Size-dependent mechanical deformation mechanisms in sub-20 nm lead crystals

Mechanical properties of small-volume metals critically affect the reliability and performance of nano-devices. Under mechanical forces, behavior of small-volume metals differs considerably from that of their bulks, but the knowledge of how the feature size give rise to the unique characteristics is currently insufficient, particularly in sub-20 nm regime. In this study, we investigate the influence of feature size on mechanical behaviors in sub-20 nm Pb crystals via performing in situ mechanical deformation tests inside TEM. Diffusion-induced pseudo-elastic deformation and dislocation-triggered plastic deformation processes were observed at atomic precision in 5-20 nm Pb particles. As size decreases, surface diffusion gradually dominates the deformation process while smallest size for twinning of Pb is determined to be 7.1 nm. Moreover, surface layers with thickness 1-6 nm are found to diffuse readily, playing a decisive role in the deformation of all Pb particles. Minimization of surface energy provides driving force for both the diffusion and size-dependent spherical surface morphology after deformation. This research directly reveals the dramatic impact of feature size on the deformation mechanisms of metallic nanocrystals, which provides significant implications for property tuning of advanced nano-devices.