| Although surface structures are commonly viewed in a simplified manner wherein all of the atoms are frozen in bulk positions, in actuality this bulk-terminated surface model is rarely appropriate. Atoms in the top few layers of almost all surfaces are displaced from their bulk positions. Surface structural modifications can range in severity from a simple layer relaxation to a significant surface atom displacement, or reconstruction. The relaxations and reconstructions of clean and adsorbate-covered metal surfaces generally occur as compensation for lost or weakened metal-metal bonding and electronic structure differences between surface and bulk atoms. Despite numerous studies of clean surfaces and adsorbate-induced reconstructions, the surface electronic structure and bonding, as well as the driving forces for many different reconstructions, are still not well understood.; Solid-state Fenske-Hall (FH) band structure calculations have been used to study the different surface structures that result from C-, N-, or O atom adsorption on the Ni(100) or Rh(100) surfaces. The adsorption of a half-monolayer of C atoms induces a “square” clock reconstruction on Ni(100), which is characterized by a significant displacement of the top-layer metal atoms from their clean surface positions. Although this same reconstruction has been reported for the N/Ni(100) surface, adsorbing O atoms on Ni(100) has very little effect on the overall surface structure. In contrast, however, when a half-monolayer of O atoms adsorb onto Rh(100), the surface undergoes an “asymmetric” clock reconstruction. The mechanisms by which the square and asymmetric clock reconstructions occur are still a matter of considerable debate by surface scientists, and as a result, many questions remain about the driving forces for these reconstructions. The results presented in Chapters 2 and 3 provide a detailed bonding picture that ultimately relates the observed surface structures to a number of factors, including the electron counts of both the metal and the adsorbate, the electronegativity of the adsorbed species, and the strength of the metal-metal bonding before adsorption. |
