Nanomeshes at Liquid Interfaces: From Free‐Standing Hole Arrays toward Metal–Insulator–Metal Architectures

Abstract

sometimes also referred to as double fishnet structures, give rise to negative refraction. [5] However, the majority of these metalinsulator-metal (MIM) structures are fabricated using top-down methods like e-beam lithography or focused-ion-beam milling. These techniques not only are very time consuming but also need sophisticated equipment making the production process expensive and hardly applicable to large areas. Especially, multilayer structures are difficult to access using classical top-down methods. Consequently, fabrication techniques suitable for the fast and cost-effective production of MIM structures still remain a challenge. Bottom-up approaches could overcome these problems by capitalizing on self-assembly strategies in the colloidal regime. [6] Tagliabue et al. prepared periodic MIM arrays with a perforated metal structure and an (unperforated) metal back reflector employing nanosphere lithography (NSL) without a transfer step. [7] For this purpose, successive e-beam evaporation of a silver or gold layer and an insulating silica layer was followed by the deposition of a gold layer through a particle template. A similar approach was used by Zhou et al. to prepare MIM structures with two perfectly aligned silver nanohole arrays, which supported magnetic cavity modes in the visible and NIR range. [8] As multiple layers are deposited onto the same particle monolayer, the number of layers in the MIM structure is limited by the height of the templating structure.Gong et al. reported a self-assembled MIM structure consisting of two stacked silver nanohole arrays. The hole arrays were consecutively prepared by NSL combined with electrochemical metal deposition. Although the self-assembly process inherently yielded misaligned hole arrays, the structures showed a negative index of refraction in the visible regime. [9] Misaligned, stacked nanohole arrays even exhibit interesting chiroptical effects, which depend on the in-plane rotational offset of the nanohole arrays. [10] To further facilitate the production of MIM structures, the direct transfer of readily prepared hole arrays is highly desirable. The transfer procedures currently available usually rely on the dissolution of thick silicon, aluminum, or copper substrates or the chromium adhesive layer under harsh conditions. [10,11] Alternatively, silver or polymer sacrificial layers have been used, which limits the freedom of fabrication regardingThe superior optical properties of metamaterials often rely on hierarchical structures in the sub-wavelength regime comprising stacks of individually patterned layers. For the fabrication of metamaterials in the visible regime, specific top-down lithographic methods have been developed. These techniques, however, have several inherent drawbacks such the lack of scalability and low production speeds. Here, a simple and modular approach for the construction of multilayer metal-insulator-metal structures based on the transfer of metal nanohole arrays via liquid interfaces is demonstrated. Gold nanohole arrays are us...

In-Plane Surface Lattice and Higher Order Resonances in Self-Assembled Plasmonic Monolayers: From Substrate-Supported to Free-Standing Thin Films

“…In order to prepare freestanding polymer films that encapsulate our plasmonic colloidal monolayers, we employed glass substrates with a sacrificial ZnO layer that was prepared using a sol−gel method. 48,49 Initially, glass slides were cleaned by sonication for 15 min in an aqueous Hellmanex solution (2 vol %). After that, the glass substrates were rinsed with MQ water, further sonicated in ethanol for 15 = 366 nm), which was placed directly above the spinning sample, was turned on in order to initiate UV curing of the NOA polymer.…”

How Colloidal Lithography Limits the Optical Quality of Plasmonic Nanohole Arrays

“…Removal of the colloidal mask by an adhesive tape reveals the nanohole arrays. 64 Scanning electron microscopy images of the produced samples (Figure 1e,f) show the hexagonal order of the nanoholes (Figure 1e), along with characteristic defects inherited from the colloidal mask, as discussed above (Figure 1f). Depending on the self-assembly conditions, monolayers with large or small grains can be produced (Figure 1a−c), as evidenced by its structural coloration resulting from constructive interference of nanoparticle arrays.…”

“… We then deposit a thin gold film (20 nm, 1 nm titanium as an adhesion promoter), silicon dioxide (20 nm), and again gold (20 nm) through the non-close-packed monolayer. Removal of the colloidal mask by an adhesive tape reveals the nanohole arrays . Scanning electron microscopy images of the produced samples (Figure e,f) show the hexagonal order of the nanoholes (Figure e), along with characteristic defects inherited from the colloidal mask, as discussed above (Figure f).…”

“…Plasmonic nanohole arrays were fabricated resulting from the resistive evaporation (Torr International Inc., THE3-KW) of 1 nm Ti (HMW Hauner, 99.995%, <3 mm granulate) as an adhesion layer and 20 nm Au (HMW Hauner, 99.99%, <3 mm granulate). 64 The study used double hole arrays, which were obtained by the additional deposition of SiO 2 via reactive evaporation of SiO (Sigma-Aldrich, powder 325 mesh) in an O 2 atmosphere (4 sccm inlet) followed by the deposition of the second titanium (1 nm) and gold (20 nm) metal film. The final structures were produced by removing the polymer colloidal particles with an adhesion tape (3M magic).…”

“…As a model system, we use double nanohole arrays, where two gold nanohole layers are placed on top of each other and are separated by a thin dielectric spacer (Figure d). Such structures are interesting for molecular sensing , and handedness detection …”