Femtosecond laser irradiation induced heterojunctions between carbon nanofibers and silver nanowires for a flexible strain sensor

We report the direct joining of carbon nanofibers (CNFs) to silver nanowire (Ag NWs) by controlled irradiation with femtosecond (fs) laser pulses. Two separate types of nano-junction dependent on joint geometry, laser fluence and irradiation time are identified in irradiated mixtures. In one type of junction, the tip of an Ag NW is melted and flows to form a bond with an adjacent CNF. The second type of junction occurs without significant heating of the Ag NW and involves the softening and flow of carbon in the CNF in response to the transfer of plasmonic energy from the Ag NW into the CNF. Bonding in a T-type joint configuration can be of either kind depending on the relative orientation of the incident optical field and the long axis of the Ag NW. FDTD simulations were used to explore this effect for different joint geometries and laser polarization. The electrical properties of a heterojunction involving a single Ag NW-CNF structure have been measured, and it is found that the junction resistance can be reduced by six orders of magnitude after laser joining. Finally, we have investigated the properties of a strain sensor based on an Ag NW-CNF hybrid nanowire network and find that this device can exhibit high sensitivity. This sensitivity occurs as nano-junctions induced by fs laser irradiation greatly reduces the initial resistance. This laser-based technique for direct nanojoining of CNF and Ag NWs may enable the design of robust nanowire structures for application in a variety of new devices.