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Metastable Induced Electron Spectroscopy of Iron Pentacarbonyl on Gold

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Title: Metastable Induced Electron Spectroscopy of Iron Pentacarbonyl on Gold
Author: Read, Stuart T
Department: Department of Chemistry
Program: Chemistry
Advisor: Rowntree, Paul
Abstract: The deposition of thin metal films is an important process for many industrial and scientific applications. Iron pentacarbonyl [Fe(CO)5] is a candidate for chemical vapour deposition of metal films as it is volatile and dissociated carbonyl ligands are easily removed from the system. Depending on substrate and reaction conditions, Fe(CO)5 can undergo thermal, photolytic and electron-induced decarbonylation. Resulting film properties such as surface uniformity and metal purity will also depend on the dissociation route and the initial structure of the precursor. Iron pentacarbonyl is known to deposit on Au(111) in a well-ordered but thermodynamically unstable film at cryogenic temperatures. Heating this structure results in an irreversible transformation of the film. This transformation was previously proposed to be the aggregation of Fe(CO)5 into clusters, however the true surface structure of this transformed film had not been directly measured. The properties of this film before and after heating are important because the transformed film is resistant to electron-induced decarbonylation. Metastable induced electron spectroscopy (MIES) uses electronically excited He atoms to probe the electronic structure at the vacuum/sample interface. The technique is sensitive to only exposed electron density and the excellent surface sensitivity of MIES provides insight into the surface structure of the Fe(CO)5/Au system not accessible to previous measurements. A unique experimental system which combines MIES, infrared reflection-absorption spectroscopy (IRRAS) and two-photon photoemission spectroscopy (2PPE) was developed as a part of this work. A quantitative model relating the MIES intensities to the fractional surface coverage of Fe(CO)5 on the surface was proposed. This model was then used to show the presence of a three-stage growth process during film deposition on the Au surface. Heating of the film results in aggregation into 3D clusters as well as regions of bare substrate and monolayer coverage. Photolysis of the Fe(CO)5 film results in incomplete decarbonylation and is more efficient for the film structure before heating. 2PPE of Fe(CO)5 proved experimentally challenging due to the efficient photolysis reaction. The application of MIES and IRRAS together to the study of this system provided structural information not previously accessible.
Date: 2014-12
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