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has been used previously as a theoretical model for glassy |
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dynamics.\cite{Vardeman2001} |
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\section{Background} |
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Hume-Rothery rules suggest that alloys composed of Copper and Silver noble fcc metals should be miscible in the solid state, because their lattice constants are within 15\% of each another. |
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\begin{figure}[htbp] |
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\begin{center} |
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\includegraphics[]{agcu_phase_diagram.pdf} |
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\caption{Equilibrium Phase Diagram for Ag-Cu binary system from reference \cite{Banhart:1992sv}. The dashed line indicates the lowest temperature required to obtain the metastable crystalline phase of the same composition.} |
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\label{fig:phasedia} |
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\end{center} |
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\end{figure} |
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Experimentally, Ag-Cu alloys are an exception to this well-known rule and are only miscible in the liquid state given equilibrium conditions. Below the eutectic temperature of \unit{779}{\celsius} and composition (60.1 wt. \% Ag,39.9 wt .\% Cu), the solid alloy Ag and Cu phase separate into a Ag and Cu rich $\alpha$ and $\beta$ phase respectively. This behavior is due to a positive heat of mixing in both solid and liquid phases. For the equatomic composition fcc solid solution, $\Delta H$ is on the order of \unit{+6}{\kilo\joule\per\mole}\cite{Ma:2005zt}. Figure \ref{fig:phasedia} shows the equilibrium phase diagram for the Ag-Cu binary system\cite{Massalski:1986kl}\cite{Banhart:1992sv}, where the dashed line indicates the temperature at which a non-equilibrium solid solution may be formed. Under these non-equilibrium conditions, a crystalline-disordered fcc $\gamma$ phase can be created. |
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Metastable alloys composed of Ag-Cu were first reported by Dewez\cite{duwez:1136} in 1960 and were created by using a "splat quenching" technique where a liquid droplet is propelled by a shock wave against a super-cooled metallic target. Because of the small positive $\Delta H$, supersaturated crystalline solutions are typically obtained rather than an amorphous phase. Higher $\Delta H$ systems, such as Ag-Ni, are immiscible even in liquid states, but they tend to form metastable alloys much more readily than Ag-Cu. If present, the amorphous Ag-Cu phase is usually seen as the minority phase in most experiments. Because of this unique crystalline-amorphous behavior, the Ag-Cu system has been widely studied. Methods for creating such bulk phase structures include splat quenching, vapor deposition, ion beam mixing and mechanical alloying. Both structural \cite{sheng:184203} and dynamic\cite{Vardeman-II:2001jn} molecular dynamics computational studies have also been performed on this system providing an excellent model for the behavior of glass forming systems. |
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Although, Ag-Cu alloys have been studied widely studied in bulk phase, this alloy has been |
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scarcely studied in nano scale materials. Most of the literature on alloyed metallic nanoparticles has dealt with the Ag-Au system, which has the useful property of being miscible on both solid and liquid phases. Nanoparticles of another miscible system, Au-Cu, have been successfully constructed using techniques such as laser ablation\cite{Malyavantham:2004cu} and the synthetic reduction of metal ions in solution\cite{Kim:2003lv}. Laser induced alloying has been used as a technique for creating Au-Ag alloy particles from core-shell particles\cite{Hartland:2003yf}. To date, attempts at creating Ag-Cu nanoparticles have used ion implantation to embed nanoparticles in a glass matrix\cite{De:1996ta}\cite{Magruder:1994rg}. These attempts have been largely unsuccessful in producing mixed alloy nanoparticles, and instead produce a phase segregated or a core-shell structure. |
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\begin{figure}[htbp] |
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\begin{center} |
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\includegraphics[width=3in]{SPR_Ag_Cu.pdf} |
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\caption{Absorption spectra, from reference \cite{gonzalo:5163}, of films containing nanoparticles of different atomic \% Ag (0 \% being pure Cu). Inset compares nanoparticles with 57 at. \% with the simulated normalized spectrum calculated by the weighted average of spectra for pure Ag and Cu particles. } |
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\label{fig:spr} |
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\end{center} |
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\end{figure} |
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One of the more successful attempts at creating Ag-Cu mixed nanoparticles used alternate pulsed laser ablation and deposition in an amorphous $\ch Al_2O_3$ matrix\cite{gonzalo:5163}. Surface plasmon resonance (SPR) of bimetallic core-shell structures typically show two distinct resonance peaks where mixed particles show a single shifted and broadened resonance peak\cite{Hodak:2000rb}. The SPR for pure silver occurs at \unit{400}{\nano\meter} and for copper at \unit{570}{\nano\meter}. Figure \ref{fig:spr} shows the absorption spectra for pure Cu and Ag $\ch Al_2O_3$ films with SPR peaks at \unit{572}{\nano\meter} and \unit{424}{\nano\meter} respectively as a reference for the pure states. For bimetallic nanoparticles with 40 at.\% Ag an absorption peak is seen between \unit{400\mbox{-}550}{\nano\meter}. With increasing Ag content, the SPR shifts towards the blue, with the peaks nearly coincident at a composition of 57 at.\% Ag. The authors cited the existence of a single broad resonance peak as evidence of a mixed alloy particle rather than a phase segregated system. Unfortunately, it was not determined whether the mixed nanoparticles were an amorphous phase or a supersaturated crystalline phase. One consequence of embedding the Ag-Cu nanoparticles in a glass matrix is that the SPR can be shifted because of the nanoparticle-glass matrix interaction\cite{De:1996ta}\cite{Roy:2003dy}. It would be useful to create free Ag-Cu nanoparticles that could be studied independent of the surrounding environment. |
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Theoretical molecular dynamics computational studies have been performed on the formation of amorphous single component nanoclusters of either gold\cite{Chen:2004ec}\cite{Cleveland:1997jb}\cite{Cleveland:1997gu} or nickel\cite{Gafner:2004bg}\cite{Qi:2001nn} by rapid cooling($\thicksim \unit{10^{12}-10^{13}}{\kelvin\per\second}$) from a liquid state. All of these studies found icosahedral ordering in the resulting structures produced by this rapid cooling which can be evidence of the formation of a amorphous structure\cite{Sachdev:1992mo}. The nearest neighbor information was obtained from pair correlation functions, common neighbor analysis and bond order parameters\cite{Steinhardt:1983mo}. It should be noted that these studies used single component systems with cooling rates that are only obtainable in computer simulations and particle sizes less than 20\AA. Single component systems are known to form amorphous states in small clusters\cite{Breaux:rz} but do not generally form amorphous structures in bulk materials. Icosahedral structures have also been reported in nanoparticles, particularly multiply twinned particles\cite{Ascencio:2000qy}. |
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Since the nanoscale Ag-Cu alloy has been largely unexplored, many interesting questions remain about the formation and properties of such a system. Does the large surface to volume ratio aid Ag-Cu nanoparticles in rapid cooling and formation of an amorphous state? Would a predisposition to isosahedral ordering in nanoparticles also allow for easier formation of an amorphous state and what is the preferred ordering in a amorphous nanoparticle? Nanoparticles have been shown to have size dependent melting transition\cite{Buffat:1976yq}, one would expect a similar trend with the glass transition temperature, because cooling rate is dependent on particle size and the glass transition temperature is dependent on cooling rate. |
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XXX stuff from ORP |
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In the sections below, we describe our |
