3.1 Mixing Elements
49
No vacancies
E F
Surface vacancies
occupied ←
Surface & bulk
vacancies
Fig. 3.12 DOSs near the Fermi level for TaCx without vacancies, with vacancies only at the surface,
and with vacancies both in the bulk and at the surface [13]
Figure 3.17a [9] schematically shows the DOS of metal- or carbon-derived electrons
near the Fermi level for 4d transition metal carbides, where the position of the Fermi
level is shown by broken lines for individual carbides. The intensity ratio among the
peaks marked (a), (b), and (c) in the figure varies with the number of d-electrons;
however, the figure indicates the position of the Fermi level relative to the electron
density of metals and carbon in transition metals. Here, electrons from carbon are
not involved in the DOS at the Fermi level of ZrC, NbC, MoC, TcC, and AgC.
For RuC, RhC, and PdC, vacancy introduction is speculated to affect the surface
term of the work function. A similar schematic DOS for 5d transition metal carbides
is shown in Fig. 3.17b [9]. Because there is no carbon-derived DOS at the Fermi
level for HfC and TaC, a vacancy will not affect the surface term, and the change
in the work function is determined by the change in the Fermi level. On the other