Immersion infrared reflection-absorption spectroscopy studies on diamond-like carbon surfaces. II. Covalent binding of nucleophiles to a-C film surfaces – type and stability

Amorphous, highly crosslinked, hydrogen-free or hydrogenated carbon films (a-C, ta-C, a-C:H, ta-C:H, etc., also referred to as diamondlike carbon (DLC) films) are commonly prepared by low-pressure physical or plasma-chemical vapor deposition. Owing to various kinds of highly reactive structural moieties generated by energetic particle bombardment, freshly prepared DLC films take up ambient oxygen under formation of functional groups, causing electrophilic surface reactivity. By chemical derivatization with amines or hydrazines bearing strongly infrared-absorbing CF3 groups, electrophilic sites can be detected and quantified with high sensitivity. In the present article this is accomplished using IR reflection-absorption spectroscopy with germanium as an immersion medium (ImIRRAS). The question of which structural moieties are responsible for the electrophilicity arises in particular with nominally H-free sputtered a-C films, containing extended domains of sp2-bonded carbon atoms. Vapor-phase derivatization with benzylamines results in the stable covalent bonding of 3 to 6 amine molecules per nm2. The disappearance of epoxy-characteristic bands and the absence of amide or imine vibrations in IR spectra, in conjunction with the stability of the bond against hydrolysis and hydrazinolysis, provide substantial evidence that ring opening reactions of epoxy groups represent a major mechanism by which amines are immobilized via C−N single bonds. However, contributions from aza-Michael addition reactions to conjugated carbonyl or carboxy groups are probable as well. Furthermore, the exposure of an amine-saturated a-C surface to the vapor of a phenylhydrazine typically results in 3 to 7 hydrazine molecules per nm2, primarily bonded as hydrazones via reaction with carbonyl groups, while the previously immobilized amine is largely retained. The lack of immobilization of (trifluoromethyl)benzene from the boiling neat liquid demonstrates that reactions of carbon-centered radicals, produced thermolytically, with the aromatic rings of employed reagents are to be excluded.