Abstract

In the past two decades it has been widely demonstrated that the optical properties of select inorganic materials may be modified by changing either their size or shape on the nanometer level (sub-nanometer to 100 nm length scale). Much of the early research on this topic focused on semiconducting nanocrystals, where it has been effectively demonstrated that reducing particle size below their Bohr radius produced a characteristic blue shift of the band gap absorption. A little over a decade after the initial work on semiconducting nanocrystals, the first scientific articles on insulating nanocrystals doped with lanthanide ions started to appear. While numerous studies have focused on examining the luminescence generated by exciting with ultraviolet (UV) light, very few have examined the upconversion phenomenon in nanocrystalline materials. Upconversion is the generation of higher energy light from lower energy radiation typically through the use of lanthanide ions doped into a solid state host. Much of the interest in upconverting nanocrystalline materials is due to their prospective application as fluorescent biological labels. In this thesis the synthesis, spectroscopic, and upconversion properties of lanthanide-doped nanocrystalline materials will be discussed. We report on our efforts to date to achieve viable upconversion luminescence from Ho 3+ doped nanocrystalline Y 2 O 3 and Gd 3 Ga 5 O 12 prepared via the combustion synthesis. These studies have determined that, while upconversion occurs in Y 2 O 3 bulk samples, it is severely reduced or nonexistent in the nanocrystal samples. This behaviour is attributed to the presence of high vibrational energies, 1500 and 3350 cm -1 , due to adsorbed atmospheric CO 3 2- and OH - anions, respectively, on the surface of the nanocrystals. A substantial increase in the upconversion efficiencies was observed in the case of the garnet (Gd 3 Ga 5 O 12 ) nanocrystals due to considerably less surface contamination. The effect of Yb 3+ Co-doping on the upconversion luminescence in the Gd 3 Ga 5 O 12 sample will also be introduced. We also evaluate the spectroscopic properties of lutetium oxide nanocrystals doped with trivalent europium (Lu 2 O 3 :Eu 3+ ) prepared by the same combustion synthesis technique. These results are compared and contrasted to those of a bulk Lu 2 O 3 :Eu 3+ sample. In the case of Lu 2 O 3 we observe significant changes in the luminescence behaviour that we attribute to the vastly different particle sizes of the two different materials. Finally, we present a new procedure for synthesizing NaYF 4 :Er 3+ , Yb 3+ nanoparticles that are capable of colloidal dispersion in non-polar organic solvents. The highly luminescent nanoparticles are synthesized via the thermal decomposition of trifluoroacetate precusors in a mixture of oleic acid and octadecene. The Er 3+ , Yb 3+ and Tm 3+ , Yb 3+ doped cubic NaYF 4 nanocrystals exhibit green/red and blue upconversion luminescence, respectively under 980 nm laser excitation with low power densities while colloidally dispersed. A brief discussion on our current attempts and future efforts towards modifying the nanoparticles surface will also be given.