Surface restoration in hygroscopic VI3 crystals via synchrotron soft X-ray irradiation

TL;DR

The paper addresses the instability of VI3 surfaces under ambient/moisture exposure and proposes a surface-conditioning approach using synchrotron soft X-ray irradiation. It employs X-ray absorption spectroscopy and X-ray photoemission spectroscopy to monitor chemical states, showing that aging drives the formation of superficial vanadates while high-flux soft X-ray exposure partially recovers the stoichiometric VI3 surface. The findings reveal that the surface degradation can be mitigated locally by irradiation, indicating a relatively weak coupling between the degraded top layer and the deeper crystal. This suggests a potential in-situ method to restore functional surface chemistry in hygroscopic 2D magnets, informing handling strategies for VI3 and related materials.

Abstract

Among van der Waals crystals, transition metal trihalide VI3 has driven attention for its magnetic and orbital properties. However, its chemical instability under ambient conditions make its exploitation challenging for technological implementation. In this context, here we show how synchrotron radiation soft X-rays partially restore stoichiometric chemical and electronic properties of VI3 crystals. By combining X-ray absorption and X-ray photoemission spectroscopies, we show as-cleaved and aged (in ultra-high vacuum conditions) chemical degradation of VI3 crystal surface, with the formation of vanadates, and its, at least partial, recovery under high-flux soft X-ray beam exposure, revealing that superficial hygroscopic contamination couples relatively weakly to the crystal surface.

Figures (4)

• Figure 1: (a) XPS survey at 800 on VI3 as-cleaved (blue) and aged (red) samples, with element-specific peaks highlighted. (b) I 3d and 4d edges for as-cleaved (blue) and aged (red) samples. (c) V 2p and O 1s edges for as-cleaved (blue) and aged (red) samples. All measurements were done with a photon flux equivalent to $2.8e9\photons\per\s$ on the mesh.
• Figure 2: XAS spectra at V L2,3 edges on as-cleaved (blue) and aged (red) samples. The photon flux for these spectra was equivalent to $2.8e9\photons\per\s$ at the gold-plated mesh grid before the endstation.
• Figure 3: (a) Subsequent XAS spettra at V L2,3 edges on the same probed area, from top to bottom, under increasing photon flux irradiation. (b) Fitted XAS spectra with reference spectra for the first spectrum at low irradiation flux (top black curve) and last measure at high irradiation flux (bottom yellow curve). (c) Evolution of the valence fraction contribution of the measured spectra as a function of irradiation flux.
• Figure 4: XPS spectra of as-cleaved, aged and irradiated samples; solid lines are fits of the experimental curves, dashed lines are the different components for V (blue/teal) and O (green) signals.