Organizational and Interfacial Examination of Adenine Films on Ag(111): Self-Assembly and Shockley State Modification

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Title: Organizational and Interfacial Examination of Adenine Films on Ag(111): Self-Assembly and Shockley State Modification
Author: Andrews, Katie Marie
Advisors: Daniel Dougherty, Committee Member
Marco Buongiorno Nardelli, Committee Member
Thomas P. Pearl, Committee Chair
Abstract: The scope of this research has been two-fold: to quantify the strength of intermolecular hydrogen bonds that dictate self-assembly of single nucleobases on a metallic platform and to identify the resultant interfacial electronic influence of an organized organic film on a surface with a Shockley-type surface state. A model system of weakly bound, physisorbed adenine molecules on the (111) face of crystalline silver has been chosen, where the dosing parameters of the molecules from a solid source are limited to the low, sub-monolayer regime when exposed to a room temperature substrate. STM topographic imaging at 300, 83, and 15 K reveal highly organized domains of two orientations composed of dimerized adenine molecules aligning with particular 2-dimensional substrate lattice vectors. In addition, this system has shown high molecular mobility and STM tip induced dissociation when the sample is either imaged at room temperature, or annealed at 340 K and subsequently imaged at 83 K. By systematically varying the bias voltage (and therefore the tunneling gap) and rescanning the same area, the small regions of alternate domains have been shown to dissociate at larger tunneling gaps than molecules in the primary domain orientation of an island. Using this qualitative study of hydrogen bond strength along with our high-resolution images, a proposed molecular structure is given with adenine dimer pair models and energy characteristics taken from the current literature. In order to probe the interfacial structure of this system and the interaction of an adsorbed nucleobase film on a surface with a known Shockley-type surface state near the Fermi level, differential conductance maps and point spectroscopy curves were taken at low temperature. The point spectra reveal an upward energetic shift of +152 meV to +82.5 ± 2 meV of the Shockley state when taken over adenine islands. A series of differential conductance maps taken over a range of energies shows free-electron like scattering in the film. A parabolic dispersion curve was obtained for the film regions and when compared to the experimentally measured dispersion curve over bare silver, gives an effective mass ratio between the two of m*A/Ag/m*Ag = 1.1 ± 0.05. Considering the characteristics of the dispersion curve and the signature of the adenine films in the point spectra, the observed interface state is believed to be a film induced modification of the native Shockley state from bare Ag(111). The chemical structure of adenine includes à €-orbitals protruding from the molecular plane as well as a large gap (~4 eV for the free molecule) between the HOMO and LUMO. It is therefore hypothesized that the mechanism behind the upward energetic shift of the surface state is a modification of the silver Shockley state, however, further investigation, both experimentally and theoretically, must be undertaken before conclusive remarks about the mechanism behind this phenomenon can be made.
Date: 2009-10-14
Degree: MS
Discipline: Physics

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