X-ray Photoemission Spectroscopy (XPS)
X-ray photoemission spectroscopy is a photon in electron out technique that provides elemental composition and chemical analysis of the surface of a sample, by analyzing the kinetic energy (KE) of ejected electrons. Using x-rays of well-defined energy (hν, in our case 1486.7 eV) and due to energy conservation the KE energy of the electrons is related to their binding energy (BE) in the material as KE=hν-BE.
To a first order, the BE of a photoelectrons is related to its origin in the material and depends on the element and on the electronic level from which it originates as shown in Figure 1. As photoemission cross sections are well-know, XPS can provide elemental composition.
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Moreover, the BE energy is slightly modified by the local chemical environment of the element. This BE shift, can help determining the chemical environment in the sample as can be seen in the C 1s spectrum of Figure 2.
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In our systems, XPS can also be used to do chemical mapping, with which not only surface composition but also the nature of the chemical bonds can be obtained as illustrated in Figure 3.
Figure 1
Figure 2
Figure 3
Ultra-Violet Photoemission Spectroscopy (UPS)
Ultra-violet photoemission spectroscopy uses photons of lower energy (typically 21.2 eV and 40.8 eV) to specifically probe the valence band of a material, but with a much better resolution than that provided by the x-ray source. An example of UPS valence band measured on Cu(100) is shown in Figure 4.
Figure 4
Figure 5
Additionally, surface work functions can be measured by applying a bias on the sample and measuring the energy width (W) of all ejected photoelectrons. The workfunction (WF) for a metal is simply: WF=hν-W as illustrated in Figure 5.
Reflected Electron Energy Loss Spectroscopy (REELS)
Reflected Electron Energy Loss Spectroscopy (REELS) is an electron in electron out spectroscopy technique that measures the loss of energy of inelastically scattered electrons.
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Typically the first energy loss is related to the first optical loss. In the case of a titanium dioxide surface, as shown in Figure 6, the first loss found ~3.0 eV below the elastic peak (at 0 eV), is related to the optical band gap of the material.
Figure 6
Angle-Resolved X-ray Photoemission Spectroscopy (AR-XPS)
Angle-Resolved X-ray Photoemission Spectroscopy (ARXPS) is a non destructive depth profiling method that enables an elemental and chemical mapping as a function of depth below the surface. This method can probe up to ~6 nm below the sample surface.
Photoelectrons exiting the sample have a limited escape-depth that is mainly dependent on their kinetic energy. Electrons exiting at normal emergence are the most bulk sensitive, while electrons escaping at grazing angles are mostly surface sensitive. In the case of the thin film A on a bulk sample B, as shown in Figure 7, comparing the relative intensity of bulk and surface contributions can help quantifying the thickness of the thin film A.
Figure 7
Sputter Depth Profiling
Sputter depth profiling is a destructive method that enables an elemental mapping as a function of depth, several microns below the surface of a sample. It consists in performing elemental analysis while sputtering layer by layer the surface of the sample using a beam of Argon ions, typically with energies of a few KeV. With proper yield calibration, the thickness of layers can be obtained as shown in Figure 8.
Figure 8