Valence Modifications in Hygroscopic VI3 Degraded Crystals via Soft X-Ray Synchrotron Radiation

Technical Summary: Valence Modifications in Hygroscopic VI3 Degraded Crystals via Soft X-Ray Synchrotron Radiation

Problem Statement
Vanadium triiodide (VI3) is a transition metal trihalide of significant interest for spin- and orbitronics due to its ferromagnetic properties down to the monolayer limit and its rich phase diagram. However, its practical application is hindered by severe chemical instability under ambient conditions. VI3 is highly hygroscopic; exposure to moisture leads to solvation, the formation of hydrated cationic vanadium ions, and a mixed valence state (3+, 4+, and 5+), ultimately destroying its crystallographic and magnetic properties. While previous studies have characterized air-exposed degradation, the specific behavior of VI3 in ultra-high vacuum (UHV) and its response to synchrotron radiation exposure remain less understood. This study addresses the chemical stability of VI3 in UHV upon aging and investigates how soft X-ray irradiation affects the valence states of degraded surfaces.

Methodology
The authors utilized a combination of X-ray Absorption Spectroscopy (XAS) and X-ray Photoemission Spectroscopy (XPS) at the APE-HE beamline of the Elettra synchrotron facility.

• Sample Preparation: Commercial VI3 crystals were handled in an argon-filled glovebox (< 0.5 ppm O2, < 0.5 ppm H2O). Two distinct sample conditions were prepared:
  1. As-cleaved: Cleaved directly in the UHV endstation (pressure < 3 × 10⁻¹⁰ mbar) with minimal surface exposure (~2 L).
  2. Aged: Cleaved in a loadlock chamber and held in high vacuum (HV, ~5 × 10⁻⁹ mbar) for 72 hours, resulting in a surface exposure of ~2600 L.
• Characterization: XAS measurements were performed at the V L2,3 edges in Total Electron Yield (TEY) mode at room temperature. XPS measurements were conducted on the V 2p, I 3d/4d, and O 1s core levels using 800 eV photons.
• Irradiation Protocol: The aged samples were subjected to varying photon fluxes, ranging from low flux (0.28 × 10¹⁰ photons s⁻¹) to high flux (2.40 × 10¹⁰ photons s⁻¹), to observe flux-dependent chemical modifications.

Key Results

1. Surface Degradation in Vacuum: Even in high vacuum, aging VI3 for 72 hours leads to significant surface contamination. XPS survey scans of aged samples show a strong reduction in I 3d/4d and V 2p signal intensities and the emergence of strong C 1s and O 1s peaks. The V 2p spectrum of aged samples exhibits features at higher binding energies, indicating a shift from the stoichiometric V3+ state to higher valence states.
2. Formation of Vanadates: XAS spectra of the aged samples reveal a shift of the main V L3 peak by 1.6 eV toward higher photon energies and a loss of pre-edge features characteristic of V3+. The lineshape closely resembles V4+ and V5+ states found in vanadates and vanadium oxides. Quantitative fitting suggests the aged surface is dominated by V5+ species with a minor V4+ contribution, consistent with the formation of surface vanadates due to interaction with residual molecular contaminants (oxygen and water).
3. X-Ray Induced Modifications: Exposure of the aged surface to high-flux soft X-rays induces reversible and irreversible changes:
   • Photoreduction/Desorption: Increasing the photon flux causes a gradual recovery of low-energy features in the XAS spectra and the re-emergence of pre-edge signals characteristic of V3+.
   • Valence Shift: Quantitative analysis shows a monotonic decrease in the V5+ fraction and an initial increase in V4+ followed by a strong enhancement of the stoichiometric V3+ component.
   • XPS Correlation: XPS confirms these changes, showing a decrease in the O 1s signal intensity (approx. 25% reduction) and a shift in the V 2p signal toward lower binding energies. The O:V ratio decreases from ~1.7:1 (aged) to ~1.3:1 (irradiated).
   • Mechanism: The authors interpret these changes as a combination of beam-induced photoreduction of high-valence vanadates and the partial desorption of adsorbed water and oxygen molecules, which exposes the underlying pristine VI3. The I 3d and 4d signals show increased intensity without energy shifts, suggesting the iodine coordination remains largely unchanged while the surface contaminants are removed.

Significance and Claims
The paper provides experimental evidence that the interaction between hygroscopic VI3 surfaces and residual contaminants (oxygen and water) in vacuum is relatively weak, allowing for partial restoration of the surface state via soft X-ray irradiation. The study demonstrates that:

• Surface degradation in VI3 leads to the formation of vanadates (V5+ and V4+) even in high vacuum conditions.
• Mild soft X-ray fluxes are sufficient to alter surface bonds, leading to the partial desorption of contaminants and a reduction of vanadate species, thereby recovering a portion of the stoichiometric V3+ signal.
• These findings suggest that low-energy radiation can locally modify the chemical environment of surface molecular species, offering a potential route for the functionalization or partial cleaning of VI3 surfaces.

The authors modestly conclude that while these effects offer a pathway to manipulate surface chemistry, a complete understanding of the microscopic mechanisms (distinguishing between pure desorption and photoreduction) requires further statistical investigation. They also note that the effect is not necessarily benign, as the role of oxide reduction in forming V3+ oxides versus restoring pristine VI3 requires further study.