Ultrasmall silica nanoparticles with controllable size were prepared by a modified Stober method. In the method, the size of the nanoparticles can be decreased to 4.8 nm by addition of Triton X-100 into the reactants combined by controlling the ammonia concentrations. The ultrasmall silica nanoparticles modified with Triton X-100 can be well deposited onto optical substrates, resulting in the formation of deep-ultraviolet antireflection coatings. The antireflection coatings demonstrated transmittance as high as 99.92% in the deep-ultraviolet band, suggesting that nearly the whole light is transmitted through the optics. These antireflection coatings will be used in deep-ultraviolet optical system such as high-peak-power laser system and photolithography. (C) 2017 Published by Elsevier B.V.