XPS Analysis of FexNiyPS3 vdW Materials Used in the Hydrogen Evolution Processes.

In response to the global demand for sustainable energy solutions, the quest for stable and cost-effective hydrogen production has garnered significant attention in recent decades. Here, the emergence of layered metal phosphorus trichalcogenides (MPX3, M: transition metal, X: chalcogen) materials and their two-dimensional counterparts with customizable composition and electronic structure holds great promise for such purposes. In the present study, we successfully synthesized large-scale and high-quality FePS3, NiPS3, and an alloyed counterpart, Fe0.5Ni0.5PS3. Subsequent systematic investigations were conducted to probe their respective electronic structures and assess their hydrogen evolution reaction (HER) properties. Remarkably, our results unveiled the successful modulation of the bandgap for FexNiyPS3, ultimately bestowing it with the most favorable HER performance for Fe0.5Ni0.5PS3 when compared to the other two samples. Furthermore, our exploration into the evolution of the X-ray photoelectron spectroscopy (XPS) spectra demonstrated that the charge conversions of metal cations play a pivotal role in the HER reactions. This critical insight further enriches our understanding of the fundamental mechanisms governing the performance of the prepared layered MPX3-based electrocatalysts, thus facilitating a comprehensive and detailed analysis of the pre- and post-HER reactions. This work not only sheds light on the intricate interplay between composition, electronic structure, and catalytic performance in the realm of novel electrocatalysts, but also contributes to the broader scientific community's pursuit of sustainable and efficient hydrogen production.

Semiconducting eutectic materials for photocatalysis and photoelectrochemistry applications: a perspective.

The world of semiconductors has drastically improved human lifestyle due to its versatile applications. The demand for new efficient semiconductors is increasing day by day, giving birth to the idea of new synthesis methods. Here, the importance of semiconductor eutectic materials has been presented, as one of the potential candidates for solar-based devices such as photo-anodes for water splitting, along with the micro (µ) pulling method (as a synthesis method for semiconductor eutectic materials). A comparison of the efficiency of the present devices with the eutectic composites has been made, showing semiconductor eutectic materials as a better alternative. In addition, possible changes that can be achieved using the µ-pulling method to enhance the photo(electro)chemical (PEC) properties have focused on semiconducting eutectic materials in this article.

Second Floor of Flatland: Epitaxial Growth of Graphene on Hexagonal Boron Nitride.

In the studies presented here, the subsequent growth of graphene on hexagonal boron nitride (h-BN) is achieved by the thermal decomposition of molecular precursors and the catalytic assistance of metal substrates. The epitaxial growth of h-BN on Pt(111) is followed by the deposition of a temporary Pt film that acts as a catalyst for the fabrication of the graphene sheet. After intercalation of the intermediate Pt film underneath the boron-nitride mesh, graphene resides on top of h-BN. Scanning tunneling microscopy and density functional calculations reveal that the moiré pattern of the van-der-Waals-coupled double layer is due to the interface of h-BN and Pt(111). While on Pt(111) the graphene honeycomb unit cells uniformly appear as depressions using a clean metal tip for imaging, on h-BN they are arranged in a honeycomb lattice where six protruding unit cells enframe a topographically dark cell. This superstructure is most clearly observed at small probe-surface distances. Spatially resolved inelastic electron tunneling spectroscopy enables the detection of a previously predicted acoustic hybrid phonon of the stacked materials. Its' spectroscopic signature is visible in surface regions where the single graphene sheet on Pt(111) transitions into the top layer of the stacking.

Unoccupied electronic band structure of pentagonal Si nanoribbons on Ag(110).

Silicon nanoribbons - one dimensional silicon structures with a pentagonal atomic structure and mixed sp2- and sp3-hybridisation - grow on Ag(110) upon deposition of silicon. These nanostructures are viewed as promising candidates for modern day electronics as they are comprised of the same element as today's semiconductor devices. Even though they have been studied extensively over the last decade, only little is known about their unoccupied band structure which is important for possible future optoelectronics, semiconductor, and spintronics applications. In order to elucidate the unoccupied band structure of the nanoribbons, k-resolved inverse photoemission spectroscopy (KRIPES) studies were performed on both nanoribbon structures reported in the literature as well as on the bare Ag(110) substrate within the energy range of E-EF = 0-6.5 eV. The obtained experimental results are compared to density functional theory (DFT) calculated band structures to assign individual spectral features to specific bands. Since even small changes in the structural model of the nanoribbons lead to a change in the calculated band structure, this comparison allows us to assess the validity of the proposed structural models.

Layer-by-Layer Decoupling of Twisted Graphene Sheets Epitaxially Grown on a Metal Substrate.

The electronic properties of graphene can be efficiently altered upon interaction with the underlying substrate resulting in a dramatic change of charge carrier behavior. Here, the evolution of the local electronic properties of epitaxial graphene on a metal upon the controlled formation of multilayers, which are produced by intercalation of atomic carbon in graphene/Ir(111), is investigated. Using scanning tunneling microscopy and Landau-level spectroscopy, it is shown that for a monolayer and bilayers with small-angle rotations, Landau levels are fully suppressed, indicating that the metal-graphene interaction is largely confined to the first graphene layer. Bilayers with large twist angles as well as twisted trilayers demonstrate a sequence of pronounced Landau levels characteristic for a free-standing graphene monolayer pointing toward an effective decoupling of the top layer from the metal substrate. These findings give evidence for the controlled preparation of epitaxial graphene multilayers with a different degree of decoupling, which represent an ideal platform for future electronic and spintronic applications.

Multichannel scanning probe microscopy and spectroscopy of graphene moiré structures.

The graphene moiré structures on 4d and 5d metals, as they demonstrate both long (moiré) and short (atomic) scale ordered structures, are the ideal systems for the application of scanning probe methods. Taking graphene-Ir(111) as an example, we present the complex studies of this graphene-metal moiré-structure system by means of 3D scanning tunnelling and atomic force microscopy/spectroscopy as well as Kelvin-probe force microscopy. The results clearly demonstrate variation of the moiré and atomic scale contrast as a function of the bias voltage as well as the distance between the scanning probe and the sample, allowing one to discriminate between topographic and electronic contributions in the imaging of a graphene layer on metals. The presented results are compared with the state-of-the-art density functional theory calculations demonstrating excellent agreement between theoretical and experimental data.

Theoretical description of X-ray absorption spectroscopy of the graphene-metal interfaces.

The present paper considers the application of the method of the near-edge X-ray absorption spectroscopy (NEXAFS) for the investigation of the graphene-based systems (from free-standing graphene to the metal-intercalation-like systems). The NEXAFS spectra for the selected systems are calculated in the framework of the approach, which includes the effects of the dynamic core-hole screening. The presented spectral changes from system to system are analysed with the help of the corresponding band-structure calculations. The obtained results are compared with available experimental data demonstrating the excellent agreement between theory and experiment. The direct correlation between the strength of the graphene interaction with the metallic substrate and the spectral distributions (shape and intensities of π* and σ* features in the C K NEXAFS spectra) is found that can be taken as a fingerprint for the description of interaction at the graphene/metal interface.

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.

Probing Active Sites on Pristine and Defective MnPX3 (X: S and Se) Monolayers for Electrocatalytic Water Splitting.

The state-of-the-art density functional theory approach was used to study the structural and electronic properties of pristine and defective MnPX3 monolayers as well as their activity toward water and hydrogen evolution reaction (HER) catalytic performance. The adsorption behavior of H2O on a pristine MnPX3 structure is of physisorption nature, whereas the adsorption energy is significantly increased for the defective structures. At the same time, the water dissociation process is more energetically favorable, and the reactivity of MnPX3 is determined by the vacancy configuration. Following Nørskov's approach, the HER catalytic performance is evaluated by calculating the hydrogen adsorption free energy on the respective MnPX3 surface. Our calculation results demonstrate that defective 2D MnPX3 with low coordinated P shows significantly higher HER performance compared to the pristine counterpart.

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.

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.

Ge(001) as a template for long-range assembly of π-stacked coronene rows.

The adsorption of coronene molecules (C(24)H(12)) on the Ge(001) surface has been studied by means of scanning tunnelling microscopy (STM). Upon room temperature deposition, the coronene molecules adsorb in an upright geometry forming compact layers patterned in rows for coverages of one monolayer and less, being the only example investigated so far in which a pure aromatic hydrocarbon forms a well-ordered monolayer on a non-passivated semiconductor surface. At half monolayer, the molecular rows consist of long chains of π-stacked molecules and the distance between molecular planes is 8 Å. This configuration is maintained upon cooling the system below the transition temperature of Ge(001) (~220 K), but the molecular layer experiences also a transition from rows perpendicular to rows parallel to the Ge dimer rows. We interpret our observations in terms of a weak bonging between molecules and substrate, which facilitates the formation of large ordered domains of molecules, revealing Ge(001) as an ideal template for the growth of this and other aromatic hydrocarbons.

Graphene on metallic surfaces: problems and perspectives.

The present manuscript summarizes the modern view on the problem of the graphene-metal interaction. Presently, the close-packed surfaces of d metals are used as templates for the preparation of highly-ordered graphene layers. Different classifications can be introduced for these systems: graphene on lattice-matched and graphene on lattice-mismatched surfaces where the interaction with the metallic substrate can be either "strong" or "weak". Here these classifications, with the focus on the specific features in the electronic structure in all cases, are considered on the basis of large amount of experimental and theoretical data, summarized and discussed. The perspectives of the graphene-metal interfaces in fundamental and applied physics and chemistry are pointed out.

Electronic and Magnetic Properties of the Graphene/Y/Co(0001) Interfaces: Insights from the Density Functional Theory Analysis.

The effect of Y intercalation on the atomic, electronic, and magnetic properties of the graphene/Co(0001) interface is studied using state-of-the-art density functional theory calculations. Different structural models of the graphene/Y/Co(0001) interface are considered: (i) graphene/Y/Co(0001), (ii) graphene/1ML-YCo2/Co(0001), and (iii) graphene/bulk-like-YCo2(111). It is found that the interaction strength between graphene and the substrate is strongly affected by the presence of Y at the interface and the electronic structure of graphene (doping and the appearance of the energy gap) is defined by the Y concentration. For the Co-terminated interfaces between graphene and the metallic support in the considered systems, the electronic structure of graphene is strongly disturbed, leading to the absence of the linear dispersion for the graphene π band; in the case of the Y-terminated interfaces, a graphene layer is strongly n-doped, but the linear dispersion for this band is preserved. Our calculations show that the magnetic anisotropy for the magnetic atoms at the graphene/metal interface is strongly affected by the adsorption of a graphene layer, giving a possibility for one to engineer the magnetic properties of the graphene/ferromagnet systems.

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.

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.

On the physisorption of water on graphene: a CCSD(T) study.

The electronic structure of the zero-gap two-dimensional graphene has a charge neutrality point exactly at the Fermi level that limits the practical application of this material. There are several ways to modify the Fermi-level-region of graphene, e.g. adsorption of graphene on different substrates or different molecules on its surface. In all cases the so-called dispersion or van der Waals interactions can play a crucial role in the mechanism, which describes the modification of electronic structure of graphene. The adsorption of water on graphene is not very accurately reproduced in the standard density functional theory (DFT) calculations and highly-accurate quantum-chemical treatments are required. A possibility to apply wavefunction-based methods to extended systems is the use of local correlation schemes. The adsorption energies obtained in the present work by means of CCSD(T) are much higher in magnitude than the values calculated with standard DFT functional although they agree that physisorption is observed. The obtained results are compared with the values available in the literature for binding of water on the graphene-like substrates.

Graphene Layer Morphology as an Indicator of the Metal Alloy Formation at the Interface.

The intercalation of different species in graphene-metal interfaces is widely used to stabilize the artificial phases of different materials, which in some cases leads to the formation of the surface alloys between atoms of the guest metal and the substrate. Here, the interfaces of graphene with Ru(0001) and Ir(111) were modified using intercalation of a thin Mn layer and investigated by means of scanning tunneling microscopy (STM) accompanied by density functional theory (DFT) calculations. It is found that Mn forms a pseudomorphic layer on Ru(0001) under a strongly buckled graphene layer. In the case of Mn intercalation in graphene/Ir(111), a buried thin layer of MnIr alloy is formed beneath the first Ir layer under a flat graphene layer. This unexpected observation is explained on the basis of phase diagram pictures for the Mn-Ru and Mn-Ir systems as well as via comparison of calculated total energies for the respective interfaces.

Adsorption of water on the pristine and defective semiconducting 2D CrPX3monolayers (X: S, Se).

The effect of vacancy and water adsorption on the electronic structure of semiconducting 2D trichalcogenide material CrPX3(X: S, Se) is studied using state-of-the-art density functional theory (DFT) approach. It is found that chalcogen vacancies play a minor role on the electronic structure of CrPX3in the vicinity of the Fermi level leading to the slightly reduced band gap for these materials, however, inducing strongly localised defect states which are placed in the energy gap formed by the valence band states. Our DFT calculations show that the interaction of water molecules with CrPX3, pristine and defective, can be described as physisorption and the adsorption energy for H2O is insensitive to the difference between pristine and chalcogen-defective surface of trichalcogenide material. These results are the first steps for the theoretical description of the ambient molecules interaction with 2D semiconducting CrPX3material, that is important for its future experimental studies and possible applications.

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.