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\section{Conclusions} |
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\label{sec:conclusion} |
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|
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Our heat-transfer calculations have utilized the best current |
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estimates of the interfacial heat transfer coefficient (G) from recent |
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experiments. Using reasonable values for thermal conductivity in both |
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the metallic particle and the surrounding solvent, we have obtained |
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cooling rates for laser-heated nanoparticles that are in excess of |
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10$^{13}$ K / s. To test whether or not this cooling rate can form |
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glassy nanoparticles, we have performed a mixed molecular dynamics |
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simulation in which the atoms in contact with the solvent or capping |
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agent are evolved under Langevin dynamics while the remaining atoms |
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are evolved under Newtonian dynamics. The effective solvent viscosity |
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($\eta$) is a free parameter which we have tuned so that the particles |
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in the simulation follow the same cooling curve as their experimental |
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counterparts. From the local icosahedral ordering around the atoms in |
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the nanoparticles (particularly Copper atoms), we deduce that it is |
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likely that glassy nanobeads are created via laser heating of |
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bimetallic nanoparticles, particularly when the initial temperature of |
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the particles approaches the melting temperature of the bulk metal |
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alloy. |
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|
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Improvements to our calculations would require: 1) explicit treatment |
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of the capping agent and solvent, 2) another radial region to handle |
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the heat transfer to the solvent vapor layer that is likely to form |
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immediately surrounding the hot |
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particle,\cite{Hu:2004lr,kotaidis:184702} and 3) larger particles in |
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the size range most easily studied via laser heating experiments. |
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|
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The local icosahedral ordering we observed in these bimetallic |
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particles is centered almost completely around the copper atoms, and |
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this is likely due to the size mismatch leading to a more efficient |
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packing of 5-membered rings of silver around a central copper atom. |
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This size mismatch should be reflected in bulk calculations, and work |
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is ongoing in our lab to confirm this observation in bulk |
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glass-formers. |
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|
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The physical properties (bulk modulus, frequency of the breathing |
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mode, and density) of glassy nanobeads should be somewhat different |
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from their crystalline counterparts. However, observation of these |
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differences may require single-particle resolution of the ultrafast |
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vibrational spectrum of one particle both before and after the |
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crystallite has been converted into a glassy bead. |
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