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 °C they begin to coarsen, reaching about 10 nm diameter at 180 °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 C 4 H 9, C 2 SH, and C 2 H 4 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 °C. In this case mass spectrometry shows the generation of alkane and thiol fragments, some S 2 and H 2 S, and oxidized sulfur-containing species, whereas XPS shows that C and S on the Au surface is much reduced.