Lanthanide-doped nanocrystals have attracted considerable interest in recent years due to a plethora of potential applications such as dual-modal detection and therapy of cancer cells, stimulators in optogenetics, energy harvesting elements of solar cells and quantum information processing. While these applications are promising, their use is hindered by a lack of understanding of fundamental physical properties.

This thesis focuses on improving the understanding of one such lanthanide doped nanocrystal, KY3F10, through the use of high-resolution infra-red spectroscopy with magnetic fields of up to 4 T, and site-selective laser fluorescence and excitation for both probing the electronic structure and energy transfer dynamics in KY3F10.

X-ray diffraction of multiple lanthanide ions doped into KY3F10 is presented (Nd3+, Sm3+, Eu3+, Dy3+, Ho3+, Er3+/Yb3+); it is shown that for lighter ions, the formation of KY3F10 is limited, with even moderate concentrations of Nd3+ quenching the growth process entirely. Site-selective laser fluorescence measurements and crystal-field analysis of KY3F10:Eu3+ are presented and show that the ion resides in a similar environment to the bulk crystal.

High-resolution infra-red absorption Zeeman spectra are presented for KY3F10 nanocrystals doped with Nd3+ and Er3+, it is shown magnetic isotropy dictates the presence of observable splittings and that Zeeman splittings are far more sensitive to changes in the magnitude of the crystal-field than the electronic energy levels. A crystal-field fit is also presented using data obtained for KY3F10:Nd3+. In addition to electronic energy levels obtained from absorption and fluorescence, the isotropic magnetic-field splittings are included in the crystal-field. The resulting parameters accurately account for the Zeeman interaction, including non-linear effects and predict isotropic splittings not included in the dataset. A superposition model analysis and sensitivity analysis of the crystal-field parameters indicate structural differences may cause differences in the bulk- and nanocrystals, and that non-superposable interactions play an important role in the determination of the B__ parameter in KY3F10. Additionally, as part of this investigation of Zeeman interactions in nanocrystals, Zeeman infra-red absorption spectra of KY3F10 doped with ytterbium/erbium, dysprosium and holmium were analysed. The use of splittings, particularly in dysprosium doped KY3F10, is discussed for use as a magnetic field sensor.

Energy transfer dynamics for KY3F10 nanocrystals doped with samarium are also analysed. Concentration-dependent fluorescence transients indicate energy transfer mechanisms that are dipole-quadrupole in nature. The inferred energy transfer rate is also considerably faster than what is reported in comparable hosts, and an argument based on the crystal structure is presented as a cause.

In addition to the work on KY3F10 nanocrystals, lanthanide-doped Y2SiO5 microcrystals, which are of interest as a crystal host in quantum information processing applications were synthesised. These were prepared using the solution combustion, solid state and sol-gel synthesis techniques. Of these, the sol-gel method yields the most reliable and high-quality X2 phase Y2SiO5 microcrystals. Absorption and laser site-selective fluorescence measurements of Nd3+, Eu3+ and Er3+ doped material, performed at cryogenic temperatures, indicate that the as-grown microcrystals are of high optical quality with inhomogeneously broadened optical linewidths that are comparable to bulk crystals at similar dopant concentrations.