Observation of nanostructured cluster formation of Tm ions in CaF2 crystals

Transmission electron microscopy, energy dispersive X-ray spectroscopy, and high-resolution scanning-transmission electron microscopy, with electron beam sizes ranging from 2 to 50 nm, were used to investigate the spatial distribution and homogeneity of doped Tm3+ ions in CaF2 host matrices with atomic resolution, in solid crystals grown from melts using the Bridgman-Stockbarger method. With the smallest size electron beam available of 2 nm, it was found that the Tm3+ ions were distributed inhomogeneously at the host sites. They took the form of sub-nm agglomerations of 3-5 atoms, rather than individual ions and the phase transition layer was 0.1 nm thick. The spatial extend of inhomogeneous Tm3+ concentration was 2.6-6 nm and originates from ionic density fluctuations in the liquid phase at the interface layer due to the local electrostatic field at the ionic sites. (C) 2008 Elsevier B.V. All rights reserved.