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|Title: ||Stereochemical Effects on the Organizational and Electronic Structure of the Tartaric Acid/Ag(111) System|
|Authors: ||Santagata, Nancy Marie|
|Advisors: ||Marco Buongiorno Nardelli, Committee Member|
Lin He, Committee Member
Christopher Gorman, Committee Co-Chair
Thomas P. Pearl, Committee Co-Chair
|Keywords: ||scanning tunneling microscopy|
Shockley-type surface state
|Issue Date: ||7-Aug-2009|
|Abstract: ||This dissertation aims to develop an understanding of the forces that drive the organization of organic molecules at metallic surfaces, namely intermolecular structure, in the limit of weak molecule-surface interactions. A model system, composed of a chiral molecule, tartaric acid (C4H6O6), and a metallic surface with unique electronic properties, Ag(111), is employed. The interfacial organizational and electronic structures of the tartaric acid/Ag(111) system has been studied in detail with low energy electron diffraction (LEED), low temperature scanning tunneling microscopy (STM) and spectroscopy (STS), differential conductance (dI/dV) mapping, and density functional theory (DFT).
Molecularly resolved STM images of both (R,R)- and (S,S)-tartaric acid on Ag(111) in the submonolayer coverage regime reveal the role of intermolecular hydrogen bonding in stereospecific domain formation. Global chirality is expressed upon the deposition of enantiopure tartaric acid; therefore, directional and anisotropic lateral interactions involving molecular chiral centers dictate domain organization despite a weak interaction with the underlying Ag(111) lattice. Further, these enantiopure films are characterized by adsorbates whose molecular axis lies parallel to the plane of the surface. In contrast, films deposited from a racemic mixture do not separate laterally into homochiral domains with global chirality. Molecularly resolved STM images, in combination with DFT simulations, confirm that these racemic films are composed of a unique paired (S,S)-/(R,R)-tartaric acid basis whose combined molecular axis is oriented perpendicular to the plane of the surface. The differences in adsorption geometry for enantiopure versus racemic tartaric acid films is therefore controlled by intermolecular interactions involving chiral centers, indicating that chirality can be utilized in the directed two-dimensional assembly of molecular components.
This dissertation also describes the modification of the Ag(111) Shockley-type surface state as a signature of tartaric acid adsorbate structure. Shockley-type surface states exist on several metal surfaces and are characterized by electron confinement by the vacuum barrier on one side and a band gap in the bulk on the other. The proximity of the surface state to the Fermi level (-67 meV for this particular surface) amplifies its role in surface chemistry, bonding, and organization. The adsorption of both enantiopure and racemic tartaric acid in the submonolayer regime induces a positive shift of the Ag(111) surface state energy after the adsorption of both forms of tartaric acid. The magnitude of the shift differs, however, for films composed of either enantiopure (E(R,R) TA=813.9 Ã‚Â± 2.9 meV) or racemic (ÃŽâ€ EDL TA=54.5 Ã‚Â± 3.5 meV) domains. These film-dependent modifications of the Shockley-type surface state are attributed to unique the adsorbate geometries, which are controlled by intermolecular interactions involving chiral centers. In sum, the combined experimental and theoretical results presented herein indicate that internal molecular structure (chirality) can be exploited for the design of rational nanostructures that possess tailored structural and electronic properties.|
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