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Right-click to download an interview with Caroline Ross at the Massachusetts Institute of Technology, Cambridge, US, talking about self-assembling nanoscale spirals (6.15 MB MP3)

"All we have to do is make the pits and nature takes care of the rest," says Caroline Ross, associate head of the Department of Materials Science and Engineering at Massachusetts Institute of Technology (MIT), as she describes how she and her collaborators in the US, Korea and Singapore produce regular arrays of nanospirals from templated block copolymers. While alternative approaches—such as electron-beam lithography—also offer the same level of control, the time needed for top-down production rapidly escalates when scaling up from a single structure to a periodic array.

Ross’s initial interest in the potential of block copolymer self-assembly stemmed from a search for fabrication methods that could produce magnetic arrays for data storage, and ways of transferring patterns from thin films. "We have also done a lot of work on designing templates that guide block copolymers into making specific structures that may be useful for an integrated circuit," says Ross. "This adds to that body of knowledge by giving us a better insight into these curved features."

Other groups have previously demonstrated the templated block copolymer production of arrays of 3D helices that coil up from the substrate, but this is the first demonstration of arrays of flat spirals that can be transferred from thin films. Ross and colleagues were also able to accurately model the self-assembly process so that they could predict the geometries resulting from different parameters.

From spaghetti to doughnuts

Block copolymers are long molecules where a strand of one polymer is joined with one (diblock copolymer), two (triblock copolymer) or more other polymers. While the long strands of polymers usually form a spaghetti-like mass of long tangled molecular strands, the immiscibility of the different polymers in block copolymers leads to domains of the two types from which microstructures emerge. An even ratio of the two polymer types can lead to interlacing lamellae structures, whereas a less balanced ratio can lead to spheres or cylinders forming within a matrix of the dominant polymer.

While the block copolymer components and ratios determine the structures they self-assemble into, simple templates can then impose long-range order on their self-assembly. Ross and her colleagues looked first at straight trenches and then arrays of circular pits. The diblock copolymer they used was polystyrene-b-polydimethylsiloxane (PS-b-PDMS) with a volume fraction of 0.335 PDMS, giving rise to a cylindrical microstructure. In a trench the cylinders align parallel with the trench edge but the researchers could also encourage the formation of doughnuts and "bulls-eye" structures of concentric rings by templating circular pits.

Chiral coils

"But the emphasis in this paper was not in making concentric rings but spirals and to do this we needed to break the symmetry of this circular pit," says Ross. This took the form of a protrusion on the edge of the pit. Curving round this notch incurs a high energy penalty so a spiral results instead. They could even force the spiral to coil in a specific direction with an asymmetric notch, or produce double spirals using a deeper notch.

"We can use a self-consistent field theory model to calculate what the lowest energy structure might be and then see how that compares with what the experiment yields, and in general they are in quite good agreement," adds Ross. "So it means we can actually do some prediction as to what notch properties to design to get a particular type of spiral."

Spiral structures have a chirality or handedness – a spiral in one direction cannot be superimposed on a spiral in the opposite direction. A number of potential applications exploit structural chirality, for example in interactions of circularly polarised light and chiral chemical synthesis. Edwin L. (Ned) Thomas at Rice University in the US, who was not involved with the current nanospiral work but who specialises in photonics research and the fabrication of polymeric photonic crystals using self-assembly - especially with block copolymers - commented, "Ross et al. have given a new twist to directed self-assembly by their clever application of a symmetry-breaking notch to induce layers to form spirals, opening up concepts for engineering chirality into nano- and micro-scale patterns."

The results may have interest for a number of devices, but as Ross tells, the work was not motivated by a specific application. "We were more interested in the science behind the self-assembly."

Full details are reported in Hong Kyoon Choi et al 2017 Nano Futures 1 015001