| 456 |
|
\subsection{Simulation Protocol} |
| 457 |
|
|
| 458 |
|
We have implemented the VSS-RNEMD algorithm in OpenMD, our |
| 459 |
< |
group molecular dynamics code. A gold slab 11 atoms thick was |
| 460 |
< |
equilibrated at 1 atm and 200 K. The periodic box was expanded |
| 461 |
< |
in the z direction to expose two Au(111) faces. |
| 459 |
> |
group molecular dynamics code. A 1188 atom gold slab was |
| 460 |
> |
equilibrated at 1 atm and 200 K. The periodic box was then expanded |
| 461 |
> |
in the z direction to expose two Au(111) faces on either side of the 11-atom thick slab. |
| 462 |
|
|
| 463 |
|
A full monolayer of thiolates (1/3 the number of surface gold atoms) was placed on three-fold hollow sites on the gold interfaces. To efficiently test the effect of thiolate binding sites on the thermal conductance, all systems had one gold interface with thiolates placed only on fcc hollow sites and the other interface with thiolates only on hcp hollow sites. To test the effect of thiolate chain length on interfacial thermal conductance, full coverages of five chain lengths were tested: butanethiolate, hexanethiolate, octanethiolate, decanethiolate, and dodecanethiolate. To test the effect of mixed chain lengths, full coverages of butanethiolate/decanethiolate and butanethiolate/dodecanethiolate mixtures were created in short/long chain percentages of 25/75, 50/50, 62.5/37.5, 75/25, and 87.5/12.5. The short and long chains were placed on the surface hollow sites in a random configuration. |
| 464 |
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|
| 483 |
|
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
| 484 |
|
\subsection{Effect of Chain Length} |
| 485 |
|
|
| 486 |
< |
We examined full coverages of five chain lengths, n = 4, 6, 8, 10, 12. As shown in table \ref{table:chainlengthG}, the trend of interfacial conductance is mostly flat as a function of chain length, indicating that the length of the thiolate alkyl chains does not play a significant role in the transport of heat across the gold/thiolate and thiolate/solvent interfaces. In all cases, the hexane solvent was unable to penetrate into the thiolate layer, leading to a persistent 2-4 \AA \, gap between the solvent region and the thiolates. While the identity of the alkyl thiolate capping agent has little effect on the interfacial thermal conductance, the presence of a full monolayer of capping agent provides a two-fold increase in the G value relative to a bare gold surface. |
| 486 |
> |
We examined full coverages of five chain lengths, n = 4, 6, 8, 10, 12. As shown in table \ref{table:chainlengthG}, the interfacial conductance is constant as a function of chain length, indicating that the length of the thiolate alkyl chains does not play a significant role in the transport of heat across the gold/thiolate and thiolate/solvent interfaces. In all cases, the hexane solvent was unable to penetrate into the thiolate layer, leading to a persistent 2-4 \AA \, gap between the solvent region and the thiolates. However, while the identity of the alkyl thiolate capping agent has little effect on the interfacial thermal conductance, the presence of a full monolayer of capping agent provides a two-fold increase in the G value relative to a bare gold surface. |
| 487 |
|
\begin{longtable}{p{4cm} p{3cm}} |
| 488 |
|
\caption{Computed interfacial thermal conductance (G) values for bare gold and 100\% coverages of various thiolate alkyl chain lengths.} |
| 489 |
|
\\ |
| 505 |
|
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
| 506 |
|
\subsection{Effect of Mixed Chain Lengths} |
| 507 |
|
|
| 508 |
< |
Previous work demonstrated that for butanethiolate monolayers on a Au(111) surface, the interfacial conductance was a non-monotonic function of the percent coverage. This is believed to be due to enhanced solvent-thiolate coupling through greater penetration of solvent molecules into the thiolate layer. At lower coverages, hexane solvent can more easily line up lengthwise with the thiolate tails by fitting into gaps between the thiolates. However, a side effect of low coverages is surface aggregation of the thiolates. To simulate the effect of low coverages while preventing aggregation we maintain 100\% thiolate coverage while varying the proportions of short (butanethiolate, n = 4) and long (decanethiolate, n = 10 or dodecanethiolate, n = 12). In systems where there is a mix of short and long chain thiolates, interfacial conductance is a non-monotonic function of the percent of long chains. The depth of the gaps between the long chains is $n_{long} - n_{short}$, which has implications for the ability of the hexane solvent to fill in the gaps between the long chains. |
| 508 |
> |
Previous work demonstrated that for butanethiolate monolayers on a Au(111) surface, the interfacial conductance was a non-monotonic function of the percent coverage. This is believed to be due to enhanced solvent-thiolate coupling through greater penetration of solvent molecules into the thiolate layer. At lower coverages, hexane solvent can more penetrate into the interfacial layer by fitting into gaps between the thiolates. However, a side effect of low coverages is surface aggregation of the thiolates. To simulate the effect of low coverages while preventing thiolate domain formation, we maintain 100\% thiolate coverage while varying the proportions of short (butanethiolate, n = 4) and long (decanethiolate, n = 10 or dodecanethiolate, n = 12). In systems where there is a mix of short and long chain thiolates, interfacial conductance is a non-monotonic function of the percent of long chains. |
| 509 |
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|
| 510 |
|
\begin{figure} |
| 511 |
|
\includegraphics[width=\linewidth]{figures/Gstacks} |
