Comparative studies of nanostructural and morphological evolution of CeO2 thin films induced by high-temperature annealing

High-temperature annealing performed at similar to 1000 degrees C or higher in an oxidizing atmosphere is a simple technique to significantly modify the as-grown nanostructures of CeO2 films, which are typically granular with high surface roughness. We evaluated the effect of high-temperature annealing on various CeO2 films deposited by three deposition methods, namely standard pulsed laser deposition (PLD), large-area pulsed laser deposition, and electron-beam evaporation. High-temperature annealing promotes high epitaxy and crystallinity, as well as stabilization of the flat (001) surfaces of CeO2. This is most commonly observed on the annealed CeO2 film grown by standard PLD, which is usually atomically smooth and pore-free. In contrast, CeO2 films grown by large-area PLD and electron-beam evaporation are characterized by similarly flat surfaces but are discontinuous due to the presence of pores. Depending on the length of the annealing time, the pores range from small-sized nanopores with widths of similar to 30-100 nm and depths of similar to 3-5 nm, up to relatively large- sized pores with widths of similar to 100-500 nm and depths of similar to 10-20 nm. The differences in the nanostructural and morphological evolution of the CeO2 films are further correlated to their as-grown properties, including grain size, orientation, and possibly the initial porosity of the film.