The use of amorphous GeSe-based chalcogenides in ovonic threshold switching selectors for highly efficient phase-change memory devices involves the formation of contacts with metal electrodes, where the nature of interfacial contact plays a crucial role in controlling the power efficiency. Here, by using a joint experimental-theoretical approach we study the key contact properties between TiN-electrode and amorphous GeSe (a-GeSe) semiconductor. Two types of stackable devices with and without amorphous carbon (a-C) interlayer film were investigated; namely, TiN/a-GeSe and TiN/a-C/a-GeSe stacks. The interfacial contact between TiN electrode and a-GeSe is characterized by a high Schottky barrier height (SBH) type contact. The insertion of the Carbon buffer layer develops a lower SBH with a-GeSe, leading to a higher leakage current and a lower VTH, in line with the experimental observation.

Trap states induced by chalcogenide vacancies in defective transition metal dichalcogenide (TMDs) monolayers are detrimental to both the charge carrier lifetime and device efficiency. To address this, chemically functionalizing the surface of defective TMD monolayers is crucial. Experimental methods such as thiol grafting on chalcogenide vacancies, oxygen passivation, and the physisorption of electroactive molecules have been explored for defect healing. Our study, using ab initio time-domain density functional theory and nonadiabatic molecular dynamics, shows that C20 molecular adsorption and oxygen passivation effectively mitigate nonradiative electron-hole recombination and enhance carrier charge lifetime in defective WSe2 beyond that of pristine WSe2 monolayers. These improvements stem from the combined effects of energy gap variations, nonadiabatic coupling, and decoherence time, resulting from either hole-trap-assisted processes or direct interactions between free electrons and holes. Our findings suggest that healing chalcogenide vacancies in defective TMDs enables precise control over charge carrier lifetime, advancing defect engineering in 2D materials.

The synthesis of nanometer‐thick (≈3 nm) gallium oxynitride (GaO x N y ) layers with a variable stoichiometry is reported. The approach primarily exploits the liquid metal chemistry (LMC) technique and promises easier integration of 2D materials onto photonic devices compared to traditional top‐down and bottom‐up methods. The fabrication follows a two‐step process, involving first liquid metal‐based printing of a nanometer‐thick layer of gallium oxide (Ga 2 O 3 ), followed a plasma‐enhanced nitridation reaction. Control over nitridation parameters (plasma power, exposure time) allows adjustment of the GaO x N y layer's composition, granting access to compounds with distinct optical properties (e.g., a 20% index variation), as demonstrated by ellipsometry and density functional theory (DFT) simulations. DFT provides a microscopic understanding of the effect of the bond polarization and crystallinity on the optical properties of GaO x N y compounds. These findings expand the knowledge of ultrathin GaO x N y alloys, which are poorly studied with respect to their gallium nitride (GaN) and Ga 2 O 3 counterparts. They also represent an essential step toward integrating such 2D materials into photonic chips and offer new opportunities to improve the performance of hybrid optoelectronic devices.

Downsizing the graphdiyne (GDY) network to shape quantum dots (QDs) will provide attractive optical and electronic properties associated with quantum confinement and edge effects. Here, it is demonstrated that quantum confinement and defect introduction allow using GDY in donor-acceptor photocatalytic systems for solar-to-hydrogen conversion. The defect-rich GDY QDs (GDYO-QDs) exhibit a blue-to-green excitation-dependent photoluminescence behavior, demonstrating their ability to harvest light over a wide energy range. Quantum-chemical calculations evidenced an increase in the electronic bandgap of GDY upon quantum confinement and defect introduction without the appearance of trap states that can hamper charge transport properties. Such a unique optical behavior of QDs is used in photocatalytic hydrogen generation through the hybridization with TiO2 as a model photocatalyst. Theoretical and experimental results demonstrate that the donor-acceptor system tremendously boosts the photocatalytic performance, reaching 5288 mu mol g-1 after 4 h of illumination at a constant rate of 1322 mu mol g-1 h-1, using a low volume of a sacrificial electron donor (6% v/v). The QDs act as efficient chromophores harvesting UV and visible light while injecting electrons into the TiO2. This work opens a new area of using GDYO-QDs as an efficient chromophore in developing donor-acceptor systems for photocatalysis and future photovoltaic devices. Introducing oxygen defects and downsizing graphdiyne to the quantum regime induces a change in its photophysical properties and light harvesting ability. Defects-rich graphdiyne quantum dots act then as chromophore that collects light and injects electrons into TiO2. The donor-acceptor system demonstrates a higher hydrogen evolution reaction using a low sacrificial electron donor concentration. image

Two-dimensional transition-metal dichalcogenide (2D-TMD) monolayers have recently attracted growing interest, thanks to their excellent optoelectronic properties, especially a moderate direct band gap in the visible spectral range and an extremely strong light-matter interaction. Herein, by means of density functional theory (DFT) and ab initio molecular dynamics (AIMD), we systematically investigate the chemical doping of the WSe2 monolayer upon non-covalent attachment of electron donor and acceptor molecules, namely, fullerenes (C20, C26, and C60), tetrathiafulvalene (TTF), 7,7,8,8-tetracyanoquinodimethane (TCNQ), and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ). Our results confirm that the physisorbed molecules stack on WSe2 via a weak van der Waals interaction; this precludes any significant damage in the basal-plane structure of the monolayer. In turn, the modifications in the carrier density in the WSe2 monolayer due to organic dopants result in a change of the work function by up to 0.41 eV. By performing AIMD calculations, we show that the effect is more pronounced upon increasing the coverage density of physisorbed TTF and TCNQ molecules. The impact of Se-vacancy (VSe) defects on the electronic properties of the WSe2 monolayer and thermodynamic stability of physisorption (including the molecular density) is also considered. Interestingly, the molecules demonstrate an ability to modulate the degree of spatial localization of VSe trap states. Moreover, the shift of the Fermi level upon molecular adsorption also enables further stabilization of charged defect states associated to a VSe vacancy. The pronounced effect of molecular adsorption on the VSe defect behavior in WSe2 might open the door for potential engineering of defect states in 2D TMDs.

The design of hybrid electrocatalysts with abundant active sites and long term stability is crucial for efficient oxygen evolution reaction (OER) application. Cobalt oxide is considered as one of the most promising electrocatalysts to replace noble metal due to its low cost, availability, and electrocatalytic activity towards the oxygen evolution reaction in alkaline media. However, nano-scale cobalt oxide suffers from severe surface self-agglomeration during the OER process, so that leading to poor activity and durability. Herein, ultra-small cobalt oxide nanoparticles are anchored on the surface of nitrogen doped porous 3D graphitic carbon nano-spheres (N-ACS@Co3O4) to increase the amount of exposed active site and avoid the self-agglomeration. The obtained electrocatalyst (N-ACS@Co3O4) is enriched with abundant oxygen vacancies and exhibits a superior OER activity (Overpotential of 237 mV at 10 mA.cm(-2)) and exceptional stability for at least 30 h in alkaline electrolyte (1 M KOH). The DFT calculations demonstrate that the strong adsorption of Co3O4 on N-doped graphene can prevent its agglomeration, and therefore improves the stability of Co3O4 nanoparticles during OER process in line with the experimental results. (c) 2023 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

This work describes a successful approach to dope copper sulfide with different amounts of Co2+ ions and combine it with TiO2 through a simple one-step hydrothermal process. Compared with the bare CuS, the syn-thesized Co-CuS@TiO2 heterostructures promote charge transport and restrict the recombination of photoexcited electrons and holes. The intrinsic properties of Co-CuS@TiO2 samples are systematically examined through experimental characterizations and density functional theory (DFT) theoretical calculations. Photocatalytic degradation tests under simulated solar light irradiation were performed using sulfamethoxazole degradation as a model emerging persistent antibiotic. The photocatalytic performance was enhanced after cobalt doping, and the heterostructure doped with 3% of Co exhibited the best degradation with an apparent rate constant of 0.0216 min-1. This sample also showed a much faster settling than bare TiO2, which indicates a much easier separation of the reaction media after being used. The enhancement of degradation is attributed to the increased light absorption and the more efficient charge transfer and separation. The plausible photocatalytic degradation mechanism of sulfamethoxazole was also proposed. This study presents a novel strategy to prepare potential photocatalysts for the elimination of emerging pollutants.

We report on the synthesis of 2D GaN materials by the so-called liquid metal chemistry and tuning of their composition between oxide and nitride materials. This technique promises easier integration of 2D materials onto photonic devices compared to traditional “top-down” and “bottom-up” methods. Our fabrication method is carried out via a two-step liquid metal-based printing method followed by a microwave plasma-enhanced nitridation reaction. The synthesis of GaN relies on plasma-treated liquid metal-derived two-dimensional (2D) sheets that were squeeze-transferred onto desired substrates. We characterized the composition and optical properties of the resulting nm-thick GaN films using AFM, XPS, and ellipsometry measurements. Finally, the optical indices measured by ellipsometry are compared with theoretical results obtained by density functional theory (DFT). Our results represent a first step toward integrating 2D materials and semiconductors into electronics and optical devices.

The choice of the ideal material employed in selector devices is a tough task both from the theoretical and experimental side, especially due to the lack of a synergistic approach between techniques able to correlate specific material properties with device characteristics. Using a material-to-device multiscale technique, a reliable protocol for an efficient characterization of the active traps in amorphous GeSe chalcogenide is proposed. The resulting trap maps trace back the specific features of materials responsible for the measured findings, and connect them to an atomistic description of the sample. The metrological approach can be straightforwardly extended to other materials and devices, which is very beneficial for an efficient material-device codesign and the optimization of novel technologies.

We theoretically and experimentally demonstrate a strong and tunable optical anisotropy in epitaxially-grown VO2 thin films. Using a combination of temperature-dependent X-ray diffraction, spectroscopic ellipsometry measurements and first-principle calculations, we reveal that these VO2 thin films present an ultra-large birefringence (Δn > 0.9). Furthermore, leveraging the insulator-to-metal transition of VO2, we demonstrate a dynamic reconfiguration of optical properties from birefringent to hyperbolic, which are two distinctive regimes of anisotropy. Such a naturally birefringent and dynamically switchable platform paves the way for multi-functional devices exploiting tunable anisotropy and hyperbolic dispersion.

Stacking layered two-dimensional materials in a type-II band alignment block has provided a high-performance method in photocatalytic water-splitting technology. The key parameters in such heterostructure configurations are the valence and conduction band offsets at the interface, which determine the device performance. Here, based on density functional theory calculations, the bandgap and band offsets at C2N/MSe2 (M = Mo, W) interfaces have been engineered. The main findings demonstrate that the C2N monolayer interacts with both MoSe2 and WSe2 monolayers through weak van der Waals interactions. These heterostructures possess a narrower indirect bandgap and a typical type-II heterostructure feature, being suitable for promoting the separation of photogenerated electron-hole pairs. The calculated Gibbs free energy of hydrogen adsorption demonstrates a reduction in the overpotential, towards the hydrogen evolution reaction, upon forming heterostructures. To further tune the bandgap values and band offsets of heterostructures, the external perturbations are included through a vertical strain and finite electric field. It is found that both the vertical strain and electric field strongly modulate the bandgap values and the magnitude of the band offsets, while the typical type-II band alignment remains preserved. It is noticeable that the band offset magnitudes of the C2N/MoSe2 and C2N/WSe2 heterostructures are more sensitive to an external electric field than to a vertical interlayer strain.

The synthesis of a supramolecular hybrid perchlorate salt using the aromatic amine 2,6-diaminopyridine afforded an unexpected crystal structure with non centrosymmetric crystallographic symmetry. The synthesized material with formula (C 5 H 8 N 3 )ClO 4 crystallize according to the monoclinic symmetry in the space group P 2 1 . The observed supramolecular crystal structure, consisting of only weak interactions in all the three dimensions, built from hydrogen bonds and through Anion...pi+ interaction between (ClO 4 ) - tetrahedrons of the mineral part and the aromatic rings of the organic molecules, is rarely observed for similar compounds. The electronic and optical properties are characterized at the theoretical level using DFT; these characterizations demonstrate a moderate direct band gap and a high optical anisotropy. Our results provide the essential information about structural, electronic and optical properties of (C 5 H 8 N 3 )ClO 4 , which are useful for guiding the future studies. (c) 2022 Elsevier B.V. All rights reserved.

Among the two half-reactions of the electrochemical water splitting, the anodic oxygen evolution reaction (OER) is a kinetically sluggish process and usually requires noble metal oxide catalysts. Therefore, the development of highly active, noble metal-free, and durable catalysts for OER is an urgent need for sustainable applications. The encapsulation of transition-metal alloyed nanoparticles in graphitic carbon layers, with core-shell features, is a viable approach for establishing an undegradable and highly efficient catalyst toward OER. Furthermore, the reactivity of the carbon shell surface can be further improved by the electron transfers between the core alloy and carbon shell, through a control of the alloyed nanoparticle compositions, or through a chemical doping. Nevertheless, it is still not clear whether the incorporation of a chemical dopant directly into the carbon shells systematically promotes the catalytic activities toward OER. To clarify this point, we synthetize trimetallic (CoNiFe) nanoparticles encapsulated in graphitic carbon shells by pyrolysis of metal-organic frameworks. We then investigate the effect of a doped graphitic carbon shell by incorporating non-metallic elements such as sulfur, phosphorus, and selenium. The main finding is that all doped CoNiFe@C core-shell catalysts exhibit an enhanced catalytic activity toward OER in the alkaline electrolyte with a low overpotential, a small Tafel slope, and long stability, which are all being comparable to those of RuO2 benchmark. Electrochemical impedance spectroscopy, X-ray photoelectron spectroscopic, and Raman measurements collected at different applied potentials during the OER process indicate that the doping of graphitic carbon shell significantly improves the interfacial electron-transfer kinetics and facilitates the adsorption of OH- ion as well as promotes the formation of metal oxyhydroxide, which positively affect the OER performances. Moreover, this improvement in OER efficiency upon incorporating of a chemical doping is rationalized by the optimization of the Gibbs free energy of OER intermediates, thereby remarkably reducing the required energy input of rate-determining step, as elucidated by the density functional theory calculations. (c) 2021 Elsevier Ltd. All rights reserved.

Graphdiyne (GDY), which features a highly π‐conjugated structure, direct bandgap, and high charge carrier mobility, presents the major requirements for photocatalysis. Up to now, all photocatalytic studies are performed without paying too much attention on the GDY bandgap (1.1 eV at the G0W0 many‐body theory level). Such a narrow bandgap is not suitable for the band alignment between GDY and other semiconductors, making it difficult to achieve efficient photogenerated charge carrier separation. Herein, for the first time, it is demonstrated that tuning the electronic bandgap of GDY via H‐substitution (H‐GDY) promotes interfacial charge separation and improves photocatalytic H2 evolution. The H‐GDY exhibits an increased bandgap energy (≈2.5 eV) and exploitable conduction band minimum and valence band maximum edges. As a representative semiconductor, TiO2 is hybridized with both H‐GDY and GDY to fabricate a heterojunction. Compared to the GDY/TiO2, the H‐GDY/TiO2 heterojunction leads to a remarkable enhancement of the photocatalytic H2 generation by 1.35 times under UV–visible illumination (6200 µmol h−1 g−1) and four times under visible light (670 µmol h−1 g−1). Such enhancement is attributed to the suitable band alignment between H‐GDY and TiO2, which efficiently promotes the photogenerated electron and hole separation, as supported by density functional theory calculations.

Large contact resistance at metal-substrate/ZnO heterostructure interfaces prevents achieving highly efficient device performance. Herein, we present a systematic study on the effect of Ni-doping in the reduction of the Schottky contact barrier at metal-substrate/ZnO heterostructure. To this end, Ni-doped zinc oxide (Ni:ZnO) thin films were deposited on glass substrate by a spray technique with different Ni-doping concentrations. X-ray Diffraction and Atomic Force Microscopy (AFM) measurements showed that Ni-doping enhances the surface uniformity as compared to the undoped-ZnO films and significantly decreases the roughness (RMS) from 35 to 17 nm. Conductive Atomic Force Microscopy (C-AFM) with a Bruker's platinum coated probe (Pt-Ir) tip results in the stabilization of a p-type Schottky contact with a small height barrier of similar to 0.4 eV, which is among the smallest values reported in literature for ZnO thin films. Our first principle calculations, which are based on the relative alignment of the band edges of the components, also confirm the reduction in the Schottky barrier height by Ni-doping in line with the experimental tendency. Both experimental and theoretical results provide a robust evidence of the potential of stabilization of a small p-type Schottky contact at metallic-substrate/ZnO interface through a Ni-doping.

The chemical engineering of chalcogens' defects (either selenium or sulfur) on the surfaces of Nonprecious transition metal chalcogenides provides an effective route to enhance their electrocatalytic activity. Yet, the precise role of such defects during the OER is not fully understood, which in turn precludes an efficient optimization of their catalytic activity. Herein, CoNiSe2 film, representing a multi-transition metal selenide model with selenium defects, is electro-deposited on a carbon cloth to investigate and optimize the intrinsic and dynamic behavior of selenium vacancies during the OER process. Interestingly, the defective CoNiSe2 with the optimized amount of selenium vacancies exhibits an advanced OER performance, including low overpotential (252 mV at 10 mA cm-2) and high stability for 30 h. The ex-situ XPS and Raman spectroscopies also indicate that the optimized amount of selenium vacancies can facilitate the pre-oxidation of Co and Ni sites. The latter leads to a rapid adsorption of OH- ions and a further stabilization of Co/NiOOH, which in turn mediates the efficiency of OER. The density functional theory (DFT) calculations show that the introduction of a Se-vacancy on CoNiSe2 surface enables to adjust the changes in Gibbs free energy between different OER steps and thereby, significantly reduce the overpotential.