Nanopatterning in Surfactant-Encapsulated Polyoxometalate Systems: A Kinetically Hindered Phase Transition Drives Structural Complexity

The construction of surfaces that combine the unique properties of polyoxometalates (POMs) with coherency over a nanoscale is valuable to fields such as magnonics and optics. By encapsulation with surfactants, POMs can be trapped at air/water interfaces, and there they form complex nanopatterns driven by the transition to a double-layer structure. In this work, we show how the interplay between the surfactant:POM ratio and a kinetic barrier in the phase transition combines to give rise to the nanopatterning effect. A series of surfactant-encapsulated POM complexes is studied, systematically varying the number of surfactants to understand how they influence the phase transition and drive structural complexity. A sharp divide between two general classes of behavior is revealed, controlled by the surfactant:POM ratio. The behavior and morphology are mapped using surface pressure-area isotherms and atomic force microscopy, and this is combined with diffraction measurements made using grazing incidence wide-angle scattering to elucidate the molecular structure of the phase transition. This understanding is utilized to explain the surface morphologies and how the kinetic stability can be exploited to modify self-assembly. In this way, controllable nonequilibrium structures such as nanoarrays can be generated for use in POM-based metamaterial surfaces.