Microstructure and optical properties of nanocrystalline CaWO 4 thin films deposited by pulsed laser ablation in room temperature

Abstract

Nanocrystalline CaWO 4 films were successfully deposited by pulsed laser ablation at various background Ar gas pressures (10–100 Pa) 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 CaWO 4 films increased with increasing Ar pressure, which was associated with a change of surface morphology. Reduced tungsten states [W 5+] or [W 4+] caused by oxygen vacancies were observed at 10 Pa. However, over 50 Pa, 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 CaWO 4 films was strongly dependent on the Ar pressure, i.e., decreased from 4.9 to 4.5 eV with the increase of Ar pressure from 50 to 100 Pa. The photoluminescence (PL) spectra was positioned in a blue-shifted region around 378 nm compared with emission at 420 nm of bulk CaWO 4 target, which clearly demonstrates the optical band-gap widening phenomena induced by quantum-size effect.