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%\documentclass[aps,jcp,twocolumn,showpacs,superscriptaddress,groupedaddress]{revtex4} % for review and submission |
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Molecular dynamics simulations of thiolate-protected and solvated |
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gold nanoparticles were carried out in the presence of a |
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non-equilibrium heat flux between the solvent and the core of the |
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particle. The interfacial thermal conductance ($G$) was computed |
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for these interfaces, and the behavior of the thermal conductance |
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was studied as a function of particle size, ligand flexibility, and |
| 47 |
> |
particle. The interfacial thermal conductance ($G$) was computed for |
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these interfaces, and the behavior of the thermal conductance was |
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studied as a function of particle size, ligand flexibility, and |
| 50 |
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ligand chain length. In all cases, thermal conductance of the |
| 51 |
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ligand-protected particles was higher than the bare metal--solvent |
| 52 |
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interface. A number of mechanisms for the enhanced conductance were |
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investigated, including thiolate-driven corrugation of the metal |
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surface, solvent ordering at the interface, solvent-ligand |
| 55 |
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interpenetration, and ligand ordering relative to the particle |
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surface. MORE HERE. |
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surface. Only the smallest particles exhibited significant |
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corrugation. All ligands permitted substantial solvent-ligand |
| 58 |
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interpenetration, and ligand chain length has a significant |
| 59 |
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influence on the orientational ordering of interfacial solvent. |
| 60 |
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Solvent -- ligand vibrational overlap, particularly in the low |
| 61 |
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frequency range ($< 80 \mathrm{cm}^{-1}$) was significantly altered |
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by ligand rigidity, and had direct influence on the interfacial |
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thermal conductance. |
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\end{abstract} |
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|
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\pacs{} |
| 167 |
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surfaces of spherical nanoparticles. Thus, as new applications of |
| 168 |
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ligand-stabilized nanostructures have been proposed, the structure and |
| 169 |
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dynamics of ligands on metallic nanoparticles have been studied using |
| 170 |
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molecular simulation,\cite{Henz2007,Henz:2008qf} NMR, XPS, FTIR, |
| 170 |
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molecular simulation,\cite{Henz:2008qf} NMR, XPS, FTIR, |
| 171 |
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calorimetry, and surface |
| 172 |
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microscopies.\cite{Badia1996:2,Badia1996,Badia1997:2,Badia1997,Badia2000} |
| 173 |
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Badia, \textit{et al.} used transmission electron microscopy to |
| 179 |
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neighboring particles.\cite{Badia1996} The molecular dynamics |
| 180 |
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simulations of Henz, \textit{et al.} indicate that at low coverages, |
| 181 |
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the thiolate alkane chains will lie flat on the nanoparticle |
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surface\cite{Henz2007,Henz:2008qf} Above 90\% coverage, the ligands |
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surface\cite{Henz:2008qf} Above 90\% coverage, the ligands |
| 183 |
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stand upright and recover the rigidity and tilt angle displayed on |
| 184 |
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planar facets. Their simulations also indicate a high degree of mixing |
| 185 |
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between the thiolate sulfur atoms and surface gold atoms at high |
| 209 |
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\protect\cite{landman:1998},~\protect\cite{vlugt:cpc2007154},~and |
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\protect\cite{hautman:4994}.} |
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\label{fig:structures} |
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+ |
\bibpunct{[}{]}{,}{n}{}{,} |
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\end{figure} |
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|
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|
| 307 |
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intra-molecular sites closer than 3 bonds. Effective Lennard-Jones |
| 308 |
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potentials were used for non-bonded interactions. |
| 309 |
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|
| 310 |
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To describe the interactions between metal (Au) and non-metal atoms, |
| 311 |
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potential energy terms were adapted from an adsorption study of alkyl |
| 312 |
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thiols on gold surfaces by Vlugt, \textit{et |
| 313 |
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al.}\cite{vlugt:cpc2007154} They fit an effective pair-wise |
| 310 |
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The TraPPE-UA force field includes parameters for thiol |
| 311 |
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molecules\cite{TraPPE-UA.thiols} as well as unsaturated and aromatic |
| 312 |
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carbon sites.\cite{TraPPE-UA.alkylbenzenes} These were used for the |
| 313 |
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thiolate molecules in our simulations, and missing parameters for the |
| 314 |
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ligands were supplemented using fits described in the supporting |
| 315 |
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information. Bonds are typically rigid in TraPPE-UA, so although |
| 316 |
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equilibrium bond distances were taken from TraPPE-UA, flexible bonds |
| 317 |
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were allowed bond stretching spring constants from the OPLS-AA force |
| 318 |
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field.\cite{Jorgensen:1996sf} |
| 319 |
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|
| 320 |
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To derive suitable parameters for the thiolates adsorbed on Au(111) |
| 321 |
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surfaces, we adopted the S parameters from Luedtke and |
| 322 |
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Landman\cite{landman:1998} and modified the parameters for the CTS |
| 323 |
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atom to maintain charge neutrality in the molecule. |
| 324 |
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|
| 325 |
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Other interactions between metal (Au) and non-metal atoms were adapted |
| 326 |
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from an adsorption study of alkyl thiols on gold surfaces by Vlugt, |
| 327 |
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\textit{et al.}\cite{vlugt:cpc2007154} They fit an effective pair-wise |
| 328 |
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Lennard-Jones form of potential parameters for the interaction between |
| 329 |
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Au and pseudo-atoms CH$_x$ and S based on a well-established and |
| 330 |
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widely-used effective potential of Hautman and Klein for the Au(111) |
| 331 |
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surface.\cite{hautman:4994} |
| 332 |
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|
| 333 |
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Additional terms to represent thiolated alkenes and conjugated ligand |
| 334 |
< |
moieties were parameterized as part of this work and are available in |
| 335 |
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the supporting information. |
| 333 |
> |
All additional terms to represent thiolated alkenes and conjugated |
| 334 |
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ligand moieties were parameterized as part of this work and are |
| 335 |
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available in the supporting information. |
| 336 |
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|
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
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% SIMULATION PROTOCOL |
| 497 |
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surface restructuring may have an impact on the interfacial thermal |
| 498 |
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conductance and is an important phenomenon to quantify. |
| 499 |
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|
| 500 |
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Henz, \textit{et al.}\cite{Henz2007,Henz:2008qf} used the metal |
| 500 |
> |
Henz, \textit{et al.}\cite{Henz:2008qf} used the metal |
| 501 |
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density as a function of radius to measure the degree of mixing |
| 502 |
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between the thiol sulfurs and surface gold atoms at the edge of a |
| 503 |
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nanoparticle. Although metal density is important, disruption of the |
