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\usepackage{setspace} |
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\usepackage{endfloat} |
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\usepackage{caption} |
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%\usepackage{tabularx} |
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\usepackage{graphicx} |
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\usepackage{multirow} |
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%authors |
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% make the title |
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< |
\maketitle/ |
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> |
\maketitle |
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|
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\begin{doublespace} |
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|
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mechanical predictions (-2.46 D~\AA)\cite{QuadrupoleCOCalc}. |
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%CO Table |
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\begin{table}[H] |
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\caption{Positions, $\sigma$, $\epsilon$ and charges for CO geometry and self-interactions\cite{Straub}. Distances are in \AA~, energies are in kcal/mol, and charges are in $e$.} |
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\caption{Positions, $\sigma$, $\epsilon$ and charges for CO geometry |
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and self-interactions\cite{Straub}. Distances are in \AA~, energies are |
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in kcal/mol, and charges are in $e$.} |
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\centering |
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\begin{tabular}{| c | c | ccc |} |
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\hline |
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& Calc. & Exp. \\ |
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\hline |
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\textbf{Pt-CO} & -1.9 & -1.4~\cite{Kelemen:1979}-- -1.9~\cite{Yeo} \\ |
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\textbf{Au-CO} & -0.39 & -0.44~\cite{TPD_Gold} \\ |
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\textbf{Au-CO} & -0.39 & -0.40~\cite{TPD_Gold} \\ |
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\hline |
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\end{tabular} |
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\end{table} |
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% Just results, leave discussion for discussion section |
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\section{Results} |
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\subsection{Diffusion} |
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< |
While an ideal metallic surface is unlikely to experience much surface diffusion, high-index surfaces have large numbers of low-coordinated atoms which have a much easier time overcoming the energetic barriers limiting diffusion, leading to easier surface reconstructions. Surface movement was divided between the parallel ($\parallel$) and perpendicular ($\perp$) directions relative to the step edge. We were then able to calculate diffusion constants as a function of CO coverage. As can be seen in Table 4, the presence and amount of CO directly affects the diffusion constants of surface platinum atoms. The presence of two 50\% coverage systems is to show how the diffusion process is affected by time. The majority of the systems were run for approximately 50 ns while the half monolayer system has been running continuously. The lowered diffusion constant at longer run times will be examined in-depth in the discussion section. |
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An ideal metal surface displaying a low-energy facet, a (111) face for |
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instance, is unlikely to experience much surface diffusion because of |
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the large energy barrier associated with atoms 'lifting' from the top |
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layer to then be able to explore the surface. Rougher surfaces, those |
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that already contain numerous adatoms, step edges, and kinks, should |
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have concomitantly higher surface diffusion rates. Tao et al. showed |
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that the platinum 557 surface undergoes two separate reconstructions |
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upon CO adsorption. \cite{Tao:2010} The first reconstruction involves a |
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doubling of the step edge height which is accomplished by a doubling |
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of the plateau length. The second reconstruction led to the formation of |
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triangular motifs stretching across the lengthened plateaus. |
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|
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As shown in Figure 2, over a period of approximately 100 ns, the surface |
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has reconstructed from a 557 surface by doubling the step height and |
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step length. Focusing on only the platinum, or gold, atoms that were |
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deemed mobile on the surface, an analysis of the surface diffusion was |
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performed. A particle was considered mobile once it had traveled more |
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than 2~\AA between snapshots. This immediately eliminates all of the |
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bulk metal and greatly limits the number of surface atoms examined. |
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Since diffusion on a surface is strongly affected by overcoming energy |
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barriers, the diffusion parallel to the step edge axis was determined |
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separately from the diffusion perpendicular to the step edge. The results |
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at various coverages on both platinum and gold are shown in Table 4. |
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|
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%While an ideal metallic surface is unlikely to experience much surface diffusion, high-index surfaces have large numbers of low-coordinated atoms which have a much easier time overcoming the energetic barriers limiting diffusion, leading to easier surface reconstructions. Surface movement was divided between the parallel ($\parallel$) and perpendicular ($\perp$) directions relative to the step edge. We were then able to calculate diffusion constants as a function of CO coverage. As can be seen in Table 4, the presence and amount of CO directly affects the diffusion constants of surface platinum atoms. The presence of two 50\% coverage systems is to show how the diffusion process is affected by time. The majority of the systems were run for approximately 50 ns while the half monolayer system has been running continuously. The lowered diffusion constant at longer run times will be examined in-depth in the discussion section. |
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|
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\begin{figure}[H] |
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\includegraphics[scale=0.6]{DiffusionComparison_error.png} |
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\caption{Diffusion parallel to the step edge will always be higher than that perpendicular to the edge because of the lower energy barrier associated with going from approximately 7 nearest neighbors to 5, as compared to the 3 of an adatom. Additionally, the observed maximum and subsequent decrease for the Pt system suggests that the CO self-interactions are playing a significant role with regards to movement of the platinum atoms around and more importantly across the surface. } |
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%Evolution of surface |
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\begin{figure}[H] |
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\includegraphics[scale=0.5]{ProgressionOfDoubleLayerFormation_yellowCircle.png} |
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< |
\caption{Four snapshots at various times a) 258 ps b) 19 ns c) 31.2 ns d) 86.1 ns. Slight disruption of the surface occurs fairly quickly. However, the doubling of the layers seems to be very dependent on the initial linking of two separate step edges. The focal point in b, appears to be a growth spot for the rest of the double layer.} |
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\caption{Four snapshots of the $\frac{1}{2}$ monolayer system at various times a) 258 ps b) 19 ns c) 31.2 ns and d) 86.1 ns. Slight disruption of the surface occurs fairly quickly. However, the doubling of the layers seems to be very dependent on the initial linking of two separate step edges. The focal point in b, appears to be a growth spot for the rest of the double layer.} |
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\end{figure} |
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