Self-Assembly of Uniform Monolayer Arrays of Nanoparticles

Five nanometer diameter gold particles encapsulated by alkanethiol molecules have been self-assembled into well-ordered monolayers on a water surface, and these nanoparticle films have been transferred intact onto various solid substrates. The method involves spreading a thin layer of an organic solvent containing the gold nanoparticles on a water subphase that has a controlled surface curvature. As the solvent evaporates, a nanoparticle monolayer free of microscopic cracks and voids nucleates at the center of the apparatus and grows radially outward until it covers practically the entire water surface. This monolayer is transferred from the water surface to a polydimethylsiloxane (PDMS) stamp pad by the Langmuir-Schaefer (LS) technique and is applied to the solid substrate by microcontact printing (μCP). Uniform centimeter-scale films have been produced with this method. The quality of the transferred films has been verified by transmission electron microscopy (TEM). The nanoparticle monolayer is a hexagonal close-packed array with a center-to-center spacing that is approximately equal to the sum of the diameter of the gold particles and twice the height of a self-assembled monolayer (SAM) of the alkanethiol molecules on Au(111). The transferred films are free of multilayer regions and of microscopic voids and grain boundaries over their entire area and exhibit crystalline order across the openings in the TEM grid (∼4000 μm²).