Versatile bottom-up assembly of metal-nanoparticle monolayer arrays via suppression of the coffee-ring effect using sandwiched substrates

Metal-nanoparticle monolayer arrays (metal-NMAs) are used in many applications, including sensors, printing, diagnostics, and photonics. In the fabrication of NMAs, applying the drop-drying method to a colloidal solution of metal nanoparticles on a substrate results in ring-like deposits formed by the pinning of the droplet at the contact line (“coffee-ring effect”). When a colloidal droplet of metal nanoparticles was sandwiched between two substrates separated by a certain distance (“sandwich method”), the top substrate caused depinning at the contact line of the sandwiched droplet, inducing the formation of well-ordered metal-NMAs. The applicability of the method was investigated using several metal nanoparticles and substrates. The mechanisms of supplying nanoparticles to the contact line and metal-NMA growth were simulated based on the Navier–Stokes equation. According to the simulations, nanoparticles were supplied to the contact line by the complex convection flows occurring in the sandwiched droplets, and the striped pattern of the metal-NMAs originated from the low concentration of the metal-nanoparticle solution. Unlike other NMA fabrication techniques, the sandwich method does not require the use of complex equipment or contaminating substances and is not limited to specific nanoparticle and substrate materials; thus, it is broadly applicable to NMA fabrication and enables the acceleration of research in nanoscience and interfacial science.