|Ph.D Student||Michaelson Shaul|
|Subject||Hydrogen Bonding and Formation Mechanism of Carbon Films|
with Nano-Diamond Character Deposited from
|Department||Department of Chemistry||Supervisor||Professor Alon Hoffman|
|Full Thesis text|
This research is focused onto elucidating the mechanism underlying the formation of carbon films with predominant nano-diamond character from energetic species. The direct current glow-discharge plasma activated chemical vapor deposition (dc GD CVD) process was used to induce nano-diamond formation from energetic hydrocarbon and hydrogen species (CH4/H2 gas mixture with a ratio of 9/91). It was established by means of Raman spectroscopy, that nano-diamond formation on silicon substrate always occurs after a time delay of ~30 min within a narrow substrate temperature window of 880±20°C. Formation of diamond nano-crystallites is always accompanied by high hydrogen retention (up to 15-20 at%, established by secondary ion mass spectroscopy). Thus, most likely, hydrogen adsorption/desorption dynamics is of primary importance during formation and stabilization of diamond nano-crystals. We propose that dense and hydrogenated amorphous carbon matrix, once created (time delay of ~30 min) allows continuous and cyclic "precipitation" of nano-crystals by means of ion preferential displacement mechanism.
Hydrogen bonding configuration within the film bulk was studied by Raman spectroscopy while hydrogen profile concentration was measured by secondary ion mass spectroscopy (SIMS). It was found that the hydrogen retention of the diamond films increases with decreasing grain size, indicating that most likely hydrogen is bonded and trapped in grain boundaries. Raman analysis shows that at least part of this hydrogen is bonded to carbon resulting in typical C-H vibrations.
Hydrogen bonding on the film surface was studied by high resolution electron energy loss spectrum (HR-EELS). DC GD nano-diamond films were investigated as-deposited and following short time ex-situ exposure to micro-wave activated atomic hydrogen. It was found that vibrational states of the as-deposited film surface are characteristic to hydrogenated amorphous carbon. The different vibrational modes, fundamentals and overtones, were directly identified through the modifications of the HR-EEL spectra induced by the isotopic exchange of H by D and 12C by 13C. Three types of peaks were identified: (1) Pure C-C related peaks; (2) Pure C-H related peaks; (3) Coupling of C-H and C-C peaks. The overtones of diamond optical phonon detected by HR-EEL spectroscopy were assigned to well-defined crystalline diamond surface. The study of hydrogen bonding to diamond surface as a function of annealing showed that Cdiamond-H bond is stable up to ~900°C. In addition, hydrogen bonded to diamond grain boundary (Csp2-H) is stable up to similar temperatures.