The evolution of the gap of a nanoscaled insulator material, namely, Gd 2 O 3 , has been observed by means of vacuum ultraviolet excitation spectra of a dopant ͑Eu 3+ ͒. The nanoparticles have been synthesized by the low energy cluster beam deposition technique and grown afterward by different annealing steps. A gap shift towards the blue is observed, similar to what is observed in semiconductor nanoparticles. Despite the strong ionic character of the material, the evolution exhibits a behavior similar to covalent materials. The evolution of the gap for Gd 2 O 3 follows the same empiric rule that has been derived for semiconductors ͑ZnO, CuBr, Si, and CdS͒. It shows that, in spite of the strong ionic character of the material ͑0.9 on the scale of Phillips͒, the amount of covalency is important enough for creating a significant delocalization of the electron with regard to its hole.
Size effects, such as structure transition, have been reported in small clusters of alkali halide compounds. We extend the study to rare earth sesquioxide (Gd(2)O(3)) clusters which are as ionic as the alkali halide compounds, but have a more complicated structure. In a clean and controlled environment (ultra high vacuum), such particles are well crystallized, facetted and tend to adopt a rhombic dodecahedron shape. This indicates the major role of highly ionic bonds in preserving the crystal lattice even at small sizes (a few lattice parameter). Based on both cathodo-luminescence and transmission electron microscopy, we report the existence of a structural transition from bcc to monoclinic at small sizes.
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