Metallic nanowires are promising for use as the conductive transparent electrodes in solar cells, touch screen panels and flat and flexible displays. At the moment such electrodes are primarily made from indium tin oxide (ITO) but this material is in short supply. It is also brittle, which means that it cannot be used in flexible devices.

Silver nanowires could be ideal alternatives to ITO in this context since thin coatings made from these wires can be bent at acute angles without breaking. However, there is a problem in that their transparency in the visible spectrum still needs to be improved. And to do this, researchers need to be able to reduce the size of the wires and increase their aspect ratio (the ratio of length to diameter). This parameter is directly proportional to the “open area” formed among the nanowires in a coating, through which the light can pass without being attenuated by the nanowires.

Nanowires less than 20 nm in diameter

Longer Ag nanowires with thinner diameters should result in films with larger open areas and thus higher light transmittances than those shorter in length and/or thicker in diameter. However, long Ag nanowires with diameters thicker than 30 nm may still significantly attenuate light at purple and blue wavelengths because of localized surface plasmon resonance, or LSPR (which depends on collective excitations of conduction electrons at the metallic surface) along the width of the wires. This attenuation could be greatly reduced by decreasing wire diameters down to 25 nm and below but it has been difficult – until now that is – to produce Ag nanowires of this size.

The new method developed by Younan Xia and colleagues produces nanowires less than 20 nm in diameter and aspect ratios of more than a 1000 by confining nanowire growth in the lateral direction. The researchers managed to do this using a synthesis process in which they add bromide ions and poly(vinylpyrrolidone) with a high molecular weight of 1 300 000g/mol to “cap” the lateral faces of the wires while slowly introducing silver nitrate into the reaction solution using a syringe pump.

Directing nanowire growth in the longitudinal direction

By optimizing the ratios between the capping agent and silver nitrate, Xia and co-workers say they were able to effectively direct nanowire growth in the longitudinal direction only. Thanks to their small diameters, they were also able to push down the transverse LSPR peak of the wires down into the ultraviolet region (below 400 nm).

“We synthesized our Ag nanowires by tightly controlling the reaction kinetics under ambient atmosphere by adding bromide ions to a polyol process,” explains Xia. “The bromide ions bind with the silver ions to form AgBr, which reduces the reaction speed by lowering the concentration of free silver ions. This slow reaction rate favours the production of penta-twinned, relatively small decahedral Ag seeds that then grow into penta-twinned nanowires thanks to the fact that they are passivated on their {100} faces by the bromide ions."

Ideal for transparent, conductive and flexible electrodes

"The ultrathin Ag nanowires we made could be cast into conductive films to replace the conventionally used ITO coatings in transparent, conductive and flexible electrodes," he tells nanotechweb.org. "Their high transparency in the visible region, their good electrical conductivity and their mechanical flexibility makes them ideal for such an application.”

The team, which includes researchers from Georgia Tech and Emory University in Atlanta, Sao Paulo University in Araraquara, the Oak Ridge National Lab in Tennessee and the Chinese Academy of Sciences in Beijing, says that it is now busy trying to improve the chemical stability of the Ag nanowires, which is a major problem for these materials. “We are trying to do this by coating the wire surfaces with an ultrathin gold sheath,” says Xia.

The work is detailed in ACS Nano DOI: 10.1021/acsnano.6b03806.