How Colloidal Lithography Limits the Optical Quality of Plasmonic Nanohole Arrays

Abstract

Colloidal lithography utilizes self-assembled particle monolayers as lithographic masks to fabricate arrays of nanostructures by combination of directed evaporation and etching steps. This process provides complex nanostructures over macroscopic areas in a simple, convenient, and parallel fashion without requiring clean-room infrastructure and specialized equipment. The appeal of the method comes at the price of imperfections impairing the optical quality, especially for arrayed nanostructures relying on well-ordered lattices. Imperfections are often generically mentioned to rationalize the discrepancy between experimental and simulated resonances. Yet, little attention is given to detailed structure–property relationships connecting typical defects directly with the optical properties. Here, we use a correlative approach to connect nano- and microscopic defects occurring from the colloidal lithography process with the resulting local optical properties. We use nanohole arrays as a common plasmonic structure known to be sensitive to lattice imperfections. Correlative optical and electron microscopies reveal the individual role of packing order, organic impurities, and solid polymer bridges. Our findings show that simple cleaning processes with solvents and oxygen plasma already improve the optical quality but also highlight how well-controlled self-assembly processes are required for predictable optical properties of such nanostructures.

High Q-Factor Plasmonic Surface Lattice Resonances in Colloidal Nanoparticle Arrays

“…The ideal 2D array of single Au nanospheres supports high-quality SLRs as reported in the literature. 2,4 In the ideal case, reducing the size of the modeled NP from 250 to 40 lattice parameter. Manjavacas and co-workers reported a dependence of the Q-factor with the geometrical parameters given by Q ≈ (LP/D) 9 , where LP is the array lattice parameter and D is the diameter of the NPs.…”

“…An interesting alternative to produce metal arrays with high quality over large areas is the use of colloids . Chemically synthesized metallic colloidal NPs already present a high degree of crystallinity, tunable optical properties, and are compatible with scalable fabrication and solution processability. − Several experimental approaches employed for the fabrication of plasmonic arrays using colloidal particles, including dip coating self-assembly, − capillary-assisted particle assembly (CAPA), − templated-assisted self-assembly, − among others, , are promising alternatives to top-down procedures. However, so far, the 2D lattices built with colloidal nanocrystals have not exhibited Q -factor values as high as their top-down counterparts.…”

Impact of Optical Cavity on Refractive Index Sensitivity of Gold Nanohole Arrays

“…Numerous plasmonic nanostructures, including dispersed or substrate-supported metal nanoparticles, 2D or 3D nanoparticle clusters, and metal films with periodic or aperiodic patterning, have been investigated so far as potential sensing elements of a biosensor [4][5][6]. Particularly interesting plasmonic systems for biosensing applications are the periodic and the short-range ordered nanohole arrays (NHAs) in thin metal films [7], since such nanostructures can be fabricated relatively easily on large-area substrates at cm 2 scale using colloidal lithography [8][9][10]. The flexibility, low cost, and high throughput of fabrication make these nanostructures advantageous compared with many other systems.…”