(Nanowerk Spotlight) Manipulating the flow of light with nanoscale precision has been a grand challenge in optics, as it requires the ability to precisely position building blocks like metallic nanoparticles on surfaces. Such control could unlock a new generation of flat optical components called metasurfaces, which use arrays of nanostructures to mold light in exotic ways. Carefully crafted layouts could enable ultrathin devices to outperform traditional optics, achieving capabilities like perfect lensing, holography, and even invisibility.
However, the dream of rationally designed metasurfaces has been stymied by the difficulty of deterministically placing nanoparticles over large areas with sufficient accuracy. Existing nanopatterning methods face a trade-off between resolution and scalability. Electron beam lithography can carve intricate patterns but struggles to individually position nanoparticles. Self-assembly techniques can organize nanoscale components but often lack the required specificity and spatial control.
In recent years, the emergence of DNA nanotechnology, particularly DNA origami, has provided a new tool for bottom-up nanofabrication. By leveraging the programmable self-assembly of DNA molecules, researchers can create complex structures with sub-nanometer precision. However, the integration of DNA origami with surfaces for nanoparticle patterning has proven challenging, limiting its impact in practical applications.
Now, a research team from Ludwig-Maximilians-Universität in Munich, Germany, has developed a synergistic approach that combines the strengths of DNA origami and electron beam lithography to overcome these hurdles. As reported in a recent issue of Advanced Functional Materials (“DNA Origami-Directed Self-Assembly of Gold Nanospheres for Plasmonic Metasurfaces”), their method allows gold nanoparticles to be positioned on silicon dioxide surfaces with nanoscale accuracy over centimeter-scale areas. This powerful technique opens the door to designing and fabricating metasurfaces with tailored optical responses, bringing us a step closer to revolutionary flat optics.
a A DNA origami structure self-assembles during thermal annealing. An anchor site for a nanoparticle is indicated in red in the center of the triangle. The plasmonic nanoparticles are conjugated via a complementary DNA sequence to the anchor site. b Liquid atomic force microscopy image of DNA origami triangles. c Scheme of AuNS placement: i) EBL is used to pattern triangular shapes in PMMA; ii) O2 plasma cleaning results in hydrophilic triangular areas on a hydrophobic background; iii) DNA origami structures attach to the hydrophilic areas via salt bridges; iv) AuNSs attach to the center of the DNA origami structure; v) in a sol-gel reaction a silicon layer is grown around the product. (Reprinted from DOI:10.1002/adfm.202404766, CC BY)
The crux of the new technique lies in using electron beam lithography to first create a patterned array of binding sites for triangular DNA origami structures on the surface. The origami nanostructures, which have a designated anchor point at their center, selectively attach to these binding sites via DNA hybridization and electrostatic interactions. Gold nanoparticles coated with complementary DNA strands then bind specifically to the origami anchor points, allowing their positioning with nanoscale accuracy. Finally, a silica coating is grown around the DNA to enhance the stability of the assembled metasurface.
Using 100 nm gold nanospheres, the researchers achieved a single particle placement yield of 74% at the target locations, with an average deviation of just 9 nm from the ideal lattice. Yields of 65% and 60% were obtained for gold nanosphere dimers and trimers respectively. This combination of high precision and relatively large area patterning marks a significant advance over prior methods.
To highlight their technique’s potential for fabricating functional optical devices, the team created metasurfaces consisting of gold nanoparticle arrays whose scattering properties depend on the polarization of incident light. By interspersing single nanoparticles with dimers and trimers oriented at different angles, they designed a surface that displays two distinct images when illuminated with orthogonal polarizations. Under one polarization, the metasurface shows the logo of the university, while at the perpendicular polarization, it depicts the logo of a research center.
This work represents a major step towards realizing optical metasurfaces with engineered properties through the deterministic arrangement of plasmonic building blocks. The key innovation lies in the synergistic combination of top-down electron beam lithography, which defines the overall nanoparticle pattern, and bottom-up DNA origami self-assembly, which enables precise positioning of individual nanoparticles on the lithographically patterned surface.
This unique integration of techniques provides a powerful and versatile platform for nanoscale patterning over macroscopic areas, improving upon the limitations of each method alone.
However, some challenges remain before this approach can be broadly applied. For example, while the current work focused on gold nanoparticles, expanding the palette to other materials like quantum dots, fluorescent molecules, or magnetic and catalytic nanoparticles could greatly extend the range of accessible functionalities. The silica coating stabilizes the structures but currently limits the minimum interparticle distances, reducing the potential for more complex optical coupling effects. With further optimization to address these limitations, the authors suggest that their DNA origami-assisted assembly method could potentially enable an even wider variety of advanced applications in areas like nanophotonics, sensing, and more.
By pushing the boundaries of nanofabrication, this research brings us closer to the long-standing goal of rationally designing and building metamaterials from the bottom up. With continued advances, this technology could one day allow us to fully harness the unique properties of metallic nanostructures and craft materials with extraordinary abilities to manipulate light, enabling sci-fi concepts like invisibility cloaks and perfect lenses to leap from the pages of fiction to reality.
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