3d Transition Metal Research Articles

2D MXenes: Synthesis, Properties, and Applications in Silicon-Based Optoelectronic Devices.

MXenes, a rapidly emerging class of 2D transition metal carbides, nitrides, and carbonitrides, have attracted significant attention for their outstanding properties, including high electrical conductivity, tunable work function, and solution processability. These characteristics have made MXenes highly versatile and widely adopted in the next generation of optoelectronic devices, such as perovskite and organic solar cells. However, their integration into silicon-based optoelectronic devices remains relatively underexplored, despite silicon's dominance in the semiconductor industry. In this review, a timely summary of the recent progress in utilizing Ti-based MXenes, particularly Ti3C2Tx, in silicon-based optoelectronic devices is provided. The composition, synthesis methods, and key properties of MXenes that contribute to their potential for enhanced device performance are focused on. Furthermore, the latest advancements in MXene applications in silicon-based solar cells and photodetectors are discussed from fundamental and applied perspectives. Finally, the key challenges and future opportunities for the integration of MXenes in silicon-based optoelectronic devices are outlined.

Cohesive, Vibrational, and Chemical Bonding Features of 3d- and 4d-Transition Metal MX (X = C, N) Compounds: A Systematic Approach

Cohesive, Vibrational, and Chemical Bonding Features of 3d- and 4d-Transition Metal MX (X = C, N) Compounds: A Systematic Approach

Defect-assisted reversible phase transition in mono- and few-layer ReS2

2D transition metal dichalcogenide (TMD) materials have attracted interest due to their remarkable excitonic, optical, electrical, and mechanical properties, which are dependent on their crystal structure. Consequently, controlling the crystal structure of these materials is essential for fine-tuning their performance, e.g., linear and nonlinear optical, as well as charge transport properties. While various phase-switching TMD materials are available, their transitions are often irreversible. Here, we investigate the mechanism of a light-induced reversible phase transition in mono- and bilayer rhenium disulfide (ReS2). Our observations, based on transmission electron microscopy, nonlinear spectroscopy, and density functional theory, reveal a transition from the ground T″ (double-distorted T) to the metastable H′ (distorted H) phase under femtosecond laser irradiation or influence of highly-energetic electrons. We show that the formation of sulfur vacancies facilitates this phenomenon. Our findings pave the way toward manipulating the crystal structure of ReS2 and possibly its heterostructures.

Unraveling 3d Transition Metal (Ni, Co, Mn, Fe, Cr, V) Ions Migration in Layered Oxide Cathodes: A Pathway to Superior Li-Ion and Na-Ion Battery Cathodes.

Li-ion and Na-ion batteries are promising systems for powering electric vehicles and grid storage. Layered 3d transition metal oxides AxTMO2 (A = Li, Na; TM = 3d transition metals; 0<x≤2) have drawn extensive attention as cathode materials due to their exceptional energy densities. However, they suffer from several technical challenges caused by crystal structure degradation associated with TM ions migration, such as poor cycling stability, inferior rate capability, significant voltage hysteresis, and serious voltage decay. Aiming to tackle these challenges, this review provides an in-depth discussion and comprehensive understanding of the TM ions migration behaviors in AxTMO2. First, the key thermodynamics and kinetics that impact TM ions migration are discussed, covering ionic radius, electronic configuration, crystal structure arrangement, and migration energy barrier. In particular, details are provided regarding the universal and specific migration characteristics of Ni, Co, Mn, Fe, Cr, and V ions in layered cathode materials. Subsequently, the impacts of these migrations on electrochemical performance are emphasized in terms of the fundamental science behind technical issues, and strategies to modulate TM ions migration for advanced cathode materials development are summarized. Besides, characterization techniques for probing the TM ions migration are present, like neutron diffraction (ND), scanning transmission electron microscopy (STEM), nuclear magnetic resonance (NMR), and others. Finally, future directions in this regard are comprehensively concluded. This review offers valuable insights into the basic design of advanced layered oxide cathode materials for Li-ion and Na-ion batteries.

High-Specific Power Flexible Photovoltaics from Large-Area MoS2 for Space Applications.

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) such as MoS2 and WSe2 are excellent candidates for photovoltaic (PV) applications. Here, we present the modeling, fabrication, and characterization of large-area CVD-grown MoS2-based flexible PV on an off-the-shelf, 3 μm-thick flexible colorless polyimide with polyimide encapsulation designed for space structures. The devices are characterized under 1 sun AM0 illumination and show a V OC of 0.180 V and a specific power of 0.001 kW/kg for a subnanometer-thick, single MoS2 monolayer absorber. Model projections indicate that the polyimide encapsulant introduces negligible absorption loss, and up to 12.97 kW/kg specific power is attainable for a 100 nm-thick MoS2 absorber layer. The devices maintain their performance after repetitive bending down to 5 mm bend radius. An increase in performance is measured after radiation exposure to 1 MeV e- fluence, which is partially attributed to defect healing. Techno-economic analysis shows that even with a lower efficiency, the specific power of a 2D PV array designed for a 6U CubeSat is 2 orders of magnitude higher, and the cost to deploy in space is 2 orders of magnitude less than that of a Si panel used in space. This indicates that the 2D TMDC-based PV has great potential for space applications.

Untangling the role of single-atom substitution on the improvement of the hydrogen evolution reaction of Y2NS2 MXene in acidic media.

The production of hydrogen (H2) fuel through electrocatalysis is emerging as a sustainable alternative to conventional and environmentally harmful energy sources. However, the discovery of cost-effective and efficient materials for this purpose remains a significant challenge. In this study, we explore the potential of the transition-metal-substituted Y2NS2 MXene as a promising candidate for hydrogen production through the hydrogen evolution reaction (HER). Using density functional theory (DFT) calculations, we first analyzed the Pourbaix diagram, and dissolution potential which showed the stability and resistance to corrosion of the sulfur termination. Later, we address the kinetic limitations of HER on bare Y2NS2 by introducing single-atom substitutions of Y atoms with 3d transition metals. Nine distinct structures were evaluated, revealing that Fe-substituted Y2NS2 exhibits the highest HER activity under acidic conditions, as indicated by volcano plot analyses. Further investigation of the bonding characteristics and electronic density of states highlights the crucial role of Fe d-orbitals and the weak interactions at the sulfur-terminated surface in enhancing the HER efficiency. These findings provide insights into the design of advanced, cost-effective materials for HER catalysis, paving the way for their application as efficient electrochemical catalysts across a wide pH range.

Two-dimensional Czochralski growth of single-crystal MoS2.

Batch production of single-crystal two-dimensional (2D) transition metal dichalcogenides is one prerequisite for the fabrication of next-generation integrated circuits. Contemporary strategies for the wafer-scale high-quality crystallinity of 2D materials centre on merging unidirectionally aligned, differently sized domains. However, an imperfectly merged area with a translational lattice brings about a high defect density and low device uniformity, which restricts the application of the 2D materials. Here we establish a liquid-to-solid crystallization in 2D space that can rapidly grow a centimetre-scale single-crystal MoS2 domain with no grain boundaries. The large MoS2 single crystal obtained shows superb uniformity and high quality with an ultra-low defect density. A statistical analysis of field effect transistors fabricated from the MoS2 reveals a high device yield and minimal variation in mobility, positioning this FET as an advanced standard monolayer MoS2 device. This 2D Czochralski method has implications for fabricating high-quality and scalable 2D semiconductor materials and devices.

A HTO-Type Nonlinear Optical Fluorophosphate with Ultrawide Bandgap.

Compounds having hexagonal tungsten oxides (HTO) topology are of intense research interests owing to their potential functional properties, such as nonlinear optical (NLO) performances. However, most of the reported HTO-type compounds exhibit narrow optical bandgaps because of the d-d electronic transition of compositional d0 transition metals and lone pair electrons effect of Se4+/Te4+, which hinder their applications in the high-energy field, such as deep-ultraviolet (deep-UV) region. In this work, a new fluorophosphate, (NH4)3[Sc3F5(PO4)](PO3F)2 exhibiting HTO-topological structures is reported. Remarkably, the compound shows an ultrawide optical bandgap (Eg(exp.) >6.2eV, Eg(cal.) = 6.724eV), surpassing those of reported HTO-type compounds. The fact is mainly ascribed to the collaborative effect of Sc3+ without destructive d-d transition, the PO4 and PO3F units having large LUMO-HOMO gaps. Furthermore, it exhibits a phase-matchable second-harmonic generation (SHG) response of ca. 0.65 × KDP, underscoring its potential use for NLO applications. This work provides new opportunities for discovering HTO-type NLO crystals with ultrawide bandgaps.

Deciphering the Role of Second Metal in M-Ni (M = Fe, Ni, and Mn) Heterobimetallic Electrocatalysts in Controlling the HAT versus Hydride Transfer Mechanism for the Dehydrogenation of Alcohols.

The second 3d-transition metal incorporation in Ni-(oxy)hydroxide has a drastic effect on alkaline OER and alcohol dehydrogenation reactivity. While Mn incorporation suppresses the alkaline OER, it greatly improves the alcohol dehydrogenation reactivity. A complete reversal of reactivity is obtained when Fe is incorporated, which shows better performance for alkaline OER with poor alcohol dehydrogenation reactivity. The role of the second 3d-metal is elusive due to the lack of systematic mechanistic studies. In this report, we thoroughly analyzed a series of M─Ni (M = Fe, Ni, Mn) (oxy)hydroxides derived from electrochemical activation of M-MOF grown on nickel foam for its electrochemical activity in alkaline OER and aliphatic, benzyl alcohol dehydrogenation. With the help of pH-dependence and kinetic isotope effect studies, the potential-determining step (PDS) and the rate-determining step (RDS) have been elucidated. The Hammett analysis revealed critical information about the transition state and offered insight into the hydrogen atom transfer (HAT) versus hydride transfer (HT) for alcohol dehydrogenation operative in various heterobimetallic electrocatalysts. Further, the superior alcohol dehydrogenation reactivity of NiMn catalyst for PET hydrolysate electro-oxidation is extended to afford valuable chemicals with concomitant production of hydrogen.

Bottom-up constructing of two-dimensional ferromagnets with high Curie temperature by assembling 5d transition metal atom@MnSr8 magnetic superatoms

Abstract The development of advanced spintronic devices requires ultrathin two-dimensional (2D) ferromagnetic (FM) materials with high Curie temperature (T C) and large out-of plane magnetic anisotropy energy (MAE). However, the number of high-T C 2D ferromagnets synthesized through top-down experimental methods is very limited. Here, we propose a bottom-up approach for constructing 2D ferromagnets with high T C by assembling magnetic superatoms. The MnSr9 superatom was first selected as building blocks to construct a series of 2D materials with square, triangular and hexagonal honeycomb lattices. First-principles studies show that all the MnSr9 self-assembled films are thermodynamically stable and exhibit ferromagnetism, unfortunately, they lack the necessary magnetic anisotropy. By substituting one Sr atom with a heavy 5d transition metal (5d-TM) atom, all these 5d-TM@MnSr8 clusters show enhanced stability and symmetry, and their self-assembled hexagonal honeycomb crystals exhibit significant magnetic anisotropy and enhanced ferromagnetism from 5d-TM atoms. Taking the PtMnSr8 superatom as an example, we have demonstrated these characteristics in detail, and the T C and out-of-plane MAE of its honeycomb structure reach up to 253 K and 3.47 meV per unit cell under biaxial tensile strain. Moreover, the PtMnSr8 honeycomb structure on hexagonal boron nitride monolayer substrate exhibit further enhanced ferromagnetism (T C ≈ 327 K) and distinctive antioxidant properties. This study highlights that assembling magnetic superatoms on suitable substrates is an effective way for constructing high-performance 2D FM materials.

Graphene-based single-atom catalysts for electrochemical CO2 reduction: unraveling the roles of metals and dopants in tuning activity.

Discovering electrocatalysts that can efficiently convert carbon dioxide (CO2) to valuable fuels and feedstocks using excess renewable electricity is an emergent carbon-neutral technology. A single metal atom embedded in doped graphene, i.e., single-atom catalyst (SAC), possesses high activity and selectivity for electrochemical CO2 reduction (CO2R) to CO, yet further reduction to hydrocarbons is challenging. Here, using density functional theory calculations, we investigate stability and reactivity of a broad SAC chemical space with various metal centers (3d transition metals) and dopants (2p dopants of B, N, O; 3p dopants of P, S) as electrocatalysts for CO2R to methane and methanol. We observe that the rigidities of these SACs depend on the type of dopants, with 3p-coordinating SACs exhibiting more severe out-of-plane distortion than 2p-coordinating SACs. Using CO adsorption energy as a descriptor for CO2R reactivity, we narrow down the candidates and identify seven SACs with near-optimal CO binding strength. We then elucidate full reaction mechanisms towards methane and methanol generation on these identified candidates and observe highly dopant-dependent activity and rate-limiting steps, divergent from conventional mechanistic understanding on metallic surfaces, calling into question whether previous design principles established on metals are directly transferrable to SACs. Consequently, we find that zinc embedded in boron-doped graphene (Zn-B-C) is a highly active catalyst for electrochemical CO2R to C1 hydrocarbons. Our work reveals the opportunities of tuning SAC reactivity via engineering dopants and metals and highlights the importance of re-elucidating CO2R reaction mechanisms on SACs towards unearthing new design principles for SAC chemistry.

Theoretical insights and design of MXene for aqueous batteries and supercapacitors: status, challenges, and perspectives.

Aqueous batteries and supercapacitors are promising electrochemical energy storage systems (EESSs) due to their low cost, environmental friendliness, and high safety. However, aqueous EESS development faces challenges like narrow electrochemical windows, irreversible dendrite growth, corrosion, and low energy density. Recently, two-dimensional (2D) transition metal carbide and nitride (MXene) have attracted more attention due to their excellent physicochemical properties and potential applications in aqueous EESSs. Understanding the atomic-level working mechanism of MXene in energy storage through theoretical calculations is necessary to advance aqueous EESS development. This review comprehensively summarizes the theoretical insights into MXene in aqueous batteries and supercapacitors. First, the basic properties of MXene, including structural composition, experimental and theoretical synthesis, and advantages in EESSs are introduced. Then, the energy storage mechanism of MXene in aqueous batteries and supercapacitors is summarized from a theoretical calculation perspective. Additionally, the theoretical insights into the side reactions and stability issues of MXene in aqueous EESSs are emphasized. Finally, the prospects of designing MXene for aqueous EESSs through computational methods are given.

Layer-number-dependent photoswitchability in 2D MoS2-diarylethene hybrids.

Molybdenum disulfide (MoS2) is a notable two-dimensional (2D) transition metal dichalcogenide (TMD) with properties ideal for nanoelectronic and optoelectronic applications. With growing interest in the material, it is critical to understand its layer-number-dependent properties and develop strategies for controlling them. Here, we demonstrate a photo-modulation of MoS2 flakes and elucidate layer-number-dependent charge transfer behaviors. We fabricated hybrid structures by functionalizing MoS2 flakes with a uniform layer of photochromic diarylethene (DAE) molecules that can switch between closed- and open-form isomers under UV and visible light, respectively. We discovered that the closed-form DAE quenches the photoluminescence (PL) of monolayer MoS2 when excited at 633 nm and that the PL fully recovers after DAE isomerization into the open-form. Similarly, the electric conductivity of monolayer MoS2 is drastically enhanced when interacting with the closed-form isomers. In contrast, photoinduced isomerization did not modulate the properties of the hybrids made of MoS2 bilayers and trilayers. Density functional theory (DFT) calculations revealed that a hole transfer from monolayer MoS2 to the closed-form isomer took place due to energy level alignments, but such interactions were prohibited with open-form DAE. Computational results also indicated negligible charge transfer at the hybrid interfaces with bilayer and trilayer MoS2. These findings highlight the critical role of layer-number-dependent interactions in MoS2-DAE hybrids, offering valuable insights for the development of advanced photoswitchable devices.

Cylindrically bent Laue analyzer in an X-ray Raman/emission spectrometer: performance tests and a comparison with spherically bent Bragg analyzers.

The performances of a spherically bent Bragg analyzer and a cylindrically bent Laue analyzer in an X-ray Raman/emission spectrometer are compared. The reflectivity and energy resolution are evaluated from the intensity of the elastic scattering and the width of the energy distribution on a SiO2 glass sample. Widely used, Bragg analyzers display excellent performance at the photon energy E ≤ 10 keV. However, at higher E, the reflectivity and the resolution gradually deteriorate as E increases, showing poor performance above 20 keV. On the other hand, the reflectivity of the Laue analyzer gradually increases at E> 10 keV, displaying excellent reflectivity and good resolution around 20 keV. The Laue analyzer is suitable for X-ray absorption spectroscopy in high-energy-resolution fluorescence-detection mode or X-ray emission spectroscopy on 4d transition metal compounds. Furthermore, the X-ray Raman features of the lithium K-edge in LiF and the oxygen K-edge feature in H2O, measured by nine Bragg analyzers (2 m radius) at E ≃ 9.9 keV and by five Laue analyzers (1.4 m radius) at E ≃ 19.5 keV, have been compared. Similar count rates and resolutions are observed.

Engineering the optoelectronic properties of ZnS (1100) surface using selected 3d transition metal dopants for enhanced Photoelectrochemical water Splitting: A DFT study

ZnS (1100) surface has emerged as a promising photocatalyst for water split- ting due to its rapid generation of electron-hole pairs upon photoexcitation and high hydrogen evolution efficiency. However, its widespread use has been limited by its response to UV spectrum and rapid recombination of charge carriers. Studies have shown that doping ZnS with transition metals can modify its band gap edge and alter its optical properties. Despite this, there are few comprehensive studies that have systematically explore the potential of doped ZnS (1100) surface for Photo-electrochemical (PEC) applications. In this work, the effects of selected transition metal (TM) dopants (Mn, Cu, Co and Fe) on the optoelectronic properties of ZnS (1100) surface using density functional theory approach has been explored. The results showed that the stability of TM dopants in ZnS (1100) surface is dependent on the d character of the TM dopant as well as their concentration and doping site. Further, it was noted that Zn-rich synthesis conditions were favorable for introduction of TM dopants compared to S-rich conditions. Notably, among the dopants studied, Cu exhibited the highest stability, whereas Co, Mn and Fe displayed decreasing levels of stability. Mn and Fe (for dopant concentration between 1–6%) induces reduction of band-gap energy by 15–60% and 19–51%, respectively, while Cu and Co dopants of similar concentrations induced a more dramatic reduction of band gap energy between 37–78% and 26–75%, respectively. Additionally, band-edge alignment analysis showed that ZnS (1100) surface doped with 4% Cu and 2% Co falls below the redox potential of water (H+/H2). Therefore, Cu and Co are anticipated to induce significant blue shift and offer improved PEC activity.

Unravelling the luminescence spectrum of garnet grossular var. tsavorite: the role of chromium (III), manganese (II) and misattribution of vanadium (II)

Gem quality garnet var. tsavorite from Tanzania is investigated by means of SEM-EDS, XPS and fluorescence spectroscopy to decipher its luminescence spectrum. The EDS analysis confirms that the main constituents of the tsavorite samples from Tanzania are those of grossularia (i.e. Ca, Al, Si, O), with V, Mn, Cr, Ti and Mg as minor but characteristic elements (particularly V, Mn and Cr) of the tsavorite variety. The XPS analysis shows that the oxidation state of Mn is +2 and that of V is +3. The luminescence of tsavorite in the yellow region (591 nm) is originated by Mn2+ in dodecahedral coordination (D2 point symmetry) while the luminescence in the red region (690-750 nm) is originated by Cr3+ in octahedral coordination (Oh point symmetry) in a strong crystal field. Contrary to what is often reported in the literature, this study demonstrates that V2+ (or other 3d transition metal ions as well) is not responsible for the red emissions of tsavorite.

Tuning structural transformations in MnCoGe System: The role of Vanadium-induced d-d hybridization

The impact of introducing the 3d transition metal vanadium (V) into the Ge site of the MnCoGe system has been investigated both theoretically and experimentally. Calculations reveal that V alters local bonding characteristics and reduces the structural transformation temperature when its concentration on the Ge site is below 25%. However, higher V content induces phase instability, making it challenging to form a single-phase MnCo(Ge, V) alloy. To address this, a combination of melt spinning and post-annealing was employed, successfully extending the V content to 10% on the Ge site. This process resulted in a complete Curie temperature window with a width of approximately 100K, and the annealed ribbons exhibited a relative cooling capacity 46% higher than that of the ingot samples. This study demonstrates a strategy for stabilizing structure and achieving magnetostructural coupling by introducing d-d hybridization to partially replace p-d hybridization, offering a pathway for developing new MM’X-based materials.

Enhancing sensitivity of C3N monolayer to CH4 molecule through doping 3d transition metal: A first-principles study

Enhancing sensitivity of C3N monolayer to CH4 molecule through doping 3d transition metal: A first-principles study

Spin coherence phenomena of an S = 1/2 copper(II) system in a polyoxometalate with a less-abundant nuclear spin.

The spin coherence phenomena of a system consisting of a mononuclear 3d transition metal S = 1/2 centre embedded in a polyoxometalate with low nuclear-spin abundance, [(n-C4H9)4N]4H2[SiW11O39Cu0.01Zn0.99], have been revealed for the first time. Variable-temperature Hahn-echo experiments using the pulsed electron spin resonance technique showed that its coherence lifetime remains at the submicrosecond level even above 100 K.

Manipulation of trions to enhance the excitonic emission in monolayer p-MoS2 and its hetero-bilayer by reverse charge injection.

Monolayer 2D transition metal dichalcogenides (TMDs) are known for their direct bandgaps and pronounced excitonic effects, which facilitate efficient light absorption and high photoluminescence (PL). In this study, we report a significant enhancement in PL emission from monolayers of p-type molybdenum disulfide (p-MoS2), fabricated on conductive substrates-such as indium tin oxide (ITO) and gold (Au). We attribute this behaviour to the reverse injection of charge carriers from substrates to p-MoS2 and the subsequent localization of electrons and holes in the substrate and p-MoS2, respectively. Such injection of charge carriers was suppressed when few-layer graphene (FLG) was used as a barrier layer. Further investigation of the PL emission characteristics from a vertically stacked hetero-bilayer (the p-n interface) of p-MoS2 and n-MoSe2 revealed a single resonant high-emission PL peak at 1.64 eV with the PL emission from this heterostructure significantly higher than that from free-standing monolayers. This finding contrasts sharply with the PL quenching often seen in hetero-bilayers with an n-n interface. These findings offer valuable insights into the fundamental optical and electronic properties of 2D TMDs and their heterostructures, which are essential for optimizing these materials for optoelectronic applications.