Resistance changes due to thermal coalescence in colloidal Au/organic linker molecule multilayer films

A decrease in the resistance of colloidal Au multilayer films was observed upon heating. These multilayer An films were fabricated by a layer-by-layer approach, using An colloids and a bifunctional linker molecule, 1,6 hexanedithiol (HD) on polymer substrates. The resistance of the film prior to heating was 1 MOmega. The films were heated at three different temperatures, 120, 160, and 180 degreesC. After heating for 12 h, the resistance decreased by 6 orders of magnitude to about 50 Omega. This decrease in resistance was faster at higher temperatures. X-ray photoelectron spectroscopy (XPS) of the unheated films revealed two S 2p peaks corresponding to the Au-S thiolate peak and an oxidized S peak. Upon heating, the relative intensity of the oxidized S peak increased and that of the Au-S peak decreased, indicating an oxidation and desorption of the linker molecules. Scanning electron microscope (SEM) images of the heated films depict coalescence of the spherical An particles into irregular shapes. The resistance decrease is believed to be due to the desorption of the linker molecule and subsequent coalescence of the An particles. This method paves a way for controlling the resistance of electrodes on flexible polymer substrates.