Plasmon tuning of nano-Au in dichroic devitrified antimony glass nanocomposites by refractive index control

Abstract

Tuning of the surface plasmon resonance (SPR) band of Au⁰ (587–611 nm) has been demonstrated by controlling the refractive index (1.8349–2.0006) of the encapsulating matrix in a new series of nano-Au⁰-antimony glass (K₂O–B₂O₃–Sb₂O₃) dichroic devitrified composites. They have been synthesized by a single-step melt-quench in situ thermochemical reduction technique without using any external reducing agent or additional processing step. Dichroic behavior is due to elliptical shape of Au⁰ nanoparticles having 13–18 nm size range and aspect ratio about 1.2 according to transmission electron microscopic image. X-ray and selected area electron diffractions manifest the growth of (1 1 1) and (2 0 0) crystallographic planes of Au.

Introduction

Metal–dielectric (glass) nanocomposites have captured sustained intellectual and practical interests over several years owing to their unusual optical and electrical properties and are widely used for real applications, such as fluorescence (photonic) and non-linear optical devices [1], [2], [3], [4]. Noble metal nanoparticles (Au or Ag) are associated with the unique surface plasmon resonance (SPR) phenomenon, which is the dynamic collective oscillation of the conduction electrons upon irradiation [1], [5]. SPR is intimately dependent on the nanoparticle size, shape, refractive index of the dielectric environment and other proximal nanoparticles (NPs) [6]. Precise tuning of the SPR band across a wide spectroscopic range can be accomplished by varying any of the aforesaid parameters [7], [8]. The effects of the medium refractive index on the optical properties of noble metal nanostructures have predominantly captured rising fundamental and technological interest in recent times. The SPR peaks generally experience a red-shift as the refractive index of the surrounding environment is increased [1], [7], [9]. This dependence of their SPR wavelengths on the surrounding refractive index forms the basis of localized surface plasmon resonance spectroscopy (LSPRS) [9] and is also utilized for medium sensing applications [10].

However, the fabrication of metal–glass nanocomposites is not simple. Conventional methods of their preparation involves multi-step techniques like melt-quench or sol–gel route fabrication followed by ion-implantation, ion-exchange and subsequent laser, ion or X-ray beam irradiation and heat treatment for long time at high temperatures in reducing atmosphere (e.g. hydrogen), to generate nanometal incorporated glasses [1], [2], [3], [4], [11]. Most of metal–glass composites to date have been comprised of spherical NPs inserted within silicate or phosphate matrices where it is very difficult to make wide variations of refractive indices. Hence it would be quite interesting to study the effect of matrix refractive index on the unusual optical properties of anisotropic metal NPs embedded within heavy metal oxide glass matrices, like antimony(III) oxide glass, by altering a wide variation of refractive index.

Earlier reports on preparation of high Sb₂O₃ containing glasses show they are yielded in tiny pieces or pulverized form [12], [13]. This is because the low field strength (0.73) of Sb³⁺ makes it a poor glass former [12]. Hence, the areas of nanometal-embedded Sb₂O₃ based glasses and nanocomposites have remained totally unexploited because of their difficulties in preparation particularly in the bulk monolithic forms which are very much essential for convenient optical applications. But, the most important advantage of using Sb₂O₃ based glass systems over conventional ones is that Sb₂O₃ is a mild reducing agent [14]. This property enables in situ reduction of Au³⁺ (HAuCl₄·xH₂O) to Au⁰ during the melting process, thereby providing for a straightforward, low-cost strategy for the fabrication of bulk metal–glass nanocomposites in a single-step. Besides other important applications like plasmon enhanced luminescence of rare-earth ions are also promising, taking advantage of the enhanced local field properties of anisotropic NPs [15], [16]. Sb₂O₃ based systems are also encouraging because they have inherent non-linear optical (NLO) properties and suggested for technological applications like optical recording media [17] due to their photosensitivity, high refractive index, non-centrosymmetric structure [13]. Plasmonic metal nanocrystals if incorporated within such Sb₂O₃ based glassy hosts are expected to attribute enhanced NLO properties. Consequently, such materials, if synthesized, are also expected to have prospective optoelectronic applications in optical switching and optical limiting devices.

Pondering over the above facts, in this Letter we demonstrate the synthesis of a new series of nano-Au⁰-embedded antimony glass dichroic nanocomposites by a simple single-step technique and establish the effect of the matrix refractive index on Au SPR band at a fixed concentration of Au. We characterize the resultant nanocomposites by UV–visible absorption spectroscopy, transmission electron microscopy (TEM), selected area electron diffraction (SAED), X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM) analyses.