High Energy Monochromatic X-Ray Source for XPS

Overlapping photoelectron lines and Auger features are a common issue in the analysis of complex materials. The standard approach of switching to an achromatic Mg X-ray source may become undesirable due to the introduction of X-ray satellites into the spectrum.

This article shows how a satellite free spectrum is excited using Kratos’ automatic monochromatic Ag Lα X-ray source to unambiguously determine the oxidation state of a Ni/Co/Mn alloy.

Introduction

XPS is commonly employed for the analysis of complex materials, enabling the quantitative determination of surface chemical species. Surface analysis of metal compounds and alloys is a key application area. A useful feature of XPS analysis of these materials is the potential to study the oxidation state of the metals and quantify the surface species. Unfortunately this type of analysis is often compromised by the presence of overlapping photoelectron lines and Auger features in the spectrum, a relatively common occurrence when the alloy has several different metallic components.

The XPS analysis of a Ni/Co/Mn alloy is one such example where this issue occurs. Figures 1 and 2 display the Al Kα monochromatic high resolution Co 2p and Mn 2p spectra. Unfortunately, the strong overlapping of Co and Ni Auger lines with the photoelectron peaks complicates these spectra. The presence of Auger features limits accurate quantification of the surface composition. Moreover, their presence may cause an erroneous assignment of the Co and Mn surface oxidation state.

Figure 1. Co 2p Spectrum recorded using the Al monochromatic X-ray source.

Figure 2. Mn 2p Spectrum recorded using the Al monochromatic X-ray source.

Switching to a different X-ray source is the standard approach to alleviate the problem of overlapping Auger features and photoelectron lines. Many instruments are equipped with achromatic Al and Mg dual anode X-ray sources as well as with the Al monochromatic source. Switching from the Al monochromatic X-ray source (1486.6 eV) to the Mg achromatic X-ray source (1253.3 eV) is a simple matter, thereby moving the Auger lines 233 eV relative to the photoelectron lines. However, the introduction of X-ray satellites, which are intrinsic to all achromatic sources, is a major downside of this approach.

Mg Kα₃ and Mg Kα₄ X-ray satellites are respectively 8.4 and 10.1 eV lower in binding energy compared to Mg Kα_1,2. Collectively, they generate a peak with approximately 12% of the intensity of the Mg Kα_1,2 peak. However, this magnitude of splitting means that Mg Kα3,4 satellites from many 2p_1/2 lines overlay with 2p3/2 lines. For instance, the Mn 2p splitting is 11.7 eV which is low enough to place the Mg Kα_3,4 satellites from the Mn 2p_1/2 line under the Mn 2p_3/2 peak envelope.

Experiment

To address these problems, Kratos’ Ag monochromatic X-ray anode can be used to record the spectra. Monochromated Ag Lα X-rays are generated at 2984.2 eV using this source. Figure 3 shows the Ag 3d_5/2 line recorded using Al and Ag X-rays. Figures 4 and 5 show the Co 2p and Mn 2p spectra recorded using the Ag monochromatic X-ray source.

Figure 3. Comparison of the Ag 3d_3/2 spectrum recorded using the Al (left) and Ag (right) monochromatic X-ray source.

Figure 4. Co 2p Spectrum recorded using the Ag monochromatic X-ray source.

Due to the displacement of Auger lines, these spectra are considerably less complex when excited by Ag Lα X-rays. This is shown by direct side by side comparison of the Mn 2p spectra recorded with Al and Ag monochromatic sources (Figure 6), but without the introduction of X-ray satellites. The removal of Auger lines results in easily interpretable spectra.

Figure 5. Mn 2p Spectrum recorded using the Ag monochromatic X-ray source.

Figure 6. Comparison of the Mn 2p Spectrum recorded using the Al (left) and Ag (right) monochromatic X-ray source.

The Mn 2p spectrum now clearly contains a single chemical component with a Mn 2p3/2 peak position of 641.0 eV – a characteristic of MnO with Mn in the +2 oxidation state. Analysis of the Ag mono excited Co 2p spectrum also shows the presence of a single Co chemical component with a Co 2p3/2 peak position of 780.5 eV – a characteristic of CoO with Co also in the +2 oxidation state.

The motorized X-ray source is equipped with an anode which is 1/3 coated with Ag and 2/3 coated with Al. When switching from Al Kα to Ag Lα, the anode is moved by motors relative to the filament, enabling the generation of X-rays from the Ag portion of the anode.

As previously mentioned, Ag Lα X-rays have an energy of 2984.2 eV which corresponds to a wavelength of 4.1544 Å. Since this wavelength is approximately half the wavelength of Al Kα X-rays, the same quartz crystal employed to monochromate Al Kα X-rays may also be employed for Ag Lα via a 2nd order diffraction. Appropriate adjustment of the position of the anode and X-ray mirror is fully automated when swapping between the different excitation sources. The AXIS Supra's motorized X-ray mirror is automatically driven to the optimum position for the Ag Lα X-rays by the Escape data system.

Figure 7. Additional core levels excited using Ag Lα X-ray lines which are not accessible using Al Kα.

Conclusion

In summary, changing to Ag monochromatic X-rays may greatly simplify the complex photoelectron spectra recorded from the Ni/Co/Mn alloy surface relative to that using Al monochromatic excitation. The spectra are recorded without the introduction of unwanted X-ray satellite lines generated by using achromatic sources. Using the recorded spectra, the surface chemistry of the alloy is now correctly interpreted. For routine analysis, an easy-to-use additional monochromated X-ray line is provided by the dual Al/Ag X-ray source. This higher energy line is capable of exciting core levels that cannot be achieved with Al, such as Si and Al 1s (see Figure 9 for more examples), or, as shown in this example, applied to simplify spectra for surface species to be easily quantified.

This information has been sourced, reviewed and adapted from materials provided by Kratos Analytical, Ltd.

For more information on this source, please visit Kratos Analytical, Ltd.