Influence of grain boundary density on the surface energy of nanocrystalline metal thin films

Abstract

Owing to the proliferation of nanomaterials, the precise measurement of surface energy (SE) has gained importance because of their large specific surface area. Grain boundary (GB) density is a critical factor determining surface characteristics of nanoscale metals. However, the relationship between GB and SE of nanocrystalline metals has not been revealed thus far due to the limited technology to measure surface energy at nanoscale. Herein, we report the role of GB density (DGB) in determining the SE of nanocrystalline Au thin films. The surface energies of nanocrystalline Au with different DGB values were characterized using atomic force microscopy (AFM) in the PeakForce quantitative nanomechanical (PF-QNM) imaging mode. The adhesion forces in the PF-QNM imaging mode, measured at the nanoscale, were converted to SE. The measured adhesion force decreased as the DGB of the nanocrystalline Au thin film increased. In addition, all-atom molecular dynamics simulations showed that the work required for separating the AFM tip from the sample surface decreased as the DGB of the Au film increased owing to the low load transfer capability at the GBs. Consequently, the SE of nanocrystalline Au (SEnc) was confirmed to be inversely proportional to the DGB, i.e., SEnc ∝ 1/ DG.B.