Spectroscopic ellipsometry analysis of metal nanoparticles-doped polymer films: mechanism of the onset of the plasmon resonance during thermal annealing

Doping polymer films with noble metal salts results during thermal annealing in the in situ synthesis of nanopar-ticles (NPs) provided the polymer acts as a reducing agent for the metal cations. The optical properties of such composite films can be probed by spectroscopic ellipsometry (SE) and are characterized by the presence of an absorption peak due to the plasmon resonance [1,2]. In the case of silver NPs, the resonance occurs at ~ 415 nm and the films appear yellowish-brown in color. In a recent publication [3], we showed, using AFM analysis and SE, that increasing the annealing time contributed to the onset of more particles whose area decreased and that the intensity of the absorption peak, modeled as a Lorentzian absorption line, was strongly correlated to the den-sity of NPs. In the mid-infrared spectral domain, the presence of additional peaks located at 1134 cm-1 and 1036 cm-1 also suggested the presence of bonds between Ag and the polymer matrix [4]. This study is focused on the mechanism of the onset of the plasmon resonance during thermal annealing. The optical properties of Ag-polymer films were studied using SE as a function of the time during the annealing of the films at a temperature higher than the glass transitions temperature. The amplitude of the absorption peak obviously obeys two-steps kinetics (Fig. 1). A sharp increase from 500 s to about 2500 s followed a slower one till the end of the experiment. On the other hand, the behavior of the resonance wavelength is much more com-plex. Corresponding to the slow kinetics, we do observe a red-shift of the resonance but during the rapid kinetic phase, a red-shift followed by a blue one is observed.