Photoelectron spectroscopy allows the determination of the elemental composition of a surface. It can distinguish between different elements as well as determine their precise bonding states. E.g. the signal of a carbon atom in a C-C bond differs from that of a carbon atom in a C-O bond.

The method is based on the detection of photoelectrons, emitted from the investigated surface under the illumination of (monochromatic) x-rays. One can distinguish between X-ray photoelectron spectroscopy (XPS, hν > 100 eV) and ultraviolett photoelectron spectroscopy (XPS, hν < 100 eV). The latter can be used to investigate the valence band of surfaces and molecular orbital energies.

The ICAN uses primarily a Versaprobe II^TM by ULVAC-Phi. It offers a small minimal beam size of < 10 µm with a spectral resolution of 0.5 eV. Additionally x-ray induced electron imaging (SXI) allows for the in-situ imaging on the µm scale.

Figure 1: Schematic diagram of the excitation process in photoelectron spectroscopy. A x-ray photon an electron from a core-shell of the atom (here 1s). The photon energy is converted into kinetic energy of the electron, which is emitted into vacuum. Knowing the photon energy and measuring the kinetic energy of the electron then allows for determination of the binding energy of the electron.

The following graph shows the detected X-ray induced secondary electrons (SXI) of a partially laser sintered TiO2 nanoparticle surface.

A laser was fired at the surface at 9 locations, varying the power of the laser. The variation of the laser power leads to a different degree of sintering of the TiO2 particles and thus to a different colour contrast.

Figure 3: An SXI of a laser-sintered TiO2 surface. The sintered surfaces are clearly visible.

With the help of the high spatial resolution of the X-ray beam, it is possible to record 2-dimensional maps of the chemical composition of a surface.

Figure 6 shows an example of this. Iron oxide and silicon nitride particles were applied to the sample as powder and measured. An area of just under 0.075 mm2 is shown. The intensity of the particles is colour-coded. The red areas show the iron oxide, while the blue areas represent silicon nitride.

Figure 6: Chemical map of Fe2O3 and SiN nanoparticles. The red coloured areas consist only of Fe2O3, the blue ones of SiN.

With the UPS it is possible to determine the work function of a surface. This is especially possible for metallic samples that have a non-vanishing electronic density of states at the Fermi energy.

For this purpose, the width of the spectrum, i.e. the energetic distance between the insertion and Fermi edges, must be determined as precisely as possible. The work function of the surface to be examined then results from the difference between the photon energy and the width of the spectrum. In the example in Figure 8, a work function of 4.4 eV results for a 50 nm thick silver film on glass.

Figure 8: Determination of the work function of a 50 nm thick silver film on glass. The work function results from the difference between the photon energy and the width of the spectrum between the input and Fermi edges.