Hemispherical Shell Nanostructures from Metal-Stripped Embossed Alumina on Aluminum Templates

Abstract

Nanostructured, and regularly patterned, noble metal films are used in many sensor devices, light emitters, and photovoltaic devices because the nanostructure patterns can be designed and fabricated to support surface plasmon resonances (SPRs) with specific energies and desirable spatially localized electromagnetic field enhancements. However, the applications require reproducible and low-cost fabrication procedures realizing large-area, uniformly nanostructured noble metal films. In this context, we propose for the first time stripping of noble metals from regularly patterned nanostructured aluminum/alumina (Al/Al2O3) templates as a novel and versatile nanofabrication procedure, and we demonstrate explicitly how to exploit the technique for developing large-area hexagonally close-packed hemispherical shell nanostructures by stripping noble metal layers from embossed templates fabricated from anodized Al. Utilizing for this process the linear relationship between anodization voltage and the resulting interpore distance in the formed oxide, it is possible to tune the radius of curvature of the resulting hemispherical shells continuously, which in turn results in tunable optical properties confirmed by reflection spectroscopy. It is demonstrated how such embossed Al/Al2O3 templates can be reused repeatedly without observable degradation of the template or the stripped metal nanostructures. This fabrication procedure allows a complete sealing of the film surface preventing oxidation or contamination of the nanostructured metal prior to its application.

Surface‐enhanced Raman microscopy of hemispherical shells stripped from templates of anodized aluminum

“…Right: applied anodization conditions and measured interpore distances, D, of the fabricated templates. * indicates that voltage ramping has been performed to achieve the voltage V, and that the anodization time, t, was sustained for the indicated time after the ramping had finished [27] Oxalic acid 0. Linear reflection spectroscopy…”

“…We have very recently introduced a novel technique that exploits low adhesion between noble metals and Al/Al 2 O 3 templates by applying epoxy resin in a stripping procedure and thereby obtaining well‐defined, hexagonally ordered hemispherical shell noble metal nanostructures (Fig. ) . This technique provides a robust, simple and unique fabrication procedure for large‐area template‐assisted production of continuously tunable nanostructures with excellent morphological stability.…”

“…1). [27] This technique provides a robust, simple and unique fabrication procedure for large-area templateassisted production of continuously tunable nanostructures with excellent morphological stability. Furthermore, the well-known problems related to surface oxide formation on Ag nanostructures are circumvented by keeping the nanostructured film sealed between the template and the epoxy resin until immediately prior to its usage, at which point the metal film is stripped off the Al/Al 2 O 3 , exposing a clean metal surface.…”

Laser Annealing as a Platform for Plasmonic Nanostructuring

“…A critical parameter in delivering practical plasmonic devices is the material preparation methods, which should allow the production of nanostructures with tunable plasmonic properties. So far, nanomaterials and nanodevice manufacturing have traditionally followed two distinct routes: (a) the top-down approach, where a process starts from a uniform material and subsequently finer and finer tools are employed to create smaller structures, like lithographic processes [35][36][37] and/or ion beam nanofabrication [38] and (b) the bottom-up approach, where smaller components of atomic or molecular dimensions self-assemble together, according to a natural physical principle or an externally applied driving force, to give rise to larger and more organized systems, like atomic layer deposition [39], cold welding [40], flash thermal annealing [41,42], pattern transfer [43] and template stripping [44][45][46]. Practically, the top-down approach offers unequalled control and reproducibility down to a few nanometres in feature size but at high cost for large area manufacturing, while the bottom-up route applies for macroscopic scale nanopatterning albeit without the fine feature and reproducibility control.…”

A potential commercial surface‐enhanced Raman scattering–active substrate: stability and usability

“…After the first step, an ordered aluminum template was formed. By using the ordered aluminum template, nanostructures of several materials were fabricated . Such nanostructures can also be good templates for preparing 2D SERS‐active substrates …”