Atomic motion on the close-packed plane of a hexagonally close-packed metal
Goldstein, Jonathan Todd
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https://hdl.handle.net/2142/18997
Description
Title
Atomic motion on the close-packed plane of a hexagonally close-packed metal
Author(s)
Goldstein, Jonathan Todd
Issue Date
1996
Doctoral Committee Chair(s)
Chiang, Tai-Chang
Department of Study
Physics
Discipline
Physics
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Physics, Condensed Matter
Language
eng
Abstract
We present, for the first time, an account of single adatom behavior on the close-packed surface of a hexagonally close-packed material. Specifically, we have explored the diffusion and related phenomena of Re and W adatoms on Re (0001). Prefactors and activation energies to diffusion of $\rm6.13(\times2.6\sp{\pm1}){\times}10\sp{-6}\ cm\sp2/s,\ 11.11\pm.43$ kcal/mol and $\rm2.17(\times2.7\sp{\pm1}){\times}10\sp{-3}\ cm\sp2/s,\ 11.04\pm.34$ kcal/mol were measured for Re and W, respectively. The large difference in prefactor values, the largest yet seen for two elements on the same surface, contrasts with the closeness of the activation energies. Furthermore, the activation energies reveal an intriguing trend. Specifically, rhenium's activation energy to diffusion as well as the ratio of its activation energy to its heat of vaporization is close to, but slightly greater than the corresponding quantities for tungsten on Re (0001), Ir (111), W (110), and W (211). Both Re and W were found to occupy only 1 of the 2 available three-fold hollow surface sites. A minimum energy difference of 2.5 $\pm$.3 kcal/mol and 2.0 $\pm$.3 kcal/mol is found between these two sites for Re and W, respectively. Temperatures for the onset of motion and of dissociation were measured for Re trimers through octamers. The trend in the temperatures displays a significant dip for tetramers. Re was not found to localize at the point of impact upon deposition onto a Re (0001) surface held at 20K. We thus present the first evidence of transient mobility obtained from direct observations of single adatoms. To supplement this study of single adatoms, we also have explored aspects of diffusion in concentrated layers. A Monte Carlo simulation study was performed in which the diffusivity of a lattice gas with nearest-neighbor exclusion is measured as a function of coverage. Through the physical mechanism of site-exclusion, the ad-ad interaction affects a diffusivity which increases with coverage to a maximum at $\theta=.1,$ and then decreases rapidly. Thermodynamics are found to be the deciding factor in the diffusivity's trend at low coverages, while the effective jump rate dominates coverages greater than.1.
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