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).

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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.

Various-sized Ag nanoparticles capped with oleylamine were synthesized by means of a thermal decomposition process for low-temperature electronic devices. The Ag nanoparticles, which had diameter of 5.1 nm to 12.2 nm, were synthesized in incubation and ripening stages related to nucleation and growth. After the Ag nanoparticles were made into ink with a proper solvent, inkjet printing and thermal sintering methods were used to form a metal thin film with thickness of 100 nm. A type of thermal sintering related to percolation transformation and surface sintering was conducted at a temperature much lower than the melting point of bulk Ag. The electrical resistivity was examined with the aid of a four-point probe system and compared with the resistivity of bulk Ag, showing that the Ag film had much higher resistivity than bulk Ag. To improve the electrical stability and properties, we applied hexamethyldisilazane (HMDS) surface treatment to the substrate and dipped the as-deposited films into methanol. Both treatments helped to diminish and stabilize the resistivity of the printed conductive films.

A various size of Sn-Cu nanoparticles were synthesized by using a modified polyol process for low temperature electronic devices. Monodispersive Sn-Cu nanoparticles with diameters of 21 nm, 18 nm and 14 nm were synthesized. In addition, the eutectic composition shift was also observed in nano-sized particles as compared with bulk alloys. By controlling the size and eutectic composition, a significant melting temperature depression of 30.3 degrees C was achieved. These melting temperature depression approaches will reduce adverse thermal effects in electronic devices and provide the synthesis guidelines for bimetallic nanoparticles with a low melting temperature.