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  \section{Introduction}
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+Excitation of the plasmon resonance in metallic nanoparticles has
+attracted enormous interest in the past several years.  This is partly
+due to the location of the plasmon band in the near IR for particles
+in a wide range of sizes and geometries.  (Living tissue is nearly
+transparent in the near IR, and for this reason, there is an
+unrealized potential for metallic nanoparticles to be used in both
+diagnostic and therapeutic settings.)  One of the side effects of
+absorption of laser radiation at these frequencies is the rapid
+(sub-picosecond) heating of the electronic degrees of freedom in the
+metal.  This hot electron gas quickly transfers heat to the phonon
+modes of the lattice, resulting in a rapid heating of the metal
+particles.
+
+Since metallic nanoparticles have a large surface area to volume
+ratio, many of the metal atoms are at surface locations and experience
+relatively weak bonding.  This is observable in a lowering of the
+melting temperatures and a substantial softening of the bulk modulus
+of these particles when compared with bulk metallic samples.  One of
+the side effects of the excitation of small metallic nanoparticles at
+the plasmon resonance is the facile creation of liquid metal
+droplets.  
+
+Much of the experimental work on this subject has been carried out in
+the Hartland and von~Plessen groups.\cite{HartlandG.V._jp0276092,Hodak:2000rb,Hartland:2003lr,Petrova:2007qy,Link:2000lr} These experiments mostly use the technique of time-resolved optical pump-probe spectroscopy where a pump laser pulse serves to excite conduction band electrons in the nanoparticle and a following probe laser pulse allows the electron-phonon coupling to be observed as a function of time. Hu and Hartland have observed a direct relation between the size of the nanoparticle and the observed cooling rate using such pump-probe techniques.\cite{HuM._jp020581+} Pleach {\it et al.} have use pulsed x-ray scattering as a probe to directly access changes to atomic structure following pump excitation.\cite{plech:195423} They further determined that heat transfer in nanoparticles to the surrounding solvent is goverened by interfacial dynamics and not the thermal transport properties of the solvent. 
+
+
+Since these experiments are often carried out in condensed phase
+surroundings, the large surface area to volume ratio makes the heat
+transfer to the surrounding solvent also a relatively rapid process.
+In our recent simulation study of the laser excitation of gold
+nanoparticles,\cite{VardemanC.F._jp051575r} we observed that the cooling rate for these
+particles (10$^{11}$-10$^{12}$ K/s) is in excess of the cooling rate
+required for glass formation in bulk metallic alloys.  Given this
+fact, it may be possible to use laser excitation to melt, alloy and
+quench metallic nanoparticles in order to form metallic glass
+nanobeads.  
+
+To study whether or not glass nanobead formation is feasible, we have
+chosen the bimetallic alloy of Silver (60\%) and Copper (40\%) as a
+model system because it is an experimentally known glass former and
+has been used previously as a theoretical model for glassy
+dynamics.\cite{Vardeman-II:2001jn} The Hume-Rothery rules suggest that
+alloys composed of Copper and Silver should be miscible in the solid
+state, because their lattice constants are within 15\% of each
+another.\cite{Kittel:1996fk} Experimentally, however Ag-Cu alloys are a
+well-known exception to this rule and are only miscible in the liquid
+state given equilibrium conditions. Below the eutectic temperature of
+779 $^\circ$C and composition (60.1\% Ag, 39.9\% Cu), the
+solid alloys of Ag and Cu will phase separate into Ag and Cu rich
+$\alpha$ and $\beta$ phases, respectively. This behavior is due to a
+positive heat of mixing in both the solid and liquid phases. For the
+one-to-one composition fcc solid solution, $\Delta H$ is on the order
+of +6~kJ/mole.\cite{Ma:2005fk} Non-equilibrium solid solutions may be
+formed by undercooling, and under these conditions, a
+compositionally-disordered $\gamma$ fcc phase can be formed.
+
+Metastable alloys composed of Ag-Cu were first reported by Duwez in
+1960 and were created by using a ``splat quenching'' technique in
+which a liquid droplet is propelled by a shock wave against a cooled
+metallic target.\cite{duwez:1136} 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}
+computational studies have also been performed on this system.
+
+Although bulk Ag-Cu alloys have been studied widely, this alloy has
+been mostly overlooked in nanoscale materials.  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:2003lr} To date, attempts at
+creating Ag-Cu nanoparticles have used ion implantation to embed
+nanoparticles in a glass matrix.\cite{De:1996ta,Magruder:1994rg} These
+attempts have been largely unsuccessful in producing mixed alloy
+nanoparticles, and instead produce phase segregated or core-shell
+structures.
+
+One of the more successful attempts at creating intermixed Ag-Cu
+nanoparticles used alternate pulsed laser ablation and deposition in
+an amorphous Al$_2$O$_3$ 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.\cite{Hodak:2000rb} The SPR for pure silver
+occurs at 400 nm and for copper at 570 nm.\cite{HengleinA._jp992950g}  
+On Al$_2$O$_3$ films, these resonances move to 424 nm and 572 nm for the pure metals. For
+bimetallic nanoparticles with 40\% Ag an absorption peak is seen
+between 400-550 nm.  With increasing Ag content, the SPR shifts
+towards the blue, with the peaks nearly coincident at a composition of
+57\% Ag. Gonzalo {\it et al.} cited the existence of a single broad
+resonance peak as evidence of a mixed alloy particle rather than a
+phase segregated system. Unfortunately, they were unable to determine
+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,Roy:2003dy}
+
+Characterization of glassy behavior by molecular dynamics simulations
+is typically done using dynamic measurements such as the mean squared
+displacement, $\langle r^2(t) \rangle$. Liquids exhibit a mean squared
+displacement that is linear in time (at long times). Glassy materials
+deviate significantly from this linear behavior at intermediate times,
+entering a sub-linear regime with a return to linear behavior in the
+infinite time limit.\cite{Kob:1999fk} However, diffusion in nanoparticles
+differs significantly from the bulk in that atoms are confined to a
+roughly spherical volume and cannot explore any region larger than the
+particle radius ($R$). In these confined geometries, $\langle r^2(t)
+\rangle$ approaches a limiting value of $3R^2/40$.\cite{ShibataT._ja026764r}  This limits the
+utility of dynamical measures of glass formation when studying
+nanoparticles.
+
+However, glassy materials exhibit strong icosahedral ordering among
+nearest-neghbors in contrast to crystalline or liquid structures.
+Local icosahedral structures are the three-dimensional equivalent of
+covering a two-dimensional plane with 5-sided tiles; they cannot be
+used to tile space in a periodic fashion, and are therefore an
+indicator of non-periodic packing in amorphous solids.  Steinhart {\it
+et al.}  defined an orientational bond order parameter that is
+sensitive to icosahedral ordering.\cite{Steinhardt:1983mo} This bond
+order parameter can therefore be used to characterize glass formation
+in liquid and solid solutions.\cite{wolde:9932}
+
+Theoretical molecular dynamics studies have been performed on the
+formation of amorphous single component nanoclusters of either
+gold,\cite{Chen:2004ec,Cleveland:1997jb,Cleveland:1997gu} or
+nickel,\cite{Gafner:2004bg,Qi:2001nn} by rapid cooling($\thicksim
+10^{12}-10^{13}$ K/s) 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{Strandburg:1992qy} 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}
+
+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} and we would expect a similar trend
+to follow for the glass transition temperature.
+
+In the sections below, we describe our modeling of the laser
+excitation and subsequent cooling of the particles in silico to mimic
+real experimental conditions.  The simulation parameters have been
+tuned to the degree possible to match experimental conditions, and we
+discusss both the icosahedral ordering in the system, as well as the
+clustering of icosahedral centers that we observed.
+