Elsevier

Applied Surface Science

Volume 426, 31 December 2017, Pages 667-673
Applied Surface Science

Full Length Article
Clustering of gold particles in Au implanted CrN thin films: The effect on the SPR peak position

https://doi.org/10.1016/j.apsusc.2017.07.233Get rights and content

Highlights

  • Formation and optical properties of Au particles in CrN films were investigated.

  • CrN layers were implanted with Au+ ions at fluences in the range of 1016 cm−2.

  • For the fluence of 2 × 1016 cm−2 gold particles of ∼200 nm in diameter were formed.

  • With increasing Au ion fluence the particles coalesce into clusters.

  • The shift of a SPR peak indicates a strong interaction between Au particles.

Abstract

We report on the formation of gold particles in 280 nm thin polycrystalline CrN layers caused by Au+ ion implantation. The CrN layers were deposited at 150 °C by d.c. reactive sputtering on Si(100) wafers and then implanted at room temperature with 150 keV Au+ ions to fluences of 2 × 1016 cm−2 to 4.1 × 1016 cm−2. The implanted layers were analysed by the means of Rutherford backscattering spectrometry, X-ray diffraction, atomic force microscopy and spectroscopic ellipsometry measurements. The results revealed that the Au atoms are situated in the near-surface region of the implanted CrN layers. At the fluence of 2 × 1016 cm−2 the formation of Au particles of ∼200 nm in diameter has been observed. With increasing Au ion fluence the particles coalesce into clusters with dimensions of ∼1.7 μm. The synthesized particles show a strong absorption peak associated with the excitation of surface plasmon resonances (SPR). The position of the SPR peak shifted in the range of 426.8–690.5 nm when the Au+ ion fluence was varied from 2 × 1016 cm−2 to 4.1 × 1016 cm−2. A correlation of the shift in the peak wavelength caused by the change in the particles size and clustering has been revealed, suggesting that the interaction between Au particles dominate the surface plasmon resonance effect.

Introduction

Transition metal nitrides, such as chromium-nitride (CrN), have been widely studied in the past both from fundamental and technological point of view due to the attractive combination of physical and chemical properties [1], [2]. CrN coatings, depending on their microstructure, exhibit favorable corrosion and oxidation resistance, high hardness and wear behaviour [3], [4]. CrN is used as a coating material for corrosion resistance and in metal forming and plastic moulding applications [5]. Also, it is often used on medical implants and tools as well as a valuable component in advanced multicomponent coating systems, such as CrAlN [6]. Recent studies [7], [8], [9], [10] have indicated that the presence of carbon or additional transition metals (W, V, Nb, Al) may change the mechanical properties dramatically and enhance the hardness of CrN coatings. In particular, addition of soft metals such as Ag within the CrN matrix has been observed to result in substantial improvement of tribological properties of CrN coatings for both ambient and vacuum environments at high temperatures [11].

Nowadays, noble metal nanoparticles (NPs), such as gold, silver and copper nanoparticles have attracted interest, because of their unique photonic, electronic and catalytic properties [12], [13]. The origin of these unique properties is the surface plasmon resonance (SPR) effect, which corresponds to a collective excitation of the free electrons localized at the surfaces of metal structures [14], [15]. This strong absorption band, usually found in the UV–vis range, is governed by the noble metal NPs morphology, namely their size and shape, as well as of the dielectric function of the surrounding medium [13], [16]. Numerous works have been dedicated to researching nanocomposite materials, which consist of noble metal particles (Ag, Au) and dielectric matrices (SiO2, ZnO, TiO2) [17], [18], [19], the films being obtained by different methods, such as sol-gel, electrochemical methods, reactive magnetron co-sputtering and ion implantation. In contrast to this, only a few studies have been performed related to the formation of embedding metallic nanoparticles in non-dielectric matrices [20], [21]. Here, the efforts were mostly focused on studying the improvement of corrosion and antibacterial properties of these layers and very little is known about the dependence of the optical properties on the structural and morphological evolutions of these nanocomposites.

In various respects, the present work is an extension of our previous optical and microstructural analyses of CrN and TiN thin films irradiated with different ions (Ar+, Xe+ and Ag+) [21], [22], [23], [24]. It was shown that the optical properties of the Ar+ or Xe+-irradiated nitride thin films can be varied due to the formation of implantation induced damage, but no substantial changes of the optical constants were observed. In contrast to this, absorption properties of TiN layers irradiated with Ag ions exhibit a contribution by the SPR effect due to the presence of silver NPs. Led by these considerations, we have carried out a series of experiments, where the CrN layers were irradiated with 150 keV Au+ ions. The aim was to determine the complex dielectric function of Au+-irradiated CrN films based on experimental results by spectroscopic ellipsometry. The influence of ion fluence and particles (Ps) clustering on the optical response of Au particles was investigated.

Section snippets

Experimental

Optimal conditions for producing layers of stoichiometric CrN phase have been achieved using d.c. reactive sputtering by properly tuning the N2 pressure, deposition rate, as well as the CrN film thickness and substrate temperature. We used a Balzers Sputtron II system with Cr target of 99.9% purity, Ar for sputtering and reactive N2 gas. The base pressure in the chamber was around 1 × 10−4 Pa, the argon and nitrogen partial pressures during deposition were 1 × 10−1 Pa and 5 × 10−2 Pa, respectively. The

Structure and phase characterization

The elemental composition, measured by RBS on as deposited chromium-nitride layers under given deposition conditions, confirmed a homogeneous Cr and N concentration over the whole layers depth, with a Cr/N ratio ∼1. [25]. Starting from these results, we determined the evaluation of the Au depth distribution within CrN layers after 150 keV Au+ ions irradiation. Fig. 1a shows a typical RBS spectrum of a CrN film implanted with a nominal Au+ ion fluence of 3.2 × 1016 cm−2. It can be seen that the Au

Conclusions

Au particles were formed in CrN layers by using ion implantation process. The concentration of gold in the layers was varied by using different ion fluences from 2 × 1016 cm−2 to 4.1 × 1016 cm−2. The films revealed significant changes of the optical properties with regard to the complex dielectric function due to the contribution of a SPR peak, characteristic for Au particles. The shift of the peak position to longer wavelengths was attributed to the coalescence of Au Ps‘ and, moreover, to the Au

Acknowledgements

This work was supported by the Ministry of Education and Science of the Republic of Serbia (Project No. III 45005) and by the German-Serbian DAAD bilateral collaboration (Project No. 451-03-01038/2015-09118/18). We would like to thank Igor Peterka for his assistance at 500 kV ion implanter in Belgrade, and Ulrich Barth for his help during the RBS experiments in Jena.

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