| 15 |
|
radius (6603 atoms) and 40 {\AA} radius (15683 atoms) were |
| 16 |
|
constructed. These initial structures were relaxed to their |
| 17 |
|
equilibrium structures at 20 K for 20 ps and again at 300 K for 100 ps |
| 18 |
< |
sampling from a Maxwell-Boltzmann distribution at each temperature. All simulations were conducted using the {\sc OOPSE} molecular dynamics package.\cite{Meineke:2004uq} |
| 18 |
> |
sampling from a Maxwell-Boltzmann distribution at each |
| 19 |
> |
temperature. All simulations were conducted using the {\sc oopse} |
| 20 |
> |
molecular dynamics package.\cite{Meineke:2004uq} |
| 21 |
|
|
| 22 |
|
To mimic the effects of the heating due to laser irradiation, the |
| 23 |
|
particles were allowed to melt by sampling velocities from the Maxwell |
| 125 |
|
|
| 126 |
|
Values for the interfacial conductance have been determined by a |
| 127 |
|
number of groups for similar nanoparticles and range from a low |
| 128 |
< |
$87.5\times 10^{6}$ $(\mathrm{Wm^{-2}K^{-1}})$ to $120\times 10^{6}$ |
| 129 |
< |
$(\mathrm{Wm^{-2}K^{-1}})$.\cite{Wilson:2002uq} Similarly, Plech |
| 130 |
< |
{\it et al.} reported a value for the interfacial conductance of |
| 131 |
< |
$G=105\pm 15$ $(\mathrm{Wm^{-2}K^{-1}})$ and $G=130\pm 15$ |
| 132 |
< |
$(\mathrm{Wm^{-2}K^{-1}})$ for Pt |
| 133 |
< |
nanoparticles.\cite{plech:195423,PhysRevB.66.224301} |
| 128 |
> |
$87.5\times 10^{6} (\mathrm{Wm^{-2}K^{-1}})$ to $130\times 10^{6} |
| 129 |
> |
(\mathrm{Wm^{-2}K^{-1}})$.\cite{XXXHartland,Wilson:2002uq} Wilson {\it |
| 130 |
> |
et al.} worked with Au, Pt, and AuPd nanoparticles and obtained an |
| 131 |
> |
estimate for the interfacial conductance of $G=130 |
| 132 |
> |
(\mathrm{Wm^{-2}K^{-1}})$.\cite{Wilson:2002uq} Similarly, Plech {\it |
| 133 |
> |
et al.} reported a value for the interfacial conductance of $G=105\pm |
| 134 |
> |
15 (\mathrm{Wm^{-2}K^{-1}})$ for Au nanoparticles.\cite{plech:195423} |
| 135 |
|
|
| 136 |
|
We conducted our simulations at both ends of the range of |
| 137 |
|
experimentally-determined values for the interfacial conductance. |
| 142 |
|
the fastest heat transfer, a value of $117\times 10^{6}$ |
| 143 |
|
$(\mathrm{Wm^{-2}K^{-1}})$ was used. Based on calculations we have |
| 144 |
|
done using raw data from the Hartland group's thermal half-time |
| 145 |
< |
experiments on Au nanospheres, the true G values are probably in the |
| 146 |
< |
faster regime: $117\times 10^{6}$ $(\mathrm{Wm^{-2}K^{-1}})$. |
| 145 |
> |
experiments on Au nanospheres,\cite{HuM._jp020581+} the true G values |
| 146 |
> |
are probably in the faster regime: $117\times 10^{6}$ |
| 147 |
> |
$(\mathrm{Wm^{-2}K^{-1}})$. |
| 148 |
|
|
| 149 |
|
The rate of cooling for the nanoparticles in a molecular dynamics |
| 150 |
|
simulation can then be tuned by changing the effective solvent |
| 157 |
|
effective solvent viscosity (again in Pa s) for an interfacial |
| 158 |
|
conductance of $117\times 10^{6}$ $(\mathrm{Wm^{-2}K^{-1}})$ is $5.7 |
| 159 |
|
\times 10^{-6}$, $7.2 \times 10^{-6}$, and $7.5 \times 10^{-6}$ |
| 160 |
< |
for 20 {\AA}, 30 {\AA} and 40 {\AA} particles. Cooling traces for |
| 161 |
< |
each particle size are presented in |
| 160 |
> |
for 20 {\AA}, 30 {\AA} and 40 {\AA} particles. These viscosities are |
| 161 |
> |
essentially gas-phase values, a fact which is consistent with the |
| 162 |
> |
initial temperatures of the particles being well into the |
| 163 |
> |
super-critical region for the aqueous environment. Gas bubble |
| 164 |
> |
generation has also been seen experimentally around gold nanoparticles |
| 165 |
> |
in water.\cite{kotaidis:184702} Instead of a single value for the |
| 166 |
> |
effective viscosity, a time-dependent parameter might be a better |
| 167 |
> |
mimic of the cooling vapor layer that surrounds the hot particles. |
| 168 |
> |
This may also be a contributing factor to the size-dependence of the |
| 169 |
> |
effective viscosities in our simulations. |
| 170 |
> |
|
| 171 |
> |
Cooling traces for each particle size are presented in |
| 172 |
|
Fig. \ref{fig:images_cooling_plot}. It should be noted that the |
| 173 |
|
Langevin thermostat produces cooling curves that are consistent with |
| 174 |
|
Newtonian (single-exponential) cooling, which cannot match the cooling |
| 236 |
|
data for both FCC solid solutions of Ag-Cu and the high-temperature |
| 237 |
|
liquid.\cite{sheng:184203} In contrast, the {\sc eam} potential does |
| 238 |
|
not reproduce the experimentally observed heat of mixing for the |
| 239 |
< |
liquid alloy.\cite{MURRAY:1984lr} Combination rules for the alloy were |
| 240 |
< |
taken to be the arithmatic average of the atomic parameters with the |
| 241 |
< |
exception of $c_i$ since its values is only dependent on the identity |
| 242 |
< |
of the atom where the density is evaluated. For the {\sc q-sc} |
| 243 |
< |
potential, cutoff distances are traditionally taken to be |
| 244 |
< |
$2\alpha_{ij}$ and include up to the sixth coordination shell in FCC |
| 245 |
< |
metals. |
| 239 |
> |
liquid alloy.\cite{MURRAY:1984lr} In this work, we have utilized the |
| 240 |
> |
{\sc q-sc} formulation for our potential energies and forces. |
| 241 |
> |
Combination rules for the alloy were taken to be the arithmetic |
| 242 |
> |
average of the atomic parameters with the exception of $c_i$ since its |
| 243 |
> |
values is only dependent on the identity of the atom where the density |
| 244 |
> |
is evaluated. For the {\sc q-sc} potential, cutoff distances are |
| 245 |
> |
traditionally taken to be $2\alpha_{ij}$ and include up to the sixth |
| 246 |
> |
coordination shell in FCC metals. |
| 247 |
|
|
| 248 |
|
%\subsection{Sampling single-temperature configurations from a cooling |
| 249 |
|
%trajectory} |
