Full Length ArticleNanoscale morphology of electrolessly deposited silver metal
Graphical abstract
Introduction
Noble metal nanostructures display fascinating phenomena such as surface-enhanced Raman scattering (SERS) [1], plasmon resonance absorption [2], and metal enhanced fluorescence [3]. These phenomena have attracted considerable attention and are becoming widely applied in several emerging technologies, including SERS microscopy [1], [4], optical sensing [5], biological labeling [6], optical wave guiding on chips [7], and nano-optics [8]. Many reports indicate that these phenomena are sensitive to size, shape, and space between nanostructures of metal [9]. Controlling the surface nanoscale morphology of metal is thus required for these applications.
Colloidal metal-particle surfaces can be used for SERS, biosensing, and related applications [4], [10]; but the instability of the metal surface due to the random agglomeration of colloidal metal limits their performance and widespread use [11]. An ideal metallization method would provide control over particle shape, size, orientation, and inter-particle spacing. Progress in this direction has been achieved. Vacuum evaporation and sputter-deposition of metal are widely used [9], [12], [13], [14], [15]. These techniques yield stable metal-particle films that are well packed, with controlled thickness [16]. Some approaches have been developed that afford improved control over particle size, shape, and surface functionalization [17], [18], [19], [20].
For some applications, specific particle sizes are required to achieve the best performance. For instance, it has been shown that silver nanoparticles surfaces having a high surface roughness, so called “hot spots,” can provide large surface enhancement when excited at optical frequencies [21], [22], [23]. Several teams have attempted to use high-temperature annealing of sputter-coated metal to control surface morphology [24], [25]. However, the procedure itself is difficult to control, and high-temperature annealing can damage supporting substrates or devices. This is particularly true for micro-devices fabricated using soft-lithography technologies [26], [27].
Electroless deposition (ED) is an alternative means for forming colloidal metallized surfaces and continuous conducting metallized surfaces under ambient conditions [28], [29]. Unlike electroplating, ED can be achieved on a non-conducting surface because the metal is deposited from a bath that contains a source of the metal as a cation co-present with a chemical reducing agent. The ED bath is formulated to be metastable, so the reaction is thermodynamically spontaneous, but kinetically slow in solution. The ED reaction can be activated at a surface by attaching a nucleating species, typically a metal cation (e.g., Sn2+, Fe3+) or metal nano-particles [30], [31], [32]. ED starts at the nucleation sites forming a layer of silver particles. Silver particles formed by initial ED serve as nucleation sites that catalyze further deposition, so the process continues forming an increasingly thick layer of metal with time. Some investigators have reported ways to control the size of silver particles formed by ED [33], [34]. Yet to date, there is little understanding of how the shape, size, and nanoscale morphology of the electrolessly deposited metal depends upon the constituents of an ED-bath and the deposition condition.
Here we report an investigation of ED of silver based on a formulation first proposed by Danscher et al. [35] Fig. 1 is a schematic of the processing parameters explored in this study. Polymeric surfaces were prepared by spin-coating the cross-linkable epoxide SU-8 onto glass substrates [36]. The polymer samples were then treated with a bifunctional amine to which gold nanoparticles (NPs) could be attached to nucleate ED. The Au NP-seeded films were immersed in an ED bath, and the resulting silvered polymer films were examined using a variety of analytical techniques, including scanning electron microscopy (SEM), to characterize the nanoscale morphology. The procedure was repeated using a wide range of buffering agents, concentration of the silver ion, deposition times, and two types of Au NP seeds. The findings reported here show that the nanoscale morphology of the deposited silver depends sensitively on the constituents of the bath, the nature of the surface-bound nucleation species, and the deposition time, and that these parameters could be used to control the nanoscale morphology and obtain metallized surfaces with targeted physical and chemical properties.
Section snippets
Preparation of polymer surfaces
Square 25 mm × 25 mm glass substrates were cleaned by immersing in aqueous 1.0 M aq. KOH (Fisher, CAS# 1310-58-3) for one hour, rinsing with copious deionized water, and drying in the oven at 100 °C for 15 min. The polymer surfaces were prepared by spin coating the cross-linkable epoxide SU-8 2035 resin (MicroChem) onto cleaned substrates (Ramp 1: 100 rpm s−1, hold at 500 rpm for 10 s; Ramp 2: 300 rpm s−1, hold at 4000 rpm for 30 s). The spin-coated films were baked at 65 °C for 3 min, followed
Electroless deposition of silver using the standard bath with gum arabic
Polymer surfaces seeded with in-situ synthesized Au NPs became reflective within 10 min. of immersion in a standard ED bath containing gum arabic and having [Ag+] = 33 mM. The deposited metal had a yellow tint. The time required to obtain a completely reflective film is referred to hereafter as the “induction period”. The length of the induction period provides insight into the activity of a given ED bath, although it cannot be simply related to the rate of the chemical reaction because the
Conclusion
This investigation shows that the size and shape of silver nanoparticles generated by ED depends strongly on numerous chemical and physical parameters, including the concentration of the silver ion, the deposition time, the carboxylate comprising the buffer, the presence or absence of gum arabic, and the nature of the underlying Au NP seed layer. By varying these conditions, a wide range of particle shapes can be obtained – varying from small spheroidal NPs; to large, highly faceted and large
Acknowledgements
This work was supported by NSF grant CHE-0809821 and NSF CAREER grant DMR/CHE-0748712.
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