Optimizing Electron Spin-Polarized States of MoSe2/Cr2Se3 Heterojunction-Embedded Carbon Nanospheres for Superior Sodium/Potassium-Ion Battery Performances

The principal challenges faced by sodium-ion batteries (SIBs) and potassium-ion batteries (KIBs) revolve around identifying suitable host materials capable of accommodating metal ions with larger dimensions and addressing the issue of sluggish chemical kinetics. Herein, a MoSe2/Cr2Se3 heterojunction uniformly embedded is fabricated in nitrogen-doped hollow carbon nanospheres (MoSe2/Cr2Se3@N-HCSs) as an electrode material for SIBs and KIBs. Cr2Se3 exhibits spontaneous antiparallel alignment of magnetic moments. Mo2+ doping is employed to regulate the electron spin states of Cr2Se3. Moreover, the MoSe2 and Cr2Se3 heterojunctions induce a lattice mismatch at the heterostructure interface, resulting in spin-polarized states or localized magnetic moments at the interface, potentially contributing to spin-polarized surface capacitance. MoSe2/Cr2Se3@N-HCSs demonstrate a high capacity of 498 mAh g-1 at 0.1 A g-1 with good cycling stability (capacity of 405 mAh g-1 and a coulombic efficiency of 99.8% after 1000 cycles). Additionally, density functional theory (DFT) calculations simulate the accumulation of spin-polarized charges at the MoSe2/Cr2Se3@N-HCSs heterojunction interface, dependent on the surface electron density of the antiferromagnetic Cr2Se3 and the surface spin polarization near the Fermi level. After regulating the electron spin states through Mo-doping, the band gap of the material decreases. These significant findings provide novel insights into the design and synthesis of electrode materials with exceptional performance characteristics for batteries.

Bipolar Resistive Switching in 2D MoSe2 Grown by Atmospheric Pressure Chemical Vapor Deposition

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) are highly promising nanomaterials for various electronic devices such as field-effect transistors, junction diodes, tunneling devices, and, more recently, memristors. 2D MoSe2 stands out for having high electrical conductivity, charge carrier mobility, and melting point. While these features make it particularly appropriate as a switching layer in memristive devices, reliable and scalable production of large-area 2D MoSe2 still represents a challenge. In this study, we manufacture 2D MoSe2 films by atmospheric-pressure chemical vapor deposition and investigate them on the atomic scale. We selected and transferred MoSe2 bilayer to serve as a switching layer between asymmetric Au-Cu electrodes in miniaturized crossbar vertical memristors. The electrochemical metallization devices showed forming-free, bipolar resistive switching at low voltages, with clearly identifiable nonvolatile states. Other than low-power neuromorphic computing, low switching voltages approaching the range of biological action potentials could unlock hybrid biological interfaces.

Engineering Phase Transition from 2H to 1T in MoSe2 by W Cluster Doping toward Lithium-Ion Battery

Phase engineering synthesis strategy is extremely challenging to achieve stable metallic phase molybdenum diselenide for a better physicochemical property than the thermodynamically stable semiconducting phase. Herein, we introduce tungsten atom clusters into the MoSe2 layered structure, realizing the phase transition from the 2H semiconductor to 1T metallic phase at a high temperature. The combination of synchrotron radiation X-ray absorption spectroscopy, Cs-corrected transmission electron microscopy, and theoretical calculation demonstrates that the aggregation doping of W atoms is the factor of MoSe2 structure transformation. When utilizing this distinct structure as an anode component, it demonstrates outstanding rate capability and durability. After 500 cycles, this results in a specific capacity of 1007.4 mAh g-1 at 500 mA g-1. These discoveries could open the door for the future development of high-performance anodes for ion battery applications.

Band Alignments, Electronic Structure, and Core-Level Spectra of Bulk Molybdenum Dichalcogenides (MoS2, MoSe2, and MoTe2)

A comprehensive study of bulk molybdenum dichalcogenides is presented with the use of soft and hard X-ray photoelectron (SXPS and HAXPES) spectroscopy combined with hybrid density functional theory (DFT). The main core levels of MoS₂, MoSe₂, and MoTe₂ are explored. Laboratory-based X-ray photoelectron spectroscopy (XPS) is used to determine the ionization potential (IP) values of the MoX₂ series as 5.86, 5.40, and 5.00 eV for MoSe₂, MoSe₂, and MoTe₂, respectively, enabling the band alignment of the series to be established. Finally, the valence band measurements are compared with the calculated density of states which shows the role of p-d hybridization in these materials. Down the group, an increase in the p-d hybridization from the sulfide to the telluride is observed, explained by the configuration energy of the chalcogen p orbitals becoming closer to that of the valence Mo 4d orbitals. This pushes the valence band maximum closer to the vacuum level, explaining the decreasing IP down the series. High-resolution SXPS and HAXPES core-level spectra address the shortcomings of the XPS analysis in the literature. Furthermore, the experimentally determined band alignment can be used to inform future device work.

Unveiling the Electrocatalytic Activity of 1T'-MoSe2 on Lithium-Polysulfide Conversion Reactions

The dissolution of intermediate lithium polysulfides (LiPS) into an electrolyte and their shuttling between the electrodes have been the primary bottlenecks for the commercialization of high-energy density lithium-sulfur (Li-S) batteries. While several two-dimensional (2D) materials have been deployed in recent years to mitigate these issues, their activity is strictly restricted to their edge-plane-based active sites. Herein, for the first time, we have explored a phase transformation phenomenon in a 2D material to enhance the number of active sites and electrocatalytic activity toward LiPS redox reactions. Detailed theoretical calculations demonstrate that phase transformation from the 2H to 1T' phase in a MoSe₂ material activates the basal planes that allow for LiPS adsorption. The corresponding transformation mechanism and LiPS adsorption capabilities of the as-formed 1T'-MoSe₂ were elucidated experimentally using microscopic and spectroscopic techniques. Further, the electrochemical evaluation of phase-transformed MoSe₂ revealed its strong electrocatalytic activity toward LiPS reduction and their oxidation reactions. The 1T'-MoSe₂-based cathode hosts for sulfur later provide a superior cycling performance of over 250 cycles with a capacity loss of only 0.15% per cycle along with an excellent Coulombic efficiency of 99.6%.

Role of processing parameters in CVD grown crystalline monolayer MoSe2

The quality of as-synthesized monolayers plays a significant role in atomically thin semiconducting transition metal dichalcogenides (TMDCs) to determine the electronic and optical properties. For designing optoelectronic devices, exploring the effect of processing parameters on optical properties is a prerequisite. In this view, we present the influence of processing parameters on the lattice and quasiparticle dynamics of monolayer MoSe₂. The lab-built chemical vapour deposition (CVD) setup is used to synthesize monolayer MoSe₂ flakes with varying shapes, including sharp triangle (ST), truncated triangle (TT), hexagon, and rough edge circle (REC). In particular, the features of as-synthesized monolayer MoSe₂ flakes are examined using Raman and photoluminescence (PL) spectroscopy. Raman spectra reveal that the frequency difference between the A_1g and E¹_2g peaks is >45 cm⁻¹ in all the monolayer samples. PL spectroscopy also shows that the synthesized MoSe₂ flakes are monolayer in nature with a direct band gap in the range of 1.50–1.58 eV. Furthermore, the variation in the direct band gap is analyzed using the spectral weight of quasiparticles in PL emission, where the intensity ratio {I(A⁰)/I(A⁻)} and trion binding energy are found to be ∼1.1–5.0 and ∼23.1–47.5 meV in different monolayer MoSe₂ samples. Hence, these observations manifest that the processing parameters make a substantial contribution in tuning the vibrational and excitonic properties.

Monolayer MoSe₂ flakes with varying shapes, including sharp triangle, truncated triangle, hexagon, and rough edge circle are synthesized using APCVD method. The lattice and quasiparticle dynamics are examined under different growth conditions.

Enhanced Li-Ion Rate Capability and Stable Efficiency Enabled by MoSe2 Nanosheets in Polymer-Derived Silicon Oxycarbide Fiber Electrodes

Transition metal dichalcogenides (TMDs) such as MoSe2 have continued to generate interest in the engineering community because of their unique layered morphology-the strong in-plane chemical bonding between transition metal atoms sandwiched between two chalcogen atoms and the weak physical attraction between adjacent TMD layers provides them with not only chemical versatility but also a range of electronic, optical, and chemical properties that can be unlocked upon exfoliation into individual TMD layers. Such a layered morphology is particularly suitable for ion intercalation as well as for conversion chemistry with alkali metal ions for electrochemical energy storage applications. Nonetheless, host of issues including fast capacity decay arising due to volume changes and from TMD's degradation reaction with electrolyte at low discharge potentials have restricted use in commercial batteries. One approach to overcome barriers associated with TMDs' chemical stability functionalization of TMD surfaces by chemically robust precursor-derived ceramics or PDC materials, such as silicon oxycarbide (SiOC). SiOC-functionalized TMDs have shown to curb capacity degradation in TMD and improve long term cycling as Li-ion battery (LIBs) electrodes. Herein, we report synthesis of such a composite in which MoSe2 nanosheets are in SiOC matrix in a self-standing fiber mat configuration. This was achieved via electrospinning of TMD nanosheets suspended in pre-ceramic polymer followed by high temperature pyrolysis. Morphology and chemical composition of synthesized material was established by use of electron microscopy and spectroscopic technique. When tested as LIB electrode, the SiOC/MoSe2 fiber mats showed improved cycling stability over neat MoSe2 and neat SiOC electrodes. The freestanding composite electrode delivered a high charge capacity of 586 mAh g-1electrode with an initial coulombic efficiency of 58%. The composite electrode also showed good cycling stability over SiOC fiber mat electrode for over 100 cycles.

Structure and electronic properties of MoSe2/PtS2 van der Waals heterostructure

Structure and electronic properties of MoSe2/PtS2 van der Waals heterostructure and their dependence on the interlayer coupling, biaxial strain and external electric field are systematically investigated by using the first-principles...

Millimeter-Scale Continuous Film of MoS2 Synthesized Using a Mo, Na, and Seeding Promoter-Based Coating as a Solid Precursor

While the chemical vapor deposition technique can be used to fabricate 2D materials in a larger area, materials like MoS₂ have limited controllability due to their lack of self-controlling nature. This article presents a new technique for synthesizing a void-free millimeter-scale continuous monolayer MoS₂ film through the diffusion of a well-controlled Mo, Na, and seeding promoter-based coating under a low-pressure N₂ atmosphere. Compared to the conventional method, this technique provides precise control of solid precursors, where MoS₂ grows next to the coating. At 800 °C, the synthesized MoS₂ showed a uniform single-layer MoS₂ film; however, a Na-free coating showed nanoscale voids and poor crystal quality, which are attributed to a higher edge-attachment barrier that slows down the MoS₂ lateral growth. The synthesized MoS₂ with Na-containing solution showed an intense PL peak with a 1.86 eV band gap. Even at the relatively low temperature of 700 °C, compared to the Na-excluded condition, MoS₂ showed almost two times higher area coverage with a comparatively larger crystal size. This finding may assist in the future development of MoS₂-based electronic and optoelectronic devices such as transistors and photodetectors.

Large-scale MoS2(1-x)Se2xmonolayers synthesized by confined-space CVD

Alloy engineering is efficient in modulating the electronic structure and physical and chemical properties of Transition metal dichalcogenides (TMDs). Here, we develop an efficient and simple confined-space CVD strategy by using a smaller quartz boat nested in a larger quartz boat for the preparation of ternary alloy MoS_2(1-x)Se_2xmonolayers on SiO₂/Si substrates with controllable composition. The effect of hydrogen ratio of the mixed carrier gas (Ar/H₂) on the resultant flakes are systematically investigated. A hydrogon ratio of 15% is demonstrated to be the most appropriate to synthesize large size (more than 400μm) single crystalline MoS_2(1-x)Se_2xalloy monolayers. The composition of the alloy can also be changed in a full range (2x= 0-2) by changing the weight ratio of Se and S powder. The as-grown monolayer MoS_2(1-x)Se_2xalloys present continuously high crystal quality in terms of Raman and PL measurements. Furthermore, to visible light (532 nm), the MoS_2(1-x)Se_2xbased photodetectors display wonderful photoresponse with a fast response of less than 50 ms. Our work may be usedful in directing the synthesis of TMDs alloys as well as their optoelectronic applications.

Synthesis of large-area monolayer and few-layer MoSe2 continuous films by chemical vapor deposition without hydrogen assistance and formation mechanism

Two dimensional (2D) MoSe2 with a layered structure has attracted extensive research due to its excellent electronic and optical properties. The controlled synthesis of large-scale and high-quality MoSe2 is highly desirable but still remains challenging. Ambient pressure chemical vapor deposition (APCVD) is an excellent method for the synthesis of 2D materials but the inevitable use of hydrogen during the growth and the easy formation of cracks in the ultrathin films still need to be solved. In the present work, we reported the synthesis of large-area continuous MoSe2 films with different layers by the APCVD method without the assistance of hydrogen on SiO2/Si substrates just by raising the reaction temperature of Se. The synthesized continuous MoSe2 films can reach several centimeters, which can be seen clearly by naked eyes, and, more importantly, the size of the monolayer film can reach up to 3 mm. The morphology, structural characteristics, and optical properties of the synthesized MoSe2 films have been investigated, demonstrating good performance and high crystallinity of the films. Raman spectra give the empirical expression of the frequency difference between E2g1 and A1g dependence of the layer number (N = 1-10 L) for CVD grown MoSe2, which is useful in layer number identification. Further, the formation mechanism of the MoSe2 continuous film is of interest as a fundamental scientific problem and needs to be studied. We proposed the wing model, boundary layer theory, and diffusion theory to account quantitatively for the formation behavior of the MoSe2 film. The presented facile growth method and theoretical model are useful to synthesize other ultrathin transition metal dichalcogenide films and understand the formation behaviors of the systems.

Promoting a Weak Coupling of Monolayer MoSe2 Grown on (100)-Faceted Au Foil

As a two-dimensional semiconductor with many physical properties, including, notably, layer-controlled electronic bandgap and coupled spin-valley degree of freedom, monolayer MoSe₂ is a strong candidate material for next-generation opto- and valley-electronic devices. However, due to substrate effects such as lattice mismatch and dielectric screening, preserving the monolayer's intrinsic properties remains challenging. This issue is generally significant for metallic substrates whose active surfaces are commonly utilized to achieve direct chemical or physical vapor growth of the monolayer films. Here, we demonstrate high-temperature-annealed Au foil with well-defined (100) facets as a weakly interacting substrate for atmospheric pressure chemical vapor deposition of highly crystalline monolayer MoSe₂. Low-temperature scanning tunneling microscopy/spectroscopy measurements reveal a honeycomb structure of MoSe₂ with a quasi-particle bandgap of 1.96 eV, a value comparable with other weakly interacting systems such as MoSe₂/graphite. Density functional theory calculations indicate that the Au(100) surface exhibits the preferred energetics to electronically decouple from MoSe₂, compared with the (110) and (111) crystal planes. This weak coupling is critical for the easy transfer of monolayers to another host substrate. Our study demonstrates a practical means to produce high-quality monolayers of transition-metal dichalcogenides, viable for both fundamental and application studies.

Terahertz Surface Emission from MoSe2 at the Monolayer Limit

The surface-charge region of bulk and monolayer MoSe₂ is analyzed directly by terahertz (THz) surface emission spectroscopy in a nondestructive way. Both surface nonlinear optical polarization and surface field-induced photocurrent contribute to the THz radiation in both bulk and monolayer MoSe₂. The first THz emission mechanism is due to the surface optical rectification and the second one is due to the photogenerated carriers accelerated by the surface depletion field. The THz radiation contribution from the surface optical rectification is basically the same for both bulk and monolayer MoSe₂ because of the same symmetry at the surface. However, the contribution from the surface field-induced photocurrent is ∼94.2% in bulk MoSe₂ and it goes down to 74.5% in monolayer MoSe₂. This is due to the larger surface depletion field in bulk MoSe₂ (∼2.54 × 10⁷ V/m) compared with that in monolayer MoSe₂ (∼5.42 × 10⁵ V/m), as such THz emission from the bulk is approximately four times larger than that from monolayer MoSe₂. This work not only proves the clear THz radiation mechanism from MoSe₂ crystals but also affords a THz technology for the surface characterization of two-dimensional materials.

Large area few-layer TMD film growths and their applications

Research on 2D materials is one of the core themes of modern condensed matter physics. Prompted by the experimental isolation of graphene, much attention has been given to the unique optical, electronic, and structural properties of these materials. In the past few years, semiconducting transition metal dichalcogenides (TMDs) have attracted increasing interest due to properties such as direct band gaps and intrinsically broken inversion symmetry. Practical utilization of these properties demands large-area synthesis. While films of graphene have been by now synthesized on the order of square meters, analogous achievements are difficult for TMDs given the complexity of their growth kinetics. This article provides an overview of methods used to synthesize films of mono- and few-layer TMDs, comparing spatial and time scales for the different growth strategies. A special emphasis is placed on the unique applications enabled by such large-scale realization, in fields such as electronics and optics.

Two-Dimensional Materials in Large-Areas: Synthesis, Properties and Applications

Large-area and high-quality two-dimensional crystals are the basis for the development of the next-generation electronic and optical devices. The synthesis of two-dimensional materials in wafer scales is the first critical step for future technology uptake by the industries; however, currently presented as a significant challenge. Substantial efforts have been devoted to producing atomically thin two-dimensional materials with large lateral dimensions, controllable and uniform thicknesses, large crystal domains and minimum defects. In this review, recent advances in synthetic routes to obtain high-quality two-dimensional crystals with lateral sizes exceeding a hundred micrometres are outlined. Applications of the achieved large-area two-dimensional crystals in electronics and optoelectronics are summarised, and advantages and disadvantages of each approach considering ease of the synthesis, defects, grain sizes and uniformity are discussed.

Morphological, optical and photovoltaic characteristics of MoSe2/SiOx/Si heterojunctions

This work reports the effect of different processing parameters on the structural and morphological characteristics of MoSe₂ layers grown by chemical vapour deposition (CVD), using MoO₃ and Se powders as solid precursors. It shows the strong dependence of the size, shape and thickness of the MoSe₂ layers on the processing parameters. The morphology of the samples was investigated by field emission scanning electron microscopy (FESEM) and the thickness of the deposited layers was determined by atomic force microscopy (AFM). Raman and photoluminescence (PL) spectroscopies were used to confirm the high quality of the MoSe₂ layers. Surface composition was examined by photoelectron spectroscopy (XPS). Moreover, the MoSe₂/SiO_x/Si heterojunctions exhibit diode behaviour, with a rectification ratio of 10, measured at ±2.0 V, which is due to the p-i-n heterojunctions formed at the p-Si/SiO_x/MoSe₂ interface. A photovoltaic effect was observed with a short circuit current density (J_sc), open circuit voltage (V_OC) and efficiency of −0.80 mA/cm², 1.55 V and 0.5%, respectively. These results provide a guide for the preparation of p-i-n heterojunctions based on few-layer MoSe₂ with improved photovoltaic response.

Synthesis of Monolayer MoSe2 with Controlled Nucleation via Reverse-Flow Chemical Vapor Deposition

Two-dimensional (2D) layered semiconductor materials, such as transition metal dichalcogenides (TMDCs), have attracted considerable interests because of their intriguing optical and electronic properties. Controlled growth of TMDC crystals with large grain size and atomically smooth surface is indeed desirable but remains challenging due to excessive nucleation. Here, we have synthesized high-quality monolayer, bilayer MoSe₂ triangular crystals, and continuous thin films with controlled nucleation density via reverse-flow chemical vapor deposition (CVD). High crystallinity and good saturated absorption performance of MoSe₂ have been systematically investigated and carefully demonstrated. Optimized nucleation and uniform morphology could be achieved via fine-tuning reverse-flow switching time, growth time and temperature, with corresponding growth kinetics proposed. Our work opens up a new approach for controllable synthesis of monolayer TMDC crystals with high yield and reliability, which promote surface/interface engineering of 2D semiconductors towards van der Waals heterostructure device applications.

Hydrogen-assisted step-edge nucleation of MoSe2 monolayers on sapphire substrates

The fabrication of large-area single crystalline monolayer transition metal dichalcogenides (TMDs) is essential for a range of electric and optoelectronic applications. Chemical vapor deposition (CVD) is a promising method to achieve this goal by employing orientation control or alignment along the crystalline lattice of the substrate such as sapphire. On the other hand, a fundamental understanding of the aligned-growth mechanism of TMDs is limited. In this report, we show that the controlled introduction of H2 during the CVD growth of MoSe2 plays a vital role in the step-edge aligned nucleation on a c-sapphire (0001) substrate. In particular, the MoSe2 domains nucleate along the [112[combining macron]0] step-edge orientation by flowing H2 subsequent to pure Ar. Systematic studies, including the H2 introduction time, flow rate, and substrate temperature, suggest that the step-edge aligned nucleation of MoSe2 can be controlled by the hydrogen concentration on the sapphire substrate. These results offer important insights into controlling the epitaxial growth of 2D materials on a crystalline substrate.

Real time optical observation and control of atomically thin transition metal dichalcogenide synthesis

Understanding the mechanisms involved in chemical vapour deposition (CVD) synthesis of atomically thin transition metal dichalcogenides (TMDCs) requires precise control of numerous growth parameters. All the proposed mechanisms and their relationship with the growth conditions are inferred from characterising intermediate formations obtained by stopping the growth blindly. To fully understand the reaction routes that lead to the monolayer formation, real time observation and control of the growth are needed. Here, we demonstrate how a custom-made CVD chamber that allows real time optical monitoring can be employed to study the reaction routes that are critical to the production of the desired layered thin crystals in salt assisted TMDC synthesis. Our real time observations reveal the reaction between the salt and the metallic precursor to form intermediate compounds which lead to the layered crystal formation. We identified that both the vapour-solid-solid and vapour-liquid-solid growth routes are in an interplay. Furthermore, we demonstrate the role H2 plays in the salt-assisted WSe2 synthesis. Finally, we observed the synthesis of the MoSe2/WSe2 heterostructures optically, and elucidated the conditions required for both lateral and vertical heterostructure syntheses.

CVD growth of molybdenum diselenide surface structures with tailored morphology

Controllable atmospheric pressure CVD has been optimized to grow transition metal dichalcogenide MoSe₂ with tunable morphology at 750 °C on a silicon substrate with a native oxide layer of 250 nm.