Photoelectron spectroscopy
What is photoelectron spectroscopy?
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 IITM 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.
Chemical analysis
The kinetic energy of the emitted electron depends on the element from which it originates, as well as from the precise bonding state of that atom. Picking up the carbon example from above, an electron from a carbon 1s shell has ~3eV kinetic energy less, if the atom is in a C=O double bon, rather than a in a C-C single bond.
In figure 2 the Au 4f signal of a thin layer of gold nanoparticles is shown. A precise analysis yields, that roughly 6% of the gold atoms in the particles are oxidized, either as Au1+ or Au3+.
Figure 2: Intensity of the Au 4f peak of a thin gold nanoparticle layer. Small amounts of the gold is oxidized, which is why to additional peaks, representing the Au1+ (orange) and Au3+ (cyan) states are needed to describe the data.
Illustrations using SXI
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.
Depth profiles and sputtering
With the existing system, it is possible to remove the surface of the sample to be examined by sputtering with argon ions.
On the one hand, this serves to clean the surface, as shown in Figure 4. By sputtering step by step, the antimony oxide layer that has formed on the surface of the sample is removed, leaving a pure, unoxidised antimony layer.
Furthermore, depth profiles can be recorded. After each sputtering step, the XPS intensity of the signal of the elements under investigation is determined. If the sputtering rate is known, the change in element concentration with increasing sample depth can be determined. An example is shown in Figure 5. It shows the intensity of Bi,Sb and Te of a BiSbTe3 layer as a function of the sputtering time. In this case, it can be seen that the concentration of tellurium and antimony decreases slightly with increasing depth.
Figure 4: Change of the Sb 3d signal with an oxidised Sb surface and stepwise cleaning by sputtering with Ar ions.
Figure 5: Depth profile of the intensity of the Sb, Te and Bi signal of a BiSbTe3 surface. By gradually removing the surface and measuring the signal intensity, a change in the composition of the layer can be detected.
2D chemical maps
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.
Surface and Quantum Well States
The high spectral resolution of He I light, depending on the system under investigation, combined with the smallest possible acceptance angle, enables the measurement of surface and quantum well states.
Figure 7 shows the monolayer dependence of these states of a crystalline magnesium layer on silicon. Depending on the layer thickness, one or more quantum well states and the increasingly sharp surface state can be seen.
Figure 7: UPS measurement of the surface state and quantum well states of a thin crystalline magnesium layer.
Determination of the work function
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.
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