| 238 |
|
form local icosahedral structures are usually on the surface of the |
| 239 |
|
nanoparticle, while the copper atoms which have local icosahedral |
| 240 |
|
ordering are distributed more evenly throughout the nanoparticles. |
| 241 |
< |
Silver, since it has a lower surface free energy than copper, tends to |
| 242 |
< |
coat the skins of the mixed particles.\cite{Zhu:1997lr} This is true even for |
| 243 |
< |
bimetallic particles that have been prepared in the Ag (core) / Cu |
| 244 |
< |
(shell) configuration. Upon forming a liquid droplet, approximately 1 |
| 245 |
< |
monolayer of Ag atoms will rise to the surface of the particles. This |
| 246 |
< |
can be seen visually in figure \ref{fig:cross_sections}. This observation is consistent with previous experimental and theoretical studies on bimetallic alloys composed of noble metals.\cite{MainardiD.S._la0014306,HuangS.-P._jp0204206,Ramirez-Caballero:2006lr} |
| 247 |
< |
Bond order parameters for surface atoms are averaged only over the neighboring |
| 248 |
< |
atoms, so packing constraints that may prevent icosahedral ordering |
| 249 |
< |
around silver in the bulk are removed near the surface. It would |
| 250 |
< |
certainly be interesting to see if the relative tendency of silver and |
| 251 |
< |
copper to form local icosahedral structures in a bulk glass differs |
| 252 |
< |
from our observations on nanoparticles. |
| 241 |
> |
Figure \ref{fig:Surface} shows this tendency as a function of distance |
| 242 |
> |
from the center of the nanoparticle. Silver, since it has a lower |
| 243 |
> |
surface free energy than copper, tends to coat the skins of the mixed |
| 244 |
> |
particles.\cite{Zhu:1997lr} This is true even for bimetallic particles |
| 245 |
> |
that have been prepared in the Ag (core) / Cu (shell) configuration. |
| 246 |
> |
Upon forming a liquid droplet, approximately 1 monolayer of Ag atoms |
| 247 |
> |
will rise to the surface of the particles. This can be seen visually |
| 248 |
> |
in figure \ref{fig:cross_sections} as well as in the density plots in |
| 249 |
> |
the bottom panel of figure \ref{fig:Surface}. This observation is |
| 250 |
> |
consistent with previous experimental and theoretical studies on |
| 251 |
> |
bimetallic alloys composed of noble |
| 252 |
> |
metals.\cite{MainardiD.S._la0014306,HuangS.-P._jp0204206,Ramirez-Caballero:2006lr} |
| 253 |
> |
Bond order parameters for surface atoms are averaged only over the |
| 254 |
> |
neighboring atoms, so packing constraints that may prevent icosahedral |
| 255 |
> |
ordering around silver in the bulk are removed near the surface. It |
| 256 |
> |
would certainly be interesting to see if the relative tendency of |
| 257 |
> |
silver and copper to form local icosahedral structures in a bulk glass |
| 258 |
> |
differs from our observations on nanoparticles. |
| 259 |
> |
|
| 260 |
> |
\begin{figure}[htbp] |
| 261 |
> |
\centering |
| 262 |
> |
\includegraphics[width=5in]{images/dens_fracr_stacked_plot.pdf} |
| 263 |
> |
\caption{Appearance of icosahedral clusters around central silver atoms |
| 264 |
> |
is largely due to the presence of these silver atoms at or near the |
| 265 |
> |
surface of the nanoparticle. The upper panel shows the fraction of |
| 266 |
> |
icosahedral atoms ($f_\textrm{icos}(r)$ for each of the two metallic |
| 267 |
> |
atoms as a function of distance from the center of the nanoparticle |
| 268 |
> |
($r$). The lower panel shows the radial density of the two |
| 269 |
> |
constituent metals (relative to the overall density of the |
| 270 |
> |
nanoparticle). Icosahedral clustering around copper atoms are more |
| 271 |
> |
evenly distributed throughout the particle, while icosahedral |
| 272 |
> |
clustering around silver is largely confined to the silver atoms at |
| 273 |
> |
the surface.} |
| 274 |
> |
\label{fig:Surface} |
| 275 |
> |
\end{figure} |
