Preferential formation of anatase in laser-ablated titanium dioxide films

This paper gives a guideline for preparation of anatase TiO2 films by pulsed laser deposition (PLD) technique, and the mechanism of preferential formation of anatase is described. The TiO2 films with different thicknesses, 50, 100 and 300 nm, were prepared on quartz-glass substrates by Nd:YAG pulsed laser deposition, and subsequently annealed in air at 500 and 1000 °C. Their structures and optical properties have been investigated. The as-deposited film with a thickness of 50 nm was a mixture of amorphous, anatase and rutile. Anatase still existed after annealing for 1 h at 1000 °C. In contrast, the anatase phase was not present in the 300 nm thick film and only the rutile phase was observed. In the film with a thickness of 100 nm, anatase was found only in the as-deposited state. It has been revealed experimentally that the anatase phase is formed when the as-deposited films have an amorphous structure. This is strongly supported by an additional experiment using a sapphire single-crystal substrate. A micromechanical calculation substantiates that the anatase formation from the amorphous phase is preferred in terms of the strain energy and Gibbs free energy.