Rare-earth upconversion nanoparticles, such as those composed of lanthanide-doped NaYF4,
can convert two or more low-energy photons at longer wavelength into one high-energy photon
at a shorter wavelength. Solar water splitting devices that incorporate them can therefore
harvest otherwise lost photons in the near-infrared region of the electromagnetic spectrum.
The conversion e ciencies of these nanoparticles have, however, been extremely poor to date,
eliminating any potential bene t (with respect to the photocurrent of water splitting devices
themselves) of being incorporated into photoelectrode devices. Towards overcoming this impasse,
the aim of this Ph.D project is to investigate the feasibility of a hybrid photoelectrode system
where upconverting nanoparticles are coupled to a plasmonic structure and conjugated with
dye molecules that absorb below bandgap photons. The synergistic enhancement e ect of
plasmonic and broadband absorption of dye molecules could signi cantly improve the conversion
e ciency of lanthanide-doped NaYF4. The dye molecules act as a sensitiser: they absorb
near-infrared light over a broad range and subsequently transfer the associated energy to the
their upconverting nanoparticles via F orster resonance energy transfer. On the other hand, the
plasmonic components enhance both the upconversion process and F orster resonance energy
transfer process, by increasing the incident electromagnetic eld intensity and the radiative
emission rate via surface plasmon resonance.
In this project, hexagonal phase NaYF4 co-doped with Yb3+ and Er3+ upconversion nanocrystals
(UCNPs) were synthesized. Plasmonic enhanced upconversion; dye-sensitised upconversion,
and nally plasmonic broadband dye-sensitised upconversion were investigated using Au nanodisk
(AuND) 2D arrays. The enhanced upconversion were observed with 26-fold and 19-fold
enhancements for green and red emission on AuND arrays, respectively. In parallel, single
infrared dye IR808 and multiple infrared dyes were conjugated to UCNPs, with 5.5-fold and
3.9-fold enhancements observed in green emission. Finally, plasmonic enhanced broadband
upconversion with 10-fold enhancement was recorded. For proof of concept, UCNPs and
three-dye-sensitised UCNPs coupled with AuND arrays were deposited on hematite-based photoelectrodes.
Photocurrent was clearly obtained from UCNPs coupled hematite on AuND arrays
when excited below the bandgap of hematite. The proof of concept established in this thesis
could not only nd applications in arti cial solar water splitting, but also has huge potential in
applications such as biological sensing, imaging and therapy based on upconversion nanoparticles.