| 616 |
|
carbons are locked at a distance of 2.77~\AA, a minimum of 6.2 kcal/mol is |
| 617 |
|
reached when the angle between the 2 CO is $\sim$24\textsuperscript{o}. |
| 618 |
|
The barrier for surface diffusion of a Pt adatom is only 4 kcal/mol, so |
| 619 |
< |
repulsion between adjacent CO molecules could increase the surface |
| 619 |
> |
repulsion between adjacent CO molecules bound to Pt could increase the surface |
| 620 |
|
diffusion. However, the residence time of CO on Pt suggests that these |
| 621 |
|
molecules are extremely mobile, with diffusion constants 40 to 2500 times |
| 622 |
|
larger than surface Pt atoms. This mobility suggests that the CO are more |
| 623 |
|
likely to shift their positions without dragging the Pt along with them. |
| 624 |
|
|
| 625 |
< |
Another possible mechanism for the restructuring is in the destabilization of strong Pt-Pt interactions by CO adsorbed on surface Pt atoms. To test this hypothesis, a number of configurations of CO in varying quantities were arranged on the upper plateaus around a step on an otherwise clean Pt(557) surface. A few sample configurations are displayed in Figure \ref{fig:SketchGraphic}, with energy curves corresponding to each configuration in Figure \ref{fig:SketchEnergies}. Certain configurations of CO, cases (e), (g) and (h) for example, can provide significant energetic pushes for Pt atoms to break away from the step-edge. |
| 626 |
< |
|
| 625 |
> |
A different interpretation of the above mechanism, taking into account the large |
| 626 |
> |
mobility of the CO, looks at how instantaneous and short-lived configurations of |
| 627 |
> |
CO on the surface can destabilize Pt-Pt interactions leading to increased step-edge |
| 628 |
> |
breakup and diffusion. On the bare Pt(557) surface the barrier to completely detach |
| 629 |
> |
an edge atom is $\sim$~43~kcal/mol, as is shown in configuration (a) in Figures |
| 630 |
> |
\ref{fig:SketchGraphic} \& \ref{fig:SketchEnergies}. For certain configurations, cases |
| 631 |
> |
(e), (g), and (h), the barrier can be lowered to $\sim$~23~kcal/mole. In these instances, |
| 632 |
> |
it becomes quite energetically favorable to roughen the edge by introducing a small |
| 633 |
> |
separation of 0.5 to 1.0~\AA. This roughening becomes immediately obvious in |
| 634 |
> |
simulations with significant CO populations, although it is present to a lesser extent |
| 635 |
> |
on lower coverage surfaces and even on the bare surfaces. In these cases it is likely |
| 636 |
> |
due to stochastic vibrational processes that squeeze out step-edge atoms. The mechanism |
| 637 |
> |
of step-edge breakup suggested by these energy curves is one the most difficult |
| 638 |
> |
processes, a complete break-away from the step-edge in one unbroken movement. |
| 639 |
> |
Easier multistep mechanisms likely exist where an adatom moves laterally on the surface |
| 640 |
> |
after being ejected so it is sitting on the edge. This provides the atom with 5 nearest |
| 641 |
> |
neighbors, which while lower than the 7 if it had stayed a part of the step-edge, is higher |
| 642 |
> |
than the 3 it could maintain located on the terrace. In this proposed mechanism, the CO |
| 643 |
> |
quadrupolar repulsion is still playing a primary role, but for its importance in roughening |
| 644 |
> |
the step-edge, rather than maintaining long-term bonds with a single Pt atom which is not |
| 645 |
> |
born out by their mobility data. The requirement for a large density of CO on the surface |
| 646 |
> |
for some of the more favorable suggested mechanisms in Figure \ref{fig:SketchGraphic} |
| 647 |
> |
correspond well with the increased mobility seen on higher coverage surfaces. |
| 648 |
|
|
| 649 |
|
%Sketch graphic of different configurations |
| 650 |
|
\begin{figure}[H] |
| 670 |
|
\label{fig:SketchEnergies} |
| 671 |
|
\end{figure} |
| 672 |
|
|
| 673 |
< |
|
| 673 |
> |
While configurations of CO on the surface are able to increase diffusion, |
| 674 |
> |
this does not immediately provide an explanation for the formation of double |
| 675 |
> |
layers. If adatoms were constrained to their terrace then doubling would be |
| 676 |
> |
much less likely to occur. Nucleation sites could still potentially form, but there |
| 677 |
> |
would not be enough atoms to finish the doubling. Real materials, where the |
| 678 |
> |
step lengths can be taken as infinite, local doubling would be possible, but in |
| 679 |
> |
our simulations with our periodic treatment of the system, this is not possible. |
| 680 |
> |
Thus, there must be a mechanism that explains how adatoms are able to move |
| 681 |
> |
amongst terraces. Figure \ref{fig:lambda} shows points along a reaction coordinate |
| 682 |
> |
where an adatom along the step-edge with an adsorbed CO ``burrows'' into the |
| 683 |
> |
edge displacing an atom onto the higher terrace. This mechanism was chosen |
| 684 |
> |
because of similar events that were observed during the simulations. The barrier |
| 685 |
> |
heights we obtained are only approximations because we constrained the movement |
| 686 |
> |
of the highlighted atoms along a specific concerted path. The calculated $\Delta E$'s |
| 687 |
> |
are the more interesting results from this investigation. When CO is not present and |
| 688 |
> |
this reaction coordinate is followed, the $\Delta E > 3$~kcal/mol. The example shown |
| 689 |
> |
in the figure, where CO is present in the atop position, has a $\Delta E < -15$~kcal/mol. |
| 690 |
> |
While the barrier height is comparable to the non-CO case, that is a nearly a 20~kcal/mol |
| 691 |
> |
difference in energies and moves the process from slightly unfavorable to energetically favorable. |
| 692 |
|
|
| 693 |
|
%lambda progression of Pt -> shoving its way into the step |
| 694 |
|
\begin{figure}[H] |
| 702 |
|
\label{fig:lambda} |
| 703 |
|
\end{figure} |
| 704 |
|
|
| 705 |
+ |
The mechanism for doubling on this surface appears to be a convolution of at least |
| 706 |
+ |
these two described processes. For complete doubling of a layer to occur there must |
| 707 |
+ |
be the equivalent removal of a separate terrace. For those atoms to ``disappear'' from |
| 708 |
+ |
that terrace they must either rise up on the ledge above them or drop to the ledge below |
| 709 |
+ |
them. The presence of CO helps with both of these situations. There must be sufficient |
| 710 |
+ |
breakage of the step-edge to increase the concentration of adatoms on the surface. |
| 711 |
+ |
These adatoms must then undergo the burrowing highlighted above or some comparable |
| 712 |
+ |
mechanism to traverse the step-edge. Over time, these mechanisms working in concert |
| 713 |
+ |
led to the formation of a double layer. |
| 714 |
+ |
|
| 715 |
|
\subsection{CO Removal and double layer stability} |
| 716 |
|
Once a double layer had formed on the 50\%~Pt system it |
| 717 |
|
remained for the rest of the simulation time with minimal |
| 736 |
|
|
| 737 |
|
|
| 738 |
|
|
| 690 |
– |
|
| 691 |
– |
|
| 692 |
– |
|
| 739 |
|
%breaking of the double layer upon removal of CO |
| 740 |
|
\begin{figure}[H] |
| 741 |
|
\includegraphics[width=\linewidth]{doubleLayerBreaking_greenBlue_whiteLetters.png} |
