Using x-ray fine structure absorption spectroscopy (XAFS), the Bunker group in the department of Physics observed that gold-core / silver-shell nanoparticles in aqueous solution at ambient temperature alloy at the sub-interface layers between the two metals.[2] The amount of inter-diffusion is highly dependent on the size of the nanoparticle with larger particles alloying on a substantially slower time scale. This alloying occurs with a timescale of approximately three days (7-9 orders of magnitude too fast to be explained by solid-state diffusion). Diffusion of atoms in a liquid droplet would alloy the metals on the order of a few microseconds. In collaboration with the Meisel group of the Notre Dame Radiation Laboratory and the Bunker group, we have helped to explain this phenomenon.  We postulated that the fast alloying times are due to vacancies present at the interface layer between the two metals. 

    To gain insight to the alloying mechanism, molecular

dynamics simulations were conducted on spherical nanoparticles composed of Ag and Au atoms interacting under the Embedded Atom Method (EAM) potential.[3] Because of the similar lattice constants between Au and Ag, the nanoparticles were constructed on an perfect FCC lattice using a average lattice constant of 4.085 Å. Gold atoms were placed inside of the core radius (rcore) and silver atoms were placed between the core radius and outer shell radius (rcore < r < rshell). We designated a 2 Å "interface" region centered on rcore where either 5% or 10% of the interfacial atoms were removed at random for the simulations that involved vacancies. For these simulations, rcore = 12.5 Å and rshell = 19.98 Å matching the smallest of the nanoparticles experimentally studied by our collaborators.

    Before starting the molecular dynamics simulations, a relatively short steepest-descent minimization was performed to relax the lattice in the initial configuration. During the initial 30 ps of each trajectory, velocities were repeatedly sampled from a Maxwell-Boltzmann distribution matching the target temperature for the run. Following this initialization procedure, trajectories were run in the microcanonical (constant-NVE)  ensemble with zero initial total angular momentum. In order to compare the structural features obtained from the NVE-ensemble molecular dynamics, additional trajectories were run using a modified Nos&eacute;-Hoover thermostat to maintain constant temperature (NVT) and zero total angular momentum. Data collection for all of the simulations started after the 30 ps equilibration period had been completed. We simulated particles with the above-mentioned interfacial vacancy density at 100 K intervals from 500 to 1200 K. Given the masses of the constituent atoms, we were able to use time steps of 5 fs while maintaining excellent energy conservation. Typical simulation times were 100 ns for nanoparticles simulated at 500-600 K and 12-24 ns for particles at 600-1200 K.

    From simulation results, we have constructed a radial density profile of the two constituents as a function of distance from the center of the nanoparticle. This density profile was obtained from the last 2 ns of the 24 ns simulations at 800 K. Remarkably, the interfacial vacancies result in a substantial smoothing of the peaks in the density profile, indicating that those nanoparticles are closer to the melting transition. The region near r=12.5 Å in the radial density profile shows the interfacial vacancies result in significantly enhanced radial diffusion of the Ag into the core region. Much of the displacement occurs along the interface (i.e., at constant r) as can be seen from the broadening of the density peaks.

1. I.Object Oriented Parallel Simulation Engine (OOPSE)
   

2. II. Alloying in bi-metallic gold-silver nanoparticles.
   

3. III. Inter-diffusion in nanoparticles and conclusion of alloying.
   

4. IV. Vibrational dynamics in gold nanoparticles.
   

5. V. Introduction to glass formation in nanoparticles.
   

6. VI. Icosahedral order in glassy nanoparticles.
   

7. VII. A new Water-metal potential model.