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

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 $K\alpha$ radiation, are reported and discussed. By comparing valence-band and carbon-$1s$ photoelectron kinetic energies, the XPS valence-band spectra $I^{\prime }\mathrm{}\left(E\right)\mathrm{}$ of diamond and graphite were rigorously affixed to the same energy scale as earlier $K$ x-ray emission spectra $\mathcal{I}\mathrm{}\left(E\right)\mathrm{}$. The two spectra—$I^{\prime }\mathrm{}\left(E\right)\mathrm{}$ and $\mathcal{I}\mathrm{}\left(E\right)\mathrm{}$—have very different energy dependences of intensity because selection rules and cross-section ratios render $\mathcal{I}\mathrm{}\left(E\right)\mathrm{}$ sensitive only to $2p$ character and $I^{\prime }\mathrm{}\left(E\right)\mathrm{}$ far more sensitive to $2s$ character. Taken together, $I^{\prime }\mathrm{}\left(E\right)\mathrm{}$ and $\mathcal{I}\mathrm{}\left(E\right)\mathrm{}$ show that the fractional $p$ character in the diamond valence band increases from ∼ 16% at the bottom of the band to ∼ 92% at the top, with an average hybridization of $\sim s^{1.2}{p}^{2.8}$. 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 $\mathrm{C}\mathrm{}\left(1s\right)\mathrm{}$ 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 $I^{\prime }\mathrm{}\left(E\right)\mathrm{}$ and $\mathcal{I}\mathrm{}\left(E\right)\mathrm{}$ for graphite resolves the valence bands cleanly into $\sigma$ and $\pi$ bands, with the spectrum $I^{\prime }\mathrm{}\left(E\right)\mathrm{}$ 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 $\left(p_z\right)\pi$ 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 $I^{\prime }\mathrm{}\left(E\right)\mathrm{}$ between that of diamond and graphite, consistent with an amorphous lattice containing both trigonal and tetrahedral bonds.