Vapor-assisted remodeling of thin gold films

In this paper we present the results of a systematic study of the remodeling of ultrathin (< 20 nm) gold films by exposure to condensable vapors of volatile solvents. The starting gold films, deposited on glass, consist of closely packed gold nanoparticles with controlled nominal thickness. Heating to relatively low temperature (60-120 degrees C) and exposure to condensable vapor carried by flowing N-2 gas lead to remodeling of the films' morphology. Electron and atomic force microscopy images show that the gold nanoparticles coalesce and form islands, interconnected networks, or holey continuous films, depending on the exposure time and the initial film thickness. Associated changes in the UV-vis spectra are similar to those seem in previous studies of thin gold films. Remodeling does not occur for films heated in a vacuum, immersed in solvent, or exposed to gases carrying saturated condensable vapor. Remodeling also occurs for other condensable vapors including water. Intermittent exposure of the films to ambient humidity followed by heating in house N2 gas also leads to remodeling. These observations are rationalized by a theoretical model of capillary condensation and partial flooding of the films' nanoscale pore spaces, leading to attractive capillary forces that draw the gold nanoparticles into intimate contact and accelerate the coalescence process.