Tech Briefs

Highly conductive films make functional circuits without adding high heat.

Printed electronics must be heated to melt all the nanoparticles together into a single conductive wire, making it impossible to print circuits on inexpensive plastics or paper. A study shows that tweaking the shape of the nanoparticles in the ink could eliminate the need for heat.

By comparing the conductivity of films made from different shapes of silver nano-structures, the study found that electrons move through films made of silver nanowires much easier than films made from other shapes such as nanospheres or microflakes. In fact, electrons flowed so easily through the nanowire films, that they could function in printed circuits without the need to melt them all together.

Silver nanowire films conduct electricity well enough to form functioning circuits without applying high heat, enabling printable electronics on heat-sensitive materials like paper or plastic. (Ian Stewart)

By suspending tiny metal nanoparticles in liquids, conductive inkjet printer “inks” can be used to print inexpensive, customizable circuit patterns on just about any surface.

These types of printed electronics could have applications not just in smart packaging, but also in making solar cells, printed displays, LEDs, touchscreens, amplifiers, batteries, and even some implantable bio-electronic devices.

Silver has become a common material for making printed electronics, and studies have measured the conductivity of films with different shapes of silver nano-structures. Experimental variations make direct comparisons between the shapes difficult, and few studies have linked the conductivity of the films to the total mass of silver used — an important factor when working with a costly material.

Silver nanostructures with different shapes were created, including nanoparticles, microflakes, and short and long nanowires. They were mixed with distilled water to make simple inks. The thin films were created with glass slides and double-sided tape. By adding a precise volume of ink into each tape well and then heating the wells — either to relatively low temperature to simply evaporate the water, or to higher temperatures to begin melting the structures together — a variety of films was developed to test.

Electrons usually flow easily through individual nanostructures, but get stuck when they have to jump from one structure to the next. Long nanowires greatly reduce the number of times the electrons have to make this jump. The resistivity of the long silver nanowire films was several orders of magnitude lower than silver nanoparticles, and only 10 times greater than pure silver.

Aerosol jets are being investigated for printing silver nanowire inks in usable circuits.

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