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https://hdl.handle.net/2142/20452
Description
Title
Direct determination of interactions at surfaces
Author(s)
Watanabe, Fumiya
Issue Date
1990
Department of Study
Materials Science and Engineering
Discipline
Materials Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Physics, Condensed Matter
Engineering, Materials Science
Language
eng
Abstract
We have obtained for the first time a fairly complete two dimensional picture of interactions between two adatoms on a surface as a function of both the interatomic distance and orientation. Four different combinations of adatoms, Re-Pd, W-Pd, Ir-Ir, and Re-Re, have been investigated on the (110) surfaces of tungsten. P(R), the probability of finding adatoms at the vector separation R is obtained from repeated observations of two adatoms at equilibrium on W(110), using a field ion microscope. Interactions are derived directly from the measured probabilities utilizing the Boltzmann relationship P(R) = C P$\sb0$(R) exp (-F(R)/kT), where P$\sb0$(R) is the probability of finding a non-interacting pair and F(R) the free energy of interaction for two adatoms separated by vector R; C is a normalization constant.
The pair interactions in two-dimensions have interesting characteristic, such as very long range effects, oscillatory behavior, and a pronounced orientational anisotropy. The magnitudes of adatom interactions derived in this study are smaller compared to binding energies estimated in more simplistic derivations of interactions at surfaces. At the closest separation, the interaction energies vary from fairly strong attraction of $\approx$85 meV for Ir, and moderate attractions of $\approx$50 meV for W-Pd and $\approx$35 meV for Re-Pd, to a repulsion of $\approx$21 meV for Re. On the contrary, the range of interactions determined in our studies is much greater than assumed in previous studies. The range varies from more than 8 A for Re-Pd and W-Pd to $\approx$14 A for Ir-Ir and Re-Re. These are quite large distances for planes with a nearest-neighbor distance of 2.74 A. In fact, the limits of 8 A and 14 A correspond to the 10-th and the 25-th nearest-neighbor sites on W(110).
We have also investigated the extent to which pair interactions play a role in cohesion of clusters. Qualitatively at least, predictions of trimer properties based on pair interactions are moderately effective for Ir, but not at all for Re. For both, many-atom effects are important however. The quantitative estimates of such contributions are $\sim$100 meV and Ir and $\sim$360 meV for Re. That many-atom effects contribute $\sim$1/3 of the cohesion for Ir and all of it for Re.
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