Exploring the interactions among lanthanides in upconversion nanoparticles toward enhancement of photoluminescence brightness

Lanthanides-based upconversion nanoparticles (UCNPs) exhibit unique optical properties such as large anti-Stokes shifts, long luminescence lifetime, and excellent photostability, offering considerable potential for emerging applications ranging from biomedical science to information technology. In practice, many of these applications will benefit from bright upconversion luminescence, especially under low excitation irradiance, which is nevertheless difficult to realize in conventional UCNPs. 

Using the Yb/Tm co-doping system as a model, my Ph.D. thesis aims to achieve a comprehensive understanding of the photoluminescence process in the UCNPs, and develop better designs towards enhanced brightness. 

Firstly, I explored the influence of the ratio between Yb³⁺ and Tm³⁺ ions on brightness by measuring the excitation-emission curves in the fully doped nanoparticles. It was found that more Yb³⁺ ions lead to lower excitation power for the same emission intensity, while a higher concentration of Tm³⁺ ions resulted in a larger number of emission photon counts at the saturation condition. Accordingly, the doping concentration of Yb³⁺ and Tm³⁺ ions can be judiciously modulated for specific applications that have different requirements for the excitation irradiance.

Secondly, I inspected the passivation effect of inert-shell coating in 8% Tm-doped NaYbF₄ nanoparticles. Both experimental and modeling results showed that a thick shell (>5 nm) is required to eliminate the surface (concentration) quenching pathway. As a result, to reach the same level of brightness, the excitation irradiance can be reduced by between 2.8-fold (for the 455 nm emission) and 6.2-fold (for the 800 nm emission) for the core-shell nanoparticles as compared to the core nanoparticles, although the emission intensity at saturation was only raised slightly. It was also demonstrated that the enhancement factor of inert-shell coating on upconversion luminescence depends crucially on the diameter of the core nanoparticles, which factor has been largely overlooked in previous studies and prevented objective benchmarking. The quantitative inspection of inert-shell coating provides valuable guidance for optimizing the brightness of UCNPs based on their power-dependent performance. 

Lastly, I investigated the potential quantum coherence among Yb³⁺ ions in these nanoparticles. A long luminescence lifetime was observed in the powder sample of NaYbF₄@NaYF₄ coreshell nanoparticles, whereas much shorter lifetimes were obtained from individual nanoparticles under the same excitation condition. Moreover, the lifetime was inversely proportional to the excitation irradiance and/or duration. I further built a Hanbury-Brown and Twiss interferometer to record the arrival times of the emission photons from single NaYbF₄@NaYF₄ nanoparticles to calculate second-order coherence at both room and cryogenic (6 K) temperatures. The cross-correlation results at zero time-delay, g²(0), presented a distinctive peak, suggesting that a collective ensemble state of Yb³⁺ ions is likely formed in a core-shell nanoparticle, yielding possible superradiance under high excitation irradiance. 

The investigations performed in this thesis provide practical references for enhancing the upconversion luminescence of UCNPs and optimizing their design for different applications. It is expected that further evaluation from a quantum mechanical perspective holds the key to major breakthroughs in creating bright UCNPs of the next generation.