Phonon-phonon and electron-phonon interactions in single crystals and nanoparticle materials.

摘要

Finite size effects in rare earth doped insulating nanoparticles and clusters of Y₂O₃:Eu3+ form the focus of this study. In addition, the effect of the unusual dispersion relations in LaF₃:Pr3+ on the phonon dynamics is the material also described.; Measurements of phonon decay rates in LaF₃:Pr3+ indicate that the frequency dependence of the phonon decay rates deviates from the usual ω5 dependence. In order to detail the effect of low lying optical modes on the phonon decay rates, a calculation of the phonon-phonon interactions based on the experimentally derived dispersion curves in this material is performed.; The finite size of a nanoparticle causes both the phonon density of states to be discrete and a gap to open up at very low frequencies. In order to investigate the effects on nonradiative relaxation, fluorescence transients of three levels in the 5D₁ manifolds of Eu3+ ions at the C site in Y₂O₃:Eu3+ nanocrystals are measured. These measurements are also performed on micron sized materials of Y₂O₃Eu3+ for comparison. The size distribution and mean size of the nanoparticles are obtained using transmission electron microscopy (TEM) images. The normal modes of elastic bodies of Y ₂O₃ are calculated to deduce the low frequency phonon spectrum of Y₂O₃ nanoparticles.; The TEM images of Y₂O₃ nanoparticles reveal that nanocrystalline materials consist of a large number of nanoparticles which form clusters. In order to investigate the effect of the cluster vibrational modes on the electronic relaxation in nanoparticles, a nanocluster is simulated by diffusion limited aggregation and the cluster vibrational modes are calculated for the simulated cluster. Coupling to a center-of-mass translational mode is proposed as a new mechanism for nonradiative relaxation. This novel mechanism predicts a more rapid relaxation rate than that expected for the radiative decay process between two narrowly separated electronic levels.; The electronic relaxation in a nanoparticle sufficiently large to support low frequency vibrational mode resonant with the energy separation between two electronic levels, is also studied. The coupling between the cluster vibrational modes and the resonant nanoparticle vibrational mode is considered.