Submitted to the Department of Chemical Engineering.
Thesis (Ph.D.)--Stanford University, 2013.
Organic functionalization of semiconductor surfaces allows established knowledge of inorganic semiconductor-based microelectronics to be combined with highly tailorable organic molecules to enable novel technologies. Such molecular level control may contribute to continued microelectronics device scaling as well as permit new device functionality that does not directly depend on device dimensions. The latter is of increasing interest as integrated circuit dimensions approach atomic length scales and physical limitations restrict continued device scaling. Application of organic functionalization requires a detailed understanding of semiconductor surface chemistry, and the work in this thesis is aimed at growing our fundamental understanding of the chemistry at these surfaces. Using primarily the Ge(100)-2×1 surface as a model system, infrared spectroscopy and X-ray photoelectron spectroscopy experiments in ultra-high vacuum and quantum chemical calculations were performed to elucidate specific features of organic functionalization chemistry. Most studies in the organic functionalization literature focus on understanding the reactivity of individual molecules. Part of this thesis is devoted to bringing together such results and using density functional theory calculations to reveal broader trends in functionalization chemistry. In particular, it is shown that reactivity of organic molecules at semiconductor surfaces often follows periodic trends. On the Ge(100)-2×1 surface, X--Ge dative bond strength for analogous functional groups increases for heteroatoms, X, located lower in a group or to the left in a period. The same trends apply on the Si(100)-2×1 surface, although dative bonding is overall more favorable on Si than on Ge. On the other hand, for ordinary covalent bonds to the Ge(100)-2×1 surface, bond strength decreases down the periodic table. To provide an understanding of their chemical origin, these trends are explained by differences in electron affinity, electronegativity, and atomic size or orbital overlap. Besides showing how atomic properties relate to surface reactivity, identification of periodic trends offers a powerful tool for predicting reactivity by extending results from one system to related systems. Developing a broad knowledge of how different organic functional groups react at a semiconductor surface provides a foundation for future applications of organic functionalization. To this end, a number of adsorbates with functional groups that have not been previously studied in detail were investigated at the Ge(100)-2×1 surface including dimethyl sulfoxide, trimethyl phosphite, and dimethyl phosphite. Dimethyl sulfoxide undergoes S--C dissociation to form surface-bound methyl and CH3SO¬ fragments and S=O dissociation which leaves adsorbed atomic oxygen at the surface. Both reaction pathways traverse through an oxygen dative-bonded intermediate, which is favored over S--Ge dative bonding due to the dipolar nature of the sulfoxide functional group. Studies of trimethyl phosphite and dimethyl phosphite represent the first detailed studies of adsorption of phosphorus-based functional groups at the Ge(100)-2×1 surface aside from phosphines. Trimethyl phosphite undergoes C--O dissociation of one of its methoxy groups to form dimethyl germylphosphonate, while dimethyl phosphite undergoes C--O and P--H dissociation forming methyl germyl phosphonate and dimethyl germylphosphonate, respectively. As with dimethyl sulfoxide, multiple dative-bonded intermediates are possible for trimethyl phosphite and dimethyl phosphite, and density functional theory calculations demonstrate that the stability of the dative-bonded intermediates plays a large role in determining the reaction pathway. Recently, much attention in organic functionalization has turned to adsorption of molecules with more than one functional group, which can adsorb while leaving one or more functional group exposed to enable further chemistry or impart specific properties. However, multifunctional molecules add a layer of complexity in achieving selective surface attachment. Two sets of homobifunctional molecules—diisocyanates and cyclohexanediamines—were studied to elucidate how changes in the molecular backbone affect adsorption. Although differing in their attachment chemistry, the results for both sets of adsorbates show that small changes in the backbone geometry or composition can affect not only whether adsorption occurs through one or both functional groups, but also the reaction pathway. Isocyanates primarily react with the Ge(100)-2×1 surface by [2+2] cycloaddition across the C=N bond, and bonding via both functional groups of a diisocyanate is favored for an alkyl versus aryl backbone. Interestingly, in all cases, [2+2] cycloaddition across the C=O bond accounts for a small percentage of products despite such reaction not being observed for monofunctional isocyanates. This result shows that bifunctionality may enable reaction pathways that do not occur for monofunctional molecules. For all cyclohexanediamine isomers, experiments show that a mixture of N dative bonded and N--H dissociated amines are observed, but the ratio of dative bonding to dissociation varies by isomer. Density functional theory calculations are employed to show that the differences in reactivity are driven primarily by small differences in strain of the adsorbate and surface-adsorbate bonds. Although achieving selective attachment of bifunctional molecules remains a challenge, these results demonstrate tools that may be used for such purposes. Overall, this thesis addresses several important topics in semiconductor organic functionalization: periodic trends in reactivity, reaction of new functional groups, the importance and role of dative bonding, and effects of molecular properties on reactivity of bifunctional molecules. Together, these studies continue to advance our understanding of fundamental chemistry at semiconductor surfaces with the ultimate aim of designing organic adsorbates from the ground up for specific applications.