Probing the Viscoelastic Properties of Stable Glass Surfaces

Nanoscale glass films are widely used in coating and display industries. However, their properties can deviate greatly from those of bulk glasses, which has been attributed to the existence of a liquid-like surface. Experimental and simulation work show a complex dependence of the surface relaxation time and elasticity on temperature. Here we use Atomic Force Microscopy (AFM) based nanoscale dynamic mechanical analysis (nDMA) to study the mechanical properties of the surfaces of various molecular glass systems. These films are produced either through physical vapor deposition to make stable glasses (SG), or through liquid cooling to produce ordinary glasses (OG). With tuning the applied force and contact area, we measure the local mechanical properties profile at various depths from the surface. The surface of all glass films (Both SG and OG) shows strong viscoelastic behavior and become liquid-like at frequencies below several Hertz, well below their glass transition temperature (Tg), corresponding to a much faster relaxation times compared to the bulk. We also observed a strong gradient of the relaxation within the surface, indicating dynamical anisotropy of the surface layer. However, temperature-dependent measurements indicate that the change in surface Tg may highly depend on the choice of the molecule. These results provide insight into the gradients of surface elasticity with high depth precision.