Fast, simple, combinatorial routes to the fabrication of reusable, plasmonically active gold nanostructures by interferometric lithography of self-assembled monolayers.

Anna Tsargorodska,Brice Darroch,Michaël L Cartron,Osama El Zubir,Tamara Basova,C Neil Hunter,Alexei V Nabok,Graham J Leggett

doi:10.1021/nn5014319

Abstract

We describe a fast, simple method for the fabrication of reusable, robust gold nanostructures over macroscopic (cm(2)) areas. A wide range of nanostructure morphologies is accessible in a combinatorial fashion. Self-assembled monolayers of alkylthiolates on chromium-primed polycrystalline gold films are patterned using a Lloyd's mirror interferometer and etched using mercaptoethylamine in ethanol in a rapid process that does not require access to clean-room facilities. The use of a Cr adhesion layer facilitates the cleaning of specimens by immersion in piranha solution, enabling their repeated reuse without significant change in their absorbance spectra over two years. A library of 200 different nanostructures was prepared and found to exhibit a range of optical behavior. Annealing yielded structures with a uniformly high degree of crystallinity that exhibited strong plasmon bands. Using a combinatorial approach, correlations were established between the preannealing morphologies (determined by the fabrication conditions) and the postannealing optical properties that enabled specimens to be prepared "to order" with a selected localized surface plasmon resonance. The refractive index sensitivity of gold nanostructures formed in this way was found to correlate closely with measurements reported for structures fabricated by other methods. Strong enhancements were observed in the Raman spectra of tetra-tert-butyl-substituted phthalocyanine. The shift in the position of the plasmon band after site-specific attachment of histidine-tagged green fluorescent protein (His-GFP) and bacteriochlorophyll a was measured for a range of nanostructured films, enabling the rapid identification of the one that yielded the largest shift. This approach offers a simple route to the production of durable, reusable, macroscopic arrays of gold nanostructures with precisely controllable morphologies.

Highlights

The field of plasmonics[1,2] has expanded rapidly over the past decade
Half of the clean coherent beam was pointed directly onto the sample surface, and the other half of the beam was pointed onto a mirror, from which it was reflected onto the sample surface where it interfered with the other half of the beam to yield a sinusoidal pattern
The angle 2θ between the mirror and sample was set to 60° to yield a period equal to the laser wavelength for fabrication of relatively large structures to demonstrate the difference in morphology

Summary

Introduction

The field of plasmonics[1,2] has expanded rapidly over the past decade. The optical properties of metal nanostructures are very different from those of bulk materials because of the strong coupling that can occur between oscillations of their surface electrons and incident electromagnetic radiation.3À9 Nanostructures formed from gold and some other metals may exhibit strong plasmon bands in their optical spectra as a consequence of this coupling, leading to pronounced coloration and dramatic changes in the optical properties of molecules that are placed in close proximity to the metal surface.[8]. In order to achieve good localized surface plasmon absorptions, it is necessary for the separation between nanostructures to be optimal.[40] Samples that were fabricated using φ = 90°, 60°, or 45° and exhibited interparticle separations of less than 125 nm (referred to hereafter as type I structures) yielded spectra that were indistinguishable from those recorded for continuous polycrystalline gold films (Figure 3a).

Results

Conclusion