Energy level alignment, self-organization, and covalent bond formation of organic molecules on inorganic surfaces
Creators: name: Viereck, Jonathan D.
Publisher: name: No Publisher Supplied
Year: 2022

description: The interface between an organic molecule and inorganic metal or semiconducting substrate serves a multifaceted role including to facilitate transfer of charge across the interface, template the growth of organic thin films, and catalyze chemical transformations of organics on surfaces. Especially as organic molecules are becoming more utilized in organic electronics due to their abundance, low cost, and high degree of tunability, it is of the utmost importance to develop a picture of these interfacial processes at the molecular level. To achieve the level of control required to study fundamental properties of the interface, we instead study model systems, made by preparing molecular thin films on a well-defined facet of a single crystal substrate in an ultrahigh vacuum (UHV) environment. Using scanning tunneling microscopy, direct and inverse photoemission spectroscopy, and temperature programmed desorption, contained within the UHV environment, aided by density functional theory, we study aspects of energy alignment, self-organization, and covalent bond formation of organic molecules on inorganic surfaces. The work presented in this thesis focuses on three different organic/inorganic systems. First we investigate two different methods to modify the alignment of a chromophore's molecular orbitals with respect to the band edges of a transition metal oxide substrate, using a monolayer of dipole-containing helical peptides. This dipole layer establishes an electrostatic potential difference between surface of the oxide and the chromophore. Two different methods are explored that use this potential to shift the energy levels of the organic molecule, and do not require the chromophore to be chemically bound to the dipole: a mixed layer of interspersed helical peptides and chromophores, and a blanket-layer of helical peptides between the chromophore and the surface. Second, we investigate the role of a metal substrate on the crystalline growth of organic thin films, by studying rubrene and a fluorinated derivative, FM-rubrene, adsorbed on the Ag(100) surface. Both molecules experience a strong interaction with the metal substrate which causes the self-assembly of each molecule on the surface to be different than that of their organic single crystal counterparts, and unable to template epitaxial growth. Additionally, the effect of fluorine functionalization is explored on the evolution of energy alignment during multilayer growth. Lastly, we investigate the role of the metal substrate on catalyzing covalent bond formation between molecular precursors. Monolayer films of organic molecules, possessing particularly chosen reactive groups, can be annealed on a metal substrate to trigger intermolecular reactions that form intermolecular bonds, and lead to the growth of extended 2D covalent structures on surfaces. We systematically investigate a novel reaction for covalent bond formation, elimination of gas phase HF, on three different metal surfaces, Cu(100), Ag(100), and Au(111) and determine that the mechanism underlying HF elimination is fundamentally different than other dehalogenation methods typically used for covalent bond formation on metal surfaces.