First-principles theoretical characterization of the electronic structure of iron and iron oxide surfaces and their interfaces with organic layers
Noh, Jung Hyun
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Organic compounds offer considerable promise in iron corrosion prevention. Attachment of organic molecules to the metal surface can alter the corrosion reactions by changing the activation barriers of the anodic and/or cathodic electrochemical reactions. Given that detailed experimental investigations of interfacial geometric and electronic structures remain challenging, computational approaches that provide quantitative structure-property descriptions of the interfacial processes can help elucidate the inhibition mechanisms of molecular modifiers with various chemical functionalities. In this dissertation, we focus on the development of an adequate theoretical platform to investigate the surfaces and interfaces involved in metal oxidation and corrosion, taking iron as the prototypical reactive transition metal element. One of the difficulties in developing appropriate theoretical models to describe organic/inorganic heterojunctions stems from the complex nature of the exposed surfaces as a function of environmental conditions. As a first step, clean iron and iron oxide surfaces were therefore examined to determine their equilibrium surface configurations in various atmospheric and electrochemical conditions. Subsequently, self-assembled monolayers were attached to those surfaces to evaluate their physical and chemical impact on the corrosion processes. We used a theoretical approach containing quantum mechanical methods and molecular dynamic simulations in order to describe the interfacial geometric and electronic structures of iron with organic modifiers. Our findings underline that long-term stability of the monolayer film can be achieved via the molecular design of self-assembled monolayers and substrate pretreatment. While in corrosion science and engineering, technologies are developed mainly to circumvent iron oxidation, it must be borne in mind that iron oxides themselves can also be of practical importance on their own, for instance, in the field of organic spintronics. In this context, the electronic and magnetic properties of interfaces between iron-based ferro-(or ferri-)magnetic electrodes and representative π-conjugated molecules have been explored. Our work underlines that not only the choice of materials but also the details of the interfacial structure, e.g., substrate crystal orientation or termination and molecular adsorption geometries, have significant impact on the nature of the hybrid interface electronic states. Our results provide guidelines to tailor the spin transfer behavior at a magnetic junction in order to achieve maximum device efficiency in applications such as non-volatile magnetic random access memories and magnetic-field sensors.