Date of Award

Spring 2015

Project Type

Dissertation

Program or Major

Physics

Degree Name

Doctor of Philosophy

First Advisor

Karsten Pohl

Second Advisor

Olof Echt

Third Advisor

Dawn Meredith

Abstract

Understanding electronic devices down to the atomic scale is essential for the development of novel organic molecule based nanotechnologies. Utilizing scanning tunneling microscopy (STM), new organic molecules can be imaged to understand their structural and electronic properties at the molecular level. 5,6,7-trithiapentacene-13-one (TTPO) is a promising organic semiconductor with potential applications in high temperature photovoltaic devices as an electron donor. My STM investigation of TTPO on the close-packed stepped Au(788) surface reveals interesting nanoscale surface structures ranging from molecular chains at low coverage to an ordered self-assembled monolayer. I have utilized density functional theory (DFT) to further probe this unique 3-D angular assembly, where the long-axis of TTPO is parallel to the gold surface, distinctive from previously observed pentacene and pentacene derivative assemblies on surfaces. It is the lateral arrangement of the underlying pentacene backbone of the molecule that is unique, causing the thiol substituent side of the molecule to be angled down towards the gold surface, with the oxygen angled away. This arrangement of the molecule orientating the dipole of the molecule away from the surface could allow for controlled tailoring of the interface. Combining imaging with density functional theory calculations allows for classification of these self-assembled structures with particular interest being directed toward the interaction between TTPO and gold at this organic-metallic interface. DFT calculations were able to reveal structures more complex than initially envisioned through STM imaging alone. Understanding of the structure of such interfaces can potentially guide nanoscale modifications for improved electrical transport and energy-conversion efficiency in future devices. This dissertation examines the self-assembly of TTPO molecules on gold from low-coverage molecular chains to monolayer coverage with the use of STM and DFT methods.

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