A first-principle study on the atomic-level mechanism of surface effect in nanoparticles

The effect of free surfaces is responsible for size effects in physical properties of nanomaterials. Fundamental origins of such surface-induced size dependence at nano-scale are investigated in this paper based on first principle simulations. Surface free energy densities and surface residual strains of face-centered-cubic (FCC) and body-centered-cubic (BCC) metallic nanoparticles are calculated, both of which increase with a decreasing particle size. Variations of electronic properties of surface atoms are further analyzed to disclose the fundamental mechanism underlying size-dependent behaviors of the two continuum quantities. It is found that more electrons of a surface atom are localized in high-energy bands when the nanoparticle becomes smaller, consequently leading to a higher free energy per surface atom as well as a larger surface energy density. Along with the electron localization in high-level bands far from nuclei, the electron cloud of surface atom exhibits an outward shift away from the ion core. Such charge redistribution induces an electrostatic attraction dominating interactions between surface atoms and inner ones, which results in a larger surface residual strain in a relatively small nanoparticle. The present research discloses atomic-level mechanisms governing the surface effect, which should provide useful guidance for tuning and manipulating overall performances of nanostructured materials.