Stable and concentrated silver nanocolloids (AgNCs) have attracted considerable attentions as “functional inks” to manufacture fine metal electrode patterns toward application of flexible electronics, using printing-based device production (or printed electronics) technologies. Recently, we developed a groundbreaking printing technique, called as “surface photo-reactive nanometal printing (SuPR-NaP)”, which allows easy manufacture of ultrafine conductive silver patterns with a high submicron resolution . It was demonstrated that the technique relies on a unique effect of nanoparticle (NP) chemisorption on a patterned photo-activated surface, as is enabled by using specific AgNCs that are obtained by thermal decomposition of oxalate-bridging silver alkyl-amine complexes . These AgNCs are composed of silver NPs (with diameter of about 13 nm) encapsulated by alkyl-amines and a slight amount of oleic acid and can be suspended at a high concentration of 40 wt% in a 4:1 mixed dispersant of n-octane and n-butanol. The peculiar nature of the AgNCs is that the high dispersion stability is maintained for a long period of time (more than a month), whereas the high surface reactivity for NP chemisorption is possible on the activated surface that is equipped with carboxylate groups in the SuPR-NaP technique [1,3].
In this presentation, we will discuss that the above-mentioned unique compatibility between the high dispersion stability and high NP chemisorption reactivity of the AgNCs are closely correlated with the choice of mixed dispersant as used for the AgNCs. We investigated the dependence of dispersion stability of AgNCs on the dispersant composition by using confocal dynamic light scattering (CDLS) technique that can probe the size distribution of colloidal particles in the concentrated AgNCs . We found that the dispersion stability dramatically depends on the dispersant compositions. Especially, we here focused on a content of methanol which is inevitably included in the AgNCs during the synthetic processes. We prepared AgNCs that contain methanol less than a few %, 10%, 20% and 30% by changing the solvent composition by controlling the drying and adding methanol content during the preparation of the AgNCs. By measuring the suspended particle size of these AgNCs by the CDLS technique, it was observed that dispersion stability got worse and the ratio of large aggregates increased in the AgNCs that contain more methanol. It was also observed that the conductivity and the degree of sintering in the electrode patterns obtained by the SuPR-NaP technique increased by adding more methanol. We will discuss the effect of solvent composition on dispersion stability, phase separation, and self-sintering properties in the AgNCs.
 T. Yamada et al., Nat. Commun. 7, 11402 (2016).  M. Itoh et al., J. Nanosci. Nanotech. 9, 6655 (2009).  K. Aoshima et al., under review.