Directing Cluster Formation of Au Nanoparticles from Colloidal Solution

Sarah M. Adams,Filippo Capolino,Regina Ragan,Salvatore Campione

doi:10.1021/la3051719

Abstract

Discrete clusters of closely spaced Au nanoparticles can be utilized in devices from photovoltaics to molecular sensors because of the formation of strong local electromagnetic field enhancements when illuminated near their plasmon resonance. In this study, scalable, chemical self-organization methods are shown to produce Au nanoparticle clusters with uniform nanometer interparticle spacing. The performance of two different methods, namely electrophoresis and diffusion, for driving the attachment of Au nanoparticles using a chemical cross-linker on chemically patterned domains of polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) thin films are evaluated. Significantly, electrophoresis is found to produce similar surface coverage as diffusion in 1/6th of the processing time with an ~2-fold increase in the number of Au nanoparticles forming clusters. Furthermore, average interparticle spacing within Au nanoparticle clusters was found to decrease from 2-7 nm for diffusion deposition to approximately 1-2 nm for electrophoresis deposition, and the latter method exhibited better uniformity with most clusters appearing to have about 1 nm spacing between nanoparticles. The advantage of such fabrication capability is supported by calculations of local electric field enhancements using electromagnetic full-wave simulations from which we can estimate surface-enhanced Raman scattering (SERS) enhancements. In particular, full-wave results show that the maximum SERS enhancement, as estimated here as the fourth power of the local electric field, increases by a factor of 100 when the gap goes from 2 to 1 nm, reaching values as large as 10(10), strengthening the usage of electrophoresis versus diffusion for the development of molecular sensors.

Highlights

It has long been established that local electromagnetic field enhancements observed in noble metal nanoparticles, as a result of excitation at the plasmon resonance, increase as nanoparticles electromagnetically couple with nanometer scale interparticle spacing.[1,2] Significant advances in controlling the nanostructure and local composition via chemical synthesis[3−6] coupled with theoretical understanding of electromagnetic coupling[2,7−12] allows for tuning optical properties across the electromagnetic spectrum
We find that electrophoretic deposition (EPD) can produce comparable areal density of Au nanoparticles on PS-b-PMMA thin films in 1/6th of the processing time with respect to random diffusion
We show Au nanoparticles that have been functionalized with thioctic acid (TA) ligand molecules and PS-b-PMMA thin films that have been treated with ethylenediamine (ED) in dimethyl sulfoxide (DMSO) solution are illustrated. 1-Ethyl-3

Summary

Introduction

It has long been established that local electromagnetic field enhancements observed in noble metal nanoparticles, as a result of excitation at the plasmon resonance, increase as nanoparticles electromagnetically couple with nanometer scale interparticle spacing.[1,2] Significant advances in controlling the nanostructure and local composition via chemical synthesis[3−6] coupled with theoretical understanding of electromagnetic coupling[2,7−12] allows for tuning optical properties across the electromagnetic spectrum While these plasmonic systems have been demonstrated to have numerous applications such as molecular sensors and efficiency-enhanced photovoltaics, scalable methods to produce nanostructures over a large area on surfaces using processes that are nonprohibitive in cost are still challenging. The ED-treated PS-b-PMMA thin films on Si were horizontally suspended in 2 mL of the cross-linker-activated Au nanoparticle solution for each cm[2] of substrate with the functionalized side of the PS-b-PMMA thin film immersed facedown and incubated at 40 °C for 120 min (in 60 min durations with refreshed nanoparticle solution) followed by IPA wash

Methods

Results

Conclusion