1. Ophelia K. C. Tsui, Department of Physics, Boston University
Binding of Polystyrene Films to Hydrogen-Passivated Silicon and Oxidation Stability of the Polystyrene/Silicon Buried Interface
Ophelia K. C. Tsui, Department of Physics, Boston University
Different thickness, h, dependences have been found of the glass transition temperature, Tg, of polystyrene (PS) nanometer films coated on hydrogen-passivated silicon (H-Si) using ellipsometry and x-ray reflectivity. Since the former had been performed in air and the latter in vacuum, it has been suggested that the different Tg(h) might be caused by oxidation of the substrate at the PS/H-Si buried interface during the ellipsometric measurement, changing the films from PS/H-Si to PS/SiOx/Si.
We verified this hypothesis by annealing PS coatings on H-Si in air then rinsing them with a good solvent of PS. We found that a residual film with refractive index similar to SiOx did form, but the thickness was 5 to 20 times that found on bare H-Si under the same annealing condition (figure below). This shows that the residual film derived from the PS/H-Si films must originate from the PS coating. Detailed analyses by
Detailed analyses by Dynamic Secondary Mass Spectroscopy (SIMS) and X-ray Photoelectron Spectroscopy (XPS) showed that the residual film was indeed a residue of the PS film, and there was no sign of oxidation of the PS/H-Si buried interface.
This result shows that the different Tg(h) observed cannot be caused by the films used being different, and calls for a refocus on the different details involved in the experiments and the likelihood that polymer in nanometer films may be significantly more hysteretic than the native polymer is as recent experiments have shown. This is also the first time PS observed to form an irreversibly adsorbed layer on Si, which had been suggested to be the cause of some mysterious elastic properties found of molten PS/Si films that had motivated this project in the first place.