Platform for photoelectron spectroscopy

X-ray photoelectron spectroscopy (XPS) involves irradiating with X-rays a sample placed in a vacuum . The X-rays cause electrons to be stripped from atoms in the surface layer of the sample (less than 10 nm ). These electrons can come from all the electron layers, including the core layers. An analyser classifies them based on their kinetic energy. By knowing the energy of the incoming X-rays and measuring the kinetic energy of the emitted electrons, we can calculate their binding energy. This allows us to identify the chemical elements from which the electrons originated.

XPS is therefore a technique for elemental surface analysis, with a detection limit of approximately 0.1 atomic percent.

XPS can be used for:

• Qualitative elemental analysis (all elements except hydrogen and helium can be detected);
• Quantitative elemental depth profiling (typically from 1 to 50 nm) by ion sputtering of the surface;
• Quantitative analysis through spectral simulation;
• Determining the chemical states of the detected elements (e.g., types of chemical bonds, oxide-to-metal ratios, etc.).

UPS:

• Ultraviolet photoelectron spectroscopy (UPS) operates similarly to X-ray photoelectron spectroscopy (XPS), with the key difference being that the sample—always placed under ultrahigh vacuum—is irradiated with ultraviolet radiation instead of X-rays. As a result, only electrons from the outer electronic shells (valence electrons) are emitted. UPS is therefore specifically suited for studying valence band structures and measuring the surface work function.

LEIS :

• In addition to XPS and UPS techniques, there is also a method known as slow ion backscattering, or Low Energy Ion Scattering Spectroscopy (LEIS). This technique enables elemental analysis of the outermost surface layer, with a depth resolution on the order of a single atomic layer. Its main advantage is its exceptional surface sensitivity, with a detection limit around 10¹² atoms/cm². In LEIS, the sample is bombarded with a monoenergetic ion beam—typically He⁺ or Ne⁺—at low kinetic energies (0.5 to 3 keV). A small fraction of the incident ions are backscattered upon colliding with surface atoms, and their kinetic energy depends on the mass of the target atoms. From this energy, the mass—and thus the identity—of the surface atoms can be inferred. While quantitative analysis using LEIS is possible, it is not straightforward and usually requires the use of reference standards.

Both XPS and UPS require ultrahigh vacuum conditions, meaning that samples must be solid materials compatible with such environments. These can include nanostructured materials, powders, or species deposited on solid substrates. In most cases, minimal sample preparation is required.

Complex Sample Preparation:

The PSASI platform also offers advanced sample preparation services. For example, treatments can be carried out under controlled atmosphere, temperature, and gas pressure using specially designed cells coupled to the characterization instruments. The samples are then transferred to the analysis chamber without exposure to ambient air, preventing contamination. These so-called quasi in situ experiments are especially valuable for studying changes in solids after specific treatments.

The surface analysis platform uses three ultrahigh vacuum instruments, each equipped with complementary scientific techniques:

1. ThermoVG Multilab ESCA 3000: This system is equipped with a dual X-ray source (Mg or Al) and is dedicated to XPS analysis for processing large batches of samples (up to six samples can be introduced simultaneously). It also includes an ion gun for atomic-scale sputtering, enabling depth profiling of thin films or nanometric layers. Additionally, the system is equipped with a furnace that allows in situ treatments at atmospheric pressure up to 600°C.

1. VSW 5000: This system is equipped with a dual X-ray source (Mg/Al) as well as a monochromatic Al source for high-resolution measurements. It allows for angle-resolved measurements (maximum φ = 80°) and heating treatment at a temperature range from -150°C to 1000°C. The system also includes a UV source for UPS, providing two helium resonance lines (hν = 21.2 eV for He I and 40.8 eV for He II). An ion gun is also included, enabling both depth profiling by ion sputtering (Ar) of the first atomic layers and characterization of the outermost surface using Low Energy Ion Scattering (LEIS).

For in situ treatments, the system is equipped with a controlled-pressure reactor (pressures ranging from 10⁻⁴ mbar to 1 bar), connected to a mass spectrometer capable of operating at high temperatures (up to Tmax = 600°C). Finally, it is also possible to apply an electric potential to the sample for electrochemical measurements.

ThermoVG Microtech: Equipped for both XPS and UPS, this system allows sample cooling down to -170°C and heating up to 1200°C. It is coupled with a differentially pumped mass spectrometer and a gas dosing system for thermal desorption spectroscopy (TDS) studies. Finally, it also includes a source for metal deposition.