Chemical Structure and Upconversion Enhancement of NaYF₄ Nanocrystals and Nanosheets
- Publication Type:
- Thesis
- Issue Date:
- 2020
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The ability of lanthanide ions to convert lower energy photons to higher energy photons through the process known as upconversion presents as an opportunity to overcome limitations in sensitivity, efficiency and selectivity present for a wide variety of applications such as bio-imaging, photovoltaics and anti-counterfeiting.
Sodium Yttrium Fluoride (NaYF₄) is an inorganic insulator with low phonon energy (360 cm⁻¹ ≈ 45 meV), wide band gap (8.5 eV) and high chemical stability. These properties make NaYF₄ an ideal optical host crystal for lanthanide dopants to produce upconversion nanoparticles (UCNPs). The shape, size and structure of UCNPs can be highly controlled allowing them to be tailored to the requirements of specific applications.
Key challenges of concentration quenching and low quantum yield still confront UCNPs before their potential can be fully realised. Accordingly, enhancing the efficiency of UCNPs and their brightness has been the focus of numerous studies. In this project these challenges facing UCNPs are addressed by material characterisation and optimisation based on a comprehensive understanding of the chemical and optical properties of the material.
Firstly, synchrotron-based XPS and NEXAFS along with EDS and ICP-MS characterisation techniques were employed on a range of UCNPs with sizes from 13 nm - 51 nm and different lanthanide (Ln³⁺) concentrations of 20% - 60% to determine how lanthanides are distributed within each nanocrystal. This analysis reveals a radial gradient distribution of Yb³⁺ and Tb³⁺ exists from the core to the surface of the NaYF₄ UCNPs, regardless of their size or lanthanide dopant concentration. The active core structure of this distribution was then systematically correlated to the optical properties of UCNPs with different sizes revealing a trend of increased optical upconversion emission efficiency by smaller sized UCNPs.
Secondly, surface plasmon coupling was achieved between core-shell UCNPs and dewetted gold nanoparticles by precisely growing NaYF₄ shell coatings of varied thickness from 5 nm - 15 nm around the optically active core UCNPs. The local surface plasmon of the gold nanoparticles could be controlled and coupled with the internal transitions of the Er³⁺ ions. Combining these inert shelled UCNPs and plasmonic gold nanoparticles produced a shell thickness dependent enhancement with five times enhanced upconversion emission from the core-shell UCNPs with a shell thickness of 10 nm.
Thirdly, bulk NaYF₄ microparticles were successfully exfoliated to make 2D NaYF₄ nanosheets. By using a simple soft exfoliation method without the need for intensive ultrasonication atomically flat optically active nanosheets down to a single monolayer were produced. Extensive characterisation of the nanosheets supported by DFT calculations reveals a phase change and 1 eV band gap reduction for this new 2D material compared to the bulk.
Finally, in an appendix to demonstrate a massive three orders of magnitude increase in emission from ultra-small Tm³⁺ doped UCNPs an anti-counterfeiting mark was designed and fabricated. This enhancement only occurs at high temperature as activated surface phonons become efficient energy pathways for energy migration. The temperature dependence of the enhancement allowed the anti-counterfeiting mark to be temperature responsive.
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