Photonic crystals in biology

Poster Session, Tuesday, June 15
Theme A1 - B702
Structure of unsupported s mall Al nanoparticles; Molecular dynamics study
Amir Chamaani 1 * Reza Darvishi Kamachali 2 Ehsan Marzbanrad 3 Alireza Aghaei 3 Yashar Behnamian 4
1 New materials Department, Materials and Energy Research Center (MERC), P.O. Box 14155-4777, Tehran, Iran
2 ICAMS, Ruhr-University Bochum, Bochum 44801, Germany
3 Ceramic Department, Materials and Energy Research Center (MERC), P.O. Box 14155-4777, Tehran, Iran
4 Chemical and Materials Engineering Department, University of Alberta Edmonton AB, T6G 2V4, Canada
Abstract-A classical molecular dynamics simulation has been used to study the structure of unsupported small Al nanoparticles. Our results
support the existence of a core-shell structure for the common truncated octahedron shape of Al nanoparticles, in terms of cohesive energy.
However, there is a critical size below which the core-shell model cannot be applied, because the entire structure of Al nanoparticles below this
size consists of merely a surface zone.
The cohesive energy of nanoparticles determines their
physical and thermodynamic properties, such as melting point,
thermal stability and structure. Different models have been
developed to account for cohesive energy, including the BE
model [1], SAD model [2], Xie’s model [3] and so on. Most
recently, the core-shell model [4-5] has been used to describe
the structure of nanoparticles, in which the core of the particle
is the same as the bulk material, and the surface shell is not
caused by large dangling bonds. This concept has already been
used in particular models to calculate the cohesive energy of
metallic nanoparticles. In these models [1, 3], the entire
structure of the nanoparticle is divided into two sections: a
bulk and a surface zone. In other words, the cohesive energy
of a nanoparticle is considered to consist of the cohesive
energies of the bulk zone and of the surface zone. This study is
a qualitative investigation of the structure of a common shape
(truncated octahedron) for Al nanoparticles.
In this study, the simulation was performed in NVT
ensemble, using a semi-emp irical potential (glue) [6]. The
expressions for and details of the potential used can be found
in aforementioned paper. The primary structure of our
nanoparticles was the truncated octahedron structure from an
ideal Al crystal. The number of atoms in each nanoparticle
was either 79, 201, 586, 1289, 2406 or 4033, with a particle
size ranging from 1–5 nm. A bulk simulation of 864 Al atoms
using periodic boundary conditions (pbc) was carried out to
serve as a comparative case. The Verlet velocity algorithm
was employed for motion equations, and a simple redial
distribution counter function was used to calculate energy
distributions. The systems were divided into the maximum
possible redial layers for which every layer included at least
one atom.
The cohesive energy versus the distance to center of Al
nanoparticles and the corresponding bulk value is shown in
Figure (1). Unlike Al bulk (864 atoms with pbc), the cohesive
energy of Al nanoparticles at a specific temperature is not
constant: receding from center to surface, cohesive energy
deviates from the bulk value. Indeed, internal atoms of large
nanoparticles (nanoparticles which consist of more than 201
atoms) have the bulk cohesive energy (-3.34 eV/atom), but
surface atoms have a lower cohesive energy. Because of this
separation between surface and interior atoms of large Al
nanoparticles, the whole structure of large nanoparticles can
be consider as having two parts: the core of the nanoparticle
(the bulk zone), and the shell of the nanoparticle (the surface
zone). The bulk zone in large nanoparticles consists of atoms
which have the bulk cohesive energy, whereas surface zone
atoms have a lower cohesive energy. Furthermore, as shown
in Figure (1), with decreasing nanoparticle size, the bulk zone
gradually decreases, and below a crit ical size it co mpletely
disappears. Under our conditions, the critical size is about 1.6
nm (201 atoms). Therefore, the entire structure of
nanoparticles below the critical size consists of only a surface
zone. These nanoparticles cannot be considered to have a
core-shell structure and it is better to define them simply as
surface materials. Recently, Qi [4-5], using mo lecular
dynamics simulation, suggested that nanoparticles of all sizes
can be regarded as having a core-shell structure, even small
ones. However, according to our results, the structure of very
small nanoparticles consists of only a surface zone; the
cohesive energies of the two smaller nanoparticles (201 and 79
atoms) are composed of only the cohesive energy of their
surface zones. Consequently, the validity of the BE and Xie
models, which are based on the core-shell model, is called into
question for small nanoparticles.
Figure 1. Cohesive energy of Al nanoparticles and bulk counterpart versus
distance to center
In summary, we investigated the structure of unsupported Al
nanoparticles in terms of the radial distribution of cohesive
energy. Our results show that the structure of nanoparticles
can be described by the core-shell model. However, there is a
critical size below which there is no bulk zone; the whole
structure of nanoparticles of this size consists of the surface
zone. Therefore, the validity of some cohesive energy models
based on the core-shell model, such as the BE and Xie models,
may not hold true for very small nanoparticles.
*Corresponding author: amir_chamani@merc.ac.ir
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(2007).
[2] W.H.Qi, M.P.Wang, J.Mater.Sci.letter. 21, 1743 (2002).
[3] D Xie, M.P.Wang and W.H.Qi, j.phys.condens.matter.16, L401
(2004).
[4] W.H.Qi, B.Huang, M.P.Wang, J. comput.theor. nanoscience. 6,
1546 (2009).
[5] W.H. Qi, S.T. Lee, chem.phys.lett. 483, 247–249 (2009).
[6] F. Ercolessi, J. B. Adams, Europhys. Lett. 26, 584 (1994).
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