Laser Nanosoldering Boosts Conductivity of Nanoelectrodes

Researchers at Jinan University and the Chinese Academy of Sciences developed a plasmon-enhanced laser nanosoldering technology to increase the electrical conductivity of silver (Ag) nanowires that are fabricated using femtosecond laser direct writing (FsLDW). This technology could provide an efficient, cost-effective way to rapidly produce large-area metal nanoelectrodes and capacitors that are uniform, flexible, and highly conductive.

FsLDW is used to build Ag nanowires for engineered patterns in 2D and 3D, and that have submicron resolution. This nanofabrication method offers many advantages, including high resolution, true three-dimensionality, and flexibility. It is widely used in the fabrication of optoelectronic devices.

There is one challenge to using this method, however; the Ag nanowires built using FsLDW are composed of aggregates of Ag nanoparticles with void or polymer inclusions that reduce electrical conductivity.

To increase conductivity and reduce the resistance of direct-write Ag nanowires, the researchers aimed to reduce the gaps and increase the contact area between the Ag nanoparticles — thereby reducing the amount of energy dissipated by the conductive electrons in the electrode.

The researchers used the photothermal effect to significantly increase the contact area of adjacent Ag nanoparticles.

Plasmon-enhanced laser nanosoldering — the method used by the researchers to enhance the electrical conductivity of the Ag nanowires — takes advantage of the structural characteristics of the FsLDW-fabricated Ag nanowires.

The nanowires are composed of aggregates of nanoparticles that are reduced by the multiphoton absorption effect. Plasmonic hot spots are generated between the nanoparticles under laser irradiation.

(a) Schematic of experimental system for plasmon-enhanced laser nanosoldering (PLNS). (b) Scanning electron microscope (SEM) image of Ag nanowires with inset showing the size distribution of Ag nanoparticles in Ag NWs. (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 PLNS with increasing laser irradiation time. (e) SEM images of the morphological changes of Ag nanowires in the PLNS process. Courtesy of Opto-Electronic Advances.

The Ag nanoparticles can be locally connected — that is, soldered — at room temperature through a plasmon-enhanced photothermal effect. This process significantly increases the contact area between nanoparticles, improving the conductivity of the nanowires. Unlike traditional annealing, in the nanosoldering method heat is localized near the hot spot, so there is no thermal damage to the substrate.

The laser nanosoldering technology does not require complicated post-processing, and it directly increases the conductivity of the Ag nanowire electrodes fabricated by FsLDW. The researchers studied the influence of laser power density and nanosoldering time on the conductivity of Ag nanowires and found that the resistance of Ag nanowires decreases appreciably when laser power density or nanosoldering time is increased. The increase in conductivity tends to be saturated, because the nanoparticles and nanogaps available for nanosoldering gradually decrease as laser irradiation time increases.

Under experimental conditions that were optimized for plasmon-enhanced laser nanosoldering, the Ag nanowires demonstrated a laser power density of 9.55 MW/cm² and a nanosoldering time of 15 minutes. Maximum conductivity was increased to 2.45 × 10⁷ S/m, which was about 39% of bulk Ag.

Metal nanowire electrodes have been widely used in photodetectors, flexible circuits, touch panels, and other devices in recent years, the researchers said. In addition to improving the conductivity of Ag nanowires, the nanosoldering method supports the use of FsLDW-built Ag nanoelectrodes as active surface-enhanced Raman spectroscopy substrates, transparent electrodes, capacitors, light-emitting diodes, and thin-film solar cells. High-performance nanoelectrodes, the researchers said, support applications in the microelectronics field.

The research was published in Opto-Electronic Advances (www.doi.org/10.29026/oea.2021.200101).