On the Coalescence of Nanoparticulate Gold Sinter Ink

We examine the mechanism by which thiol-protected gold nanoparticle inks can sinter at surprisingly low temperatures. At room temperature the sample is comprised of randomly close-packed gold nanoparticles of about 2.3 nm diameter with a ligand shell of about 0.2 nm effective thickness. As the particles are heated through 80 degrees C they begin to coarsen, reaching about 10 nm diameter at 180 degrees C. Upon further heating, rapid sintering and grain growth occurs at a temperature that depends on environment and heating rate. Sintering in vacuum requires a higher temperature than in oxidizing environments. Mass spectrometry in the former case is consistent with volatile species such as C4H9, C2SH, and C2H4 being displaced, whereas XPS shows that the exposed surface of the Au is rich in C and S. However, when sintering is performed in the presence of even trace O-2, it is the Au-S bond that is cleaved, and the sintering temperature is lowered by up to 50 degrees C. In this case mass spectrometry shows the generation of alkane and thiol fragments, some S-2 and H2S, and oxidized sulfur-containing species, whereas XPS shows that C and S on the Au surface is much reduced.