- XPS Theory
Typically, spectral peaks from photelectron emission exhibit a symmetrical lineshape, representing a range of energies characterised by the full-width at half maximum. For example, the oxygen peak in figure 1.
Figure 1: (Left) Symmetric O 1s XPS peak and (Right) Asymmetric Ni 2p XPS peak
Metals and other conductive materials such as graphene, however, may in fact produce asymmetric peaks (such as the nickel peak in figure 1). This is the result of final state effects within the metal, caused by the conduction band.
When we think about metals/conductive materials we think of the energy levels as bands. The valence band (equivalent to the HOMO) in nickel consists of the 4s and 3d electrons. The LUMO is the conduction band, a continuum of unfilled molecular energy levels (figure 2).
Figure 2: Nickel valence energy levels and conduction band
Figure 3: Shake-up processes in conducting material
We have discussed shake-up peaks in another section, wherein a photoemission can excite the resulting ion to an energy state slightly above the ground state, typically a few eV. When our LUMO is a conduction band, however we have a continuum of possible shake-up structures which may leave the ion in a variety of states (figure 3).
The result of this is that the final peak possesses an asymmetric character, due to a multitude of final states each having lost a different amount of kinetic energy (and hence increasing effective binding energy of the photoemission).
The degree of asymmetry depends heavily on the density of states (DOS) near the Fermi level. High DOS means there is more of a continuum for shake-up processes.
Figure 4: Comparison of asymmetry of Pt vs Au and DOS near the Fermi level
This is best visualised by looking across the first row of the transition metal series – as we fill the d-band we see more and more symmetry in our resulting photoemission peaks.
Figure 5: Asymmetry and Fermi DOS in first row TMs.
For more on asymmetry – see part 5 of our fundamentals of XPS course!