Chapter Conclusions and Recommendations 6.1 Conclusions In this thesis, I demonstrated the effects of surface, surface coatings, and plasmonic effects of metallic nanoshells on the fluorescence of the UC nanoshells. The results showed the UC fluorescence enhancement could be achieved by surface coatings and plasmonic effects. UC nanoshells with hcp crystal structure were successfully synthesized. The interior cavity and UC shell thickness were ~7 nm and ~4 nm, respectively. The effects of surface on the fluorescence of the UC nanoshells were investigated. The results demonstrated the total emission intensity of the UC nanoshells significantly decreased when compared to that of the corresponding solid nanoparticles (~15 nm) since the nanoshells had a higher UC active volume-normalized surface area. However, it was demonstrated that the total emission intensity of the UC nanoshells could significantly be increased by surface coatings of undoped NaYF4. The surface coatings of ~3 nm undoped NaYF4 on both the inner and outer surfaces of the UC nanoshells led to an emission enhancement to ~19 and ~5 times compared to that of the UC nanoshells and the solid UC (~15 nm) nanoparticles, respectively. To study the plasmonic effects on the fluorescence properties of UC nanoshells, Au-Ag metallic nanoshells were synthesized via galvanic replacement reaction between Ag templates and HAuCl4. The shell thickness of the Au–Ag nanoshells was controlled by HAuCl4 concentration. The shell thickness increased from ~5 nm to ~10 nm for the equiaxed Au-Ag nanoshells 103    (~39-nm interior cavity) and ~5 nm to ~8 nm for the triangular prismatic AuAg nanoshells (~52-nm interior edge length). Their LSPR extinction peak was tunable from ~670 nm to ~840 nm. Investigations on the nanoshell formation showed the transformation from Ag templates to Au-Ag nanoshells was sizedependent. The assembly of Au-Ag nanoshell layer/silica film/UC nanoshell layer was prepared to investigate the plasmonic effects of Au-Ag nanoshells on the fluorescence of UC nanoshells. This study showed the fluorescence of the UC nanoshells was either enhanced or quenched by Au-Ag nanoshells, depending on the silica film thickness and the surface coverage % of the Au-Ag nanoshell layer in the assembly. For fixed silica thickness, the UC fluorescence showed nonlinear changes with the surface coverage of Au-Ag nanoshells. At Au-Ag surface coverage lower than 22%, the total UC emission intensity increased with the surface coverage % due to the increase of local field intensity enhancement. However, when the surface coverage of Au-Ag nanoshell layer further increased from 22% to 46%, the UC fluorescence intensity decreased due to the photothermal effects of Au-Ag nanoshells. For the assembly with a fixed surface coverage of Au-Ag nanoshell layer, the transitions from UC fluorescence quenching to enhancement and subsequent unenhanced UC emission intensity were observed with increasing thickness of silica film. This study demonstrated a maximum UC fluorescence enhancement could be obtained by controlling the distance from Au-Ag nanoshells to UC nanoshells and the surface coverage % of Au-Ag nanoshell layer. 104    6.2 Recommendations for future study This work showed the surface effects of UC nanoshells significantly decreased the UC emission intensity compared with their solid nanoscale counterparts. By using the protection of undoped NaYF4 shells and plasmonic effects, their UC emission intensities could be enhanced. However, these enhanced emission intensities of nanostructures were still only ~1 – % compared with that of their solid bulk counterparts. Thus, it warrants further study for the emission enhancement, for example, using more efficient host and investigation on the distribution of Yb and Er ions in NaYF4 host. The UC nanoshells had the interior cavity ~7 nm and shell thickness ~4 nm, making it a good object to study the surface effects due to their large surface area. The formation mechanism of the UC nanoshells involved vacancy diffusion, likely due to the Kirkendall effect and Ostwald ripening mechanism. However, the diffusion of ions in the host during the formation of such small nanoshells is not yet well-understood yet. The diffusion of ions should be investigated in future study. This study demonstrated the concentration and distance dependences of plasmon-enhanced UC fluorescence. An optimum fluorescence enhancement was obtained by controlling the distance between the UC nanoshells and Au-Ag nanoshells and the concentration of Au-Ag nanoshells on the substrates. However, the UC fluorescence quenching efficiency due to non-radiative losses to Au-Ag nanoshells was not quantitatively measured because of the complex UC process. Further study should investigate the concentration- and distance-dependent non-radiative losses. 105    . distance from Au- Ag nanoshells to UC nanoshells and the surface coverage % of Au- Ag nanoshell layer. 105  6. 2 Recommendations for future study This work showed the surface effects of UC nanoshells. the plasmonic effects of Au- Ag nanoshells on the fluorescence of UC nanoshells. This study showed the fluorescence of the UC nanoshells was either enhanced or quenched by Au- Ag nanoshells, depending. nanoshells, Au- Ag metallic nanoshells were synthesized via galvanic replacement reaction between Ag templates and HAuCl 4 . The shell thickness of the Au Ag nanoshells was controlled by HAuCl 4 concentration.