Determination of the occupancy of valence bands in graphite, diamond and less-ordered carbons by X-ray photo-electron spectroscopy

Abstract

The profile of the occupied parts of the valence-band of elemental carbon in a number of distinct forms has been estimated by measuring the intensity of the photo-electrons emitted as a result of AlK_α and MgK_α irradiation. The widths of the bands for graphite (31 ± 2 eV), for diamond (33±3 eV), and for the less-ordered forms (carbonized poly-vinylidene chloride cellulose char, carbon fibre (PAN), vitreous carbon, powdered nuclear-grade graphite) are larger than expected theoretically, and on the basis of previous soft X-ray spectroscopic measurements. In line with recent views on the consequences of “amorphization”, it is established that the major part of the density-of-states curve is largely unaltered in going from the crystalline to the disordered solid. It is also established that the valence band of coronene is very similar to that of graphite. Our measurements of the binding energies of core electrons yield two main conclusions: (i) the C_1s binding energies in diamond and graphite are very similar, the respective values being 284.0 ± 0.3 and 284.3 ± 0.5 eV; (ii) chemisorption of oxygen at prismatic (but not at basal) faces of single-crystals of graphite is readily detectable from the O_1s peak, the width of which indicates that at least two types of chemical linkages exist for the bound oxygen.