Metal / Organic and Metal / Inorganic Interfaces: Interfacial Bond Energies, Structure and Energy-Level Alignment
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Metals deposited on surfaces play an important role in many applications, such as heterogeneous catalysts, fuel cells, photovoltaics, microelectronics and coatings. Further insight into the energetics and structure of metals deposited on surfaces can lead to more efficient catalysts and photovoltaics. The first part of this dissertation details the results from studying the adsorption of Ca onto poly[2-(2-ethylhexyloxy)-5-methoxy-1,4-phenylene cyano¬vinylene] (MEH-CN-PPV), poly(9,9-di-n-hexyl-2,7-fluorene) (PDHF) and poly(9,9-di-n-hexyl-2,7-fluorene vinylene) (PDHFV), polymers of interest for organic photovoltaics. Insights into interfacial binding were obtained using microcalorimetry, X-ray photoelectron spectroscopy (XPS) and low energy ion scattering spectroscopy (LEIS). The second part details the results from studying Ag adsorption onto Fe3O4(111) using the same techniques, and the third part describes a new method for analyzing signal intensities measured during thin film growth by electron spectroscopies, such XPS and Auger electron spectroscopy (AES), to determine quantitative details of film morphology, specifically extending the well-known "hemispherical cap model" to angles of detection other than normal to the surface. For the adsorption of Ca on polymer surfaces, it is found that whenever the polymer contains atoms other than hydrogen or carbon, Ca reacts aggressively with these heteroatoms with high heat. For MEH-CN-PPV, Ca was shown to initially react with the cyano group and form Ca(CN)2 clusters (Hads = 436 kJ/mol). There is also XPS evidence for Ca reacting with some of the ether groups to make Ca alkoxides at low coverage. After that, Ca reacted with other Ca on the surface to form 3D islands of Ca(s) (Had = 464 kJ/mol) until ~15 ML of Ca coverage, where the polymer surface is completely covered by Ca. For both PDHF and PDFHV, which ideally contain only hydrogen and carbon, the amount of Ca that reacted aggressively with the polymer was 10-fold less than the amount observed for polymers with heteroatoms. Its initial high heat of adsorption (253-315 kJ/mol) is attributed to Ca reacting with impurities or defects in either PDHF or PDFHV. After reaction with defects or impurities, Ca forms 3D Ca(s) islands which eventually grow together to make continuous film. For Ag on Fe3O4(111), Ag was found to adsorb with an initial heat of adsorption ~230 kJ/mol and increase to within a few percent of the heat of sublimation of bulk Ag (285 kJ/mol) by ~2 ML Ag. Using AES, low energy ion scattering spectroscopy (LEIS), and modeling the growth of Ag on the surface as hemispherical caps, it was found that Ag had a particle density of ~4 x 1012 particles/cm2, from which the heat of adsorption versus particle diameter was extracted.
- Chemistry