Size-dependent elastic properties of thin films: surface anisotropy and surface bonding

Surface eigenstress and eigendisplacement models were used to investigate the surface stress,surface relaxation and surface elasticity of thin films with different surface orientations.Molecular dynamics simulations and first-principles calculations were conducted on face-centered cubic Au films with the focus on relaxation induced nonlinear initial deformation.The simulation results verify the theoretical predictions of the size dependency of surface energy density and surface stress,and the nonlinear scaling law of the size-dependent Young’s modulus of thin films.The mechanism of the size-dependent behaviors was further explored at the atomic bonding level with the charge density field.The Au atomic bonding at surfaces is enhanced compared to its interior counterpart and therefore the nominal Young’s modulus of the Au thin films is larger when the film thickness is smaller.