Novel behaviors/properties of nanometals induced by surface effects

Surface effect is believed as one of the most important origins from which the novel properties of nanomaterials derive. Although this effect has been investigated for decades, the understanding of the essential correlations between materials' structures and their unique properties still has a long way to go. Recently, the innovation of aberration-correction techniques in electron microscopy, as well as the fast-developing in situ techniques, has made a big step toward unveiling the mysterious mechanisms underlying the unusual behaviors. In this review, we summarize the surface effect-induced extraordinary phenomena of nanometals that were uncovered recently, including peculiar mechanical behaviors, unusual thermal instabilities, remarkable electromigrations, unconventional structure evolution, and phase separations. All these findings apparently give an in-depth understanding of the novelties that appeared only in nanometals, such as the rubber-like or liquid-like deformation behaviors in mechanics, the size-dependent melting and wetting behaviors in thermodynamics and surface science, the atomic-scale welding and mass conveying in electrics, and the size- or composition-dependent phase segregations in kinetics and metallography. Such abundant knowledge not only extends the classical theories established on bulk materials but also can provide valuable instructions for future applications of nanometals such as the design of versatile functional nanodevices like sensors and actuators. By using the state-of-the-art characterization techniques, tremendous progress has been made toward approaching the truth on what factors essentially govern the discrepancies between nanometals and their bulk counterparts. Moreover, this review also opens up a discussion on several surface effect-related controversies that have been retained for a long period. Finally, a brief perspective is presented on the basis of upcoming new techniques in electron microscopy, giving an imagination of viewing and measuring the surface structures and properties directly. (C) 2018 Elsevier Ltd. All rights reserved.