Ally S. Y. Chan

Facet stability in oxygen-induced nanofaceting of Re(1231)

The stability of the various facets in oxygen-induced faceting of Re(1231) has been studied by low-energy electron diffraction, scanning tunneling microcopy, and synchrotron-based high-resolution X-ray photoemission spectroscopy. When Re(1231) is annealed at 800-1200 K in oxygen (10(-7) Torr), the surface becomes completely covered with nanometer-scale facets, and its morphology depends on the substrate temperature and oxygen exposure. Especially, the (1121) facet competes with the (1011) facet in determining the surface morphology, and the stability of each facet relies on oxygen coverage. Using density functional theory, the O-Re binding energies on the facets for various oxygen concentrations are calculated to explain how the oxygen coverage affects the anisotropy of surface free energy, which in turn determines the morphology of the faceted surface.

Near-edge absorption fine structure and UV photoemission spectroscopy studies of aligned single-walled carbon nanotubes on Si(100) substrates

We report near-edge absorption fine structure (NEXAFS) and UV photoemission spectroscopy (UPS) studies of aligned single-walled carbon nanotube films on Si(100) substrates. Orientation of the films was detected in the NEXAFS spectra, with the intensity of the π* core exciton at 284.4 eV showing a strong dependence on nanotube alignment with respect to the polarization of the incident radiation. At lower angles of incidence, the intensity of the π* peak was higher for all orientations, which we attribute to the greater accessibility of the π* orbitals. UPS spectra of the films showed little angular dependence and included features consistent with the total density of states of graphite. As a result of the nanotube curvature and the distribution of nanotube chiralities, the UPS spectra are similar to angle-integrated graphite spectra.