--- trunk/COonPt/firstTry.tex	2013/03/18 21:20:12	3879
+++ trunk/COonPt/firstTry.tex	2013/03/19 15:01:59	3880
@@ -86,6 +86,17 @@ systems and the lack  of reconstruction of the Au syst
 is sufficient to explain the reconstructions observed on the Pt 
 systems and the lack  of reconstruction of the Au systems.
 
+
+The mechanism and dynamics of surface reconstructions of Pt(557) 
+and Au(557) exposed to various coverages of carbon monoxide (CO) 
+were investigated using molecular dynamics simulations. Metal-CO 
+interactions were parameterized from experimental data and plane-wave 
+Density Functional Theory (DFT) calculations.  The large difference in 
+binding strengths of the Pt-CO and Au-CO interactions was found to play 
+a significant role in step-edge stability and adatom diffusion constants. 
+The energetics of CO adsorbed to the surface is sufficient to explain the 
+step-doubling reconstruction observed on Pt(557) and the lack of such 
+a reconstruction on the Au(557) surface.
 \end{abstract}
 
 \newpage
@@ -161,7 +172,7 @@ Au-Au and Pt-Pt interactions.\cite{EAM} The CO was mod
 Coulomb potential.  For this work, we have used classical molecular
 dynamics with potential energy surfaces that are specifically tuned
 for transition metals.  In particular, we used the EAM potential for
-Au-Au and Pt-Pt interactions.\cite{EAM} The CO was modeled using a rigid
+Au-Au and Pt-Pt interactions.\cite{Foiles86} The CO was modeled using a rigid
 three-site model developed by Straub and Karplus for studying
 photodissociation of CO from myoglobin.\cite{Straub} The Au-CO and
 Pt-CO cross interactions were parameterized as part of this work.
@@ -174,8 +185,8 @@ parameter sets. The glue model of Ercolessi {\it et al
 methods,\cite{Daw84,Foiles86,Johnson89,Daw89,Plimpton93,Voter95a,Lu97,Alemany98}
 but other models like the Finnis-Sinclair\cite{Finnis84,Chen90} and
 the quantum-corrected Sutton-Chen method\cite{QSC,Qi99} have simpler
-parameter sets. The glue model of Ercolessi {\it et al}. is among the
-fastest of these density functional approaches.\cite{Ercolessi88} In
+parameter sets. The glue model of Ercolessi {\it et al}.\cite{Ercolessi88} is among the
+fastest of these density functional approaches. In
 all of these models, atoms are treated as a positively charged
 core with a radially-decaying valence electron distribution. To
 calculate the energy for embedding the core at a particular location,
@@ -600,14 +611,14 @@ which did exhibit doubling featured in Figure \ref{fig
 \subsection{Mechanism for restructuring}
 Since the Au surface showed no large scale restructuring in any of 
 our simulations, our discussion will focus on the 50\% Pt-CO system 
-which did exhibit doubling featured in Figure \ref{fig:reconstruct}. A 
+which did exhibit doubling. A 
 number of possible mechanisms exist to explain the role of adsorbed 
 CO in restructuring the Pt surface. Quadrupolar repulsion between 
 adjacent CO molecules adsorbed on the surface is one possibility.  
 However, the quadrupole-quadrupole interaction is short-ranged and 
 is attractive for some orientations.  If the CO molecules are ``locked'' in 
 a specific orientation relative to each other, through atop adsorption for 
-example, this explanation would gain credence. The energetic repulsion 
+example, this explanation would gain credence. The calculated energetic repulsion 
 between two CO molecules located a distance of 2.77~\AA~apart 
 (nearest-neighbor distance of Pt) and both in a vertical orientation, 
 is 8.62 kcal/mol. Moving the CO to the second nearest-neighbor distance 
@@ -615,35 +626,36 @@ The barrier for surface diffusion of a Pt adatom is on
 from a purely vertical orientation also lowers the repulsion. When the 
 carbons are locked at a distance of 2.77~\AA, a minimum of 6.2 kcal/mol is 
 reached when the angle between the 2 CO is $\sim$24\textsuperscript{o}. 
-The barrier for surface diffusion of a Pt adatom is only 4 kcal/mol, so 
+The calculated barrier for surface diffusion of a Pt adatom is only 4 kcal/mol, so 
 repulsion between adjacent CO molecules bound to Pt could increase the surface 
 diffusion. However, the residence time of CO on Pt suggests that these 
 molecules are extremely mobile, with diffusion constants 40 to 2500 times 
-larger than surface Pt atoms. This mobility suggests that the CO are more 
-likely to shift their positions without dragging the Pt along with them.
+larger than surface Pt atoms. This mobility suggests that the CO molecules jump
+between different Pt atoms throughout the simulation, but will stay bound for 
+significant periods of time. 
 
 A different interpretation of the above mechanism, taking into account the large 
 mobility of the CO, looks at how instantaneous and short-lived configurations of 
 CO on the surface can destabilize Pt-Pt interactions leading to increased step-edge 
 breakup and diffusion. On the bare Pt(557) surface the barrier to completely detach 
-an edge atom is $\sim$~43~kcal/mol, as is shown in configuration (a) in Figures 
+an edge atom is $\sim$43~kcal/mol, as is shown in configuration (a) in Figures 
 \ref{fig:SketchGraphic} \& \ref{fig:SketchEnergies}. For certain configurations, cases 
-(e), (g), and (h), the barrier can be lowered to $\sim$~23~kcal/mole. In these instances, 
+(e), (g), and (h), the barrier can be lowered to $\sim$23~kcal/mole. In these instances, 
 it becomes quite energetically favorable to roughen the edge by introducing a small 
 separation of 0.5 to 1.0~\AA. This roughening becomes immediately obvious in 
-simulations with significant CO populations, although it is present to a lesser extent 
-on lower coverage surfaces and even on the bare surfaces. In these cases it is likely 
+simulations with significant CO populations. The roughening is present to a lesser extent 
+on lower coverage surfaces and even on the bare surfaces, although in these cases it is likely 
 due to stochastic vibrational processes that squeeze out step-edge atoms. The mechanism 
-of step-edge breakup suggested by these energy curves is one the most difficult 
+of step-edge breakup suggested by these energy curves is one of the most difficult 
 processes, a complete break-away from the step-edge in one unbroken movement. 
 Easier multistep mechanisms likely exist where an adatom moves laterally on the surface 
-after being ejected so it is sitting on the edge. This provides the atom with 5 nearest 
+after being ejected so it ends up alongside the ledge. This provides the atom with 5 nearest 
 neighbors, which while lower than the 7 if it had stayed a part of the step-edge, is higher 
 than the 3 it could maintain located on the terrace. In this proposed mechanism, the CO 
 quadrupolar repulsion is still playing a primary role, but for its importance in roughening 
 the step-edge, rather than maintaining long-term bonds with a single Pt atom which is not 
 born out by their mobility data. The requirement for a large density of CO on the surface 
-for some of the more favorable suggested mechanisms in Figure \ref{fig:SketchGraphic} 
+for some of the more favorable suggested configurations in Figure \ref{fig:SketchGraphic} 
 correspond well with the increased mobility seen on higher coverage surfaces.
 
 %Sketch graphic of different configurations
@@ -674,9 +686,9 @@ would not be enough atoms to finish the doubling. Real
 this does not immediately provide an explanation for the formation of double 
 layers. If adatoms were constrained to their terrace then doubling would be 
 much less likely to occur. Nucleation sites could still potentially form, but there 
-would not be enough atoms to finish the doubling. Real materials, where the 
-step lengths can be taken as infinite, local doubling would be possible, but in 
-our simulations with our periodic treatment of the system, this is not possible. 
+would not be enough atoms to finish the doubling. For a non-simulated metal surface, where the 
+step lengths can be assumed to be infinite relative to atomic sizes, local doubling would be possible, but in 
+our simulations with our periodic treatment of the system, the system is not large enough to experience this effect. 
 Thus, there must be a mechanism that explains how adatoms are able to move 
 amongst terraces. Figure \ref{fig:lambda} shows points along a reaction coordinate 
 where an adatom along the step-edge with an adsorbed CO ``burrows'' into the 
@@ -684,33 +696,31 @@ are the more interesting results from this investigati
 because of similar events that were observed during the simulations. The barrier 
 heights we obtained are only approximations because we constrained the movement 
 of the highlighted atoms along a specific concerted path. The calculated $\Delta E$'s 
-are the more interesting results from this investigation. When CO is not present and 
+are provide a strong energetic support for this modeled lifting mechanism. When CO is not present and 
 this reaction coordinate is followed, the $\Delta E > 3$~kcal/mol. The example shown 
 in the figure, where CO is present in the atop position, has a $\Delta E < -15$~kcal/mol. 
-While the barrier height is comparable to the non-CO case, that is a nearly a 20~kcal/mol 
-difference in energies and moves the process from slightly unfavorable to energetically favorable. 
+While the barrier height is comparable for both cases, there is nearly a 20~kcal/mol 
+difference in energies and makes the process energetically favorable.
 
 %lambda progression of Pt -> shoving its way into the step
 \begin{figure}[H]
 \includegraphics[width=\linewidth]{lambdaProgression_atopCO_withLambda.png}
-\caption{A model system of the Pt(557) surface was used as the framework
- for exploring energy barriers along a reaction coordinate. Various numbers,
- placements, and rotations of CO were examined as they affect Pt movement.
- The coordinate displayed in this Figure was a representative run.  relative to the energy of the system at 0\%, there
- is a slight decrease upon insertion of the Pt atom into the step-edge along
- with the resultant lifting of the other Pt atom when CO is present at certain positions.}
+\caption{ Various points along a reaction coordinate are displayed in the figure. 
+The mechanism of edge traversal is examined in the presence of CO. The approximate
+barrier for the displayed process is 20~kcal/mol. However, the $\Delta E$ of this process
+is -15~kcal/mol making it an energetically favorable process.}
 \label{fig:lambda}
 \end{figure}
 
-The mechanism for doubling on this surface appears to be a convolution of at least 
+The mechanism for doubling on this surface appears to require the cooperation of at least 
 these two described processes. For complete doubling of a layer to occur there must 
 be the equivalent removal of a separate terrace. For those atoms to ``disappear'' from 
 that terrace they must either rise up on the ledge above them or drop to the ledge below 
-them. The presence of CO helps with both of these situations. There must be sufficient 
-breakage of the step-edge to increase the concentration of adatoms on the surface. 
-These adatoms must then undergo the burrowing highlighted above or some comparable 
+them. The presence of CO helps with the energetics of both of these situations. There must be sufficient 
+breakage of the step-edge to increase the concentration of adatoms on the surface and
+these adatoms must then undergo the burrowing highlighted above or some comparable 
 mechanism to traverse the step-edge. Over time, these mechanisms working in concert 
-led to the formation of a double layer.
+lead to the formation of a double layer.
 
 \subsection{CO Removal and double layer stability}
 Once a double layer had formed on the 50\%~Pt system it 
@@ -740,9 +750,10 @@ It is possible with longer simulation times that the 
 \begin{figure}[H]
 \includegraphics[width=\linewidth]{doubleLayerBreaking_greenBlue_whiteLetters.png}
 \caption{(A)  0~ps, (B) 100~ps, (C) 1~ns, after the removal of CO. The presence of the CO
- helped maintain the stability of the double layer and upon removal the two layers break
- and begin separating. The separation is not a simple pulling apart however, rather 
- there is a mixing of the lower and upper atoms at the edge.}
+helped maintain the stability of the double layer and its microfaceting of the double layer 
+into a (111) configuration. This microfacet immediately reverts to the original (100) step 
+edge which is a hallmark of the (557) surface. The separation is not a simple sliding apart, rather 
+there is a mixing of the lower and upper atoms at the edge.}
 \label{fig:breaking}
 \end{figure}
 
@@ -770,8 +781,18 @@ In this work we have shown the reconstruction of the P
 
 
 \section{Conclusion}
-In this work we have shown the reconstruction of the Pt(557) crystalline surface upon adsorption of CO in less than a $\mu s$. Only the highest coverage Pt system showed this initial reconstruction similar to that seen previously. The strong interaction between Pt and CO and the limited interaction between Au and CO helps explain the differences between the two systems. 
+The strength of the Pt-CO binding interaction as well as the large 
+quadrupolar repulsion between CO molecules are sufficient to 
+explain the observed increase in surface mobility and the resultant 
+reconstructions at the highest simulated coverage. The weaker 
+Au-CO interaction results in lower diffusion constants, less step-wandering, 
+and a lack of the double layer reconstruction. An in-depth examination 
+of the energetics shows the important role CO plays in increasing 
+step-breakup and in facilitating edge traversal which are both 
+necessary for double layer formation.
 
+
+
 %Things I am not ready to remove yet
 
 %Table of Diffusion Constants
