Block copolymers are composed of two or more chemically distinct polymer chains connected via covalent bonds. By tuning their molecular weight, chemical compositions and the volume fraction of the blocks, researchers can make periodic nanopatterns such as arrays of parallel lines and hexagonal dot patterns on a variety of surfaces. The good thing is that, compared to commercial optical lithography techniques, nanopatterning via self-assembling BCP is much cheaper and has a much higher spatial resolution.

Directed self-assembly (DSA) of BCP is even better in this respect in that it can produce long-range order and orientation in the nanopatterns and reduce the number of defects at the same time. DSA can for example be used to produce complex patterns such as bands, jogs, T-junctions, concentric circles, square arrays and even some 3D nanopatterns. However, although promising, little work has been done in this field so far.

Sequential deposition of each dot array

A team led by Erik Luber and Jillian Buriak of the University of Alberta and the National Research Council Canada recently employed an array of hexagonal silica dots prepared via self-assembly of PS-b-PDMS BCPs to direct the self-assembly of a top layer of a thin film of the same BCP. The researchers converted the commensurate, two-layered pattern they produced to a honeycomb pattern of silicon dots that had double the density of the original dots. Now, the researchers have gone a step further and have produced nanoscale structures using the DSA of two incommensurate BCP dot arrays via sequential deposition of each dot array.

“We did this by first depositing the bottom BCP layer as a thin film using a technique called spin coating, followed by solvent annealing to form a highly ordered hexagonal BCP lattice,” explains Luber. “We then converted this BCP lattice to an array of silica dots using reactive ion etching. We repeat this process on top of the same substrate using a different BCP with a different lattice spacing. The alignment of the two layers is completely self-directed and is the result of energy minimization between the layers.”

Making moiré superstructures never seen before in directed self-assembly

The researchers found that the layers preferentially align in a way that produces moiré superstructures never seen before in DSA. These structures contain large, multi-micron-sized grains with preferred majority phases. “Although it is currently unclear what types of applications will make use of these BCP moiré superstructures, one promising application may be photonic metasurfaces with tuneable optical properties,” says Luber.

“As reported in our work, published in ACS Nano DOI: 10.1021/acsnano.7b00322, we can modify the resultant moiré structure by not only changing the pitch ratio of the two dot lattices but also the dot height and size,” he tells nanotechweb.org. “We will now be exploring the effect of dot height while keeping all other parameters constant. This study should shed more light on how the moiré patterns form, and help us design these patterns in the future.”