Light-facilitated dewetting in amorphous selenium thin films

Properties of glass thin films deviate from bulk for various materials, such as polymers, molecular glasses, and metallic glasses. When the film thickness is below a certain threshold, fast surface dynamics plays a strong role. Here, we study dewetting and glass transition temperature (T_g) of amorphous selenium (a-Se) thin films. A unique property of a-Se is that it forms random chain length and exhibits structural flexibility through bond breakage and formation of new bonds or over-coordinated sites upon above bandgap light irradiation. We investigate light-induced fluidity in a-Se and its effect on the length scale and the magnitude of enhancement in surface dynamics. Isothermal dewetting experiments are performed on a-Se films of different thicknesses below and above T_g. The hole-growth rate satisfies non-slip dewetting conditions. Thus, the growth velocity is directly related to the film's viscosity. Both the viscosity and activation energy in thin films are enhanced compared to the bulk. Films that start to deviate from bulk can be as thick as 100 nm, while this length scale turned out to be about 30 nm in most organic glasses. By applying various illumination condition, we study the relationship between the film structure and enhanced surface dynamics.