Mixed metal fluoride nanoparticles of the MYF family (M=Na, K, and Ba) are an excellent choice as the host for lanthanide-doped upconverting nanomaterials since they are able to accommodate rare earth ions at the Y³⁺ sites. Moreover, these fluoride materials have relatively low phonon energies, thereby minimising non-radiative relaxation, ensuring good fluorescence quantum efficiencies for the principal emitting levels. However, despite many studies, reports on the corresponding fluoride nanomaterials are minimal, describing the detailed optical properties such as the effect of optimum pump wavelength on optical properties and the excited-state dynamics for lanthanide-doped nanoparticles, low-temperature optical spectroscopy, which is crucial in order to enhance their optical performance for further technological applications. Therefore, the work of this thesis is dedicated to understanding and tackling these challenges with mixed metal fluoride nanoparticles based on the KYF family.

In the first experimental studies, we investigated the effect of infrared laser excitation wavelength and core@shell nanoarchitectures on up-conversion fluorescence and luminescence thermometry of Yb³⁺/Er³⁺ co-doped KY₃F₁₀ upconverting nanoparticles. High-resolution Yb³⁺ excitation spectra were measured by monitoring the Er³⁺ ⁴S₃/₂→⁴I₁₅/₂, ⁴F₉/₂→⁴I₁₅/₂ upconversion fluorescence, with the highest fluorescence yield obtained at 10254 cm⁻¹ (975 nm) for both core and core-shell nanoparticles. We observed a five-fold increment in Er³⁺ upconversion intensity using resonant excitation compared to excitation at 980 nm for both nanoparticles. The maximum thermal sensitivity of 1.51 %·K⁻¹ (300 K) and temperature uncertainty of 0.113 K for KY₃F₁₀:Yb³⁺/Er³⁺ core nanoparticles were achieved by tuning the excitation wavelength to 975 nm in resonance with optical transitions of the Yb³⁺ ion. It was found that core@shell nanoarchitectures and laser excitation wavelength had no evident influence on thermometric performance except enhancement in the upconversion emission intensity.

Secondly, we studied upconversion fluorescence and colour tunability properties of Er³⁺/Yb³⁺ codoped β−KYF₄ and β−NaYF₄ nanoparticles by tuning the laser excitation wavelength. The Yb³⁺ ²F₇/₂→²F₅/₂ absorption spectra exhibit absorption maxima at 10237 cm⁻¹ (977 nm) for β−NaYF₄ and 10267 cm⁻¹ (974 nm) for β−KYF₄ nanoparticles. The Er³⁺ upconversion fluorescence spectra consist of the ²H₁₁/₂, ⁴S₃/₂, and ⁴F₉/₂→⁴I₁₅/₂ transitions for either 974, 977 or 980 nm laser excitation in both materials. We observed an enhancement in the upconversion intensity by a factor of up to 20 for β−KYF₄ and 1.5 fold for β−NaYF₄ under resonant excitation compared with off-resonant excitation at 980 nm. The CIE chromaticity coordinates of β−KYF₄ nanoparticles are (0.6836, 0.3151) with a highest red colour purity of 99.70% (centred at 617 nm) along with colour coordinate temperature (CCT) value (4700 K) were in good agreement to coordinate of National Television System Committee (NTSC). The absolute upconversion quantum yields are 1.18% and 2.34% under low power density (1.65 W.cm⁻²).

Furthermore, we explored phase-dependent fluorescence and thermometry properties of cubic (α) and hexagonal (β) phases of KYF₄:Yb/Er upconverting nanoparticles. The Yb³⁺ absorption spectra of these two nanoparticles exhibit similar absorption maxima at 10268 cm⁻¹ (974 nm). The green and red fluorescence of the hexagonal phase (β) was around 100 and 2000 times more intense than that of the cubic (α) phase of KYF₄:Yb/Er nanoparticles. The red to green ratio (R/G) was 50:1 and 2:1 for the β−KYF₄:Yb/Er and α−KYF₄:Yb/Er nanoparticles. Using the FIR technique from the thermally coupled ²H₁₁/₂ and ⁴S₃/₂ levels, a very high thermal sensitivity of 2.039 and 1.655 %·K⁻¹ at physiological temperature was achieved for α−KYF₄:Yb/Er and β−KYF₄:Yb/Er nanoparticles. In addition, the high sensitivity of α− KYF₄:Yb/Er can be explained using the classical Judd-Ofelt (J-O) theory.

Finally, high-resolution spectroscopy and a crystal field analysis was conducted for KY₃F₁₀: Er³⁺ nanoparticles and K₂YF₅: Er³⁺ microparticles. A total of 49 crystal-field energy levels, distributed amongst 13 multiplets of the Er³⁺ ion, have been deduced for the C₄ᵥ point group symmetry site of the Er³⁺ ion-doped KY₃F₁₀ nanoparticles. A parametrised, single-electron crystal-field calculation provides an excellent approximation to the experimental data with optimised crystal fit parameters that are comparable to the bulk KY3F10:Er³⁺ crystal. Simulated spectra, based upon wave-functions derived from the crystal-field calculations, unequivocally demonstrate that excited state absorption is the predominant upconversion mechanism in this material– agreeing well with upconversion excitation spectra obtained for Yb³⁺ co-doped samples. Similarly, we constructed an energy level scheme for 39 crystal-field states amongst 7 multiplets of the Er³⁺+ ion, which have been deduced for the C2v point group symmetry site of Er³⁺ ion doped K₂YF₅ microparticles. Furthermore, a comprehensive crystal-field model for Er³⁺ ion doped K₂YF₅ microparticles through the use of experimental energy levels was deduced from absorption and fluorescence spectroscopy.