Direct approach to high-resolution, square-lattice alternating nanodot array by breaking hexagonal symmetry of block copolymer spheres.

Expanding the morphological spectrum of block copolymer (BCP) self-assembly remains a notable challenge in BCP-based bottom-up nanofabrication. We present a simple method to fabricate unconventional yet crucial structures of sphere-forming BCPs in thin films using solvent vapor. By precisely controlling solvent uptake, we transform the final lattice structure of spheres, modifying the thermodynamically stable lattice of BCPs. Molecular dynamics simulations reveal that increased solvent uptake elongates the spheres, raising interfacial energy and causing sphere splitting. The additional layers generated from the sphere splitting present a nonconventional lattice, typically not observed in thin films. Using these structures, we fabricate bimetallic nanodot arrays, where two different metal components are positioned alternatingly. This array exhibits higher catalytic activities compared to the homometallic nanopatterns, with the ultralow mass of noble metals below 300 nanograms per square centimeter, highlighting their potential as electrochemical catalyst platforms.