Microstructure and optical properties of nanocrystalline CaWO4 thin films deposited by pulsed laser ablation in room temperature

Nanocrystalline CaWO4 films were successfully deposited by pulsed laser ablation at various background Ar gas pressures (10-100Pa) without substrate heating or after annealing treatment. The effects of Ar pressure on microstructure, surface morphology, chemical composition and optical properties were investigated by XRD, HR-TEM, FE-SEM, XPS, UV-vis and PL analyses. The crystallite size of CaWO4 films increased with increasing Ar pressure, which was associated with a change of surface morphology. Reduced tungsten states [W5+] or [W4+] caused by oxygen vacancies were observed at 10Pa. However, over 50Pa, the atomic concentration of all the constituent elements was almost constant, especially [Ca]/[W] ratio, which was nearly unity. The optical energy band-gap of CaWO4 films was strongly dependent on the Ar pressure, i.e., decreased from 4.9 to 4.5eV with the increase of Ar pressure from 50 to 100Pa. The photoluminescence (PL) spectra was positioned in a blue-shifted region around 378nm compared with emission at 420nm of bulk CaWO4 target, which clearly demonstrates the optical band-gap widening phenomena induced by quantum-size effect.