Abstract: A monolayer film of nanoparticles can formed from a fluid mixture by combining nanoparticles dispersed in water with a water-miscible organic solvent and a molecular ligand comprising a head group with affinity for the nanoparticle, and introducing the fluid mixture to a substrate in the presence of an air/fluid interface, thereby causing a monolayer film of nanoparticles to form on the substrate. Such monolayers films can include metallic nanoparticles such as gold, and possess substantially uniform spacing over at least a one centimeter length scale. The films are metasurfaces effective to cause a range of linear refractive index of from around unity at 700 nm to 10 at 1500 nm. Such metasurfaces might find utility, for example, as ultrafast (picosecond scale) switches at wavelengths useful for telecommunications.