Thermal stability of perfluorinated molecular monolayers immobilized on pulsed laser deposited amorphous carbon surfaces

Amorphous carbon (a-C) films grown by Pulsed Laser Deposition were optimized to get smooth surfaces with sp3-rich hybridization, sp3/(sp2+sp3) = 0.45-0.65. Under UHV annealing conditions, some sp3-to-sp2 conversion mechanism becomes efficient above 350°C. Covalent immobilization of linear alkene molecular layers was performed by a thermally-assisted (160-300°C) process, using perfluorodecene (PFD) in the gas phase, with no physical or chemical surface preparation of a-C films. Quantitative XPS analysis indicates the immobilization of a dense molecular monolayer. Thermal stability of the grafted PFD monolayer is investigated by using UHV annealing steps to derive the desorption rate constant k(T) and the temperature corresponding to half initial coverage (T1/2 = 460°C). The slower desorption rate of perfluorodecene monolayers immobilized on a-C as compared with a-Si:H surfaces reveals the role of the covalent C-C interface bond in the robustness of the molecule / semiconductor assembly.