The chemisorption of CO on the (111) surface of Ir at room temperature and below has been investigated using both low-energy electron diffraction (LEED) and thermal desorption mass spectrometry. The CO is adsorbed rapidly onto the (111) Ir forming a well-ordered (√3×√3) R30° overlayer structure after an exposure of approximately 2 to 3 Langmuirs. For larger exposures the overlayer compresses continuously as additional CO is adsorbed until a (2√3×2√3) R30° structure is formed. The surface coverage of CO corresponding to the (√3×√3) R30° structure is one-third monolayer, and mass spectrometry has shown that the surface coverage corresponding to the (2√3×2√3) R30° structure is 7/12 monolayer, where monolayer coverage is based on the number of surface Ir atoms. The isosteric heat of adsorption at one-third monolayer coverage was measured by monitoring the intensity of the overlayer LEED beams as a function of CO pressure and surface temperature, and it was found to be 35±1 kcal/mole. Thermal desorption revealed the existence of two apparent binding states of the CO at saturation coverage. At low coverage (ϑ≲0.4), the CO desorbs in a single peak, whereas the second state builds in at a higher coverage. The appearance of the second state in the thermal desorption spectra coincides with the transformation between the two overlayer structures. The probability of adsorption of the CO is nearly constant for ϑ≲0.2 and has a value of essentially unity, whereas the adsorption probability at larger surface coverages is much smaller, e.g., the value has dropped by more than an order of magnitude by the time ϑ=0.4 has been reached. Models are suggested for the observed surface structures, and the results are compared with other data of CO chemisorption on the close-packed surface of other Group VIII transition metals.
We studied the solid-phase reaction between a thin Ni film and a single crystal Ge(001) or Ge(111) substrate during a ramp anneal. The phase formation sequence was determined using in situ X-ray diffraction and in situ Rutherford backscattering spectrometry (RBS), while the nature and the texture of the phases were studied using X-ray pole figures and transmission electron microscopy. The phase sequence is characterized by the formation of a single transient phase before NiGe forms as the final and stable phase. X-ray pole figures were used to unambiguously identify the transient phase as the-phase, a non-stoichiometric Ni-rich germanide with a hexagonal crystal structure that can exist for Ge concentrations between 34% and 48% and which forms with a different epitaxial texture on both substrate orientations. The complementary information gained from both RBS and X-ray pole figure measurements revealed a simultaneous growth of both the-phase and NiGe over a small temperature window on both substrate orientations. V
Phase formation and growth kinetics have been investigated with lateral diffusion couples in Cu-Si and Cu-Ge systems. Analytical electron microscopy was used to determine the crystal structures and chemical compositions of the growing phases. CusSi is found to be the dominant phase in the Cu-Si system. The growth of the silicide follows a (time) 1'2 dependence with an activation energy of 0.95 eV in the temperature range of 200-260 "C. Cu3Ge is the only phase observed in Cu-Ge lateral diffusion couples with its length up to 20 ,um. The growth of CusGe is a diffusion controlled process at a rate similar to that of Cu$i. The activation energy of CusGe growth is 0.94 eV at 200-420 "C. In Cu-silicide or Cugermanide formation, Cu appears to be the dominant diffusing species.
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