X-ray photoemission studies of diamond, graphite, and glassy carbon valence bands

Abstract

The high-resolution x-ray photoemission spectra (XPS) of the total valence bands of atomically clean diamond, graphite, and glassy carbon, obtained with monochromatized Al $K\ensuremath{\alpha}$ radiation, are reported and discussed. By comparing valence-band and carbon-$1s$ photoelectron kinetic energies, the XPS valence-band spectra ${I}^{\ensuremath{'}}(E)$ of diamond and graphite were rigorously affixed to the same energy scale as earlier $K$ x-ray emission spectra $\mathcal{I}(E)$. The two spectra---${I}^{\ensuremath{'}}(E)$ and $\mathcal{I}(E)$---have very different energy dependences of intensity because selection rules and cross-section ratios render $\mathcal{I}(E)$ sensitive only to $2p$ character and ${I}^{\ensuremath{'}}(E)$ far more sensitive to $2s$ character. Taken together, ${I}^{\ensuremath{'}}(E)$ and $\mathcal{I}(E)$ show that the fractional $p$ character in the diamond valence band increases from \ensuremath{\sim} 16% at the bottom of the band to \ensuremath{\sim} 92% at the top, with an average hybridization of $\ensuremath{\sim}{s}^{1.2}{p}^{2.8}$. The spectra agree well with the density of states of Painter et al., but indicate a valence-band width of 24.2(10) eV rather than their 20.8 eV. The $\mathrm{C}(1s)$ binding energy of 284.68(20) eV in graphite agrees well with a recent theoretical estimate of 284.4(3) eV by Davis and Shirley. Analysis of ${I}^{\ensuremath{'}}(E)$ and $\mathcal{I}(E)$ for graphite resolves the valence bands cleanly into $\ensuremath{\sigma}$ and $\ensuremath{\pi}$ bands, with the spectrum ${I}^{\ensuremath{'}}(E)$ of the former resembling that of diamond, but with a stronger $2s$ admixture ($s{p}^{2}$ vs $s{p}^{3}$). The XPS cross section of the $({p}_{z})\ensuremath{\pi}$ bands was very low, as expected by symmetry. The bandwidth of 24(1) eV somewhat exceeded Painter and Ellis's calculated value of 19.3 eV. Glassy carbon showed an ${I}^{\ensuremath{'}}(E)$ between that of diamond and graphite, consistent with an amorphous lattice containing both trigonal and tetrahedral bonds.