Metallic Behavior Research Articles

Phase-field investigation of dendrite suppression strategies for all-solid-state lithium metal batteries

All-solid-state Li metal batteries are widely considered as the most promising technologies to realize the increasing safety and capacity requirements for the next generation of Li batteries. However, it is experimentally demonstrated that the penetration of stiff solid electrolytes by Li dendrites seems even more severe than that of liquid electrolytes and the abnormal failure behavior of soft Li metal penetrating hard SEs is quite elusive. In the present work, a multi-coupling phase field model of Li electrodeposition is further developed to incorporate the elasto-plastic mechanical behavior and the significant size effect of Li metal. Then, the established approach is utilized to specifically evaluate the different mechanisms of Li electrodeposition and stripping in liquid electrolytes and hard solid electrolytes with various governing factors including applied voltage, external stress, ionic conductivity, SEI inhomogeneity and interfacial porosity. Especially, we summarize the accurate quantification of the regularity of external stress mechanisms which determine whether the external stress can either promote or demote the nonuniformity of Li/ solid electrolytes interface. This research provides a theoretical perspective of mechanical mechanism to reveal the conflict of experimental optimization strategies and may be instructive for the Li/ solid electrolytes interface design of all-solid-state Li metal batteries.

On the grain level deformation of BCC metals with crystal plasticity modeling: Application to an RPV steel and the effect of irradiation

Capturing realistic deformation behavior in BCC metals at the polycrystal scale is an important aspect of predicting the material’s strength and failure. Furthermore, local deformation/strength heterogeneity also influences the lifetime and its assessments in reactor pressure vessel (RPV) steels, especially with accumulating irradiation doses during long-term operational conditions. This work utilizes a micromorphic crystal plasticity (CP) model for BCC materials with the capability to address temperature-dependent stress–strain response and irradiation effects relevant to RPV materials. The microstructure of the investigated material was characterized prior to and during testing using electron microscopy experiments, which are used for finite element model generation and simulation result validation. To analyze the validity of the model to predict strain localization under monotonic tensile loading, micro digital image correlation (DIC) was employed jointly with the CP simulations. Different modeling choices, such as the use Schmid/non-Schmid laws and gradient based CP modeling, greatly affected the capability of the model to represent similar magnitudes in strain localization and grain reorientation. The requirement for experimental measurements, including image based analyses, on a microstructural level as a validation tool for CP modeling is clearly demonstrated.

Assessing the complexation of dissolved organic matter with heavy metals (Cu2+, Pb2+) in leachate from an old Japanese landfill site using fluorescence quenching.

Dissolved organic matter (DOM) in landfill leachate impacts the toxicity, bioavailability, and migration of heavy metals. The present study investigated the complexation of heavy metals (Cu2+ and Pb2+) with DOM from two landfill leachate samples, representing an old landfill site containing incineration residues and incombustible waste. The logarithms of the stability constant (log KM) and percentage of complexed fluorophores were calculated using both the Ryan-Weber non-linear model and the modified Stern-Volmer model, yielding good agreement. The log KM values (at pH = 6.0 ± 0.1) calculated using both methods for the two sampling points were 5.02-5.13 and 4.85-5.11 for Cu2+-DOM complexation, and 5.01-5.13 and 4.46-4.87 for Pb2+-DOM complexation, respectively. Log KM was slightly higher for binding of DOM with Cu2+ than Pb2+, and the quenching degree was stronger for complexation with Cu2+ (28.5-30.6% and 38.0-45.9%) than Pb2+ (6.5-7.1% and 10.0-15.4%) in both leachate samples. While log KM values were similar, differences in the contributions of functional groups and molecular composition led to varying degrees of quenching. This study reveals the potential for heavy metal binding by DOM in landfill leachate with a unique solid waste composition and emphasizes variations in fluorescence quenching between Cu2+ and Pb2+ despite similar log KM levels. These findings may be useful for assessing heavy metal behavior in landfill leachate and its impacts on the surrounding environment.

Adsorption behavior of heavy metals onto microplastics derived from conventional and biodegradable commercial plastic products

This study extensively explored the adsorption behavior of heavy metals (Pb+2, Ni+2, Cu+2, Zn+2, and Cd+2) onto microplastics (MPs). The particle sizes of MPs ranged from 0.149 to 0.25 mm. The microplastics were generated from commercial products manufactured from both conventional (polyethylene (PE) bottle, polystyrene (PS) spoon, and polyethylene terephthalate (PET) egg carton) and biodegradable (polylactic acid (PLA) spoon, and polylactic acid (PLA) egg carton) plastics. The study also considered the influence of solution pH on the adsorption capacity of heavy metals. Regarding the adsorption potential for Cu+2, the ranking was as follows: PLA-egg (1408 μg·g−1) > PLA-spoon (735 μg·g−1) > PE-bottle (315 μg·g−1) > PET-egg (283 μg·g−1) > PS-spoon (237 μg·g−1). PLA MPs showed the highest adsorption capacity due to the lower thermal stability and higher presence of surface oxygen functional groups. Moreover, the adsorption capacities of the five metals onto PLA-spoon and PLA-egg decreased in the following order: Pb (1785 μg·g−1) > Zn (1267 μg·g−1) > Cd (748 μg·g−1) > Cu (735 μg·g−1) > Ni (722 μg·g−1), and Pb (1520 μg·g−1) > Ni (1412 μg·g−1) > Cu (1408 μg·g−1) > Zn (1118 μg·g−1) > Cd (423 μg·g−1), respectively. The SEM-EDS, FTIR and XPS results demonstrated that surface oxygen-containing functional groups play an important role during the adsorption process. This study extended its analysis to quantify the metal content of the post-adsorption MPs, revealing uneven adsorption of heavy metals onto the MPs. This implies that the diversity of commercial plastic products may result in significant variations in their ability to adsorb heavy metals, underscoring the importance of effectively managing discarded commercial plastic products.

Transformation of typical heavy metals during preparation of porous carbon from coal gasification fine slag

To clarify the fate of typical heavy metals during preparation process of porous carbon, we produced porous carbon using a two-step strategy and investigated the transformation behavior of heavy metals including V, Cr, Mn, Cu, Zn and Ba in the process. A modified Community Bureau of Reference (BCR) sequential extraction procedure was implemented to reveal the occurrence state of heavy metals. The results clearly showed that the transformation of selected heavy metals in activated carbon was mostly determined by the occurrence state, volatile nature and operating procedure. In addition to the new aluminosilicate minerals, abundant pore structure also had evident dependence on solidifying heavy metals in porous carbon. The content of heavy metals in activated carbon were considerably lower than YKFS, and V, Cr and Cu were more accumulated in porous carbon. It is also important to note that Zn possessed markedly lower content and REI value (3.61–56.17 %) in activated carbon and porous carbon owning to the volatile nature of Zn and acid solubility of Zn-base minerals. The Risk Assessment Code (RAC) evaluation conducted that the acid leaching operation effectively decreased the hazardous properties of heavy metals. The research results can provide significant guidance for value-added utilization of gasification fine slag in an environmentally friendly.

Characterization and modeling of biaxial plastic anisotropy in metallic sheets

The anisotropic behavior of cold-rolled sheet metals has been extensively studied, typically characterized by uniaxial loading tests in different directions to determine yield strengths and plastic flow (Lankford r-values). However, biaxial principal stress states often focus solely on yield loci in the RD/TD (rolling/transverse) directions, with limited studies on other loading directions. This study analyzes the relationship between the principal stress yield loci and the material principal anisotropic stress directions based on the Hill48 yield function. Biaxial tensile tests are conducted on DP590 high-strength steel using cruciform specimens cut in various directions different from the sheet RD/TD directions. The evolution of the anisotropic yield locus and plastic flow of DP590 under biaxial tensile directions beyond the traditional RD/TD anisotropic directions is revealed. Furthermore, a modified Hill48 model that considers any principal stress direction of loading is proposed, accurately describing the continuous evolution of equi-biaxial tension stress, near-plane strain stress, and plastic flow direction for different principal stress directions of loading. The issues of failing to predict plastic flow under biaxial loadings in the original Hill48 model, and the degraded representation of existing constitutive models when accounting for out-of-plane anisotropy are addressed by decoupling the relationship between anisotropic parameters and stress state. In addition, the proposed calibration strategy of anisotropic parameters, using experimental data from various principal stress directions of loading, effectively captures the anisotropic evolution caused by changes in principal stress directions. Comprehensive evaluation and verification of the plasticity model should consider yield locus for any principal stress directions of loading, and also the normal and shear planes.

Electrochemical-Mechanical Coupling Experiments on Solid-State Battery Materials and Interfaces

The interaction of mechanical and electrochemical effects in the areas of thermodynamics, kinetics, and transport are known, but accurately measuring those couplings can be complicated by the difficulty of getting a uniform stress at a solid-solid interface where surface roughness, voids and inclusion result in inhomogeneous contacts.1 This is a particular challenge for solid-state batteries where both the active material and electrolyte are in the solid phase. In addition, cycling of metal anodes (e.g., Na and Li) results in processes such as dendrite and void formation in which electrochemical and mechanical can be amplified.2 This talk will focus on two improvements to typical experiments in which electrochemical-mechanical coupling strongly influences results: (1) The use of a solid-state reference electrode for solid Na / solid electrolyte interfacial studies, and (2) the use of a nanoindentation platform to assess electrochemical-mechanical coupling behaviors in thin-film solid-state materials.For topic (1), we will discuss our use of a solid Na metal reference electrode to inform the cycling of Na metal against a Na-β″-Al2O3 solid electrolyte using cycling protocols of commercial relevance (e.g., 5 mAh/cm2, <5 MPa external pressure).2 Our work identifies the current density at which stripping shifts from stable to unstable, and shows that large (100s of mV) of interfacial overpotential can persist for hours in some stripping test protocols, which, according to models indicates that >80% of the initial electrochemical active interfacial area has been lost. We will also present recent results on how the applied pressure affects the interfacial behavior of Na metal / Na-β″-Al2O3 interfaces, as well as the role of applied pressure and the cycling protocol for an interface that starts as only current collector / Na-β″-Al2O3.For topic (2), we will present on our electrochemical-mechanical coupling studies of thin-film (i.e., <1 um thick) V2O5 samples on an Ohara Li+-conducting glass ceramic disc. This platform allows us to simultaneously control electrochemical behavior (e.g., Li insertion and removal, current) and the applied force, to study thermodynamic and kinetic electrochemical-mechanical couplings. This platform enables the application of a uniform stress on the thin-film V2O5 sample.1. Joule 7, 652–674, April 19, 2023.2. ACS Appl. Mater. Interfaces 2023, 15, 42, 49213–49222

Understanding Organic Components of Solid Electrolyte Interphase on the Lithium Metal Anode

Lithium ion batteries have become the dominant form of energy storage used in consumer electronics and, recently, electric vehicles. However, high costs have prevented widespread deployment of lithium ion batteries for applications other than portable electronics, and the safety issues associated with liquid organic electrolytes remain to be addressed. In order to enable the greater utilization of electric vehicles, allow for grid scale energy storage, and meet the demands of new electronic applications, new materials for high energy density batteries must be developed. High capacity electrode materials like lithium metal have the potential to facilitate these technologies, but lithium metal electrodes are presently limited by significant side reactions, poor quality deposition, and the potential to form hazardous dendrites. Therefore, it is important to develop a clear understanding of the surface reactivity and growth behavior of the lithium metal at the interface with the electrolyte in order to enable stable long-term cycling.The products that form as a result of electrolyte decomposition reactions at the electrode interface are known to be extremely important in determining the final cell performance, yet characterizing the organic components of the solid electrolyte interphase (SEI) proves to be challenging with typical techniques. In this presentation we will discuss a combination of in operando and ex situ techniques developed in efforts to more comprehensively characterize the reaction intermediates and organic SEI products of electrolyte reduction reactions. Specifically, we employ electron paramagnetic resonance (EPR) spectroscopy to identify radical intermediates and clarify electrolyte reduction mechanisms. Furthermore, SEI product formation with respect to electrochemical potential and time will be described using data obtained from in operando ATR-FTIR. ATR-FTIR is a powerful method for characterizing organic species, but typical materials such as Ge, Si, and ZnSe react with lithium metal during deposition. In this talk, the development of a diamond based operando cell and its application to the characterization of organosulfur and fluorinated electrolytes will be discussed. With the understanding developed from these two techniques we provide new insights into the formation and chemical nature of SEI components that promote stable cycling of lithium metal electrodes.

Voltammetric Investigations of Low-Coordinate Manganese As an Electrocatalyst for Organic Coupling and Functionalization

Earth-abundant, first row, low coordinate transition metal complexes are considered promising candidates for the electrocatalysis of many organic reactions. The use of transition metal catalysts is no stranger to the field of synthetic organic chemistry; however, many of the existing catalysts require the use of rare and expensive metals such as platinum, palladium, ruthenium, and iridium. In this study, we seek to identify and understand the mechanism by which an Mn(I) complex may act as an electrocatalyst for simple organic coupling and C-H functionalization reactions, beginning with the radicalization of benzyl bromide and benzyl chloride as representative organic halide substrates. The careful design and evaluation of this manganese complex has the potential to offer an effective non-toxic 3d metal electrocatalyst, provided that common transition metal behaviors, such as disproportionation or carbon-metal bond formation, are avoided. Initial characterization of the complex by cyclic voltammetry (CV) demonstrates general reversibility of the catalyst, while additions of benzyl halide exhibit the execution of the electron transfer and catalytic step. Electrochemical simulations run in parallel with CV experiments provide supplementary guidance into the relationship between the substrate and catalyst. Additional studies conducted using bulk electrolysis, mass spectrometry, nuclear magnetic resonance, and density functional theory (DFT) calculation will help to further characterize the mechanism and supporting details. Elucidation of the catalytic mechanism will allow us to use this electrocatalyst in a series of synthetic organic conditions and aid in our advancement toward sustainable organometallic catalysis.

(Invited) Is the [5,5] C120-D5d Fullertube a Metal?

Single-walled nanotubes (SWNTs) with a roll-up vector (n = m) have long been predicted to exhibit metallic behavior at infinite nanotube lengths.1 Moreover, in contrast with normal metals, armchair nanotubes e.g., [5,5] should result in exceptional ballistic transport properties.2 However, the question remains at what finite length of a SWNT or fullertube does metallic character begin, if at all. From a computational viewpoint, Cioslowski has shown that the HOMO-LUMO bandgap decreases to ~1 eV in an oscillatory fashion in approaching [5,5] C200 fullertubes.3 Although the HOMO-LUMO gap has been extensively used to predict the kinetic stability of fullerenes and metallofullerenes with less than 100 carbon atoms, Fowler has argued that the HOMO-LUMO gap can not be used to predict the kinetic stability of large fullerenes.4 More recently, Lilijeroth and coworkers have measured the experimental and DFT HOMO-LUMO gap for graphene nanoribbons (GNR) and find that GNRs with lengths of 5 nm have bandgaps of ~0.1 eV.5 These workers also point out that DFT calculated HOMO-LUMO bandgaps are always overestimated relative to experimental values. In this presentation, I will present the evidence in support of metallic character for [5,5] C120-D5d.6,7

Tuning the ground state of cuprate superconducting thin films by nanofaceted substrates

Anisotropic transport properties have been assessed in a number of cuprate superconductors, providing evidence for a nematic state. We have recently shown that in ultra-thin YBa2Cu3O7−δ films, where nematicity is induced via strain engineering, there is a suppression of charge density wave scattering along the orthorhombic a-axis and a concomitant enhancement of strange metal behavior along the b-axis. Here we develop a microscopic model, that is based on the strong interaction between the substrate facets and the thin film, to account for the unconventional phenomenology. Based on the atomic force microscopy imaging of the substrates’ surface, the model is able to predict the absence (presence) of nematicity and the resulting transport properties in films grown on SrTiO3 (MgO) substrates. Our result paves the way to new tuning capabilities of the ground state of high-temperature superconductors by substrate engineering.

The structural, electronic and magnetic properties of [formula omitted] anti-perovskite

A comprehensive exploration of the structural, electronic, and magnetic attributes of anti-perovskite Fe3ZnC carbides was carried out using Density Functional Theory (DFT) and Monte Carlo Simulation (MCS). These anti-perovskite materials possess a unique structure where cation and anion positions are interchanged within the perovskite framework. Our study involves a comparative analysis of the electronic band structures and density of states (DOS) for Fe3ZnC, considering prior theoretical and experimental research. Understanding these anti-perovskite materials' band structures and DOS is pivotal for their effective utilization in magnetic sensors and magnetic refrigeration applications. Our results indicate that Fe3ZnC displays ferromagnetic metallic behavior, particularly when applying the Generalized Gradient Approximation (GGA). Notably, there is a significant overlap between the valence (VB) and conduction (CB) bands. Furthermore, MCS predicts a second-order ferromagnetic-to-paramagnetic transition in the anti-perovskite Fe3ZnC compound, characterized by a notably high Curie temperature. These insights advance our understanding of these materials, paving the way for their effective utilization in magnetic technologies.

Ab-initio investigation of stability and electronic properties of new yttrium-based solid solution of double transition metal YMXTx (M= Ti and zr; X= C and N; Tx= H, O, and F) MXenes

In this study, an extensive examination was carried out to explore the stability, and physical characteristics of a novel solid solution comprising two transition metals, namely YMXTx (M = Ti and Zr; X = C and N; Tx = H, O, and F) MXenes. The Quantum-Espresso computational package which is based on density functional theory (DFT) and pseudopotential techniques has been employed for performing calculations. The results obtained from the investigation of the structural, dynamic, and mechanical stability revealed that the YTiX and YZrX MXenes exhibited remarkable stability. These findings strongly suggest the feasibility and potential success of experimental synthesis of these materials. Furthermore, the result of cohesive energy indicated that Nitride-based MXenes (Y(Ti/Zr)N) are more stable than Carbide-based MXenes (Y(Ti/Zr)C). By performing comprehensive calculations of the total density of states and band structures, using various approximations such as LDA, GGA, and GGA + HSE06, the investigation revealed a metallic behavior in YMX MXenes. Additionally, the analysis of the TDOS revealed the presence of a pseudogap close to the Fermi level. This observation provides further evidence for the structural stability of the investigated mentioned MXenes. After undergoing saturation with different functional groups, it was observed that the YTiNH2 and YTiNF2 MXenes displayed semiconducting behavior, whereas YZrNO2 is classified as a non-conductive material, and the remaining monolayers exhibit metallic properties. Analyses of charge density and electron localization functions confirmed the metallic behavior of the studied MXenes. Modifying the electronic band gap of 2D MXenes, while also preserving structural stability, could lead to enhancing the application of these materials in optoelectronic devices.

Adaptation of the BaAl4 Type to Small Cations: The Case of NaGa4.

Single-phase NaGa4 samples were prepared by annealing stoichiometric element mixtures at 200 °C, 300 °C, and 450 °C in closed tantalum ampoules. No compositional homogeneity range was detected. While single crystals annealed at 200 °C feature a fully ordered crystal structure, a crystal annealed at 300 °C reveals a defect with mutual exchange of Na atoms and Ga2 dumbbells. The structure motif caused by the violation of translational symmetry resembles the CeMg2Si2 type of structure. The NaGa4 structure constitutes a branch of the BaAl4 type distinguished by a notably high c/a ratio (space group I4/mmm; a=4.2261(1) Å, c=11.2633(6) Å, c/a=2.67). This distortion enables the adaption of the BaAl4-type to small cations, when the further shortening of the apical-apical distance d(Ga-Ga) is not possible any more. Therefore, NaGa4 offers an adaption alternative to the monoclinic distortion observed in CaGa4 and YbGa4. Conductivity measurements validate the metallic behavior anticipated by NMR measurements in earlier studies.

Water Effect on the Electronic Properties and Lithium-Ion Conduction in a Defect-Engineered LiFePO4 Electrode

Defect-engineering accelerates the conduction of lithium ions in the cathode materials of lithium-ion batteries. However, the effects of defect-engineering on ion conduction and its mechanisms in humid environments remain unclear in the academic discourse. Here, we report on the effect of vacancy defects on the electronic properties of and Li-ion diffusion in a LiFePO4 material in humid environments. The research findings indicate that vacancy defects reduce the lattice constant and unit cell volume of LiFePO4. Additionally, the water molecules occupy the Li-ion vacancies, leading to an increase in the lattice constant of LiFePO4. The computational results of the electronic properties show that the introduction of water molecules induces a transition in LiFePO4 from a semiconductor to a metallic behavior, with a transfer of 0.38 e of charge from the water molecules to LiFePO4. Additionally, the migration barrier for Li ions in the H2O + LiFePO4 system is found to be 0.50 eV, representing an 11.1% increase compared to the pristine LiFePO4 migration barrier. Our findings suggest that water molecules impede the migration of Li ions and provide important insights into the effect of defect-engineering on electronic properties and ion conduction under humid conditions.

Adsorption and migration behaviors of heavy metals (As, Cd, and Cr) in single and binary systems in typical Chinese soils

In this study, the competitive adsorption and migration behaviors of arsenic (As), cadmium (Cd), and chromium (Cr) in typical Chinese soils were investigated. It was observed that Hainan, Shanxi, and Zhejiang Mengjiadai soils exhibited the highest adsorption capacities for As (563 μg/g), Cd (653 μg/g), and Cr (383 μg/g), respectively. Heavy metals (HMs) adsorption capacities were predicted by Extreme gradient boosting (XGBoost) models, and the Shapley additive explanation (SHAP) was employed to elucidate the effect of soil physicochemical properties on target values. Due to redox and complexation reaction, the primary factor affecting adsorption has changed from free state manganese (Mn) in single As system to antimony (Sb) in As/Cd and As/Cr systems. Furthermore, the maximum adsorption capacity (Qm) of As increased by 49.4 % with the addition of Cd into Heilongjiang soil. Finally, the migration process of HMs in Heilongjiang, Hebei, and Hainan soils was simulated by column experiments. With a relatively large dispersion coefficient (D = 29.630 cm2/h) and small retardation factor (Rh = 0.030), Cr penetrated fastest in Heilongjiang soil. This research demonstrates that both the types and coexistence of HMs may affect the HMs behaviors in soil.

Optical and conduction mechanism study of lead-free CsMnCl3 perovskite

Perovskite research is increasingly focused on lead-free, all-inorganic single crystals due to their lower biological toxicity and greater material stability. While lead-based perovskites show promise, their environmental and health risks are concerning. Lead-free alternatives provide a safer, more sustainable option with enhanced durability for practical applications. Nevertheless, this paper presents a structural, optical, and conduction study of lead-free CsMnCl3 perovskite. The material was prepared using slow evaporation method, the obtained compound crystallizes in a hexagonal system with the space group R-3m. The absorption spectrum of CsMnCl3 is analyzed, and the band gap is estimated to be 4.56 eV. The electrical study is crucial for the identification of the allows us to classify CsMnCl3 as a semiconductor, with conductivity increasing with temperature, as expected. The temperature-dependent electrical conductivity (dc) reveals a transition from semiconductor to metallic behavior at 383 K and from metallic to semiconductor behavior at 403 K. The study of the conductivity in the semiconductor phase indicates that above 383 K, the conductivity of CsMnCl3 is governed by a single polaron hopping, and that below 403 K. In summary the comprehensive study of the structural, optical, and electrical properties of CsMnCl3 provides valuable insights into its semiconductor characteristics and charge transport mechanisms, highlighting its potential for sustainable and environmentally friendly applications.

Study on interface characteristics and electron spin manipulation of two-dimensional [formula omitted] (001) heterostructures

Among numerous strongly correlated electron materials, cobalt ions in perovskite oxide SrCoO3 (SCO) possess rich valence states, playing a crucial role in determining the functionality of the material. Meanwhile, thin film preparation techniques such as epitaxial growth and pulsed laser deposition have accelerated the research progress of two-dimensional SCO films. In this study, we primarily investigate the interface characteristics and electron spin of two-dimensional SrCoO3/LaAlO3 (SCO/LAO) (SCO/LAO) (001) heterostructures using density functional theory, revealing the influence of atomic layer thickness and doping on the heterostructure. Our research results indicate that SCO/LAO heterostructures exhibit metallic behavior and ferromagnetism, and doping with oxygen vacancies and Nb2+ ions leads to significant changes in electronic properties and magnetism of the heterostructure. These findings provide insights for the experimental synthesis of two-dimensional heterostructures and suggest that two-dimensional SCO/LAO (001) heterostructures possess rich interface properties with tremendous potential applications in electronic devices.

Electron-phonon interaction, magnetic phase transition, charge density waves, and resistive switching in VS2 and VSe2 revealed by Yanson point-contact spectroscopy

VS2 and VSe2 have attracted particular attention among the transition metals dichalcogenides because of their promising physical properties concerning magnetic ordering, charge density wave (CDW), emergent superconductivity, etc., which are very sensitive to stoichiometry and dimensionality reduction. Yanson point-contact (PC) spectroscopic study reveals metallic and nonmetallic states in VS2 PCs, as well as a magnetic phase transition, was detected near 20 K. The rare PC spectra, where the magnetic phase transition was not visible, show a broad maximum of around 20 mV, likely connected with electron-phonon interaction (EPI). The PC spectra of VSe2 demonstrate metallic behavior, which allowed us to detect features associated with EPI and CDW transition. The Kondo effect appeared for both compounds, apparently due to interlayer vanadium ions. Besides, the resistive switching was observed in PCs on VSe2 between a low resistive, mainly metallic-type state, and a high resistive nonmetallic-type state by applying bias voltage (about 0.4 V). Reverse switching occurs by applying a voltage of opposite polarity (about 0.4 V). The reason may be the alteration of stoichiometry in the PC core due to the displacement of V ions to interlayer under a high electric field. The observed resistive switching characterizes VSe2 as a potential material, e.g., for non-volatile resistive RAM, neuromorphic engineering, and other nanoelectronic applications. Per contra, VS2 attracts attention as a rare layered van der Waals compound with magnetic transition.

From MAX to MXene: A case study of Sc2TlC MAX phase, Sc2C pristine MXene, and surface-functionalized Sc2CT2 (T=O, F) MXenes

The MXenes-based two-dimensional (2D) materials have become a hotspot of recent research due to their exciting physical behavior. In this first-principles study, we investigate the Sc2TlC MAX phase and its 2D derivatives, focusing on the transformation of Sc2TlC into a Sc2C 2D sheet and the subsequent surface functionalization of Sc2C with oxygen (O) and fluorine (F). Investigations of the electronic structures reveal that the Sc2TlC MAX phase is metallic and nonmagnetic, while the conversion to the Sc2C pristine monolayer and surface functionalized Sc2CO2 induces significant changes in its electronic structure but retains its metallic behavior. On the other hand, functionalization of the Sc2C monolayer with F2 (such as Sc2CF2) evolves semiconductor-like behavior with an energy gap of magnitude 1.325 eV for the up-spin state and 1.227 eV for the down-spin state. The optimal optical absorption of ultraviolet (UV) for Sc2TlC MAX phase and Sc2C, Sc2CO2, and Sc2CF2 MXenes have been found as 13.8 × 105 cm−1, 49.17 × 104 cm−1, 81.39 × 104 cm−1, and 69. ×104 cm−1, respectively. Investigations of the optical properties reveal considerable reflection and absorption capabilities, particularly upon exposure to low-energy light photons, suggesting their potential for optoelectronic and energy harvesting applications.