Structures, energetics and magnetic properties of Au&lt;sub&gt;n&lt;/sub&gt;SFe&lt;sub&gt;m&lt;/sub&gt; and Au&lt;sub&gt;n&lt;/sub&gt;Fe&lt;sub&gt;m&lt;/sub&gt; clusters for m = 1-5 and n = 1-5, 12 and 32

H Woldeghebriel

doi:10.4314/mejs.v6i2.109711

Abstract

Simulations of nanoscale systems are important from the understanding point of view of the physics and chemistry involved in describing observed phenomenon. This paper presents the results of systematic theoretical investigation of the structural and magnetic properties of gold-iron complexes at small size scale. Ab initio calculations are performed for AunFem with and without the presence of a sulfur atom, i.e., the clusters AunSFem, where n, m = 1 - 5. The study also includes Au6SFe and Au12Fe in order to investigate how the fully wrapped Fe atom responds to the Au atoms .The study mainly focuses on the geometrical and magnetic changes with a step by step removal of the sulfur atom as a function of the cluster size. It is found that average Au-Fe bond length is increased with increasing number of atoms in the cluster. An increase of Au-Fe bond length would have increased the magnetic moment of the Fe atom, but due to the hybridization of Au and Fe orbitals, the moment converges to about 3.00 μB. This value is higher than the magnetic moment of Fe atom in bulk gold. An enhanced magnetic moment is found on Fe atom even if it is fully wrapped by the Au12 octahedral cluster. From this study it is found that, the cluster stability is increased on the addition of a single sulfur atom to the AunFem clusters. A special stability is observed in Au4SFe, Au6SFe, Au12SFe and Au4SFe2 clusters as the sulfur atom in these clusters is found to be doubly bonded. Generally, the systematic studies on the small sized clusters show an enhanced magnetic moment on the iron atoms bonded to the gold atoms as compared to the corresponding bare iron clusters. This indicates that, the magnetic moment of iron atoms can be enhanced by a complete coating with gold atoms for practical applications. This complete coating can prevent iron from oxidation and may also prevent coalescence of iron clusters and formation of thromboses. The coupling of iron atoms in this work remains ferromagnetic irrespective of the number of gold atoms in the cluster.Keywords: Cluster, Magnetic moment, Coating, Nanoparticles.

Highlights

The presence of unfilled d shells of transition metal clusters has important consequences: d electrons give rise to more directional bonds and there are many low-lying excited states corresponding to the various possibilities to arrange the electrons in the empty d states.Properties, such as the stability of the cluster can often be discussed in terms of shells of atoms, relating the number of atoms needed to form a compact symmetric structure to an enhanced stability
The number of Fe dopant atoms is increased in steps and the same calculation is repeated for each cluster
Geometrical structures, binding energies and magnetic properties are presented as a function of the number of gold atoms in the cluster for each Fe atom doped in the cluster

Summary

INTRODUCTION

The presence of unfilled d shells of transition metal clusters has important consequences: d electrons give rise to more directional bonds and there are many low-lying excited states corresponding to the various possibilities to arrange the electrons in the empty d states. Sun et al (2004) studied the effect of gold coating on the optical properties of a nanosilica cluster using time-dependent Density Functional Theory (DFT) They observed a reduction of the optical gap which makes it possible to absorb near infrared light (NIR). (3) They can be oxidized, which in turn will weaken their magnetic property In another experimental study, gold-coated acicular and spherical shaped iron nanoparticles were characterized using transmission electron microscopy (TEM) and alternating gradient magnetometry. Sun et al (2006) reported the first theoretical investigation of the structural and magnetic properties of gold-coated iron nanoparticles in various size ranges. Motivated by the practical application of gold-coated clusters, the author has studied computationally the interaction of small gold clusters with iron clusters

COMPUTATIONAL DETAILS

RESULTS AND DISCUSSION

Geometry and Magnetization

Binding Energy

CONCLUSION