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

Sol-gel grown aluminum oxide is a highly porous material composed of a netw ork of nano-crystalline gamma-Al2O3 particles. The sol-gel process allows t he incorporation of trivalent rare-earth ions into the crystal structure. T hese ions can be used to study, via optical techniques, the structure and d ynamics of the nanoparticles in this material. Here we report on optical st udies of Yb3+-doped gamma-Al2O3. We find non-exponential decay curves for t he F-2(5/2) to F-2(7/2) Yb3+ fluorescence and a strong temperature dependen ce 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 channe ls. 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.