Diameter-dependent nanojoint formation and grain refinement in femtosecond laser nanojoining of AgNWs

Femtosecond (fs) laser has emerged as an effective technique for nanojoining. However, the role of nanowire diameter and the associated electric field distribution under fs laser irradiation in nanojoint formation and polycrystalline evolution remains unclear. In this study, AgNW nanojoints were fabricated via fs laser irradiation, and the electric field intensity distribution was simulated using the three-dimensional finite-difference time-domain (FDTD) method. Nanojoint formation was examined across nanowires of different diameters under consistent laser processing conditions. High-resolution transmission electron microscopy (TEM) revealed that smaller-diameter nanowires (e.g., 60 nm) facilitate nanojoint formation with a relative joint cross-sectional area reaching 544%, significantly exceeding those of 200 nm (73%) and 300 nm (19%) nanowires. Moreover, regions with higher electric field intensity exhibited refined polycrystalline grains, with an average size of 4.42 nm at the nanojoint interface-substantially smaller than that at non-bonded regions (15.69 nm and 26.27 nm). These findings provide new physical insights into the role of localized electromagnetic enhancement in joint morphology and grain structure regulation during fs laser nanojoining.