X -Ray Absorption Fine Structure (EXAFS and XANES) and Microscopic Investigations of Supramolecular Systems.
Date of Award
Doctor of Philosophy (PhD)
Robin L. McCarley
Erwin D. Poliakoff
Self-assembled monolayers of n-alkanethiols, CH3-(CH 2)x-SH, on Au, Ag, and Cu have been studied with Grazing incidence EXAFS (GIXAFS) at the sulfur K-edge. Characterization of the bonding of self-assembled monolayers is of great interest because of their ability to tailor physical and chemical properties of surfaces. For both pentanethiol and decanethiol monolayers on Ag and Cu, the three-fold hollow site is found to be the most probable sulfur binding site. However, observations for octadecanethiol indicate that the three-fold-hollow site is not the exclusive binding site. In addition, the possible existence of disulfide bonds on the metal surface (adsorbed dialkyldisulfides) is not supported by the data. In addition, careful examination of the systems by XANES clearly demonstrate the peculiar behavior of octadecanethiol on surfaces. Preliminary results from monolayers on Au are also reported and constitute the first ever reported EXAFS data for alkanethiol monolayers on gold. These findings represent a great advance for the investigation of thin films on surfaces, and also present the possibility to study mixed monolayers as a complement to scanning probe microscopy. Recently, dendritic systems, and especially dendrimer-metal complexes, have been shown to have outstanding properties which make them suitable for applications in catalysis and biology. Our long-term plans include investigations of adsorbed dendrimer-metal complexes on metal surfaces through the use of grazing incidence EXAFS. Preliminary X-ray absorption studies of poly(propylene imine) dendrimer-copper(II) complexes of generation 1--5 in powder form and methanolic solution are presented here. XANES allows determination of the oxidation state of the copper species and the geometry of the copper complex whereas EXAFS gives us information about bond distances, coordination number and nature of the neighbor atoms. When reacted with NaBH4, these dendrimer-copper complexes undergo reduction which leads to the formation of Cu(0) nanoclusters. We investigated these clusters by EXAFS, XANES, ultra violet-visible spectroscopy and high resolution transmission electron microscopy (HR-TEM) and discuss their geometry, composition and size depending on the dendrimer generation. The synthesis of metallic nanoclusters by the dendrimer route has been successful to produce nanoclusters of reduced size and increased monodispersity.
Floriano, Pierre Nicolas, "X -Ray Absorption Fine Structure (EXAFS and XANES) and Microscopic Investigations of Supramolecular Systems." (2001). LSU Historical Dissertations and Theses. 284.