To fabricate a low cost, highly conductive ink for inkjet printing, we synthesized a gram scale of uniformly sized Sn nanoparticles by using a modified polyol process and observed a significant size-dependent melting temperature depression from 234.1 °C for bulk Sn to 177.3 °C for 11.3 nm Sn nanoparticles. A 20 wt% of Sn nanoparticles was dispersed in the 50% ethylene glycol: 50% isopropyl alcohol mixed solvent for the appropriate viscosity (11.6 cP) and surface tension (32 dyn cm(-1)). To improve the electrical property, we applied the surface treatments of hydrogen reduction and plasma ashing. The two treatments had the effect of diminishing the sheet resistance from 1 kΩ/sq to 50 Ω/sq. In addition, conductive patterns (1 cm × 1 cm) were successfully drawn on the Si wafer using an inkjet printing instrument with conductive Sn ink. The maximum resistivity for an hour of sintering at 250 °C was 64.27 µΩ cm, which is six times higher than the bulk Sn resistivity (10.1 µΩ cm).

To steadily apply conductive inks that contain Cu nanoparticles (NPs) to inkjet printing of patterns at temperatures below 150 °C, the size of the Cu NPs must be reduced. Therefore, we obtained Cu NPs in the range of 9-33 nm, and we studied how their size changes. The variation of the chemical reaction rate changed the size of the Cu NPs for two main reasons. First, the fast transition rate of the Cu precursors at high pH values raises the supersaturation level of the Cu precursor above that of a process with a slow transition rate. The high supersaturation level is generally attributed to the small Cu nuclei and the slow growth caused by their density. Second, the high viscosity of the reaction solution, which occurs because polyvinyl pyrrolidone (PVP) causes an increase in the repulsive force, slows the growth of the Cu NPs at high pH values. The recrystallization temperature of the 9 nm Cu NPs was reduced to 108 °C, and a low specific resistivity of 45 μΩ cm was achieved using the conductive ink prepared with 9 nm Cu NPs at 120 °C. This temperature is significantly lower than those reported for other Cu NP inks. Hence, Cu NP conductive ink could considerably reduce costs because of its apparently low temperature, resolving the main bottleneck of inkjet printing on flexible (polymeric) substrates.

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