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📚 X-ray Spectroscopy: Tuning Surface Sensitivity in NEXAFS

🎯 Introduction to NEXAFS and Surface Sensitivity

Near-Edge X-ray Absorption Fine Structure (NEXAFS) is a powerful spectroscopic technique used to investigate the electronic and structural properties of materials. A critical aspect of NEXAFS is the ability to tune its surface sensitivity, which means controlling the depth from which information is collected from the sample. This control is achieved through various electron yield (EY) detection modes.

📈 Electron Yield (EY) Detection Modes in NEXAFS

Different electron yield detection modes allow for varying degrees of surface sensitivity by collecting different types of emitted electrons.

1. Total Electron Yield (TEY) 📊

• Detection Principle: All electrons emerging from the sample surface are detected.
• Electron Type: Includes low-kinetic energy secondary electrons, which have a longer inelastic mean free path (IMFP).
• Sampling Depth: Approximately 10 nanometers (nm).
• Application: Suitable for studying general surface and bulk properties. It provides a more averaged signal from a thicker layer.

2. Partial Electron Yield (PEY) 📊

• Detection Principle: Surface sensitivity is enhanced by suppressing lower kinetic energy electrons.
• Mechanism: A retarding potential is applied to filter out electrons with kinetic energies below a certain threshold.
• Signal Quality: Offers a better signal-to-background ratio compared to TEY, especially for investigating adsorbates on surfaces.
• Sampling Depth: Intermediate, typically around 2 to 6 nm.

3. Auger Electron Yield (AEY) 📊

• Detection Principle: Only elastically scattered Auger electrons are recorded.
• Surface Sensitivity: Provides the best surface sensitivity among the three detection techniques.
• Requirement: Necessitates an electron energy analyzer for selective detection.
• Sampling Depth: Shortest, approximately 1 nm, making it ideal for probing the topmost surface layers.

💡 Comparison of Sampling Depths

| Detection Mode | Sampling Depth (Approx.) | Surface Sensitivity | | :------------- | :----------------------- | :------------------ | | TEY | 10 nm | Lower | | PEY | 2-6 nm | Intermediate | | AEY | 1 nm | Highest |

🛠️ Mechanism of Partial Electron Yield (PEY) Detection: Micro-Channel Plate (MCP) Detectors

Micro-channel plate (MCP) detectors are key components used for PEY detection.

Key Components:

• High Transmission Metal Grids ('Mesh'): Used for electron retardation.
• Micro-Channel-Plate Assembly: For electron multiplication.
• Collector: Gathers the amplified electron signal.

1️⃣ Operation Process:

1. Grounding and Retardation: The first mesh is typically grounded. The second mesh operates at a retarding voltage (-Uret).
2. Electron Rejection: This -Uret voltage rejects all electrons with kinetic energies less than Uret.
   • 💡 Tip: Uret is usually set 100-120 eV lower than the energy of the corresponding Auger peaks.
3. Electron Passage and Acceleration: Electrons with energies above Uret pass through these two meshes. They are then accelerated towards the channel-plate assembly by a small positive voltage applied to a contact just before the uppermost channel plate.
4. Signal Amplification: The electron signal is amplified by the channel-plate arrangement through the application of a voltage on a contact just behind the bottom-most channel plate.
5. Signal Collection: The amplified electron output from the MCP is collected by a collector, which is connected to a floating battery box for current measurement.
   • ✅ This precise control allows for filtering low-energy secondary electrons, thereby enhancing surface sensitivity.

⚠️ Importance of Vacuum Conditions in NEXAFS Experiments

Operating under (ultra) high-vacuum (UHV) conditions is fundamental for NEXAFS experiments.

Reasons for UHV:

1. Photons In (Incident Soft X-ray Beam):
   • Soft X-rays are easily absorbed by atmospheric gases.
   • UHV prevents absorption of the incident soft X-ray beam by air, ensuring the beam reaches the sample without significant loss.
2. Electrons Out (Emitted Electrons):
   • Electrons emitted from the sample surface can lose energy or scatter upon collision with gas molecules.
   • UHV preserves the emitted electrons, preventing energy loss and scattering, which would otherwise reduce detection efficiency and cause spectral distortions.

🧪 Multi-Technique Endstations

NEXAFS measurement setups are often integrated with complementary surface analytical techniques to provide a comprehensive characterization of sample surfaces.

Integrated Techniques:

• X-ray Photoelectron Spectroscopy (XPS): Provides elemental composition, chemical states, and electronic structure.
• Ultraviolet Photoelectron Spectroscopy (UPS): Probes valence band electronic structure.
• Low Energy Electron Diffraction (LEED): Determines surface crystallography and long-range order.

Examples of Endstations:

• HESGM beamline @ BESSY II (Berlin): An example of an integrated XPS/NEXAFS endstation.
• FlexPES beamline @ MAX-IV (Lund): Another example of a combined XPS/NEXAFS endstation.

Typical Components of an Endstation:

1. Load Lock Chambers: For sample introduction without breaking the main chamber vacuum.
2. Preparation Chamber: Equipped with evaporators and a LEED system for sample preparation and characterization.
3. Analysis Chamber: Contains XPS/UPS electron analyzers and the NEXAFS detection setup.
4. Distribution Chamber with Park-Station: For storing samples under UHV conditions for extended periods.

🚀 Advanced Techniques: Ambient Pressure NEXAFS

Traditional vacuum conditions pose challenges for in situ and operando experiments, which require samples to be studied under realistic working environments (e.g., in gas atmospheres, liquids, or at high pressures).

Challenges with Vacuum:

• Incompatibility with 'fantastic' sample environment equipment designed for in situ and operando XAS experiments.

Key Enabling Technology: Si3N4 Membranes

• Innovation: Fabrication of extremely thin silicon nitride (Si3N4) membranes (approx. 150 nm thick).
• Function: These membranes act as windows in NEXAFS cells, allowing X-rays to pass through while maintaining an ambient pressure environment for the sample.

APE-HE Cell (Ambient Pressure Experiment - High Energy) @ Elettra

• Design: The cell features two independent electrical contacts: one on the sample and one on the membrane.
• Operation:
  1. The membrane is polarized with a positive bias voltage.
  2. The TEY signal from the sample is recorded by measuring the drain current.
  3. This allows for recording XAS spectra of the sample while it is kept at ambient pressure (up to 1 bar) and exposed to a desired gas or gas mixture.
• Temperature Control: Typically controllable within the RT-350°C range.
• Similar Setups: Also found at ISISS @ BESSY II and BL 9.3.2 @ APS.

🔬 Practical Application: In Situ Ambient Pressure NEXAFS

Example: Surface Redox Chemistry in Co3O4 Coatings

• Study Goal: To investigate the surface redox chemistry of Co3O4 coatings, composed of ~10 nm nanoparticles, prepared by pulsed laser deposition.
• Experimental Setup: Ambient pressure NEXAFS (using an APE-HE cell) was employed.
• Procedure:
  1. Thin film coatings were pretreated in either Helium (He) or Hydrogen (H2) up to 150°C.
  2. Reactivity towards Carbon Monoxide (CO) and Oxygen (O2) was monitored by NEXAFS during subsequent gas exposures.
• Key Findings:
  • Samples pretreated in He showed reactivity only at high temperatures.
  • Samples pretreated in H2 were reactive even at room temperature (RT).
• Significance: These in situ experiments provided critical insights into how pretreatment significantly affects the material's behavior towards reactive gases. The Co L3-edge in situ experimental results were also supported by theoretical simulations, enhancing the reliability of the findings.
• ✅ Such studies are invaluable for understanding surface reaction mechanisms in fields like catalyst development, energy storage, and sensor technologies, providing data closer to real-world conditions.