Study on the Role of Additives in Non-Cyanide Baths for the Fabrication of Anisotropic Metallic Nanostructures

Abstract

Metal nanostructures are one of the most significant components of nanotechnology due to their physical and chemical properties like the catalytic activity and electron transport that allow them suitable for nano device fabrication. Moreover, anisotropic metallic nanostructures are of importance in optical, diagnostic and therapeutic applications. A large variety of bottom-up methods and techniques involving templates or capping agents have been used during the last decade to synthesize many types of anisotropic nanoparticles/structures. Electro deposition techniques being cost effective have an advantage over other fabrication techniques such as nano-imprint lithography, focussed ion beam lithography and vacuum based depositions. The electro depositions include both electroplating and electroless plating techniques. In electroplating of nanostructures, the limiting factor would be nanoprobe fabrication and positioning of the same. On the other hand, electroless plating is cost effective and therefore, can be considered for the mass production of metal coated surfaces. Gold plating is valuable in industries that involve semi-conductors and micro-fabrication. Electro less gold plating involves the deposition of soft gold on silicon or metallic substrate [1]. Eventhough cyanide bath is the most widely used for electroless gold plating, its use can lead to toxicity and environmental degradation. Moreover, it is observed that cyanide can attack the interface residing between the substrate and the photoresist thereby, causing superfluous gold depositions. Therefore, there is a requirement for the development of more efficient non-cyanide baths that could be used for electro less plating. In the present study, non-cyanide sulphite and thiosulphate bath formulations are prepared and gold plating has been performed. The role of different additives in the bath formulations and their effects on gold nanostructures have been investigated. In this study, two non-cyanide gold solutions (Bath A and Bath B) are prepared in house and one commercial cyanide gold plating solution (Bath C) is used [2]. Bath A is formulated using sodium tetrachloroaurate dehydrate (NaAuCl4.2H2O), sodium sulphite (Na2SO3), sodium thiosulphate pentahydrate (Na2S2O3.5H2O), disodium phosphate (Na2HPO4) and 2-Mercaptobenzothiazole. The composition of Bath B is similar to that of Bath A except for 2-Mercaptobenzothiazole is replaced by sodium L-ascorbate. In both bath formulations A and B, thiosulphate-sulphite mixed ligand acts as a complexing agent. Both Na2SO3 and Na2S2O3 are used simultaneously to impart more stability to the bath systems. The 2-Mercaptobenzothiazole acts as stabilizing agent in Bath A. The Na L-ascorbate is expected to influence the crystalline structure of gold nano-deposits in addition to being acting as a reducing agent in Bath B. The zeta potential of bath formulations is measured using NanoPlus HD particle size & zeta potential analyser. It is observed that zeta potential of Bath A is +2 mV, Bath B is +37.35 mV and Bath C is -34.35 mV. From the values, it can be concluded that Bath C and Bath B are more stable than Bath A formulated in the experiments. It is also found that the addition of sodium L ascorbate resulted in + 37.35 mV. The metallic substrates of Cu, Ag and Ni of 2 x 2 cm size are electroless plated in bath solutions for 2 minutes at 60 °C. After which the surface morphology of the metallic substrates are characterized using SEM technique. Figure 1 (a) shows the SEM analysis of a bare gold metallic substrate and Figure 1 (b) shows the SEM analysis of electroless gold plated on copper metallic substrate. In case of Bath C, the morphology and dimensions of the resulting nanostructures are found to be strongly dependent upon temperature, plating time and copper metallic substrate. At 60 °C and 2 mins, gold nanocubes with sides less than 500 nm is formed. It is found that increasing the plating time may vary the dimensions of the gold nanocubes along several triangular and square facets leading to the formation of a polyhedral as shown in figure 1 (b). This may have formed due to a capping agent present in the commercial cyanide gold plating solution (Bath C). Similarly, electroless plating of silver metallic substrate in Bath A produced star shaped gold nanostructures implying the effect of both bath composition and respective metallic substrate. In order to investigate the anisotropy of metallic nanostructures formed on the surfaces, XRD analysis is also performed for bare metallic substrates and electroless gold plated metallic substrates. This study gives an insight on the role of additives in non-cyanide baths for the fabrication of anisotropic metallic nanostructures. Further research is in progress with respect to different bath compositions and metallic substrates paving way for fabricating anisotropic gold nanostructures towards sensing applications. Figure 1