Electronic Correlations in FexNiyPS3 Van der Waals Materials: Insights from Angle-Resolved Photoelectron Spectroscopy and DFT.

Recently layered antiferromagnetic materials with different magnetic orderings attract increased attention. It was found that these properties can be preserved down to the monolayer limit opening large perspectives for their applications in (opto)spintronics and sensing, however, lacking the experimental results on electronic structure studies. Here the results of angle-resolved photoelectron spectroscopy (ARPES) studies accompanied by DFT calculations for FexNiyPS3 layered van der Waals (vdW) alloys are presented, addressing the effects of electronic correlations in these materials. It is demonstrated that in the case of FePS3 the top of the valence band is formed by the hybrid Fe 3d-S 3p states and is of pure S 3p character for NiPS3, respectively, whereas for the mixed Fe-Ni-based vdW alloy the electronic structure is a sum of contributions from the parent compounds. The obtained results give a clear understanding of the nature of the insulating state in studied MPX3 materials and pave the way on their applications in different areas.

Realization of a New Graphene-Ferromagnet Interface with Dirac Linear Band Dispersion.

The integration of graphene in spintronics applications requires its close contact with ferromagnetic materials, promoting effective spin injection. At the same time, the linear energy vs wave-vector dependence for the charge carriers in the vicinity of the Fermi level for graphene has to be conserved. Here, motivated by recent theoretical predictions, we present the experimental realization on the synthesis of graphene/ferromagnetic-Mn5Ge3/semiconducting-Ge heterostructures using the intercalation of Mn in the epitaxial graphene/Ge interfaces. Different in situ and ex situ methods confirm the formation of such heterosystems, where graphene is in close contact with ferromagnetic Mn5Ge3, as the Curie temperature reaches room temperature. Despite the expected small distance between graphene and Mn5Ge3 causing the strong interaction at interfaces, our angle-resolved photoelectron spectroscopy experiments for the formed graphene/Mn5Ge3 interfaces confirm the linear band dispersion around the Fermi level for the carriers in graphene. These findings open up an interesting perspective for the integration of graphene in modern semiconductor technology with possible implications for spintronics device fabrication.

A Stress Self-Adaptive Structure to Suppress the Chemo-mechanical Degradation for High Rate and Ultralong Cycle Life Sodium Ion Batteries.

Transition-metal phosphides (TMPs) as typical conversion-type anode materials demonstrate extraordinary theoretical charge storage capacity for sodium ion batteries, but the unavoidable volume expansion and irreversible capacity loss upon cycling represent their long-standing limitations. Herein we report a stress self-adaptive structure with ultrafine FeP nanodots embedded in dense carbon microplates skeleton (FeP@CMS) via the spatial confinement of carbon quantum dots (CQDs). Such an architecture delivers a record high specific capacity (778 mAh g-1 at 0.05 A g-1 ) and ultra-long cycle stability (87.6 % capacity retention after 10 000 cycles at 20 A g-1 ), which outperform the state-of-the-art literature. We decode the fundamental reasons for this unprecedented performance, that such an architecture allows the spontaneous stress transfer from FeP nanodots to the surrounding carbon matrix, thus overcomes the intrinsic chemo-mechanical degradation of metal phosphides.

Dual Character of the Insulating State in the van der Waals Fe1-xNixPS3 Alloyed Compounds.

The electronic structure of the alloyed transition-metal phosphorus trichalcogenide van der Waals Fe1-xNixPS3 compounds is studied using X-ray absorption spectroscopy and resonant photoelectron spectroscopy combined with intensive density functional theory calculations. Our systematic spectroscopic and theoretical data demonstrate the strong localization of the Fe- and Ni-ions-derived electronic states that leads to the description of the spectroscopic data as belonging simultaneously to Mott-Hubbard and charge-transfer insulators. These findings reveal Fe1-xNixPS3 as unique layered compounds with dual character of the insulating state, pointing to the importance of these results for the description and understanding of the functionality of this class of materials in different applications.

Mixed Insulating State for van der Waals CoPS3.

Large-scale high-quality van der Waals CoPS3 single crystals are synthesized using a chemical vapor transport (CVT) method. The crystallographic structure and electronic properties of this layered material are systematically studied using different spectroscopic methods (XPS, NEXAFS, and resonant photoelectron spectroscopy) accompanied by density functional theory (DFT) calculations. All experimental and theoretical data allow assignment of this material to the class of mixed Mott-Hubbard/charge-transfer insulator with Udd ≅ Δ. All obtained results can enrich the information on the new class of van der Waals materials, transition metal phosphorus trichalcogenides, and help to further effectively exploit their electronic, optical, and transport properties, which are important for adopting this kind of materials into different application areas, such as spintronics and catalysis.

Mott-Hubbard insulating state for the layered van der Waals [Formula: see text] (X: S, Se) as revealed by NEXAFS and resonant photoelectron spectroscopy.

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 [Formula: see text] (X: S, Se) materials. Using systematic resonant photoelectron spectroscopy studies we observed strong resonant behavior for the peaks associated with the [Formula: see text] final state at low binding energies for these materials. Such observations clearly assign [Formula: see text] 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.

Topological Quasi-2D Semimetal Co3Sn2S2: Insights into Electronic Structure from NEXAFS and Resonant Photoelectron Spectroscopy.

The electronic structure of the natural topological semimetal Co3Sn2S2 crystals was studied by using near-edge X-ray absorption spectroscopy (NEXAFS) and resonant photoelectron spectroscopy (ResPES). Although, the significant increase of the Co 3d valence band emission is observed at the Co 2p absorption edge in the ResPES experiments, the spectral weight at these photon energies is dominated by the normal Auger contribution. This observation indicates the delocalized character of photoexcited Co 3d electrons and is supported by the first-principle calculations. Our results on the investigations of the element- and orbital-specific electronic states near the Fermi level of Co3Sn2S2 are of importance for the comprehensive description of the electronic structure of this material, which is significant for its future applications in different areas of science and technology, including catalysis and water splitting.

Correlations in the Electronic Structure of van der Waals NiPS3 Crystals: An X-ray Absorption and Resonant Photoelectron Spectroscopy Study.

The electronic structure of high-quality van der Waals NiPS3 crystals was studied using near-edge X-ray absorption spectroscopy (NEXAFS) and resonant photoelectron spectroscopy (ResPES) in combination with density functional theory (DFT) approach. The experimental spectroscopic methods, being element specific, allow one to discriminate between atomic contributions in the valence and conduction band density of states and give direct comparison with the results of DFT calculations. Analysis of the NEXAFS and ResPES data allows one to identify the NiPS3 material as a charge-transfer insulator. Obtained spectroscopic and theoretical data are very important for the consideration of possible correlated-electron phenomena in such transition-metal layered materials, where the interplay between different degrees of freedom for electrons defines their electronic properties, allowing one to understand their optical and transport properties and to propose further possible applications in electronics, spintronics, and catalysis.