Photonic crystals in biology

Poster Session, Tuesday, June 15
Theme A1 - B702
Melting evolution and diffusion behavior of bimetallic CuAu nanoparticles studied molecular
dynamics simulations
Serap Senturk Dalgic 1 * and Unal Domekeli 1
1 Department of Physics, Trakya University, Edirne, 22030, Turkey
Abstract-Molecular dynamics calculations have been performed to study the melting evolution and atomic diffusion behavior of bimetalic
CuAu nanoparticles with the number of atoms ranging from 1286 to 26066 (diameters around 3–9 nm) The interactions between atoms are
described using the quantum Sutton – Chen (Q-SC) many – body potential. The obtained results reveal that the melting temperatures of
nanoparticles are inversely proportional to the reciprocal of the nanoparticle size, and are in good agreement with the predictions of the
liquid-drop model. The melting process can be described as occurring in two stages, firstly the stepwise premelting of the surface region, and
then the abrupt overall melting of the whole nanoparticles. The heat of fusion of nanoparticles are also inversely proportional to the
reciprocal of the nanoparticle size. In addition, surface energies and entropies have been calculated for each sized CuAu nanoparticle at
melting point. The diffusion is mainly localized to the surface region at low temperatures and increases with the reduction of nanoparticle
size, with the temperature being held constant.
Bimetallic nanoparticles are of particular interest in
applications because the nanoparticle properties can vary
dramatically not only with size, as occurs in pure
nanoparticles, but also with chemical composition. Cu-Au
is a well-known model bimetallic alloys system, which is
famous for the existence of a temperature-induced orderdisorder
transition and the capability of forming
thermodynamically stable long period superlattice structure
[1]. On the other hand, size and shape effects on formation
enthalpy and melting process of Cu-Au nanoparticles have
been studied in recent years and results have been reported
in literature [2-6].
In this work, we have studied the melting evolution and
diffusion behavior of bimetallic CuAu nanoparticles using
molecular dynamics calculations with the quantum Sutton-
Chen (Q-SC) potential. The whole melting process is traced
from the changing of the local structure, atomic diffusion
and thermodynamics properties such as the atomic energy,
surface energy, heat of fusion and entropy. All spherical
nanoparticles started with geometries constructed from a
large cubic FCC (L1 0
for CuAu) crystal structure using a
series of spherical cutoff centered at a core of cubes. The
stable structure at 0 K is obtained through the initial
configurations annealed fully at T=300 K and then cooled
to T=0 K at a cooling rate 0.5 K/ps. In order to get an
energy-optimized structure during heating at a given
temperature for bulk systems it is performed molecular
dynamics under constant temperature and constant pressure
conditions (NPT) with a periodic boundary conditions. For
the nanoparticles, we used the constant volume and
constant temperature (NVT) molecular dynamics without
the periodic boundary conditions. The temperature is
controlled by Nose-Hoover thermostat. Newtonian
equations of motion are integrated using the Leapfrog
Verlet method with a time step 2 fs. For the simulation of
the bulk system and nanoparticles the heating process
consists of a series of MD simulations with temperature
increments T=50 K and relaxing time 100ps. However,
for a temperature near the melting region, we used smaller
temperature increment, T=10 K, and the relaxation time
still keeps 100 ps.
The experimental observation of the melting phenomena
on a particle is just the surface liquid skin resulted by
surface premelting. Using molecular dynamic simulation,
the melting process and melting mechanism of
nanoparticles can be analyzed size. In Figure 1, we show
snapshot views of the MD sample with 2708 (D=4nm) at
different temperature. It is observed that CuAu particle with
D=4nm diameter melts at approximately 800 K.
T=300K T =500K T=800K
Figure 1. Snapshot views of the MD sample with N=2708
(D= 4 nm) at a series of temperatures during heating.
To conclude, the melting evolution of bimetallic CuAu
nanoparticles with the number of atoms ranging from 1286
to 26066 under heating condition is investigated by the
molecular dynamics simulations using Q-SC potentials. It
has been found that the melting point is inversely
proportional to the reciprocal of the nanoparticle size and
the results are in good agreement with the theoretical
analysis from the liquid drop model [7]. The evolution of
atomic configuration and energy with temperature reveals
that the melting process of CuAu nanoparticles can be
described in two stages, the stepwise premelting from the
surface region and the sudden melting in the interior region.
Owing to the two stages melting, the heat of melting for
nanoparticles should involve two parts. As the particle size
decreases, the heat of fusion, surface energy and entropy
decreases correspondingly. We have also studied the
structural evolution of CuAu nanoparticles with different
temperature. Radial distribution functions are adopted to
explore structural transition. The atomic diffusion in
nanoparticles is mainly localized to the surface regions
before the melting temperature and increases with the
reduction of particle size at the same temperature.
*Corresponding author: dserap@yahoo.com
13 657 (2005).
[2] W.H.Qi, B.Y. Huang, M.P. Wang, Physica B 404 1761
(2009).
[3] S. Darby, T. V. Mortimer-Jones, R.L.Johnston and C.
Roberts, J. Chem. Phys. 116, 1536 (2002).
[4] J. L. Rodriguez-Lopez, J. M. Montejono-Carrizales, M. Jose-
Yacaman, Appl. Surf. Sci. 219, 56 (2003).
[5] F. Delogu, Nanotechnology 18 (2007) 235706.
[6] Y.J. Li, W.H.Qi, B.Y.Huang, M.P.Wang, S.Y.Xiong, J. Phys.
and Chem. Sol. Article in press.
[7] K.K. Nanda, S.N. Sahu, S.N. Behera, Phys. Rev. A 66,
013208 (2002).
6th Nanoscience and Nanotechnology Conference, zmir, 2010 341