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The prospects offered by nanotechnologies heavily depend on their ability to nanostructure materials at high resolution, low cost, and on macroscopic samples. Plasmonics, nanophotonics, and optoelectronics are among the most active research fields that require such nanostructuring.
In the innovative approach described in this work, a layer of nanoparticles floating at the air/water interface is transferred by simple contact onto a nanostructured substrate, simultaneously filling all the holes forming the pattern.
Electron microscope images show silver nanocubes (55 nm per side) arranged with precision and at multiple scales, with a resolution equal to their size. The regular arrangement of these “meta-atoms” on a surface endows it with optical functionalities for creating ultra-thin optical components.
© Muhammad L. Fajri et al.
Generally, the benefits of nanostructuring matter are demonstrated through experiments that require the crafting of a particular sample via a so-called top-down approach, which involves carving the material through deposition, lithography, and etching steps. However, the cost and complexity of the nanofabrication processes raise deep questions about the industrial viability of the resulting materials, especially when considering the need for macroscopic quantities of these materials for practical applications.
An alternative approach, called bottom-up, consists of combining colloidal synthesis with directed self-assembly, allowing the fabrication of nanomotifs (small networks of colloids assembled and/or sintered) using high-quality materials, with realistic prospects for macroscopic-scale production at a reasonable cost.
The reference technique for directed assembly involves sliding a drop of highly concentrated colloidal solution over the substrate, which is a nanostructured elastomer with embedded molding holes. As the drop moves, the nanoparticles descend into the holes and become trapped in a complex dynamic process involving multiple forces (capillary, friction, adhesion), which is unfortunately highly prone to instability and limits this technique to the production of 1D meta-atoms at low density.
As a result, the creation of colloidal materials in high-resolution, defect-free nanomotifs respecting arbitrarily complex geometries was unattainable with this dynamic deposition technique.
In a recent study, researchers from the Interdisciplinary Center for Nanoscience in Marseille (CINaM, Aix-Marseille University / CNRS), the Research Center on Heteroepitaxy and its Applications (CRHEA, CNRS / Côte d'Azur University), and the Institute of Microelectronics Nanosciences of Provence (IM2NP, Aix-Marseille University / CNRS / University of Toulon) have significantly improved the performance of this type of approach.
The innovative idea was to use a floating layer of nanoparticles at the water/air interface as a starting point instead of a concentrated drop. This allows both a very high concentration and excellent stability (absence of flow-induced interactions that cause instability) and high mobility.
This floating layer is transferred by simple contact onto the nanostructured substrate, simultaneously filling all the holes forming the pattern. Thanks to this quasi-static assembly method, which improves upon the previous technique, the shape, density, and organization of the motifs no longer determine the success or failure of the fabrication process.
In this work, researchers demonstrate the ability to obtain 2D meta-atoms (colloidal motifs deposited on a glass slide) with true freedom regarding their geometry and complexity (for example, they create Tetris-shaped motifs or form the word CNRS), which was not possible previously.
This success, combined with the ability to produce such arrangements periodically over large areas, makes this new technique a breakthrough innovation in the field of metasurfaces for optics and nanophotonics. As a proof of concept, they propose a metasurface that, thanks to its geometric phase, "sculpts" the reflection of circularly polarized light in a very particular way, unmatched by traditional materials.
This innovative approach to assembling meta-atoms is the starting point for entirely bottom-up metasurface production and will ultimately serve as a powerful toolbox for creating and studying new nano-objects that are extremely difficult, if not impossible, to synthesize chemically. These results are published in the journal ACS Nano.
Reference:
Designer Metasurfaces via Nanocube Assembly at the Air−Water Interface,
ACS Nano, published August 19, 2024.
Doi: 10.1021/acsnano.4c06022
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