A method of forming a monolayer film of nanoparticles includes forming 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.