Non-radiative relaxation of Yb3+ in highly porous γ-Al2O3

Sol-gel grown aluminum oxide is a highly porous material composed of a network of nano-crystalline gamma-Al2O3 particles. The sol-gel process allows the incorporation of trivalent rare-earth ions into the crystal structure. These ions can be used to study, via optical techniques, the structure and dynamics of the nanoparticles in this material. Here we report on optical studies of Yb3+-doped gamma-Al2O3. We find non-exponential decay curves for the F-2(5/2) to F-2(7/2) Yb3+ fluorescence and a strong temperature dependence of the decay rate between 4 and 300 K. We show that this behavior is not caused by confinement but is indicative of non-radiative relaxation channels. The non-radiative relaxation channel is provided by OH molecules at the surface of the gamma-Al2O3 nanoparticles, demonstrating the importance of surface effects in nanocrystalline materials. (C) 1999 Elsevier Science B.V. All rights reserved.