C -plane chromia on c -plane sapphire

Chromium oxide thin films were atomic layer deposited from CrO2Cl2 and CH3OH on α-Al2O3(0 0 1) (c -sapphire) substrates at 420 °C. On the basis of X-ray reflection evidence, the thickness of the films in a series prepared ranged from 4 to 400 nm. As it was ascertained by reflection high-energy electron diffraction, the films grew epitaxially. Electron probe microanalysis showed that O/Cr atomic ratio in them was close to 1.5 and that the residual impurities were present in insignificant amounts. The rms roughness of the films measured by atomic-force microscopy and X-ray reflection was of the order of 1 nm or less. X-ray diffraction and Raman scattering data suggested that the films were strained, and that the strain was tensile in the c direction and compressive in the c plane. According to the surface Brillouin scattering measurements, the elastic properties of the films differed only slightly from those for single-crystal bulk chromia. The films, when surface activated with Pt nanoislands, had a moderate response to CO in air with the transition times in the range of tens of seconds. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)