The structure and composition of metal surfaces

Transmission electron microscopy (TEM) has become a valuable tool in the study of the structure and composition of metal surfaces, though its surface-sensitivity is known to be lower than that for reflection high energy electron diffraction (RHEED), low energy electron diffraction (LEED) and Auger electron spectroscopy. In the present work evaporated films of gold and clean and oxidized surfaces of copper single crystals have been studied using TEM. The gold films were used to test a dynamical theory analysis of coherent (Bragg) and diffuse background scattering involving first order Laue zone diffracted beams, with moderately good agreement between theory and experimental results being obtained. Monolayer thickness changes have been imaged in dark field on (111) Au films and a comparison of calculated and experimental values for bright field intensity changes identifies hillocks or depressions in the surfaces. Severe limitations are imposed on the imaging of surface steps by diffraction conditions, specimen thickness and cleanliness, and objective aperture size. Although surface features producing preferential nucleation of cuprous oxide on low index copper faces could not be analysed by the dark field technique mentioned above, detailed information is provided by two-beam dark field and weak beam imaging using copper reflections. The three-dimensional shape of the cuprous oxide is determined by weak beam imaging employing both copper and cuprous oxide reflections, showing that islands on (100) Cu specimens, grown at 250 to 400° C in 1 x 10⁻³ torr of oxygen, have about 28% of the oxide above the substrate surface. A transfer device for transporting specimens under ultrahigh vacuum conditions from a U.H.V. chamber to a high resolution electron microscope has proved of value in the study of oxidized copper surfaces. By combining RHEED, in the U.H.V. system, and TEM analysis not only is cuprous oxide studied but the higher form, cupric oxide, is also detected and its morphology and epitaxy ascertained.