The 2D materials toolkit includes diverse methods — either top down or bottom up — by which pristine monolayers can be prepared. However, although we can produce materials with exquisite atomic-scale ordering, neatly patterned 2D mesoscale structures represent elusive targets. This may now be set to change thanks to a creative templating method that furnishes mesoscale-ordered conducting polymers, as Xinliang Feng's team describe in Angewandte Chemie International Edition.

It is not obvious how one might construct a template with regular features in the range of 10–50 nm — a regime larger than most well-defined synthetic molecules, and one not conveniently accessed from the top down. Operating from the bottom up requires very large assemblies but also, as Feng notes, “synergic control over molecular building blocks in different dimensions to simultaneously achieve 2D superstructures as well as periodic mesoporous arrays”.

The authors' hierarchical template combines two architectures: a self-assembled bilayer serves as the canvas, to both sides of which are anchored monodisperse and highly ordered micelles. The bilayer comprises perfluorocarboxylic acid molecules — linear amphiphiles with hydrophilic heads and fluorous tails that have similar aspect ratios. Protruding from the 2D fluorous plane are polar acid heads that form hydrogen bonds with surrounding ethanol and water molecules. Outer surfaces of the sheet are ideal binding sites for superstructures with polar exteriors, including micelles prepared from diblock copolymers comprising hydrophobic (poly(styrene)) and hydrophilic (poly(ethylene glycol)) regions.

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The hydrophilic block is flexible and occupies more space than the hydrophobic block. The different aspect ratios of the two blocks favour micelle formation, with poly(ethylene glycol) groups occupying the exterior not just because of their size but also owing to the polar solvent used. Remarkably, the weak interactions holding molecules in the bilayer and the micelles together still allow order, such that micelles cover both sides of the sheet in a hexagonal array expected for spherical particles.

The pictures do not lie. Indeed, cryo-transmission electron microscopy, which has revolutionized imaging of soft materials, produces detailed vistas of these systems, demonstrating that molecules comprising the bilayers and spheres assemble orthogonally. Aside from the different aspect ratios, the use of hydrophobic, hydrophilic and fluorous phases deters promiscuity between unlike molecules.

With templates in hand, Feng and co-workers sought to direct the formation of a 2D conducting polymer around the mesoscale features. Poly(aniline) fit the bill nicely, as aniline is a liquid that readily fills voids between micelles above and below the bilayers. The addition of persulfate initiates poly(aniline) formation in a mild process that preserves the soft template, which can then be dissolved to leave behind two 13–45-nm-thick poly(aniline) nanosheets — one for each side of the template. Significantly, the sheets feature a hexagonal array of pores in locations previously occupied by micelles. According to X-ray scattering and microscopy analysis, the pores are spaced approximately 30 nm apart and are uniformly sized (tunable between 7–18 nm depending on the copolymer length).

Self-assembled fabrication of ordered mesoporous conducting polymers is a great challenge that has plagued researchers for more than two decades

Just how good are these nanosheets? When packed into a film, their in-plane electrical conductivity (41 S cm−1 — a record for a poly(aniline)) surpasses that of aniline polymerized without a template, which takes the form of nanoparticle films with low conductivity (12 S cm−1). The exceptional electrical performance of mesoporous poly(aniline) is ascribed to strong in-plane π–π stacking that augments conduction. The conductivity perpendicular to the plane is much lower, and such anisotropy is desirable in electronics applications.

The significance of these materials notwithstanding, the greater challenge, says Feng, is “to synthesize large-area, defect-free, single-crystal-like 2D mesoscale-ordered conducting polymers, which would hold promise in applications, for example, as monolayer membranes for separation and sensing”. He then laments that “the self-assembled fabrication of ordered mesoporous conducting polymers is a great challenge that has plagued researchers for more than two decades after the first discovery of mesoporous materials”. It is safe to say that we will not have to wait another two decades before this elegant methodology affords a suite of new multifunctional materials.