Comparison between single monomer versus gas mixture for the deposition of primary amine-rich plasma polymers

Primary amine-based plasma polymer films (NH2-PPFs) attract great attention due to their potential for various biomedical applications. In this context, in order to better understand the growth mechanism of such coatings, we investigate the impact of the precursor mixture on plasma chemistry and ultimately, on PPFs properties. PPFs are synthesized from both cyclopropylamine (CPA) and ammonia/ethylene (AmEt) mixture in low pressure (2.7 Pa) inductively-coupled plasma discharges, keeping the N/C ratio constant in the precursor flow rate. Optical emission spectroscopy (OES) is performed to study the plasma phase while PPFs chemistry is investigated by Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy (combined with chemical derivatization). The results show that, for similar energetic conditions, the use of CPA allows a better nitrogen incorporation in the film compared with the use of the AmEt mixture. This is attributed to the initial presence of CN bonds in the CPA molecule. In addition, it is shown that primary amine retention decreases when the power increases due to a strong dehydrogenation of the monomers that promotes the formation of multiple and conjugated CN bonds. Using OES, etching reactions of the growing PPFs surfaces are highlighted and are shown to strongly influence the plasma chemistry.