| In the Fischer-Tropsch Synthesis, hydrogenation of carbon monoxide is used to make hydrocarbons with iron as the catalyst. Sulfur from coal deactivates iron. Manganese increases the activity of iron in the presence of sulfur. Different alloys of iron and manganese (100% Fe, 100% Mn, 50% Fe-50% Mn) were made by Okasaki's method of electro-compaction. Thin films were then made by evaporation of the bulk alloy with silicon as the substrate. Our primary goal was to make atomic resolution images of manganese using a scanning tunneling microscope (STM). However, we did not get atomic-scale images for thin films deposited on Si. Every effort was then made to improve the stability of the instrument. A double stage vibration isolation stage was built. Other aspects like the etching of sharp Pt/Ir tips was achieved by a new approach that incorporated a double etching process.; While using the STM for analysis of the bulk thin films, a high percentage of manganese films showed Mn-like surface features. Images from an atomic force microscope, on the other hand, showed that the grain size of the bulk iron thin films was less than that of manganese. The size of the grains (50 nm to 150 nm) observed was in agreement with the University of Utah researchers, Jensen et al. Due to the affinity of Si towards oxygen, we concluded that Si was not a good choice for depositing thin films of iron or manganese as atomic resolution was not achieved.; We then switched to graphite (Highly Oriented Pyrolytic Graphite single crystal, HOPG) as our substrate. I was fortunate to be a part of the University of Louisville team that observed all the six carbon sites of graphite well resolved with exact dimensions of 2.45 A (in agreement with the standard result) between {dollar}alpha{dollar}- and {dollar}alpha{dollar}-, {dollar}beta{dollar}- and {dollar}beta{dollar}-, and 1.4 A between the nearest neighbors {dollar}alpha{dollar}- and {dollar}beta{dollar}-. Contributions from {dollar}alpha{dollar}- and {dollar}beta{dollar}- sites varied with the bias. At an intermediate bias between 0 and 0.5 V, equal contributions from both {dollar}alpha{dollar}- and {dollar}beta{dollar}-sites were observed. The asymmetry factor as defined by Tomanek et al. (the contribution from {dollar}alpha{dollar}- and {dollar}beta{dollar}- sites) showed a minimum {dollar}approx{dollar}0.3 V.; On graphite, thin films of Fe, Mn, and 50% Fe-50% Mn were deposited and studied using an STM. Even on graphite, a large thickness (a few hundred A) of adsorbate-metal thin film did not provide us with well-resolved atomic scale images. At lower thickness ({dollar}approx{dollar}10 A), well-resolved atomic scale images were observed. Usually, metal films are difficult to image while using an STM. To get around this problem, dissimilar metals were used to image oxides of iron over a graphite substrate. Iron deposited on graphite showed a lattice spacing of {dollar}approx{dollar}3 A, in agreement with Galloway et al. The Mn lattice was successfully imaged with 3.33 and 4.44 A unit vectors and with 83.4{dollar}spcirc{dollar} between the unit vectors. Also, catalytically active sites like steps, mounds, point defects, and clusters were successfully imaged. (Abstract shortened by UMI.)... |
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