Tuning Lanthanide Luminescence in Core-Shell Nanostructures for Technological Applications

基於核殼型稀土摻雜納米結構的發光調控及應用研究

Student thesis: Doctoral Thesis

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Award date5 Jun 2018

Abstract

Downshifting, upconversion, and downconversion luminescence that exploit f-f and/or f-d transitions in lanthanide ion have been extensively studied in the past few decades. Recent research focuses on lanthanide luminescence is mainly shifted to lanthanide-doped core–shell nanostructures comprising spatially confined dopant ions. By coating an inert shell to lanthanide-doped core nanoparticles, the emission intensity can be substantially enhanced. More intriguing hierarchical active core/active shell nanostructure can not only provide an elegant solution to the problem of uncontrolled cross-relaxation processes between incompatible lanthanide ions but also permits the integration of dissimilar luminescence process into a single nanoparticle. Furthermore, by the combination of core−shell nanostructural engineering and energy migration, unprecedented control of lanthanide luminescence can be achieved. These advances in core−shell nanofabrication have enabled unprecedented control over nanoparticle optical properties, which in turn has greatly enhanced the capability of lanthanide luminescence in various technological applications.

The thesis begins with a review of recent progress in the development of core–shell lanthanide-doped nanoparticles with particular emphasis on the nanostructured materials and their applications in the fields of biomedicine. Moreover, the precise control over complex energy transfer among lanthanide ions in doped in core–shell nanoparticles opened up exciting new opportunities for various technological applications such as anti-counterfeiting and photovoltaic, which are also summarized in this chapter.

In chapter 4, we studied how emission enhancement factor due to the surface coating is affected by the concentration of Yb3+ sensitizer and excitation power in NaYF4:Yb/Er (8-38/2 mol%)@NaYF4 core–shell nanoparticles. We found that the enhancement factor varies due to the complex electronic transitions involved in the upconversion processes. In general, the enhancement factor declines with decreasing the Yb3+ concentration and increasing the excitation power, as a result of indeterministic surface quenching processes in the naked core nanoparticles. Our findings are vital for rational design of UCNPs to cater for diverse applications.

In chapter 5, we experimentally demonstrated quantum cutting in core–shell nanoparticles doped with a set of lanthanide ions in separate layers to eliminate deleterious cross-relaxations. We prepared NaGdF4:Ce@NaGdF4:Nd (or Nd/Yb)@NaGdF4 core–shell nanostructures. Ce3+ ion was employed as the sensitizer and then transfer its energy to Gd3+ ions. The excitation energy migrated through a Gd3+ sublattice and finally captured by Nd3+ and then Yb3+ ions, resulting in tunable downconversion emission from Nd3+ and Yb3+ ions. We also demonstrated the use of the quantum cutting nanoparticles as spectral converters to improve the solar cell performance since the original unused UV portion of the solar energy was again harvested by the downconversion nanoparticles on the top of the conventional c-Si solar cell. As a result, a 1.2-fold and 1.4-fold increase in short-circuit current and power conversion efficiency were achieved respectively. These studies will enhance our ability to control spectral downconversion using lanthanide ions and pave the way for application in extraterrestrial power plants and cost-effective photodetectors operating at short wavelengths.

In chapter 6, we developed a novel strategy for encrypting anti-counterfeiting patterns by taking advantage of highly tunable lanthanide luminescence. The pattern was fabricated by photolithography technique with the mixture of luminescence core–shell nanotaggants and SU-8 as the photoresist. The luminescent core–shell nanotaggants were encrypted to be excited with distinct wavelengths (980 nm, 808 nm, and 254 nm). Decryption was achieved by examining the temporal color response of the pattern to those three wavelengths of illuminations and results in a covert sequence. The covert color sequence can secure a high covert level of authentication because a genuine pattern can be guaranteed only if the desired three luminescence color sequence is exactly right. In addition, this technique also features forensic level of authentication since extra luminescent information can be extracted by a spectrometer. We further employed a serial contact printing processes to fabricate spatially encoded anti-counterfeiting patterns. Under 980 or 808 nm excitation, the spatially encoded nanotaggant in different graphical pattern can respond and give the corresponding emission output. Our invented encryption technique, which introduces new optical codes for authentication, can substantially increase the difficulty of duplication and thus to provide extra high-level security protection in many business applications.

    Research areas

  • Lanthanide, Upconversion, Downconversion, Luminescence, Core–shell, Nanoparticles, Anti-counterfeiting, Photovoltaic, Solar cell