Authors: Sofiya Hlynchuk
Source: Deep Blue (University of Michigan) (article, 2019-01-01)
Field: Materials Science — Materials Chemistry

Functional Organic Monolayers on Semiconductor Surfaces This thesis describes wet chemical surface functionalization strategies that introduce organic groups onto semiconductor surfaces silicon (Si) and gallium phosphide (GaP). The overarching motivation is to develop tailored interfaces for particular electrical and photoelectrochemical applications. This thesis employs concepts developed by previous group members to design semiconductor surfaces with specific wetting properties, with better adhesion to photoresists, and with molecular sensitizers for sensitization. This thesis also demonstrates a new avenue for functionalizing GaP surfaces. In chapter 2, surface functionalization strategies on Si are developed that yield surficial hydroxyl and amine functional groups. These functionalities alter the wetting properties of Si while also acting as reactive handles for surface reactions. These organic monolayers were characterized by grazing angle attenuated total reflectance infrared (GAATR-IR) and X-ray photoelectron (XP) spectroscopies. The qualities of the interface were assessed by measuring surface recombination velocities of photogenerated charge carriers by microwave photoconductivity. The net results show that it is possible to achieve three distinct surface properties on Si: hydrophilicity, secondary reactivity, and good electronic passivation. In chapter 3, a specific demonstration of Si surface functionalization is presented. The objective is to improve adhesion between a photoresist film and a Si surface under humid (wet) conditions. Si surfaces with a monolayer consisting of terminal alkene groups were prepared and characterized by GAATR-IR and XP spectroscopies. The adhesion between SU8 photoresist and alkene-terminated Si surfaces was probed using the nanoindentation method and the chemical integrity of adhesion was studied determined after exposure to strongly alkaline conditions. The chemical structure of several types of Si/SU8 interfaces were additionally characterized using sum frequency generation (SFG) vibrational spectroscopy. Overall, the experimental data illustrate that a purposely functionalized Si surface can yield Si/SU8 contacts with desirable properties. The second portion of this thesis focuses on the surface chemistry of GaP. Chapter 4 describes the sensitization of p-GaP photocathodes in the presence of physisorbed dye. Freshly etched p-GaP(100) and p-GaP(111)A electrodes were loaded with several triarylmethane dyes by soaking the electrodes in an aqueous solution of dye. Dye coverages were evaluated using XP and Auger electron spectroscopies. The magnitude of sensitization currents were probed by measuring steady-state photoelectrochemical responses. The cumulative findings showed low dye loading and cathodic degradation were common occurrences when sensitization was attempted with bare GaP. The data suggest that avoiding these issues requires developing p-GaP electrodes where the dye is covalently attached and the underlying GaP surface is otherwise passivated. In chapter 5, the idea of deliberately functionalizing GaP surface with a method that is agnostic to crystallographic surface type is investigated. Functionalization of GaP(100) and GaP(111)A using thermal activation of alkenes is described. Alkene grafting reactioins were evaluated under various temperatures, reactions times, and surface pretreatments. Although functional groups were introduced on various GaP surfaces successfully, low surface coverages were routinely observed. This aspect limited the ability of this methodology to yield surface passivation layers that inhibited chemical oxidation in ambient conditions. However, this reaction did provide a path to higher dye loadings. Quaternary amines were introduced to GaP surfaces to attract anionic dye. High dye loading was detected by XP spectroscopy. Still, the sensitization currents remained low. Some potential reasons are presented in the text. Finally, chapter 6 provides a summary of the major conclusions of the cumulative work, with additional details regarding possible future experiments. Monolayer ; Semiconductor ; Organic semiconductor ; Materials science ; Nanotechnology ; Optoelectronics Machine Learning in Materials Science ; Surface and Thin Film Phenomena ; Molecular Junctions and Nanostructures