Large area metasurfaces
A unique class of second class of applications which might benefit from DNA origami-based fabrication are those which require vast areas of substrates to be patterned at the nanoscale, but requiring little integration and tolerant to high defect densities. One such application is planar optical metasurfaces - arrays of subwavelength light scatterers (referred to as meta-atoms) whose overall optical properties depend on the size, shape, composition and spatial arrangement of the scatterers. The structural color of a butterfly wing provides a striking example of an optical effect arising from a natural metasurface. Artificial metasurfaces include clusters of metal nanoparticles that passively interact with light [1] (fig (Origami Metasurface) b), graphene nanodisks whose properties can be actively controlled using an electric field [2](fig (Origami Metasurface) c) as well as surfaces that mimic butterfly wings [3] (fig (Origami Metasurface) d). Metasurfaces promise unprecedented control over light and while there are niche applications that require small area devices, most transformative applications are envisioned to cover extremely large areas such windows, roofs and solar panels. Unfortunately current top-down techniques cannot economically be applied over large areas and existing bottom-up nanofabrication techniques (e.g. block co-polymers or nanosphere lithography) lack the design flexibility necessary to create meta-atoms of arbitrary desired shape. However, a DNA origami-based approach could be ideal for this purpose.
Figure (Origami Metasurfaces)a summarizes a potential framework for manufacturing metasurfaces using DNA origami. Much like a top-down process flow there are two steps: The first "patterning" step involves organizing DNA origami on a surface (arrow 1-3 in fig (Origami Metasurfaces)a). It is followed by a second "pattern transfer'' step during which geometric information in the origami is converted into meta-atoms by attaching nanoparticles, etching or material growth (arrows 4-8 in fig (Origami Metasurfaces) a). The lithography driven DOP (arrow 1 fig (Origami Metasurfaces) a) would only be used for prototyping purposes and economically viable large area patterning could be achieved using existing origami crystallization techniques [4,5] (arrow 2 fig (Origami Metasurfaces) a) or random immobilized (arrow 3 fig (Origami Metasurfaces) a). While the patterning of the surface with origami is an important part of this framework, the main challenge will be pattern transfer. The easiest method for this would be using DNA origami as a breadboard to scaffold nanoparticles (arrow 4,5 in Fig (Origami Metasurfaces) a). Over the last decade several different designs of metal-nanoparticle clusters have been realized on DNA origami [6,7] however no optical metasurfaces has been created with such clusters. Further, defined clusters of dielectric nanoparticles (such as silica or silicon nanospheres) have never been realized on DNA origami.
An intriguing alternative to using DNA origami as a scaffold is its potential to direct the formation of meta-atoms by sculpting bulk inorganic material. Within such an approach DNA origami immobilized on a surface would be viewed analogous to features formed in polymer after top-down patterning. However, polymer features function well as a resist because they are chemically robust and at least a few tens of nanometers thick; a single layer of DNA origami, on the other hand, is only about 2 nanometers thick and chemically fragile. Thus, existing top-down etching or liftoff processes can't be used to transfer the DNA origami pattern into an inorganic material unless it is made thicker or more chemically robust. Figure (Origami Metasurfaces)e illustrates a potential selective area material growth that could enable conversion of DNA origami into a silica structure of identical shape to act as an etch mask. Other analogous chemistries and atomic layer deposition methods could also be used to template growth of materials like titanium dioxide, aluminum dioxide or platinum on the DNA origami. The structures formed in this fashion can not only be regarded as an etch mask but also a meta-atom in and of itself depending on the material, aspect ratio as well as the reliability of reproducing the geometric features of the DNA origami.
(Origami Polymer): Two schema for converting an immobilized DNA origami into a geometrically well-defined polymer island that can, in turn, be used as mask for metal liftoff or dry etching. (Top) using ATRP or RAFT initiators on DNA origami or (bottom) using DNA origami to direct the phase separation of polymer blend.
It might also be possible to create polymer features directly on top of the origami to act as an etch or liftoff mask; Figure (Origami polyer) illustrates two examples of this approach. The first potential polymer-based pattern transfer technique involves using ATRP or RAFT polymerization [] to directly grow polymer from initiators immobilized on DNA origami (fig (Origami Polymer).top). The main advantages of this method is that the relevant polymerization protocols are well established, and DNA-coupled ATRP and RAFT initiators are commercially available. A second potential polymer-based pattern transfer technique involves using DNA origami to direct phase separation of a polymer blend (fig (Origami Polymer).bottom), and then selectively remove one of the polymers []. Once the polymer structure is formed it is used exactly like a top-down mask for etching or metal liftoff.
Reference: