Density-of-states effect on surface and lattice vibrational modes in hydrogenated polycrystalline diamond

Energy-loss spectra and inelastic excitation functions of spᵐ0.3em0exCHₓ stretching and mixed bending-lattice modes have been measured by high-resolution electron energy-loss (HREEL) spectroscopy for hydrogenated and deuterated polycrystalline diamond films. The comparison between the excitation functions recorded for surface stretching modes of sp³- and sp²-hybridized CHₓ species demonstrates that vibrational excitation functions are strongly governed by the density of states of the local environment in which the probed species are embedded. In particular, any band gap leads to a strong enhancement of the number of backscattered electrons. The effect of the second absolute band gap of diamond (observed at incident electron energies neighboring 130.3em0exeV) on the excitation functions associated with mixed bending-lattice modes was analyzed in order to characterize to what extent lattice modes are mixed with hydrogen-termination bending modes. The loss observed at 1500.3em0exmeV for hydrogenated and deuterated polycrystalline diamond films is attributed to a dominant lattice mode, while the losses observed at 165 and 1800.3em0exmeV for hydrogenated polycrystalline diamond are attributed to surface modes, mainly involving the bending vibration of the hydride groups.