In electronic circuits, thin metal films are used to connect components such as transistors and resistors together. As these devices continue to shrink, there is a constant need for new techniques that are able to accurately pattern metal at nanoscale dimensions. One strategy is to spray-print patterns directly onto the circuit using a metallic ink — a low-cost approach that promises higher resolution and speed compared to current methods.

A major challenge in spray-printing processes, however, is formulating suitable metallic inks. Now, Teng-Yuan Dong and colleagues from the National Sun Yat-Sen University in Taiwan1 have developed a one-step method to produce small silver nanoparticles that show great potential for metallic inks because of their uniform thermal properties.

The researchers created silver nanoparticles using a capping agent — a thin monolayer of long organic molecules known as decanoates — to control the particle size. First, a solution of silver ions was mixed with the decanoate capping agent, where the oppositely charged species are attracted to each other. Then, hydrazine — a mild reducing agent and electron donor — was added to the mix. This donor caused small particles of silver, protected by the capping agent, to precipitate out of solution as uniform spheres, with an average diameter of just eight nanometers.

According to Dong, these silver nanoparticles can be converted into conductive films, patterns, lines and joints by utilizing one of the unique features of nano-sized particles: the drastically reduced melting temperature.

Fig. 1: Silver nanoparticles can be spray-printed as metallic ink, then transformed into high-quality films through a mild annealing process.

After depositing the nanoparticles onto silicon, the researchers transformed the metallic ink into a high-quality, conductive silver film by heating at 300 °C (Fig. 1). This temperature is sufficiently high to melt away the capping agents to give a pure metal film, but is much lower than that needed in typical metal patterning techniques — a significant processing improvement.

The success of this spray-printing technique relies on producing nanoparticles of uniform size. “It is important to control the size of the silver nanoparticles to be able to convert them into conductive films at a constant curing temperature,” says Dong.

Currently, the scientists are working to convert their well-defined silver nanoparticles into self-organized two- and three-dimensional arrays.