Mott–Hubbard insulating state for the layered van der Waals $$\hbox {FePX}_3$$ (X: S, Se) as revealed by NEXAFS and resonant photoelectron spectroscopy

Abstract

A broad family of the nowadays studied low-dimensional systems, including 2D materials, demonstrate many fascinating properties, which however depend on the atomic composition as well as on the system dimensionality. Therefore, the studies of the electronic correlation effects in the new 2D materials is of paramount importance for the understanding of their transport, optical and catalytic properties. Here, by means of electron spectroscopy methods in combination with density functional theory calculations we investigate the electronic structure of a new layered van der Waals $$\hbox {FePX}_3$$ FePX 3 (X: S, Se) materials. Using systematic resonant photoelectron spectroscopy studies we observed strong resonant behavior for the peaks associated with the $$3d^{n-1}$$ 3 d n - 1 final state at low binding energies for these materials. Such observations clearly assign $$\hbox {FePX}_3$$ FePX 3 to the class of Mott–Hubbard type insulators for which the top of the valence band is formed by the hybrid Fe-S/Se electronic states. These observations are important for the deep understanding of this new class of materials and draw perspectives for their further applications in different application areas, like (opto)spintronics and catalysis.

To the Stability of Janus Phases in Layered Trichalcogenide MPX3 Crystals: Insights from Experiments and Theory

“…Both of them can be represented in hexagonal unit cells (Figure f). Then, every unit cell contains three MPX 3 single layers having D 3 d symmetry (Figure c), which are stacked in a different way . The optical images of the FePS 3– x Se x ( x = 0, 1.5, and 3) bulk crystals and their photographs are shown in the upper row of Figure .…”

“…In our previous studies, ,,− we have shown that PBE + U + D2 performs very good when studying structures and electronic and magnetic properties of MPX 3 materials, including pristine FePS 3 and FePSe 3 . Here, we employ the same approach to get better insight into the properties of the mixed FePS 1.5 Se 1.5 material under study.…”

“…Then, every unit cell contains three MPX 3 single layers having D 3d symmetry (Figure 1c), which are stacked in a different way. 25 The optical images of the FePS 3−x Se x (x = 0, 1.5, and 3) bulk crystals and their photographs are shown in the upper row of Figure 2. One can clearly see that all samples demonstrate very high quality with well-ordered planes and steps oriented with respect to each other either at 120 or 60°angles between the crystalline edges as one expects from the hexagonal-like shape for the grown crystals.…”

“…In addition, we performed electronic structure calculations by means of density functional theory (DFT). All results are presented in comparison with those of the parent FePS 3 and FePSe 3 materials, 25 whose synthesis and characterization were repeated in this work for consistency reason. Our experimental and theoretical results demonstrate that a uniform random distribution of S and Se is observed in FePS 1.5 Se 1.5 , confirming the absence of the Janus phase in these 3D crystals, which can be assigned to the compensated local dipole moments in the single 2D layers.…”

The Valence Band Structure of the [Ni(Salen)] Complex: An Ultraviolet, Soft X-ray and Resonant Photoemission Spectroscopy Study

“…However, even in this case, the information that is obtained about the electronic structure of the valence band from the VB PE spectra is still very limited. Additional information about the electronic structure of the valence band can be obtained using the method of resonant photoelectron spectroscopy (ResPES), which is especially useful in the study of compounds of transition and rare earth metals—particularly, 3d metals [ 22 , 23 , 24 ]. This method is implemented by measuring the VB PE spectra at photon energies in the vicinity of absorption resonances in the NEXAFS spectrum of an atom of the polyatomic system under study.…”