superlattice artists conception

Using a method combining top-down lithography and bottom-up, DNA-assisted self-assembly, a Northwestern University–led team created superlattices consisting of layers of differently shaped plasmonic nanoparticles, with tunable optical properties. [Image: Northwestern University] [Enlarge image]

In recent years, scientists and engineers have had considerable success building metasurfaces dotted with arrays of plasmonic nanoparticles, to create materials with sometimes exotic optical properties. Far more difficult has been taking the construction to the third dimension—assembling nanoparticle lattices built up from the surface, and including particles of multiple shapes and sizes. Now, a team from Northwestern University, USA, offers a method for building such plasmonic “superlattices”: throw a little DNA into the mix (Science, doi: 10.1126/science.aaq0591).

The researchers believe that the technique offers some new design options for building and controlling materials with highly customized light-matter interactions. As a demo, they used the approach to construct a broadband absorber, the properties of which can be dynamically tuned by tweaking the surface chemistry.

Top-down meets bottom-up

The method identified by the Northwestern team combines top-down basic lithography with bottom-up self-assembly, with an assist from the DNA. The team began by depositing a 300-nm-thick polymer layer atop a gold-coated silicon substrate, and then lithographically etching precisely spaced pores into the polymer, down to the gold layer beneath. The researchers then tied DNA molecules with a specific terminal sequence of nucleotides to the gold surfaces at the bottom of each pore. The terminal sequence acted as a “sticky end,” ready to pair up with the right complementary fragment of DNA.

Next, the team modified colloidal gold nanoparticles of different shapes with DNA fragments with a specific set of sequences on either end, and used those solutions to vertically build up a stack of nanoparticles within each polymer pore, layer by layer. For each nanoparticle shape, one end of the attached DNA was designed to grab onto to the sticky-end sequence presented by the previous layer, while the other end created a different sticky end tuned to connect to the next layer of DNA-treated nanoparticles.

At the end of the process, the researchers chemically dissolved the polymer cladding. The result was a gold substrate hosting a uniform superlattice of stacked, differently shaped nanoparticles (see image above).

Chemically tunable

A particularly interesting facet of the technique is that, after assembly, the nanoparticles remain held together by strands of DNA. That means that, for a given arrangement, the researchers can tweak the spacing between the particles by adding different concentrations of ethanol, which chemically kinks the nucleotide bonds in the DNA to a predictable degree depending on concentration.

To demonstrate how that might work in creating tunable optical devices, the team took a crack at building a tunable broadband absorber in the visible wavelength range—a device, according to the team, “not yet realized experimentally and difficult to envision making by conventional lithography or assembly.” The researchers first used finite-difference time-domain modeling to converge on an architecture that would have the appropriate, large-magnitude tunability of wavelength and amplitude. The winning design consisted of a superlattice including a layer of 105-nm-diameter circular disks, beneath a 76-nm-edge cube, topped by a 60-nm-diameter sphere—all arranged into a square array with a 200-nm periodicity.

The Northwestern researchers then used their hybid lithography-DNA building technique to realize the numerically modeled design, and measured the absorption spectra of the resulting metasurface with an inverted microscope. As predicted, they were able to tune the surface’s visible-light absorption across a dramatic 75-percent range by treating it with different solvent concentrations—and even to see the changes in optical response play out in color changes from maroon to green to brown on the surface.

Diverse possibilities?

The team believes that the flexibility of the system, and its ability to create tunable structures from the bottom up, “should dramatically increase the diversity of structures and compositions that can now be explored by theorists and experimentalists to access new and useful optical properties.” One of the team leaders, Northwestern chemistry professor Chad Mirkin, suggested in a press release that, using the approach, scientists “will be able to build an almost infinite number of new structures with all sorts of interesting properties.”

“Architecture is everything when designing new materials,” said Mirkin. “We now have a new way to precisely control particle architectures over large areas.”