Ultrathin Gold Nanowire Cold Welding Induced by the Collapse of Carbon Nanotube Walls

The cold welding of two Au nanowires (AuNWs) encapsulated in carbon nanotubes (CNTs) is observed in situ using a transmission electron microscope (TEM) at room temperature. It is observed that under electron irradiation, the radial shrinkage and collapse of CNT walls lead to the squeezing of the two AuNWs encapsulated within CNTs, bringing their ends closer to each other. Upon further irradiation, the ends of the two AuNWs make contact and fuse together. However, instances of failed welding are observed, with the distance between the two AuNWs identified as a significant factor determining a successful welding. Specifically, when the tip-to-tip distance is too large, the amorphous carbon generated from the amorphization of CNT walls cannot provide sufficient squeezing force. Additionally, the acceleration voltage of the TEM is identified as another critical factor affecting the welding, as the acceleration voltage decreases, the welding is difficult to carry out. The compressive stresses generated by the collapse of CNT walls on AuNWs are calculated using gold’s compressive stress–strain curve and strain in the diameter of AuNWs. It is estimated that a stress of at least 539 MPa is required for the successful cold welding of AuNWs. Furthermore, it is also calculated for the first time that the force generated by amorphous carbon resulting from CNT amorphization is approximately over 384 MPa.