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

Energy-loss spectra and inelastic excitation functions of sp(m) CHx 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 sp(3)- and sp(2)-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 13 eV) 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 150 meV for hydrogenated and deuterated polycrystalline diamond films is attributed to a dominant lattice mode, while the losses observed at 165 and 180 meV for hydrogenated polycrystalline diamond are attributed to surface modes, mainly involving the bending vibration of the hydride groups.