Analysis Features

Reflected Electron Energy Loss Spectroscopy

Reflected Electron Energy Loss Spectroscopy

Reflection Electron Energy Loss Spectroscopy (REELS) involves firing a beam of electrons at the surface and measuring the kinetic energy of the scattered electrons. It provides valence electronic information from a similar range of depths as XPS (~0-10nm) and in some cases can also detect and quantify hydrogen, which XPS cannot. REELS is ideal for analysing metal oxides, semiconductor films or organic materials with conjugated bonding configurations.

Electronic information

In REELS a beam of energetic electrons (typically 1 keV) is scattered from the surface. Most of those electrons reflect directly from the surface without losing any energy and are detected at a kinetic energy which is the same as the beam energy. These elastically scattered electrons form the most intense part of the REELS spectrum, which defines the 0eV position on the energy loss scale. Some electrons do interact with the surface and give up some of their energy, to promote electrons from occupied valence levels to unoccupied conduction band states, for example. Because the sample has a finite band gap, the incoming electrons cannot give up just any amount of energy to the surface; they must give up at least the energy equivalent of the band gap, very useful information for semiconductor samples. On the REELS spectrum, this effect appears at a gap between the peak due to elastically scattered electrons and the structure due to inelastically scattered electrons. Gaining information on the unoccupied states and the band gap makes REELS data a complement to valence band data from XPS and UPS.

REELS data from PFO and polystyrene

For materials with conjugated carbon bonding systems, XPS and REELS provide interesting complementary information. In the example shown here, the OLED material poly(9,9-di-n-octylfluorenyl-2,7-diyl), more simply known as PFO, was analysed with REELS. The material is composed only of carbon and hydrogen. The XPS spectrum shows the core-level peak associated with ionization from the C1s level, but closer inspection of the spectrum reveals two very weak peaks to higher binding energy. These peaks are due to the main C1s photoelectron giving up some energy to promote an electronic transition from the highest occupied molecular orbitals (HOMO) in PFO into the lowest unoccupied molecular orbitals (LUMO). The HOMO and LUMO have pi and pi* character respectively.

In the REELS spectrum from the same sample, the same two peaks can be identified but they are much stronger and sharper. The shift of these peaks on the energy loss scale relative to the elastic peak gives the analyst a direct measure of relative energies of the HOMO and LUMO levels and so REELS can be used to build energy level diagrams in materials such as OLEDs. As an interesting comparison, because polystyrene has a completely different bonding structure compared to PFO, we can see that the pi-pi* structure in a REELS spectrum is also completely different.

Hydrogen detection

Another interesting capability of REELS is its ability to detect and quantify hydrogen. When analysing a polymer, for example, XPS could be used to detect and quantify atoms such as carbon, oxygen or nitrogen but it cannot see the hydrogen. A combined XPS/REELS analysis, therefore, would enable a more complete compositional analysis of the polymer. 1keV REELS was used to analyse a range of polymers. For polymers with conjugated carbon bonding, such as PET and polystyrene (PS), the pi-pi* shake-up features are seen, giving the HOMO-LUMO electronic information previously discussed above the PFO. Approximately 1.8eV shifted from the primary elastic peak, most of the polymers also show a small peak due to hydrogen. (The only spectrum without this hydrogenic peak was acquired from PTFE, which has no hydrogen.)

REELS spectra from polymers with different H content

The origin of the hydrogen peak is analogous to the ISS process, but instead of a noble gas ion scattering from a heavier element, an electron is scattering from relatively low mass hydrogen. The primary elastic peak in the polymer analysis is composed of electrons which has have scattered elastically from atoms such as carbon, oxygen or nitrogen. When the electrons strike the hydrogen atoms, however, because the hydrogen is so much lighter, the hydrogen recoils and a process similar to ISS takes place. The energy of the electrons scattered from the hydrogen is therefore shifted from those electrons scattered from the other atoms. The relative intensities of the hydrogen peak and the primary elastic peak is proportional to the relative amounts of hydrogen and other atoms, such as carbon, oxygen and nitrogen. A simple peak deconvolution of the two peaks can therefore be used to quantify the hydrogen.

Typical Uses

  • Batteries & fuel cells
  • Nanomaterials
  • Plasma-treatments
  • Catalysts
  • Ceramics
  • OLEDs
  • Semiconductor materials