Thin films of cerium oxide were prepared using e-beam evaporation. The starting material was CeO
pellets from Balzers (99.99% purity). The films were prepared in a Leybold L560 box coater pumped by a turbomolecular pump. The system was initially pumped to a base pressure of 1 ×10 Pa. Before deposition, the material was slowly outgassed, with a shutter blocking the vapor from the substrates. The films were deposited on unheated substrates. The substrates were rotating during the deposition, and the source-to-substrate distance was 40 cm. The deposition rate was 0.4 nm/s. The evaporation rate and thickness of the films were controlled by a quartz crystal thickness monitor. For different purposes of film characterization, the films were simultaneously deposited on BK7 glass (for X-ray diffraction, XRD), tantalum substrates (for X-ray photoelectron spectroscopy, XPS), fused silica substrates (for optical measurements), and alumina substrates (for gas-sensing measurements). After the films were deposited, they were removed fromthe coating chamber and exposed to the ambient atmosphere. Subsequently, all samples were annealed in air at 500 Cfor2h in order to thermally stabilize the films prior to sensing measurements.