|
We present the spectroscopic study of the mechanisms of excitation transfer between rare earth ions excited by energy transfer from SnO2 nanocrystals in silica. Bulk samples of pure and Er-doped silica with SnO2 nanoparticles were prepared by a sol gel technique and further thermal sintering process. Transmission electron microscopy (TEM) reveals the formation of spherical nanoclusters with a size distribution strongly determined by erbium doping. Small angle neutron scattering (SANS) experiments confirm and detail the TEM data evidencing the existence of a interphase region at the cluster boundaries where a SnOlike phase compensates the structural mismatch between the crystalline lattice in SnO2 nanoparticles and the amorphous silica network. The analysis of the SANS patterns show what kind of modification of the interphase morphology of SnO2 nanoparticles in silica brings to the passivation of interfacial defects. Surface states, which may preclude the exploitation of UV excitonic emission, are reduced after doping by rare earth ions. We demonstrate, by means of transmission-electron-microscopy and small-angle-neutron-scattering data, that a smooth interphase with a non negligible thickness takes the place of the fractal and discontinuous boundary observed in undoped material. The time resolved photoluminescence spectra of erbium in the infrared region show the spectral profile ascribable to ions in a ordered environment. Moreover, the absence of the broad contribution of the radiative decay of erbium ions dispersed in the silica amorphous matrix indicates that the excitation transfer follows paths enveloped in the interphase region. The spectroscopic analysis allows us to conclude that the excitation is transferred from ion to ion within a quasi-crystalline region where each site is surrounded by a different distribution of PL quenching sites which are responsible for the multi-exponential decay kinetics.
|
