STRUCTURAL ANALYSIS AND INTERACTIONS OF C60 ON Ag(111)AND Pb(111)

Open Access
Author:
Li, Hsin-I
Graduate Program:
Physics
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
December 08, 2009
Committee Members:
  • Renee Denise Diehl, Dissertation Advisor
  • Renee Denise Diehl, Committee Chair
  • Milton Walter Cole, Committee Member
  • Roy F Willis, Committee Member
  • Kristen Ann Fichthorn, Committee Member
Keywords:
  • vacancy
  • hoc
  • lead
  • silver
  • fullerene
Abstract:
The geometry of adsorbed C60 influences its collective properties. We report the dynamical low-energy electron diffraction and scanning tunneling microscopy studies to determine the geometry of a C60 monolayer, Ag(111)-(2√3 × 2√3)30 ◦-C60 and related density functional theory calculations. The stable monolayer has C60 molecules in vacancies that result from the displacement of surface atoms. C60 bonds with hexagons down, with their mirror planes parallel to that of the substrate. The results indicate that vacancy structures are the rule rather than the exception for C60 monolayers on close-packed metal surfaces and closely related to the anneal. Low-energy electron diffraction (LEED) indicates that the monolayer structure of C60 on Pb(111) comprises two coexisting incommensurate structures with nonsymmetry epitaxial rotations near 20 ◦ relative to the Pb(111) lattice. These structures are observed in scanning tunneling microscopy (STM) as Moir´e superstructures having periods of about 46°Aand 34°A. The Moir´e images and LEED patterns are consistent with two higher-order commensurate (HOC) structures that were identified using the hexagonal number sequence method. These structures are close to predictions from the Novaco-McTague theory of epitaxial rotation, assuming a weakly corrugated substrate potential. As a consequence of the fullerenes within the Moir´e structures having different local environments, the energetic alignment of the molecular resonances is also modulated, with shifts measured by tunneling spectroscopy of up to 20 meV. LEED experiments and grand canonical Monte Carlo simulations were carried out to study the adsorption of Xe on a substrate composed of a monolayer of C60 molecules on a Ag(111) surface. LEED adsorption isobars indicated that the adsorption occurs in steps, with the Xe initially adopting a structure having the same unit cell as the C60. Isosteric heats corresponding to the first two steps were measured to be 234 ± 8 and 204 ± 14 meV, respectively. For the simulations, the interaction potential of Xe with the composite substrate was modeled as the sum of two parts: the Xe-Ag part was computed using an ab initio van der Waals potential that varies as an inverse-distance cubed and the Xe-C60 part was computed using a spherically averaged C60 potential [E. S. Hernandez et al., J. Low Temp. Phys. 134, 309 (2004)]. The resulting adsorption potential is highly corrugated, with the most attractive sites located in the threefold hollows between the C60 molecules, forming a honeycomb array. The simulations (at temperatures ranging from 55 to 90K) show that these attractive sites are filled first, followed by adsorption in two types of secondary sites, where a competition exists due to steric hindrance. The thermodynamic properties of film growth obtained in the simulation are in good agreement with the experiment.