Abstract
Temperature dependent studies of a selection of materials including erbium-doped yttrium aluminum garnet (Er:YAG), zinc nitride nanocrystals(NCs) and manganese-doped cadmium selenide (CdMnSe) NCS have been undertaken in order to determine their fundamental optical properties. The study is based upon the measurement of their photoluminescence (PL) and PL transient decay transient over the temperature range of 300K to 5K. For the Er:YAG samples, two different sample types are studied as a function of erbium concentration that are a fast-cooled (mono-phase) and a slow-cooled (bi-phasic) polycrystalline material. Due to the presence of emission upconversion in these materials the emission dependence on excitation power is also studied. It is found that for high Er concentrations (40-50%) energy transfer upconversion (ETU) occurs that may be of use for assisting population inversion at the 4I11/2 level for the laser 3µm emission. Generally it is confirmed that the single phase and bi-phasic materials possess slightly different optical properties and that the material production must therefore be carefully controlled in order to realize optimized materials for optical applications. Zinc nitride NC materials are studied for the first time with four samples representing a range of NC diameters characterized. These materials were highly susceptible to oxidation which presented a significant challenge in their handling and study. Strong emission was observed across the visible spectral region though the origin of this it was found probably included trap state emission and for the smallest NC samples organic ligand emission. The PL was found to shifting to higher energies as the size of the NC is decreased as expected due to increased quantum confinement and in line with the Brus equation. Two of the samples (8.9nm and 2.7nm diameter NCs) display a temperature dependence of the optical properties in line that seen in other semiconductor NCs such as PbS. The other samples displayed anomalous behavior that could be due to ligand emission (2.5nm NC sample) or higher energy trap states caused by localized oxidation (3.8nm NCs). A study of temperature dependent optical properties of CdMnSe NCs was focused around the role of NC shape and type. Core only, core/shell and dot-in-rod samples were studied and all found to display a blue shift as temperature is reduced from 300K to ~5K of between ~24meV to 58meV. The core only NCs display a different luminescent behavior to that of the core-shell and dot-in-a-rod samples. The PL is related to the recombination of confined excitons within the NC, together with a contribution from what are most probably trap states located at the surface of the NC. In these samples no contribution from the Mn2+ ion is found suggesting that the dopant ions are not fully incorporated into the NC but may reside on the surface.