Nanovoid formation through the annealing of amorphous Al₂O ₃ and WO₃ films

A high density of nanovoids between 2 and 13 nm in size was produced during the crystallization process when amorphous Al₂O₃ and WO₃, which are 20-30% lower in density than their crystalline phases, were annealed. By contrast, no formation or growth of nanovoids was observed during the crystallization of amorphous TiO₂, which has a density 2% lower than its crystalline phase. These results suggest that large differences in density between amorphous and crystalline oxides can be utilized to produce nanoporous structures.