Laser ablation time-of-flight mass spectrometry (LA-TOF-MS) of “nitrogen doped diamond-like carbon (DLN) nano-layers”

Nitrogen-doped diamond-like carbon (DLC) layers (a-C:H:N, N-DLC or DLN) were prepared by the plasma-enhanced chemical vapor deposition (PECVD) technique, using a RF capacitive discharge (13.56 MHz), at low pressures (20 Pa), produced from a mixture of methane, nitrogen and hexamethyldisiloxane (HMDSO), deposited on single-crystalline silicon wafers placed on steel samples. The films, of differing deposition times, were subjected to laser ablation time-of-flight (LA-TOF) mass spectrometric measurements, using different commercial instrumentation to characterize their structures. The analysis of mass spectra was made and the following positively singly charged species were detected and identified: C_n⁺ (n=4–30), Si_n⁺ (n=2, 3), Si_nH⁺ (n=2, 3), SiOK⁺, Si₃H₄⁺, Si₂N⁺, Si₂NH₂⁺, and Si₃C⁺. The later three species could reflect the presence of nitrogen–silica and carbon–silica chemical bonds in the structure of the DLN layer. The stoichiometry of all species was confirmed by isotopic pattern simulation. In the negative detection mode, the C_n⁻ (n=2–12) clusters were observed. The findings are discussed in the light of the current research concerning analysis of the DLN thin layers and it is concluded that namely Si₂N⁺, Si₂NH₂⁺ and Si₃C⁺ species are reflecting the chemical structure of the DLN layer. LA-TOF-MS was found useful supplementary method for the characterization of DLN nano-layers.