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

Energy-loss spectra and inelastic excitation functions of $s{p}^m{\mathrm{CH}}_x$ 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 $s{p}^3$- and $s{p}^2$-hybridized ${\mathrm{CH}}_x$ 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 $13\mathrm{eV}$) 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 $150\mathrm{meV}$ for hydrogenated and deuterated polycrystalline diamond films is attributed to a dominant lattice mode, while the losses observed at 165 and $180\mathrm{meV}$ for hydrogenated polycrystalline diamond are attributed to surface modes, mainly involving the bending vibration of the hydride groups.

Figure 1

(Color online) Energy-loss spectra recorded at $E_0=3$, 6, 8, and $13\mathrm{eV}$ for the ex situ hydrogenated diamond films. Intensities are given in counts/s. The complete spectra presented in (a) are artificially shifted by $50\mathrm{counts}∕\mathrm{s}$ from each other. The stretching band region $330–400\mathrm{meV}$ is expanded in (b) using the same intensity scale. The overtone and combination band region $400–800\mathrm{meV}$ is magnified in (c), where the measured intensities have been multiplied by 5 and the spectra have been arbitrarily shifted vertically from one another.

Figure 2

(Color online) Energy-loss spectra recorded at $E_0=4.5$, 9, and $13\mathrm{eV}$ for the in situ hydrogenated diamond films. For energy-loss ranges and used intensity scales, see the caption of Fig. 1.

Figure 3

(Color online) Energy-loss spectra of the “bare” reconstructed curve $\left(a\right)$, in situ deuterated curve $\left(b\right)$, in situ curve $\left(c\right)$ and ex situ hydrogenated $\left(d\right)$ curve diamond films recorded at $E_0=13\mathrm{eV}$. The elastic peak intensity has been arbitrarily normalized to 200 for all spectra.

Figure 4

(Color online) Specular excitation functions of ex situ hydrogenated diamond films, recorded for energy losses (a) at 362, 350, and $380\mathrm{meV}$ belonging to the stretching band, and (b) at 165, 150, and $139\mathrm{meV}$ belonging to the bending-lattice band together with the harmonic modes at 300 (upper line) and $450\mathrm{meV}$ (lower line). See text for details.

Figure 5

(Color online) Specular excitation functions of bending-lattice modes recorded (a) for in situ hydrogenated diamond films and for the energy losses at 164, 180, 150, 300 (upper line), and $454\mathrm{meV}$ (lower line), and (b) for in situ deuterated films and for the losses at 98, 152, and $299\mathrm{meV}$.