Concomitant effects of the substrate temperature and the plasma chemistry on the chemical properties of propanethiol plasma polymer prepared by ICP discharges

In this work, the plasma polymerization of propanethiol was investigated aiming to give new insights into the growth mechanism of such material. The plasma polymers films (PPF) were synthesized using the two plasma mode production of ICP discharges, namely the capacitive (E) and the inductive (H) mode. Using the E mode, the atomic sulfur content in the PPF (at.%S) was found to be higher (~ 40%) than in the precursor (25%) which was explained by the trapping of molecules presenting a high S/C ratio in the PPF network. This explanation is validated by aging experiments revealing a strong decrease of at.%S likely due to the release of the trapped species. In contrast, using the H mode, at.%S is significantly lower (17%-25%) and stable under aging. This different behavior regarding chemical properties of the PPF as a function of the discharge mode used for their synthesis was understood by considering the concomitant effect of the substrate temperature (Ts) and the plasma chemistry for both ICP modes. It is shown that in the H mode, Ts ranges from 60 to 90 °C compared to 30-35 °C in E mode. This induces a decrease of the residence time of the sulfur-based molecules at the growing film interface and, ultimately a decrease of at.%S. On the other hand, experiments carried out for similar Ts but using both modes reveal the importance of the plasma chemistry on the chemical composition of the films. Indeed, in these conditions, at.%S was correlated to the amount of H2S in the discharge which is therefore identified as the trapped sulfur-based molecules. Our data allow highlighting the concomitant effect of both substrate temperature and plasma chemistry in order to understand the evolution of the chemical properties of propanethiol PPF prepared in the E and H mode of an ICP discharge.