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Thesis (M.S. in mechanical engineering)--Washington State University, December 2009. / Title from PDF title page (viewed on Feb. 4, 2010). "School of Mechanical and Materials Engineering." Includes bibliographical references (p. 42-45).

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Many deductions made about STM images are based upon the model of Tersoff and Hamann, in which images are given in principal by a combination of surface atomic positions and local charge density. There is a now a need for a fuller understanding of this technique in order to explain experimental evidence which indicates that the tip and sample can interact strongly during normal imaging. In order to investigate the fundamental STM imaging process, a method for deducing the tunnel barrier height has been developed which is based on corrugation height measurements of constant current topographs. From experiments on clean Cu(100), values of the tunnel barrier height have been shown to be somewhat below the workfunction (~ 1-2.5eV) but are in good agreement with other reports of atomically resolved barrier height data. At large values of the tunnel conductance (~ 1μS), a fall-off (based upon extrapolation of large separation data) in the corrugation heights is observed with increasing conductance. This effect is quantitatively explained using a Molecular Dynamics simulation of the tip approaching the sample. The simulation gives a good estimate of both the absolute tip-sample separation and site-dependent tip-surface forces. Distributions of corrugation heights indicate that variations in both tip geometry and chemistry are likely to occur in practice and strongly influence the phenomena described above. Similarly, it is found that increased local tunnel barrier heights are measured when the Cu(100) surface is modified with small numbers of single halogen atoms. This data has been used to estimate the contributions to the increase in local barrier height of both adsorbate induced dipoles and geometric topography. Values for the charge transfer between the surface and adsorbate have been established. The process of tip-induced adsorbate manipulation has also been demonstrated at room temperature.

A dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of requirements for the degree of Master of Science. Johannesburg, October 2016. / The adsorption of molecules on a metal surface is core in heterogeneous catalysis. Surface sensitive techniques such as low energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD) are key tools to study adsorption geometries and structures of molecules and atoms on a metal surface. As our first model system we investigated the dissociation of NO on Ir{100}. The LEED experimental results showed a p(2 X 2) diffraction pattern at 300 K using. In this study two options were explored: phase mixing where dissociated nitrogen and oxygen are on the same unit cell, as well as phase separation where both nitrogen and oxygen form their own separate unit cell which results in a p(2 x 2) unit cell. Calculations were done on atop, bridge and hollow sites, with only perpendicular parameters and vibrational amplitude being varied initially. Results for phase mixing calculations gave the lowest R-factor of 0.70 ± 0.11 for atop site. We further considered phase separation for hollow and bridge sites for nitrogen and oxygen respectively because these two sites were found to be the most stable sites using DFT from previous studies. The lowest R-factors were 0.37 ± 0.06 for nitrogen c(2x2) and 0.24 ± 0.13 for oxygen p(2 X 1) For oxygen significant row pairing of iridium atoms stabilized the structure as mentioned in previous studies. Therefore from our results it is evident that phase separation models the experimental data better than phase mixing. Nitrogen and oxygen form c(2 X 2) and p(2 X 1) overlayer structures respectively which in combination result in a p(2 X 2) pattern that is in agreement with experimental results. The second system involves enantio-selectivity and chiral resolution at the organic-inorganic interfaces. The d-serine molecule was adsorbed on the Cu{110} surface. Density functional theory (DFT) calculations were used as a benchmark for our CLEED calculations. LEED experiments showed a (- 1 + 2: 40) overlayer pattern for d-serine adsorbed on Cu{110} surface. Three structures from DFT calculations with the lowest energy were used for CLEED calculations. These structures differed by the way they bond to the surface and molecular interactions. Calculations were carried out on these three structures and the structure with intra-dimer bonding was the best structure. The searches for this structure were further optimized by introducing pairing of the atoms in the row reconstruction on the copper surface and angle search. The lowest value obtained was 0.37 ± 0.09, which suggests that further understanding of this system is needed. The ultra-high vacuum (UHV) chamber was fully commissioned and is now ready for TPD and XPS studies. / LG2017

Thesis. 1977. Ph.D.--Massachusetts Institute of Technology. Dept. of Mechanical Engineering. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND ENGINEERING. / Vita. / Includes bibliographical references. / Ph.D.