X-ray Photoemission Spectroscopy Service
Figure 1. Photoemission setup of the group Monney at the University of Fribourg
Figure 2. Temperature dependent XPS spectra of the 4f7/2 core level of 1T-TaS2
Our photoemission setup is a machine in constant development, as a tool for fundamental research. However, our team can do XPS as a service to the community, be it at the Faculty of Science and Medicine of Fribourg or outside. We typically do this on a collaborative basis, or as remunerated service.
i. UHV: XPS requires an ultra-high vacuum (UHV) environment i.e. a pressure during the measurements below 10-10 mbar. The sample studied must be compatible with this constraining environment, i.e. during its introduction under vacuum it must not pollute the photoemission chamber. It is therefore necessary to pay attention to its composition. To control this, we have an introduction chamber that can reach low 10-8 mbar in a few hours (Figure 1). The test sample can be introduced beforehand to see if it will be possible to study it in photoemission.
ii. Dimensions: Our X-ray tube produces a typical beam of about (1 x 3) mm2 at 1486.6 eV : it is therefore preferable to have a sample of sufficient size to take advantage of the entire beam. This allows to optimize the acquisition time. Optimal size: a surface between 1 and 10 mm2. For technical reasons (focus to the analyzer), the thickness must be between 1 and 5 mm.
iii. What we can learn: With our setup, it is possible to resolve fine details of the core levels (see Figure 2) with a resolution of the order of 300 meV. With XPS, it is possible to perform a "basic" surface chemical analysis of a material but also to better understand the nature of the chemical bonds specific to each atoms, their chemical order or to obtain information on the atomic structure of the solid of interest. In our case, an advanced quantitative chemical analysis (< 5%) is very complicated because it generally requires measurements on reference substrates as well as a good knowledge of the atomic structure of the studied material.
iv. Time constraints: it takes about 30 to 60 min to realize a large spectrum allowing a qualitative overview of the chemical composition of a sample. To make an accurate measurement (with the best achievable energy resolution) on a core level of interest, it will take about 1h depending on the effective cross section of the chemical element considered.
To study a sample in detail, it takes about ten hours of measurements i.e. a complete day of measurements.