Barre 22-23 – Salle 317
Mathieu Frégnaux – Institut Lavoisier de Versailles – UVSQ
Photoelectron spectroscopy (PES) is an analytical method that uses photons (commonly ultraviolet and X-ray) to study the properties of matter by measuring the orbital energies of electrons. Relying on the photoelectric effect PES is a very precise and accurate technique for measuring these electron energies. It is used to infer surface information about materials. X-ray photoelectron spectroscopy (XPS) focuses on core electrons and informs on surface elemental composition and chemical environments (bonds, oxidation states). Whereas ultraviolet photoelectron spectroscopy (UPS) is used in solid state and condensed matter physics to measure the electron density of states for organic and inorganic materials. Their complementarity provides information on both the chemistry and physics of the analyzed samples.
In this presentation, I will talk about current challenges for PES and its associated techniques: XPS mapping, PES on nanomaterials (nanopowder), in situ PES and operando PES. Based on concrete cases from CEFS2 briefly described below, I will discuss solutions that can be proposed at the laboratory scale and at synchrotron facilities.
Major PES constraints are associated with ultrahigh vacuum, low analysis depth (≤ 10 nm) and lateral resolution imposed by the X-ray beams above tens of microns. Nevertheless, I will show how recent evolution in spot size and mathematical processes (Principal Component Analysis) makes XPS imaging compatible with the characterization of micrometric patterns. Indeed, I will develop examples based on 2D materials or transparent conductive oxides (TCO) which are well-adapted for such acquisition mode.
Current developments, such as NAP-XPS (Near Ambient Pressure) overcomes the UHV constraints and allows to analyze the reactivity of surfaces under gas exposure. We conducted experiments in NAP conditions at synchrotron SOLEIL (TEMPO beamline) to study the top surface evolution of a particular Sr based perovskite oxide under oxidizing or reducing atmosphere. In situ monitoring was performed in the chamber up to 5 mbar in order to exhibit the strong evolution of the first nanometers of SrVO3 substrates. The use of various wavelengths from 100 to 700 eV above core levels allowed to estimate the thickness of the modified region.
Similarly, PES appears particularly suitable for nanomaterial characterization since the escape depth of photogenerated electron in the matter is in the nanometer range. However, we demonstrated that several surface analysis tools, such as REELS, XPS and UPS might be helpful to get insights about substrate affinity, conductivity (band diagram reconstruction) and surface termination (chemical bonds) of diamond nanoparticles.
In the characterization communities, “operando” measurements represent a hot topic. Such approaches cover the modifications of a sample subjected to an external stimulus (optical, electrical, magnetic…). The development of experimental PES setups allowing the study of materials under stimulus remains a major challenge. In this context, we were recently able to apply a bias voltage on a perovskite solar cell (lateral heterojunction configuration) in the XPS chamber. This study gives strong insights to highlight the potential ion migration which is known to deteriorate the device performance and stability.