Uniform two-dimensional crystals of polystyrene nanospheres fabricated by a surfactant-assisted spin-coating method with polyoxyethylene tridecyl ether.

Spin-coated self-assemblies of colloidal particles have been developed recently as an attractive close-packed monolayer of the particles for a variety of applications， but they are limited by the small area of their monolayers， especially given their low uniformity and monolayer coverage on large-area substrates. We report several noteworthy characteristics of a close-packed monolayer of polystyrene nanospheres (PS NSs) fabricated using a simple and inexpensive spin-coating method with a PS NS suspension mixed using the nonionic surfactant polyoxyethylene tridecyl ether (PEO-TDE). In our study， we show that the PEO-TDE surfactant offers excellent wettability， surface tension， and a slow solvent evaporation rate of the PS NS suspension， similar to the conventional surfactant Triton X-100. We demonstrate that the relatively high monolayer coverage with reduced defects is produced when introducing the PEO-TDE surfactant. Specifically， monolayer coverage of more than 95% on a Si substrate was achieved， which is much better than that with the typical Triton X-100， and is one of the highest coverage rates realized by a spin-coating method. This excellent uniformity of the PS NS monolayer with high monolayer coverage is mainly attributed to the relatively low viscosity of the PS NS suspension， even at high concentrations of PEO-TDE. Moreover， the PEO-TDE surfactant provides highly uniform monolayers on a large-scale glass substrate even for large-sized PS NSs. We also highlight the fact that the PEO-TDE surfactant has another advantage in that the spin-coating process of the PS NS suspension can be done under common ambient laboratory conditions， unlike those required for the highly toxic Triton X-100. We therefore conclude that PEO-TDE can be a useful surfactant during the fabrication of close-packed monolayers for various applications owing to its simple and straightforward control of PS NSs， its uniform and high surface coverage， and due to the safety of the fabrication process.