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Ehrlich, Gert Goldstein, Jonathan Todd 2011-04-20T20:44:01Z 2011-04-20T20:44:01Z 10000-01-01 1996 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. Submitted by Carolyn Mead (cmead2@illinois.edu) on 2011-04-20T20:44:01Z No. of bitstreams: 1 1996_goldstein.pdf: 4846742 bytes, checksum: 585794f4c6868670d5ec69af40a6954f (MD5) Made available in DSpace on 2011-04-20T20:44:01Z (GMT). No. of bitstreams: 1 1996_goldstein.pdf: 4846742 bytes, checksum: 585794f4c6868670d5ec69af40a6954f (MD5) Previous issue date: 1996 Restriction data tranferred 2014-07-01T11:12:09-05:00 Original Data Group with Access UIUC Users [automated] Release Date: none Reason: Thesis Item marked as restricted to the 'UIUC Users [automated]' Group (id=2) by Carolyn Mead (cmead2@illinois.edu) on 2011-04-20T20:44:01Z Item is restricted indefinitely. Thesis U of I Only 4007598 http://hdl.handle.net/2142/18854 en 1996 Jonathan Todd Goldstein single adatom behavior W adatoms Re adatoms Atomic motion on the close packed plane of a hexagonally close packed metal Dissertation / Thesis text Physics Physics University of Illinois at Urbana-Champaign Dissertation Ph.D.