Characterization of a Keggin polyoxometalate on metal and semiconductor surfaces.

摘要

This thesis contains studies relating to the interaction between a Keggin polyoxometalate, α-[SiW₁₂O₄₀]4−, and surfaces including Ag, Au, and Si.; First, electrochemical scanning tunneling microscopy is utilized in a specific way to obtain information regarding a monolayer of α-[SiW ₁₂O₄₀]4− on Ag(111). The information obtained is on the nanometer size scale while also being time resolved on the millisecond time scale. At a qualitative level, behavior of individual α-[SiW ₁₂O₄₀]4− molecules is observed within and at the edge of the monolayer. Furthermore, quantitative information is obtained by analyzing step edge and island edge positions. These results show that α-[SiW₁₂O₄₀]4− is substantially less mobile on the surface than are Ag atoms in the absence of the monolayer. In addition, the potential dependence of the surface motion is essentially the opposite with a α-[SiW₁₂O₄₀]4− monolayer present relative to the monolayer free case. The α-[SiW ₁₂O₄₀]4− molecules exhibit increased motion at more negative potentials.; Second, surface sensitive vibrational spectroscopies are utilized to elucidate details of the strong interaction between α-[SiW₁₂O ₄₀]4− and Ag surfaces. Assignment of vibrational modes yields valuable information regarding the nature of the surface interactions, particularly as the potential is varied. The results indicate that vibrational modes arising primarily from different parts of the molecule exhibit different potential dependent behaviors. These results are also correlated to electrochemical results which show peculiar behavior for α-[SiW₁₂O₄₀ ]4− on Ag surfaces. Notably, these results indicate that the interaction of α-[SiW₁₂O₄₀]4− with Au surfaces is substantially different than the interaction of α-[SiW₁₂O₄₀]4− on Ag surfaces.; Finally, studies of α-[SiW₁₂O₄₀]4− on Si surfaces are undertaken. In the first route, the Si surface is derivatized with a multicationic layer and α-[SiW₁₂O ₄₀]4− is assembled on this layer via electrostatic attraction. Characterization of this layer indicates that there is a substantial amount of α-[SiW₁₂O₄₀]4− present on the surface and that the molecule is intact. In the second route, the Si surface is etched and then reacted with α-[SiW₁₂O ₄₀]4−, with the goal being covalent attachment of α-[SiW₁₂O₄₀]4− directly to the Si surface. Utilizing a specific deposition scheme, the results show that α-[SiW₁₂O₄₀]4− is likely present on the surface as a very thin layer.