Abstract

This thesis presents a detailed investigation into the spectroscopic properties of inorganic nanocrystals doped with trivalent rare earth ions. We focus on their upconversion luminescence, emission of radiation at higher energy than the pump wavelength, and evaluate the fundamental mechanisms of upconversion in the nanocrystal. We evaluate the spectroscopic properties of sesquioxide nanocrystals doped with trivalent erbium (M 2 O 3 :Er 3+ , where M = Y, Lu, or Sc) prepared by the propellant synthesis technique. Characteristic green, red, and near-infrared Er 3+ emission is observed following excitation with 488 nm in all samples under investigation. The overall luminescence intensity of the sesquioxide nanocrystals is lower compared to the microcrystalline material (bulk) as a result of the presence of high vibrational energies, 1500 and 3350 cm -1 , due to adsorbed CO 3 2- and OH - anions, respectively, which significantly increase the rate of multiphonon relaxation. The garnet (Gd 3 Ga 5 O 12 :Er 3+ ) nanocrystals, however, have considerably less surface adsorbed species, which consequently increases the luminescence intensity drastically. The upconversion of red (n exc = 650 nm) and near-infrared (n exc = 800 or 980 nm) radiation into UV, blue, green, and red emission is studied for Er 3+ ions doped in various sesquioxide (Y 2 O 3 , Lu 2 O 3 , and SC 2 O 3 ) and garnet (Gd 3 Ga 5 O 12 ) nanocrystals over a wide range of temperatures and dopant concentration is investigated. We present, for the first time, upconversion in a trivalent rare earth (RE 3+ ) doped nanocrystalline material, specifically Y 2 O 3 :Er 3+ . We show that replacing the Y 3+ cation has significant consequences on the upconversion. The upconverted luminescence of Lu 2 O 3 :Er 3+ nanocrystals have intensities that are 100x greater compared to identically doped nanocrystalline Y 2 O 3 :Er 3+ . Furthermore, Sc 2 O 3:Er 3+ nanocrystals show an enhanced red emission, which is greater than Y 2 O 3 :Er 3+ nanocrystals (with identical Er 3+ concentration) due to the smaller unit cell resulting in increased interaction between Er 3+ ions. The upconversion is observed to be dependent on the method of preparation. We explore nanocrystalline Y 2 O 3 :Er 3+ prepared via the propellant synthesis technique and a controlled hydrolysis synthesis (or wet chemical synthesis) where we observed quite diverse upconversion behavior attributed to the vastly different morphological properties of the two different nanocrystalline materials. Additionally, we investigate the effect of Yb 3+ co-doping on the upconversion luminescence of Y 2 O 3 :Er 3+ nanocrystals prepared via the two distinct synthesis routes, and observe a significant change in the mechanisms of upconversion. In the sesquioxides, the upconversion properties of the nanocrystalline material are diverse from the bulk counterpart. Finally, we attempt to ascertain if any spectroscopic changes occur in nanosized Lu 2 O 3 :Nd 3+ , Y 2 O 3 :Sm 3+ and Y 2 O 3 :Dy 3+ prepared via combustion synthesis. In all cases, the size of the particles affects the luminescence behavior