Low-energy electron scattering on deuterated nanocrystalline diamond films-a model system for understanding the interplay between density-of-states, excitation mechanisms and surface versus lattice contributions.

Electron energy loss spectrum, elastic reflectivity and selected vibrational excitation functions were measured by High Resolution Electron Energy Loss Spectroscopy (HREELS) for deuterated nanocrystalline dc GD CVD diamond films. The electron elastic reflectivity is strongly enhanced at about 13 eV, as a consequence of the second absolute band gap of diamond preserved up to the surface for D-nano-crystallites. The pure bending modes δ(CD(x)) at 88 meV and 107 meV are dominantly excited through the impact mechanism and their vibration excitation functions mimic the electron elastic reflectivity curve. Pure diamond phonon mode ν(CC) can be probed through the resolved fundamental loss located at 152 meV and through the multiple loss located at 300 meV. In addition to the well-known 8 eV resonance, two supplementary resonances located at 4.5 eV and 11.5 eV were identified and clearly resolved for the first time. A comprehensive set of data is now available on low-energy electron scattering at hydride terminated polycrystalline diamond films grown either by HF (microcrystalline) or dc GD (nanocrystalline) chemical vapour deposition. The careful comparison of the vibrational excitation functions for hydrogen/deuterium termination stretching modes ν(sp(3)-CH(x)) and ν(sp(3)-CD(x)), for hydrogen termination bending modes δ(CH(x)) mixed with diamond lattice modes ν(CC), for deuterium termination bending modes δ(CD(x)), and for multiple loss 2ν(CC) demonstrates the close interplay between three characteristics: (i) the density-of-states of the substrate, (ii) the vibrational excitation mechanisms (dipolar and/or impact scattering including resonant scattering) and (iii) the surface versus lattice character of the excited vibrational modes. This work shows clearly that excitation function measurement provides a powerful and sensitive tool to clarify loss attributions, involved excitation mechanisms, and surface versus lattice characters of the excited vibrational modes.

Hydrogen bonding configuration and thermal stability of ambient exposed and in situ hydrogenated polycrystalline diamond surfaces studied by high resolution electron energy loss spectroscopy.

In this work we report on an investigation of hydrogen bonding and thermal stability on the surface of poly-crystalline diamond by high resolution electron energy loss spectroscopy (HR-EELS). Diamond films were grown on silicon substrates from CH(4)/H(2) as well as from CD(4)/D(2) gas mixtures by hot filament chemical vapor deposition (HF-CVD). The impact of ex situ ambient exposure on hydrogen bonding and its thermal stability was examined for: (i) as deposited films from a CH(4)/H(2) gas mixture; (ii) the same sample treated ex situ in micro-wave activated hydrogen plasma; and (iii) as deposited films from a CD(4)/D(2) gas mixture. In order to clarify the changes in the hydrogen bonding configuration detected on the different surfaces as a function of thermal annealing in situ hydrogenation by thermally activated atomic hydrogen was performed and examined. This study provides direct evidence that the exposure to ambient conditions and low temperature vacuum annealing have a pronounced effect on the hydrogen-carbon bonding configuration onto the poly-crystalline diamond surfaces.