Size and surface effects on chemically-induced joining of Ag conductive inks
Direct write (DW) technologies offer a potential avenue towards reducing existing long lead times required for fabrication of prototypes and evaluation of new materials and integrated circuit designs. Nanostructured metallic DW inks are widely utilized due to their excellent electrical conductivity which dictates the performance of printed sensors and circuits. Besides controlling the surface chemistry and rheology of these inks to control the print resolution, the essential joining of these conductive nanomaterials to create functional sensors and devices remains a challenge. Low-temperature interconnecting techniques are required to minimize the deleterious heat effects on the shape of printed conductive networks and electronic components. Therefore, the reduced melting temperatures characteristic of metallic nanomaterials offer opportunities to bridge this technological gap. Amongst the reported low-temperature sintering approaches, the chemically-induced sintering method emerges as a readily scalable approach with relatively low energy input. By uncovering the underlying relationship between particle size effects and the role of the ionic salts used to induce sintering under ambient conditions, we aim to elucidate the compatible chemistries (i.e., size and salt) to achieve optimal chemically-induced sintering. The sintering of polyacrylic acid-modified Ag nanoparticle-based DW inks of different size distributions (bimodal vs. monodispersed distribution) and average sizes (e.g., 5 nm and 17 nm) was systematically investigated to elucidate the role of a commonly used salt, NaCl, in the chemically-induced sintering process. An observed enhancement in our ink conductivities by about 4 orders of magnitude for the monodispersed 17 nm and bimodally distributed 9 and 170 nm particles resulted mainly from the sintering between neighboring Ag nanoparticles. In the case of monodispersed 5 nm nanoparticles, a low electrical resistivity was observed despite an increase in grain size which indicated successful sintering. The low measured electrical resistivity was mainly due to significant AgCl formation that had a lower electrical conductivity. Our results show that the chloride ions played an active role in triggering first the oxidative decomposition of Ag to Ag+ resulting in AgCl formation which subsequently led to the sintering between neighboring Ag nanoparticles. The difference in the results for the 5 nm and 17 nm monodispersed Ag nanoparticles was attributed to the differences in the size-dependent surface reactivities and relative amounts (i.e., packing density, thickness) of the polymeric PAA coating.