Physics and Astronomy, Department of


Date of this Version

Fall 10-2011


A DISSERTATION Presented to the Faculty of The Graduate College at the University of Nebraska In Partial Fulfilment of Requirements For the Degree of Doctor of Philosophy, Major: Physics & Astronomy, Under the Supervision of Professor Axel Enders. Lincoln, Nebraska: October, 2011

Copyright 2011 Geoffrey A. Rojas


The study of the electronic properties and geometrical arrangement of 5, 10, 15, 20-tetraphenyl-21H, 23H-porphine on metal is presented. The systems were analyzed using both scanning tunneling microscopy and photoelectron spectroscopy and compared across surfaces to determine how the interface chemistry between the metal and molecule affect the self-assembly and band structure of the adsorbed species. The molecules are found to self-assemble and grow on the Ag(111) surface in a manner described by similar models to weakly bound metal/metal surface systems. The CH-pi bonds between molecules are found to largely determine the relative inter-molecular arrangement, while the more isotropic van der Waals interactions drive the self-assembly. The 2H-TPP however remains isolated and equally dispersed despite any increases in coverage, observed motion, or annealing on the Cu(111) surface, indicating an electrostatic repulsion between adsorbates. Through calculation, spectroscopic observations of state shifts and mapping of the local work function, the limiting factor in the inter-molecular repulsion is found to be due to a combination of charge transfer between molecule and surface and perturbation of the surface electrons due to frontier orbital overlap. By comparing this molecule across surfaces and temperatures, the complex interplay between band structure matching, charge transfer, surface barriers, and self-assembly is described. Controlling the charge transferred to the adsorbed species by the underlying metal, these properties are tailored without changing the atomic constituents or general band structure of the adsorbed species.

Adviser: Professor Axel Enders