Thin film properties have drawn a lot of attention because they are shown to deviate from their bulk counterparts for various materials, such as polymers, molecular glasses and some metallic glasses. When the film thickness is below a certain threshold, fast surface dynamics plays a strong role in the film properties. The material of interest here is amorphous selenium (a-Se), which is a metallic glasses former that can be used as a high-efficient photon sensor. A unique property of a-Se is its light-induced structural flexibility. Upon above bandgap irradiation, electrons can be excited in a-Se and they could induce bond breakage, new bond formation and some over coordination sites. The project we study here is the effect of light on altering a-Se surface length scale and the magnitude of enhanced surface dynamics. We measure the glass transition temperature (T<sub>g</sub>) with ellipsometry and dewetting velocity under microscope in a-Se thin films, both under white light and with above-bandgap light filtered. To be specific, isothermal dewetting measurements are performed on a-Se films of different thicknesses below and above T<sub>g</sub>. It turns out the hole-growth rate satisfies non-slip dewetting conditions. Thus, the growth velocity is directly related to the film viscosity. Both the viscosity and activation energy in thin films are shown to be enhanced compared to the bulk. The film that starts to appear deviation from bulk can be as thick as 100 nm. It is an anomalous behavior because this length scale was measured to be around 10 to 30 nm in most organic glasses. By applying various illumination condition, we study the relationship between the film structure and degree of enhanced surface dynamics.

Light-facilitated dewetting in amorphous selenium thin films

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.