| 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 exists 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 |
| 428 |
> |
of adsorbates 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. 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 |
| 469 |
|
the doubling to occur. In this section we give data on dynamic and |
| 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:energies} 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 |
|
\subsection{Diffusion} |
| 686 |
|
%breaking of the double layer upon removal of CO |
| 687 |
|
\begin{figure}[H] |
| 688 |
|
\includegraphics[width=\linewidth]{doubleLayerBreaking_greenBlue_whiteLetters.png} |
| 689 |
< |
%: |
| 648 |
< |
\caption{(A) 0 ps, (B) 100 ps, (C) 1 ns, after the removal of CO. The presence of the CO |
| 689 |
> |
\caption{(A) 0~ps, (B) 100~ps, (C) 1~ns, after the removal of CO. The presence of the CO |
| 690 |
|
helped maintain the stability of the double layer and upon removal the two layers break |
| 691 |
|
and begin separating. The separation is not a simple pulling apart however, rather |
| 692 |
|
there is a mixing of the lower and upper atoms at the edge.} |
