Structure and thermal stability of Au–Fe alloy nanoclusters formed by sequential ion implantation in silica

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Abstract

Sequential ion implantation of Au and Fe ions has been performed in silica. Despite the fact that the two species are not miscible in the bulk, structural characterizations show that the nanoparticles produced are a Au–Fe alloy. The crystalline structure of the nanoparticles is fcc with a lattice parameter of 0.395 nm. The nanocomposite is ferromagnetic at 3 K, with a magnetic moment per Fe atom equal to 1.4μB and it shows a hysteresis loop with a coercive field of 24 mTorr. Due to alloying, the optical absorption spectrum does not exhibit the typical surface plasmon resonance of gold nanoparticles. The sample exhibits a change of its optical and structural properties when is annealed at 600 °C.

Introduction

In the last decades a strong effort in materials research has been devoted to the study of the synthesis and properties of nanocomposites, that is, materials formed by nanoparticles dispersed in a matrix. This interest is due to the fact that nanocomposites exhibit novel and enhanced properties associated both to the particular features of the nanoparticles and to their interaction with the matrix. Moreover, the nanocomposites show promising properties for a wide range of applications like electronic, opto-electronic, information storage, mechanics, biotechnology, energy storage, catalysis, gas sensing, etc. Ion implantation technique has been extensively applied to obtain metal–dielectric nanocomposites [1], [2]. This technique allows an easy control of the nanostructure (both size and dispersion) by properly tuning the implantation conditions. Also the composition of the particles can be varied by sequentially implanting two species [1], [3], [4]. The growth of nanostructure with binary composition has a major interest for applications because most of the best performing materials are alloys. Binary nanoparticles can be present as alloys but also as core–shell structure that can have new properties due to the interaction or combination of the properties of the two parts of the nanoparticle. The nanostructure of the binary nanoparticles depends both on the chemical reactivity of the implanted species [5], [6] and on the alloy formation heat.

Most of the studies on binary implantations involve metallic elements that are miscible. Recently it has been shown that sequential implantation of metals, like Co and Cu, which are not miscible in the bulk phase diagram, give rise to alloy nanoparticles [7], [8]. In this work we present experimental results on the sequential implantation of Au and Fe metals that are not miscible in the bulk. The interest of investigating this type of system is in the combination two metals with different application areas: while Fe or Fe-based nanocomposites are among the most investigated magnetic material, Au nanocomposites have peculiar optical properties, like the surface plasmon resonance or the enhanced third-order optical non-linearity [1]. Au–Fe nanoparticles could combine two kinds of properties: magnetic and optical. Bulk Au–Fe alloys can be obtained by different out-of-equilibrium methods [9] showing interesting magnetic, magneto-transport and magneto-optical properties [9], [10], [11], [12], [13], [14], [15]. In particular the L10 ordered structure has been observed in thin films [11]. Nanoparticles with this structure, typically characterized by a large magnetocrystalline anisotropy, could be very interesting for magnetic recording applications. However most works related to Au–Fe nanoparticles have considered core–shell or onion-like nanostructures [16], [17], [18]. Only recently Sato et al. have reported the formation by electronic beam evaporation technique of Fe–Au nanoparticles with complex structure [19]. In this work we will show the possibility of obtaining Au–Fe alloy nanoparticles by sequential implantation technique. The nanostructure, the optical and magnetic properties and the thermal stability of these novel materials will be illustrated and discussed.

Section snippets

Experimental

Fused silica slides (Herasil 1 by Heraeus) were sequentially implanted with Au+ and Fe+ ions at the same fluence of 9 × 1016 ions/cm2 at the INFN–INFM laboratories in Legnaro (Italy). The implantation energies were 190 keV for Au and 90 keV for Fe to obtain the same projected range (about 70 nm), in order to maximize the overlap between the concentration depth profiles of both species. The current density was below 2 μA/cm2 to avoid sample heating. For comparison, single implants of gold and iron were

Results and discussion

Fig. 1 shows the RBS spectrum of the Au–Fe as-implanted sample. The calculated total retained metal fluence, using RUMP code [20], gives values (7 × 1016 Au/cm2 and 8 × 1016 Fe/cm2) smaller than the implanted fluences and the total Au/Fe ratio, 0.87, is smaller than the expected ratio. Considering the high fluences used, this could probably be related to sputtering effects. Indeed, in Fig. 2 the bright-field cross-sectional TEM micrograph of the Au–Fe as-implanted sample shows that the largest

Conclusions

Nanocomposites formed by Au–Fe alloy nanoparticles dispersed in silica matrix can be obtained by sequential implantation technique. These nanoparticles have fcc structure and are rich in gold. The solid solution begins to decompose at temperatures above 600 °C in reducing atmosphere. The magnetic and optical properties of the bimetallic Au–Fe nanoparticles are different from those of only-Au or Fe nanoparticles. In particular: (i) the surface plasmon resonance typical of pure Au nanoclusters is

Acknowledgement

This work was financially supported by MICROPOLYS – FIRB Italian project.

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