Date of Award
Doctor of Philosophy
Patricia A. Thiel
We reported sulfur adsorption, structure, and effects on coarsening on both Ag(111) and Ag(100) single crystal surfaces. Experiments were performed in ultra high vacuum (UHV) using variable temperature scanning tunneling microscopy (VT-STM) and Auger electron spectroscopy (AES). Sulfur was deposited prior-to, during, and after the deposition of Ag at temperatures usually below 300 K. Comparison with clean Ag(111) and Ag(100) surfaces provides a way to determine how sulfur affects the atomic-scale mass transport on both surfaces.
Experimental data show that a well-ordered, self-organized dot-row structure appears after adsorption of S on Ag(111) at 200 K. This dot-row motif, which exhibits fixed spacing between dots within rows, is present over a wide range of coverage. The dots are probably Ag3S3 clusters with adsorbed S in the spaces between dots. Dynamic rearrangements are observed. Trace amounts of adsorbed S below a critical coverage on the order of 10 millimonolayers have little effect on the coarsening and decay of monolayer Ag adatom islands on Ag(111) at 300 K. In contrast, above this critical coverage, coarsening is greatly accelerated. This critical value appears to be determined by whether all S can be accommodated at step edges. Accelerated coarsening derives from the feature that the excess S residing on the terraces produces significant populations of metal-sulfur complexes which are stabilized by strong Ag-S bonding. Furthermore, below room temperature, and at coverages above the threshold, an ordered sulfur structure develops. This structure contains long rows of Ag3S3 trimers as its dominant motif, and its development coincides with inhibition of coarsening.
We also show that adsorbed S on Ag/Ag(100) causes the ripening mechanism change from island diffusion/coalescence, i.e., Smoluchowski ripening (SR) to Ostwald ripening (OR) at S coverages from 0.03 ML to 0.21 ML at 300 K. The Ag island decay rate for OR increases with increasing S coverage. Ag islands change from square to round with increasing S coverage, and change orientation at S coverage higher than 0.16 ML. No more coarsening occurs when S coverage increases to 0.27 ML, where ordered structures form.
Shen, Mingmin, "Sulfur adsorption, structure, and effects on coarsening on Ag(111) and Ag(100)" (2009). Graduate Theses and Dissertations. 11010.