Laser nanosoldering increases conductivity of silver nanowire electrodes

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(a) Schematic of experimental system for plasmon-enhanced laser nanosoldering. (b) SEM image of silver nanowires with inset showing the size distribution of silver nanoparticles. (c) Plasmon-enhanced electric field as a function of interparticle gap for light polarization direction parallel and vertical to the interparticle axis. (d) Schematic illustration of plasmon-enhanced laser nanosoldering with increasing laser irradiation time. (e) SEM images of the morphological changes of silver nanowires in the new process. (Image: OEA)

Researchers have proposed a laser nanosoldering process for enhancing the electrical conductivity of silver nanowire electrodes. 

In recent years, such electrodes have been widely used in new photodetectors, flexible circuits, solar cells, touch panels, and more.

An existing technique for building silver nanowires for engineered patterns in two and three dimensions is femtosecond laser direct writing, which is based on multi-photon absorption induced photoreduction and offers submicron resolution.

This technique however results in voids or polymer coatings between the silver nanoparticles that make up the wires, leading to poor electrical conductivity. 

In order to increase the conductivity of the nanowires, it is therefore necessary to reduce the gap and increase the contact area between the nanoparticles.

This would reduce the energy dissipation of the conductive electrons in the electrode.

Two research groups, from the Institute of Photonics Technology of Jinan University and the Chinese Academy of Sciences’ Institute of Physics and Chemistry, have therefore proposed an optical method for enhancing the electric conductivity of silver nanowires produced using femtosecond laser direct writing, using plasmon-enhanced laser nanosoldering.

Described in Opto-Electronic Advances, the method generates plasmon ‘hot-spots’ among the silver nanoparticles, triggering a photothermal effect that locally solders the nanoparticles, with aggregates of these nanoparticles then growing into larger particles. The process significantly increases the contact area between the nanoparticles while reducing the gap between them, increasing the conductivity of the nanowire.

The new technique requires no complicated post-processing and directly increases the conductivity of the silver nanowire electrodes fabricated by femtosecond laser direct writing, according to the researchers.

Further study of the influence of laser power density and nanosoldering time on the conductivity of silver nanowires revealed that their resistance decreases significantly with an increase in either parameter. 

By optimising the conditions of the new process, using a laser power density of 9.55MW/cm2 and a nanosoldering time of 15 minutes, the researchers were able to increase the conductivity of silver nanowire to 2.45×107S/m, which is around 39 per cent of that of bulk silver. 

This researchers have determined that the new method provides an efficient, controllable and low-cost method to improve the conductivity of silver nanowires, and promotes the application of femtosecond laser direct writing of silver nanowire electrodes as active SERS substrates, transparent electrodes, capacitors, light-emitting diodes, and thin-film solar cells.

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