364 |
|
1200~K were performed to confirm the relative |
365 |
|
stability of the surfaces without a CO overlayer. |
366 |
|
|
367 |
< |
The different bulk melting temperatures (1337~K for Au |
368 |
< |
and 2045~K for Pt) suggest that any possible reconstruction should happen at |
367 |
> |
The different bulk melting temperatures (1337~K for Au\cite{Au:melting} |
368 |
> |
and 2045~K for Pt\cite{Pt:melting}) suggest that any possible reconstruction should happen at |
369 |
|
different temperatures for the two metals. The bare Au and Pt surfaces were |
370 |
|
initially run in the canonical (NVT) ensemble at 800~K and 1000~K |
371 |
|
respectively for 100 ps. The two surfaces were relatively stable at these |
378 |
|
Because of the difference in binding energies, nearly all of the CO was bound to the Pt surface, while |
379 |
|
the Au surfaces often had a significant CO population in the gas |
380 |
|
phase. These systems were allowed to reach thermal equilibrium (over |
381 |
< |
5 ns) before being run in the microcanonical (NVE) ensemble for |
382 |
< |
data collection. All of the systems examined had at least 40 ns in the |
381 |
> |
5~ns) before being run in the microcanonical (NVE) ensemble for |
382 |
> |
data collection. All of the systems examined had at least 40~ns in the |
383 |
|
data collection stage, although simulation times for some Pt of the |
384 |
|
systems exceeded 200~ns. Simulations were carried out using the open |
385 |
|
source molecular dynamics package, OpenMD.\cite{Ewald,OOPSE} |
393 |
|
\subsection{Structural remodeling} |
394 |
|
The surfaces of both systems, upon dosage of CO, began |
395 |
|
to undergo remodeling that was not observed in the bare |
396 |
< |
metal system. The surfaces to which no CO was exposed |
397 |
< |
did experience minor roughening of the step-edge, but the |
396 |
> |
metal system. The surfaces which were not exposed to CO |
397 |
> |
did experience minor roughening of the step-edge because |
398 |
> |
of the elevated temperatures, but the |
399 |
|
(557) lattice was well-maintained throughout the simulation |
400 |
|
time. The Au systems were limited to greater amounts of |
401 |
|
roughening, i.e. breakup of the step-edge, and some step |
402 |
|
wandering. The lower coverage Pt systems experienced |
403 |
|
similar restructuring but to a greater extent when |
404 |
|
compared to the Au systems. The 50\% coverage |
405 |
< |
Pt system formed double layers at numerous spots upon its surface. |
405 |
> |
Pt system was unique among our simulations in that it |
406 |
> |
formed numerous double layers through step coalescence, |
407 |
> |
similar to results reported by Tao et al.\cite{Tao:2010} |
408 |
|
|
409 |
|
|
410 |
|
\subsubsection{Step wandering} |
411 |
< |
The 0\% coverage surfaces for both metals showed |
412 |
< |
minimal movement at their respective run temperatures. |
413 |
< |
As the coverage increased, the mobility of the surface |
414 |
< |
also increased. Additionally, at the higher coverages |
415 |
< |
on both metals, there was a large increase in the amount |
416 |
< |
of observed step-wandering. Previous work by |
417 |
< |
Williams\cite{Williams:1993} highlighted the entropic |
418 |
< |
contribution to the repulsion felt between step-edges, |
419 |
< |
and situations were that repulsion could be negated, or |
420 |
< |
overcome, to allow for step coalescence or facet formation. |
411 |
> |
The 0\% coverage surfaces for both metals showed minimal |
412 |
> |
movement at their respective run temperatures. As the CO |
413 |
> |
coverage increased however, the mobility of the surface, |
414 |
> |
adatoms and step-edges alike, also increased. Additionally, |
415 |
> |
at the higher coverages on both metals, there was more |
416 |
> |
step-wandering. Except for the 50\% Pt system, the step-edges |
417 |
> |
did not coalesce in any of the other simulations, instead preferring |
418 |
> |
to keep nearly the same distance between steps as in the |
419 |
> |
original (557) lattice. Previous work by Williams et al.\cite{Williams:1991, Williams:1994} |
420 |
> |
highlights the repulsion that exists between step-edges even |
421 |
> |
when no direct interactions are present in the system. This |
422 |
> |
repulsion arises because the entropy of the step-edges is constrained, |
423 |
> |
since step-edge crossing is not allowed. This entropic repulsion |
424 |
> |
does not completely define the interactions between steps, |
425 |
> |
which is why some surfaces will undergo step coalescence, |
426 |
> |
where additional attractive interactions can overcome the |
427 |
> |
repulsion\cite{Williams:1991} and others will not. The presence and concentration |
428 |
> |
of adsorbates, as shown in this work, can affect these step interactions, potentially |
429 |
> |
leading to a new surface structure as the thermodynamic minimum. |
430 |
|
|
431 |
|
\subsubsection{Double layers} |
432 |
|
Tao et al. have shown experimentally that the Pt(557) surface |
433 |
< |
undergoes two separate reconstructions upon CO |
434 |
< |
adsorption.\cite{Tao:2010} The first involves a doubling of |
435 |
< |
the step height and plateau length. Similar behavior has been |
436 |
< |
seen to occur on numerous surfaces at varying conditions: Ni(977), Si(111). |
437 |
< |
\cite{Williams:1994,Williams:1991,Pearl} Of the two systems |
438 |
< |
we examined, the Pt system showed a greater propensity for |
439 |
< |
reconstruction when compared to the Au system. The amount |
440 |
< |
of reconstruction is correlated to the amount of CO |
433 |
> |
undergoes two separate reconstructions upon CO adsorption.\cite{Tao:2010} |
434 |
> |
The first involves a doubling of the step height and plateau length. |
435 |
> |
Similar behavior has been seen to occur on numerous surfaces |
436 |
> |
at varying conditions: Ni(977), Si(111).\cite{Williams:1994,Williams:1991,Pearl} |
437 |
> |
Of the two systems we examined, the Pt system showed a greater |
438 |
> |
propensity for reconstruction when compared to the Au system |
439 |
> |
because of the larger surface mobility and extent of step wandering. |
440 |
> |
The amount of reconstruction is correlated to the amount of CO |
441 |
|
adsorbed upon the surface. This appears to be related to the |
442 |
< |
effect that adsorbate coverage has on edge breakup and on the surface |
443 |
< |
diffusion of metal adatoms. While both systems displayed step-edge |
444 |
< |
wandering, only the Pt surface underwent the doubling seen by |
445 |
< |
Tao et al. within the time scales studied here. |
446 |
< |
Only the 50\% coverage Pt system exhibited |
447 |
< |
a complete doubling in the time scales we |
448 |
< |
were able to monitor. Over longer periods (150~ns) two more double layers formed on this interface. |
449 |
< |
Although double layer formation did not occur in the other Pt systems, they show |
450 |
< |
more lateral movement of the step-edges |
451 |
< |
compared to their Au counterparts. The 50\% Pt system is highlighted |
440 |
< |
in Figure \ref{fig:reconstruct} at various times along the simulation |
441 |
< |
showing the evolution of a step-edge. |
442 |
> |
effect that adsorbate coverage has on edge breakup and on the |
443 |
> |
surface diffusion of metal adatoms. While both systems displayed |
444 |
> |
step-edge wandering, only the 50\% Pt surface underwent the |
445 |
> |
doubling seen by Tao et al.\cite{Tao:2010} within the time scales studied here. |
446 |
> |
Over longer periods, (150~ns) two more double layers formed |
447 |
> |
on this interface. Although double layer formation did not occur |
448 |
> |
in the other Pt systems, they show more step-wandering and |
449 |
> |
general roughening compared to their Au counterparts. The |
450 |
> |
50\% Pt system is highlighted in Figure \ref{fig:reconstruct} at |
451 |
> |
various times along the simulation showing the evolution of a step-edge. |
452 |
|
|
453 |
|
The second reconstruction on the Pt(557) surface observed by |
454 |
|
Tao involved the formation of triangular clusters that stretched |
455 |
|
across the plateau between two step-edges. Neither system, within |
456 |
< |
the 40~ns time scale, experienced this reconstruction. |
456 |
> |
the 40~ns time scale or the extended simulation time of 150~ns for |
457 |
> |
the 50\% Pt system, experienced this reconstruction. |
458 |
|
|
459 |
|
\subsection{Dynamics} |
460 |
|
Previous atomistic simulations of stepped surfaces dealt largely |
462 |
|
\cite{Williams:1991,Williams:1994}. Consequently, the most common |
463 |
|
technique utilized to date has been Monte Carlo sampling. Monte Carlo gives an efficient |
464 |
|
sampling of the equilibrium thermodynamic landscape at the expense |
465 |
< |
of ignoring the dynamics of the system. Previous work by Pearl and |
466 |
< |
Sibener\cite{Pearl}, using STM, has been able to show the coalescing |
465 |
> |
of ignoring the dynamics of the system. Previous experimental work by Pearl and |
466 |
> |
Sibener\cite{Pearl}, using STM, has been able to capture the coalescing |
467 |
|
of steps on Ni(977). The time scale of the image acquisition, |
468 |
< |
$\sim$70 s/image provides an upper bound for the time required for |
468 |
> |
$\sim$70~s/image provides an upper bound for the time required for |
469 |
|
the doubling to occur. In this section we give data on dynamic and |
470 |
|
transport properties, e.g. diffusion, layer formation time, etc. |
471 |
|
|
473 |
|
\subsubsection{Transport of surface metal atoms} |
474 |
|
%forcedSystems/stepSeparation |
475 |
|
The movement or wandering of a step-edge is a cooperative effect |
476 |
< |
arising from the individual movements, primarily through surface |
466 |
< |
diffusion, of the atoms making up the steps. An ideal metal surface |
476 |
> |
arising from the individual movements of the atoms making up the steps. An ideal metal surface |
477 |
|
displaying a low index facet, (111) or (100), is unlikely to experience |
478 |
|
much surface diffusion because of the large energetic barrier that must |
479 |
< |
be overcome to lift an atom out of the surface. The presence of step-edges |
480 |
< |
on higher-index surfaces provide a source for mobile metal atoms. |
479 |
> |
be overcome to lift an atom out of the surface. The presence of step-edges and other surface features |
480 |
> |
on higher-index facets provide a lower energy source for mobile metal atoms. |
481 |
|
Breaking away from the step-edge on a clean surface still imposes an |
482 |
|
energetic penalty around $\sim$~40 kcal/mol, but this is significantly easier than lifting |
483 |
|
the same metal atom vertically out of the surface, \textgreater~60 kcal/mol. |
484 |
|
The penalty lowers significantly when CO is present in sufficient quantities |
485 |
|
on the surface. For certain distributions of CO, the penalty can fall as low as |
486 |
|
$\sim$~20 kcal/mol. Once an adatom exists on the surface, the barrier for |
487 |
< |
diffusion is negligible ( \textless~4 kcal/mol) and these adatoms are |
487 |
> |
diffusion is negligible ( \textless~4 kcal/mol for a Pt adatom). These adatoms are |
488 |
|
able to explore the terrace before rejoining either the original step-edge or |
489 |
|
becoming a part of a different edge. It is a more difficult process for an atom |
490 |
|
to traverse to a separate terrace although the presence of CO can lower the |
500 |
|
between saved configurations of the system (typically 10-100 ps). An atom that was |
501 |
|
truly mobile would typically travel much greater distances than this, but the 2~\AA~cutoff |
502 |
|
was used to prevent swamping the diffusion data with the in-place vibrational |
503 |
< |
movement of buried atoms. Diffusion on a surface is strongly affected by |
503 |
> |
movement of buried atoms. Diffusion on a surface is strongly affected by |
504 |
|
local structures and in this work, the presence of single and double layer |
505 |
|
step-edges causes the diffusion parallel to the step-edges to be different |
506 |
|
from the diffusion perpendicular to these edges. Parallel and perpendicular |
507 |
|
diffusion constants are shown in Figure \ref{fig:diff}. |
508 |
|
|
509 |
+ |
The lack of a definite trend in the Au diffusion data is likely due |
510 |
+ |
to the weaker bonding between Au and CO. This leads to a lower |
511 |
+ |
coverage ({\it x}-axis) when compared to dosage amount, which |
512 |
+ |
then further limits the affects of the surface diffusion. The correlation |
513 |
+ |
between coverage and Pt diffusion rates conversely shows a |
514 |
+ |
definite trend marred by the highest coverage surface. Two |
515 |
+ |
explanations arise for this drop. First, upon a visual inspection of |
516 |
+ |
the system, after a double layer has been formed, it maintains its |
517 |
+ |
stability strongly and is no longer a good source for adatoms. By |
518 |
+ |
performing the same diffusion calculation but on a shorter run time |
519 |
+ |
(20~ns), only including data before the formation of the double layer, |
520 |
+ |
provides a $\mathbf{D}_{\perp}$ diffusion constant of $1.69~\pm~0.08$ |
521 |
+ |
and a $\mathbf{D}_{\parallel}$ diffusion constant of $6.30~\pm~0.08$. |
522 |
+ |
This places the parallel diffusion constant more closely in line with the |
523 |
+ |
expected trend, while the perpendicular diffusion constant does not |
524 |
+ |
drop as far. A secondary explanation arising from our analysis of the |
525 |
+ |
mechanism of double layer formation show the affect that CO on the |
526 |
+ |
surface has with respect to overcoming surface diffusion of Pt. If the |
527 |
+ |
coverage is too sparse, the Pt engages in minimal interactions and |
528 |
+ |
thus minimal diffusion. As coverage increases, there are more favorable |
529 |
+ |
arrangements of CO on the surface allowing the formation of a path, |
530 |
+ |
a minimum energy trajectory, for the adatom to explore the surface. |
531 |
+ |
As the CO is constantly moving on the surface, this path is constantly |
532 |
+ |
changing. If the coverage becomes too great, the paths could |
533 |
+ |
potentially be clogged leading to a decrease in diffusion despite |
534 |
+ |
their being more adatoms and step-wandering. |
535 |
+ |
|
536 |
|
\subsubsection{Dynamics of double layer formation} |
537 |
|
The increased diffusion on Pt at the higher |
538 |
|
CO coverages plays a primary role in double layer formation. However, this is not |
539 |
|
a complete explanation -- the 33\%~Pt system |
540 |
|
has higher diffusion constants but did not show |
541 |
< |
any signs of edge doubling. On the |
542 |
< |
50\%~Pt system, three separate layers were formed over |
543 |
< |
150~ns of simulation time. Previous experimental |
541 |
> |
any signs of edge doubling in the observed run time. On the |
542 |
> |
50\%~Pt system, one layer formed within the first 40~ns of simulation time, while two more were formed as the system was run for an additional |
543 |
> |
110~ns (150~ns total). Previous experimental |
544 |
|
work gives insight into the upper bounds of the |
545 |
|
time required for step coalescence.\cite{Williams:1991,Pearl} |
546 |
|
In this system, as seen in Figure \ref{fig:reconstruct}, the first |
547 |
|
appearance of a double layer, appears at 19~ns |
548 |
|
into the simulation. Within 12~ns of this nucleation event, nearly half of the step has |
549 |
< |
formed the double layer and by 86 ns, the complete layer |
549 |
> |
formed the double layer and by 86~ns, the complete layer |
550 |
|
has been flattened out. The double layer could be considered |
551 |
|
``complete" by 37~ns but remains a bit rough. From the |
552 |
|
appearance of the first nucleation event to the first observed double layer, the process took $\sim$20~ns. Another |
563 |
|
\begin{figure}[H] |
564 |
|
\includegraphics[width=\linewidth]{ProgressionOfDoubleLayerFormation_yellowCircle.png} |
565 |
|
\caption{The Pt(557) / 50\% CO system at a sequence of times after |
566 |
< |
initial exposure to the CO: (a) 258 ps, (b) 19 ns, (c) 31.2 ns, and |
567 |
< |
(d) 86.1 ns. Disruption of the (557) step-edges occurs quickly. The |
566 |
> |
initial exposure to the CO: (a) 258~ps, (b) 19~ns, (c) 31.2~ns, and |
567 |
> |
(d) 86.1~ns. Disruption of the (557) step-edges occurs quickly. The |
568 |
|
doubling of the layers appears only after two adjacent step-edges |
569 |
|
touch. The circled spot in (b) nucleated the growth of the double |
570 |
|
step observed in the later configurations.} |
611 |
|
This combination of growth and decay of the step-edges is in a state of |
612 |
|
dynamic equilibrium. However, once two previously separated edges |
613 |
|
meet as shown in Figure 1.B, this nucleates the rest of the edge to meet up, forming a double layer. |
614 |
< |
From simulations which exhibit a double layer, the time delay from the initial appearance of a nucleation point to a fully formed double layer is $\sim$35 ns. |
614 |
> |
From simulations which exhibit a double layer, the time delay from the initial appearance of a nucleation point to a fully formed double layer is $\sim$35~ns. |
615 |
|
|
616 |
|
A number of possible mechanisms exist to explain the role of adsorbed |
617 |
|
CO in restructuring the Pt surface. Quadrupolar repulsion between adjacent |
642 |
|
of Pt atoms was then examined to determine possible barriers. Because |
643 |
|
the movement was forced along a pre-defined reaction coordinate that may differ |
644 |
|
from the true minimum of this path, only the beginning and ending energies |
645 |
< |
are displayed in Table \ref{tab:energies}. These values suggest that the presence of CO at suitable |
645 |
> |
are displayed in Table \ref{tab:rxcoord} with the corresponding beginning and ending reaction coordinates in Figure \ref{fig:lambdaTable}. These values suggest that the presence of CO at suitable |
646 |
|
locations can lead to lowered barriers for Pt breaking apart from the step-edge. |
647 |
|
Additionally, as highlighted in Figure \ref{fig:lambda}, the presence of CO makes the |
648 |
|
burrowing and lifting of adatoms favorable, whereas without CO, the process is neutral |
650 |
|
|
651 |
|
%lambda progression of Pt -> shoving its way into the step |
652 |
|
\begin{figure}[H] |
653 |
< |
\includegraphics[width=\linewidth]{lambdaProgression_atopCO.png} |
653 |
> |
\includegraphics[width=\linewidth]{lambdaProgression_atopCO_withLambda.png} |
654 |
|
\caption{A model system of the Pt(557) surface was used as the framework |
655 |
|
for exploring energy barriers along a reaction coordinate. Various numbers, |
656 |
|
placements, and rotations of CO were examined as they affect Pt movement. |
661 |
|
\label{fig:lambda} |
662 |
|
\end{figure} |
663 |
|
|
664 |
+ |
\begin{figure}[H] |
665 |
+ |
\includegraphics[totalheight=0.9\textheight]{lambdaTable.png} |
666 |
+ |
\caption{} |
667 |
+ |
\label{fig:lambdaTable} |
668 |
+ |
\end{figure} |
669 |
|
|
670 |
|
|
671 |
+ |
|
672 |
+ |
\begin{table}[H] |
673 |
+ |
\caption{} |
674 |
+ |
\centering |
675 |
+ |
\begin{tabular}{| c || c | c | c | c |} |
676 |
+ |
\hline |
677 |
+ |
\textbf{System} & 0.5~\AA & 2~\AA & 4~\AA & 6~\AA \\ |
678 |
+ |
\hline |
679 |
+ |
A & 6.38 & 38.34 & 44.65 & 47.60 \\ |
680 |
+ |
B & -20.72 & 0.67 & 17.33 & 24.28 \\ |
681 |
+ |
C & 4.92 & 27.02 & 41.05 & 47.43 \\ |
682 |
+ |
D & -16.97 & 21.21 & 35.87 & 40.93 \\ |
683 |
+ |
E & 5.92 & 30.96 & 43.69 & 49.23 \\ |
684 |
+ |
F & 8.53 & 46.23 & 53.98 & 65.55 \\ |
685 |
+ |
\hline |
686 |
+ |
\end{tabular} |
687 |
+ |
\label{tab:rxcoord} |
688 |
+ |
\end{table} |
689 |
+ |
|
690 |
+ |
|
691 |
|
\subsection{Diffusion} |
692 |
|
The diffusion parallel to the step-edge tends to be |
693 |
|
much larger than that perpendicular to the step-edge. The dynamic |
706 |
|
%breaking of the double layer upon removal of CO |
707 |
|
\begin{figure}[H] |
708 |
|
\includegraphics[width=\linewidth]{doubleLayerBreaking_greenBlue_whiteLetters.png} |
709 |
< |
%: |
648 |
< |
\caption{(A) 0 ps, (B) 100 ps, (C) 1 ns, after the removal of CO. The presence of the CO |
709 |
> |
\caption{(A) 0~ps, (B) 100~ps, (C) 1~ns, after the removal of CO. The presence of the CO |
710 |
|
helped maintain the stability of the double layer and upon removal the two layers break |
711 |
|
and begin separating. The separation is not a simple pulling apart however, rather |
712 |
|
there is a mixing of the lower and upper atoms at the edge.} |