Organic Reactions of Gallium Phosphide and Silicon Surfaces for Stability and Dye Sensitization

The primary aim of this thesis is to develop wet-chemical functionalization strategies for GaP(111)A, GaP(111)B, and Si(111) for defect passivation and secondary attachment of molecular species. Analyses were performed that probe the physicochemical and photoelectrochemical properties of modified single crystalline semiconductor surfaces. The motivation for these studies was to augment the interfacial properties of covalent inorganic semiconductors with bulk optoelectronic properties suitable for solar energy conversion processes.;Gallium phosphide (GaP) is a potential photocathode material for dye sensitized solar cell applications. However, GaP photocathodes are limited in water by severe oxidation and dissolution. Although passivation strategies have been identified previously that stabilize GaP surfaces, no method to attach molecular species that does not involve a defective native oxide has been yet shown. Further, definitive proof that defects could be removed by surface modification is lacking.;Silicon surface chemistry is also of interest because of wide use in photovoltaics and microelectronics. Although much is known about Si surface chemistry, there is presently no demonstration of Si surfaces with densely packed monolayers with low surface defect density, the capability for secondary attachment, and stability in wet environments. Such interfacial properties would enhance compatibility for wet-process steps such as atomic layer deposition or spin casting of aqueous metal oxo cluster solutions.;To this end, this thesis describes the following advancements. Chapter 2 describes Williamson ether-type reaction that allows rational attachment of modified Coomassie Blue dye to freshly etched GaP(111)B through atop P atoms. In Chapter 3, GaP(111)A electrodes after chlorination/Grignard reaction sequence exhibited enhanced stability, pH-insensitive band edge energetics, and lower density of surface states.;Chapter 4 contains data demonstrating Si(111) surfaces with simultaneously low defect density, high stability, and high hydrophilicity. In Appendix A, additional work is presented that illustrates how these surface chemistries could be applied to GaP nanowires. Appendix B illustrates the influence of surface groups on sensitization trough band edge energy changes. Appendix C summarizes attempts to produce super hydrophobic Si(111) through termination with --CF3 groups.