesis, I employed gold nanorods (GNRs) as a plasmonic nanostructure to enhance the UC luminescence from CaF₂:Yb³⁺,Er³⁺ UCNCs by precisely controlling the distance between them. In addition, I studied plasmon-modulated UC emission properties, such as polarization state, at single nanoparticle level. Finally, I performed finite-difference time domain (FDTD) simulations to understand the physical mechanisms responsible for experimental observations. The main results are summarized as following. 1. Mono-dispersed, sub-10-nm CaF₂:Yb³⁺,Er³⁺ UCNCs were synthesized via a sodium co-doping co-precipitation route. GNRs were grown in aqueous solution using a seed-mediated method and the aspect ratio of the GNRs was finely tuned by anisotropic oxidation to align the longitudinal and transverse LSPR wavelengths of the GNRs with the red and green emission bands of the CaF₂:Yb³⁺,Er³⁺ NCs, respectively. In order to systematically investigate the influence of the GNRs-UCNCs distances on the UC luminescence intensity, I attached the UCNCs on the surface of GNRs covered with dielectric spacers of different thicknesses. For the spacer thicknesses below 9 nm I utilized a layer-by-layer self-assembly method to coat the GNRs with polyelectrolyte shells. I observed a significant UC luminescence quenching for the spacer thickness below 2 nm and a moderate enhancement for the spacer thicknesses between 2 and 9 nm. However, the non-uniform coating of the polyelectrolyte layers prevented us from achieving more conclusive results about an exact spacer thickness for an optimal plasmonic enhancement. To obtain a series of spacer thicknesses in the region between 9 and 35 nm, I coated the GNRs with silica layers of various thicknesses via a modified Stober method. The UCNCs were then adsorbed to the GNRs through electrostatic attraction to form GNR@SiO₂@CaF₂:Yb³⁺,Er³⁺ hybrid nanostructures. The largest intensity enhancement of the green UC emission was achieved with a factor of 3.0 for the case of 19-nm-thick silica spacer while that for the red emission was 6.9 when the thickness of the silica spacer was 23 nm.2. I measured both scattering and fluorescence properties of an individual GNR@SiO₂@CaF2:Yb³⁺,Er³⁺ hybrid nanostructure using a home-built dark-field microscope system. The scattered light was found to be polarized along the long axis of the GNR core. This is due to the excitation of the longitudinal LSPR of the GNR, which can be regarded as an electric dipole along the long axis of the GNR. In the fluorescence measurements, the polarization of the excitation laser was kept either parallel or perpendicular to the long axis of a selected single hybrid nanostructure, and the polarization states of both red and green fluorescence emissions were investigated. I found that the interplay between the two emission bands of the UCNCs and the two orthogonal plasmonic modes of the GNR influenced the polarization response of the UC luminescence. To explain this phenomenon, I put forward a theoretical analysis based on Forster resonance energy transfer theory. 3. I performed three-dimensional (3D) FDTD simulations to understand the underlying physical mechanisms responsible for the experimental observations. The calculated plasmonic electric field intensity profiles were used to reveal the enhancement distribution around a GNR@SiO₂ nanostructure. I further simulated the UC emission properties of the UCNCs around a GNR@SiO₂ nanostructure by modelling each UCNC as a Hertzian dipole that was located at different positions with respect to the GNR. The calculated radiation for the dipole was found to be polarized and the results were in good agreement with the experiments. Moreover, the 3D far-field emission patterns of a dipolar source placed at different distances from the GNR were computed. With decreasing the distance in between, the polarization state of the fluorescence emission resembled more and more that of the plasmonic dipole radiation pattern of a bare GNR, further corroborating the plasmon-modulated polarized UC emissions.Department of Applied PhysicsM.Phil., Department of Applied Physics, The Hong Kong Polytechnic University, 2016MPhi