Atomic-Studies of Molecules on Palladium{111} Surface

Open Access
Author:
Zhang, Rong
Graduate Program:
Chemistry
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
April 08, 2008
Committee Members:
  • Paul S Weiss, Thesis Advisor
Keywords:
  • Scanning Tunneling Microscopy
  • Thiophene
  • Palladium
Abstract:
We use a low-temperature scanning tunneling microscope (STM) to conduct an atomic-scale study of thiophene on Pd{111} at 4 K. In one set of experiments, we characterized the Pd{111} single crystal. Two distinct features of subsurface impurities in a normally clean Pd are resolved under topographic images. One is ascribed to S, and the other to either C or O. The vibrational spectrum of the Pd substrate lattice was measured by inelastic electron tunneling spectroscopy with the STM. We find that the vibrational energy of the Pd surface phonon is consistent with previous experimental results and theoretical predictions. In another set of experiments, we investigated the system of thiophene adsorbed on Pd{111} at 4 K. Thiophene was found preferentially bound to the upper step edges of Pd substrate due to a combination of molecule-substrate interactions and intermolecular attractive dipole-dipole interactions. By resolving the underlying substrate lattice in topography, we are able to determine the adsorption sites and orientations of thiophene molecules on Pd. We also use the STM as an operative tool to study molecular motions induced by tunneling electrons. We believe that tunneling-electron-induced molecular mo- tions (rotation, translation, dissociation) involve the inelastic electron scattering and relaxation of C-H stretch vibrations to lower energy modes of the molecules that leads to excitation of other degrees of freedom in thiophene. The vibrational relaxation of a single molecule was monitored by time-dependent changes in the current level.