Two-Dimensional Empty Liquid Composed of a Patchy Metal Nanoparticle Network Structure for Transparent Plasmonic Devices

Plasmonic coloration derived from metal nanoparticles and arrays has attracted great attention in the fields of color filters, sensors, and various photonic/optoelectronic devices. However, there are cases in which localized surface plasmon resonance (LSPR) coloration becomes an obstacle, and transparent plasmons are needed. In this study, we report an extremely transparent network structure film at an air-water interface composed of patchy spherical gold nanoparticles with two different ligands (oleylamine and mPEG-thiol). These films exhibit high surface pressure as a liquid film at an extremely low areal density of similar to 10% while maintaining extremely transparent plasmonic colors even at high areal densities, reaching 40-60%. Such states of materials are regarded as "empty liquids" (liquid states with a vanishing density), which were predicted in theory but found in only a few cases experimentally in the past. The extremely transparent coloration of two-dimensional empty liquid films obtained in this study is reasonably interpreted by finite-difference time-domain (FDTD) model simulations of network assembled structures with shorter gap distances. These findings will be crucial for the design of high-density transparent LSPR metamaterials for certain applications, such as plasmonic solar cells and smart windows, in the future.