We have systematically studied the evolution of the optical absorption of free standing PS films during thermal oxidation at 200degC in air. Our experiment results show the evolution of transmission curve is quite complicated, which red-shifts first and then blueshifts during thermal oxidation. At the same time, the transmission at the low energy decreases first and then increases. We propose an explanation as follows: (1) the energy gap associated with each crystallite should increase during thermal oxidation process, due to the quantum confinement effect; (2) the energy gap should decrease with increase in oxygen termination atoms. Both the increasing of the gap due to the quantum confinement effect and the decreasing of the gap due to the Si-O bond formation cause a complicated evolution of optical absorption.
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