Atomic motion on the close packed plane of a hexagonally close packed metal

We present, for the first time, an account of single adatom behavior on the close-packed surface of a hexagonally dose-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 6.13(x2.6±1 )x10-6 cffil. Is, 11.11±.43 kcal/mol and 2.17(x2.7±1)x10-3 cffil/s, 11.04±.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±.3 kcal/mol and 2.0±.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 aRe (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 nearestneighbor exclusion is measured as a function of coverage. Through the physical mechanism of site-exclusion, the ad-ad interaction affects a diffusivity which iv increases with coverage to a maximum at 8 =.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.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Physics

Year dc:date

2011

Author dc:creator

• Goldstein, Jonathan Todd

Contributors dc:contributor

• Ehrlich, Gert

• single adatom behavior
• W adatoms
• Re adatoms

Statement dc:rights

• 1996 Jonathan Todd Goldstein

Language dc:language

en

Identifier

4007598

OAI identifier oai:identifier

oai:www.ideals.illinois.edu:2142/18854