ESCA is a widely used technique for studying chemical and electronic structure of organic materials. More precisely, the method is very useful for the study of surfaces and interfaces. In the case of UPS, the photoelectron inelastic mean free path is less than ten Angstroms.
The well known basic equation used in interpreting photoelectron spectra is:
Eb= hv-Ekin^SP (6)
Where EB is the binding energy, hv is the photon energy, ф^ spectrometer specific constant (the work function of the spectrometer). Assuming that due to the removal of an electron
from orbital i the rest of the electron system is not affected (frozen approximation), Eb corresponds to orbital energies – s(i). However, the remaining electrons in the environment can screen the photohole, which induces an additional relaxation contribution and impacts the measured Eb value. Changes in the valence electron density induces small, but significant, shift of the core level binding energy, called chemical shift. Hence, charge transfer and chemical bond formation can be probed using XPS. UPS is used for valence electronic study because the photoionisation cross-section for electrons is orders of magnitude higher in the valence band region for UPS and the photon source (He lamps) has high resolution. The source of photons is either HeI (hv = 21.2 eV) or HeII radiation (hv = 40.8 eV). These energies allow for mapping the valence electronic states of organic materials. The UPS spectra give information about the electronic structure of the material and its work function. It also measures the change A of the work function after coverage (Figure 9).
Fig. 9. Shows the principle of UPS for the study of an interface: a – clean metal, b – metal covered with an organic monolayer.
The UPS spectrum of a clean metal substrate can be seen in Figure 9a. Electrons below the Fermi level are excited by the uv light and emitted into vacuum. The kinetic energy Ekin distribution of the emitted electrons is called the UPS spectrum and reflects the density of the occupied states of the solid.
Only photoelectrons whose kinetic energy is higher than the work function фM of a sample can escape from the surface, consequently фM can be determined by the difference between the photon energy and the width of the spectrum (Figure 9 a). The width of the spectrum is given by the energy separation of the high binding energy cutoff (Ecutoff) and the Fermi energy (Eb = 0):
A change in work function, A, then can be tracked by remeasuring the Ecutoff after deposition of an organic monolayer.
Possible shift of the cutoff and thus of the vacuum level suggests the formation of an interfacial dipole layer A [Crispin, Solar Energy Materials & Solar Cells, 2004; Kugler et al., Chem. Phys. Lett., 1999; Seki, Ito and Ishii, Synthetic Metals, 1997] (Figure 9 b).
In this case the small binding energy onset corresponds to the emission from the highest occupied molecular orbital (HOMO) and the high binding energy (low kinetic energy) cutoff corresponds to the vacuum level at the surface of the organic layer.
Therefore as said above we can visualise the relative position of the energy levels at the interface, and examine the difference of the vacuum level between the metal and organic layer which corresponds to A (Figure 10).
UPS is a very powerful tool to detect the presence-or not – and to measure the interface dipole and therefore to understanding of the energy-level alignment at interfaces organic material/electrode.
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