Metallic 1T Phase Research Articles

Switchable Photoresponse Mechanisms Implemented in Single van der Waals Semiconductor/Metal Heterostructure.

van der Waals (vdW) heterostructures based on two-dimensional (2D) semiconducting materials have been extensively studied for functional applications, and most of the reported devices work with sole mechanism. The emerging metallic 2D materials provide us new options for building functional vdW heterostructures via rational band engineering design. Here, we investigate the vdW semiconductor/metal heterostructure built with 2D semiconducting InSe and metallic 1T-phase NbTe2, whose electron affinity χInSe and work function ΦNbTe2 almost exactly align. Electrical characterization verifies exceptional diode-like rectification ratio of >103 for the InSe/NbTe2 heterostructure device. Further photocurrent mappings reveal the switchable photoresponse mechanisms of this heterostructure or, in other words, the alternative roles that metallic NbTe2 plays. Specifically, this heterostructure device works in a photovoltaic manner under reverse bias, whereas it turns to phototransistor with InSe channel and NbTe2 electrode under high forward bias. The switchable photoresponse mechanisms originate from the band alignment at the interface, where the band bending could be readily adjusted by the bias voltage. In addition, a conceptual optoelectronic logic gate is proposed based on the exclusive working mechanisms. Finally, the photodetection performance of this heterostructure is represented by an ultrahigh responsivity of ∼84 A/W to 532 nm laser. Our results demonstrate the valuable application of 2D metals in functional devices, as well as the potential of implementing photovoltaic device and phototransistor with single vdW heterostructure.

Metallic 1T-MoS2 coupled with MXene towards ultra-high rate-capabilities for supercapacitors

A schematic diagram showing the synthesis of the 1T-MoS2/Ti3C2Tx heterostructure and the fast charge–discharge networks in the heterostructure.

Highly Efficient Electrocatalytic N2 Reduction to Ammonia over Metallic 1T Phase of MoS2 Enabled by Active Sites Separation Mechanism.

The 1T phase of MoS2 has been widely reported to be highly active toward the hydrogen evolution reaction (HER), which is expected to restrict the competitive nitrogen reduction reaction (NRR). However, in this work, a prototype of active sites separation over 1T‐MoS2 is proposed by DFT calculations that the Mo‐edge and S atoms on the basal plane exhibit different catalytic NRR and HER selectivity, and a new role‐playing synergistic mechanism is also well enabled for the multistep NRR, which is further experimentally confirmed. More importantly, a self‐sacrificial strategy using g‐C3N4 as templates is proposed to synthesize 1T‐MoS2 with an ultrahigh 1T content (75.44%, named as CNMS, representing the composition elements of C, N, Mo, and S), which yields excellent NRR performances with an ammonia formation rate of 71.07 µg h–1 mg–1 cat. at −0.5 V versus RHE and a Faradic efficiency of 21.01%. This work provides a promising new orientation of synchronizing the selectivity and activity for the multistep catalytic reactions.

Two-Dimensional Nanofluidic Membranes toward Harvesting Salinity Gradient Power.

The salinity gradient between seawater and river water has been identified as a promising, clean, renewable, and sustainable energy source that can be converted into electricity using ion-selective membranes in a reverse electrodialysis (RED) configuration. However, the major hindrance to current salinity gradient power (SGP) conversion is its poor energy efficiency due to the use of low-performance membrane processes, which affords power for neither miniaturized devices nor industrial-level applications. Nanofluidics, which combines strong confinement and surface charge effects at the nanoscale, contributes to novel transport properties, including excellent ion selectivity and high ion throughput; thus, nanofluidics may lead to technological breakthroughs and act as an emerging platform for harnessing SGP. Recently, two-dimensional (2D) materials have provided impressive energy extraction performance and further insight into fundamental transport mechanisms and theoretical feasibility. To reach the commercialization benchmark and real-world applications, an array of nanopores and channels that can be scaled up to industrial sizes is in high demand; additionally, it remains challenging to develop macroscale nanofluidic membranes that meet the "selectivity versus throughput" dual requirement. In the first section, we start with our understanding of the underlying mechanism of ion-channel interactions and transport characteristics in nanofluidic channel systems from the microscale to the macroscale. We review our recent efforts in this field by constructing a heterojunction with asymmetric ion transport behavior that generates rectification of the ion flux and creates an osmotic diode, which is composed of two nanofluidic layers with opposite polar charges and different chemical compositions. Another efficient way to improve the performance of the system is introducing charged functional materials intercalated into laminar 2D nanosheets. The intercalated nanofluidic material can be explained by two classical models to account for the synergistic effects that (i) improve the stability and mechanical properties of 2D materials with a fixed interlayer spacing and (ii) provide space charge for modulating ion diffusion; both of these effects contribute to its considerable energy conversion performance. Further, layer-by-layer membranes are superior to traditional membranes consisting of a simple stack because they retain their repulsion effect toward co-ions, largely strengthening the efficiency of ion separation and conversion. In particular, we highlight our views on the role of the 2D phase structure (e.g., semiconductor 2H phase and metallic 1T phase) in which the two phases differ from each other in physical and chemical properties, including ionic conductance, surface charge, and wetting, thereby presenting a state-of-the-art avenue for controlling ion transport. In view of the nature of 2D materials, we also report improved osmotic energy harvesting by exploiting the photoinduced heat gradient and electrons that increase ion mobility and surface charge, respectively. Finally, we point out specific research topics in which a combined project can certainly come into the limelight. For example, we discuss the combination of SGP with desalination systems and water splitting. We expect that this Account will stimulate further efforts toward functionalized 2D nanoporous materials and facilitate interdisciplinary efforts in chemistry, material engineering, environmental science, and nanotechnology.

Synergistic Effect of Metal Cations and Visible Light on 2D MoS2 Nanosheet Aggregation.

Aggregation significantly influences the transport, transformation, and bioavailability of engineered nanomaterials. Two-dimensional MoS2 nanosheets are one of the most well-studied transition-metal dichalcogenide nanomaterials. Nonetheless, the aggregation behavior of this material under environmental conditions is not well understood. Here, we investigated the aggregation of single-layer MoS2 (SL-MoS2) nanosheets under a variety of conditions. Trends in the aggregation of SL-MoS2 are consistent with classical Derjaguin-Landau-Verwey-Overbeek (DLVO) colloidal theory, and the critical coagulation concentrations of cations follow the order of trivalent (Cr3+) < divalent (Ca2+, Mg2+, Cd2+) < monovalent cations (Na+, K+). Notably, Pb2+ and Ag+ destabilize MoS2 nanosheet suspensions much more strongly than do their divalent and monovalent counterparts. This effect is attributable to Lewis soft acid-base interactions of cations with MoS2. Visible light irradiation synergistically promotes the aggregation of SL-MoS2 nanosheets in the presence of cations, which was evident even in the presence of natural organic matter. The light-accelerated aggregation was ascribed to dipole-dipole interactions due to transient surface plasmon oscillation of electrons in the metallic 1T phase, which decrease the aggregation energy barrier. These results reveal the phase-dependent aggregation behaviors of engineered MoS2 nanosheets with important implications for environmental fate and risk.

Phase-Transition Mo1-xVxSe2 Alloy Nanosheets with Rich V-Se Vacancies and Their Enhanced Catalytic Performance of Hydrogen Evolution Reaction.

Alloys of transition-metal dichalcogenide can display distinctive phase evolution because of their two-dimensional structures. Herein, we report the colloidal synthesis of Mo1-xVxSe2 alloy nanosheets with full composition tuning. Alloying led to a phase transition at x = 0.7 from the semiconducting 2H phase MoSe2 to the metallic 1T phase VSe2. It also produced significant V and Se vacancies, which became the richest in the 2H phase at x = 0.3-0.5. Extensive spin-polarized density functional theory calculations consistently predicted the 2H-1T phase transition at x = 0.7, in agreement with the experimental results. The vacancy formation energy also supports the formation of V and Se vacancies. Alloying in the 2H phase enhanced the electrocatalytic performance toward hydrogen evolution reaction (HER) at x = 0.3 (in 0.5 M H2SO4) or 0.4 (in 1 M KOH). The Gibbs free energy along the HER pathway indicates that this maximum performance is due to the highest concentration of active V and Se vacancy sites.

Catalytic-conversion behavior of MoS2 for polysulfides by nickel introduction and phosphorous-doping in advanced lithium-sulfur batteries

Lithium-sulfur batteries (LSBs) have aroused remarkable concern as promising energy storage equipments owing to the high theoretical capacity and energy density. Efficient conversion of lithium polysulfides into S as well as Li2S/Li2S2 in the charge and discharge process is a crucial factor to improve the performance of LSBs. Among all the competing polysulfides conversion catalysts, MoS2 is considered as the one of the strongest competitors. Herein, the dual effects of nickel introduction and phosphorus doping on the morphology, structure and electrochemical properties of MoS2 materials are detailedly investigated. It has been found that the Ni-P coordinated sites on the MoS2 can provide the affluent sites for adsorption and conversion of polysulfides. Meanwhile, the phase transition of MoS2 from 2H phase to metallic 1T phase can be distinctly improved owning to nickel introduction, which enhances the electrochemical performances of LSBs. Besides, the P0.1-Mo0.9Ni0.1S2/CNT/S displays a good initial capacity of 742 mAh g−1 with a low fading rate of 0.075% per cycle at 1C as well as the discharge capacity of 463 mAh g−1 at the 500th cycle. Therefore, the strategy of Ni introduction and heteroatom doping can effectively inhibit Polysulfides shuttle and promote the performance of LSBs.

Edge-Rich Bicrystalline 1T/2H-MoS2 Cocatalyst-Decorated {110} Terminated CeO2 Nanorods for Photocatalytic Hydrogen Evolution.

Developing all-solid-state Z-scheme systems with highly active photocatalysts are of huge interest in realizing long-term solar-to-fuel conversion. Here we reported an innovative hybrid of {110}-oriented CeO2 nanorods with edge-enriched bicrystalline 1T/2H-MoS2 coupling as efficient photocatalysts for water splitting. In the composites, the metallic 1T phase acts as an excellent solid state electron mediator in the Z-scheme, while the 2H phase and CeO2 are the adsorption sites of the photosensitizer and reactant (H2O), respectively. Through optimal structure and phase engineering, 1T/2H-MoS2@CeO2 heterojunctions simultaneously achieve high charge separation efficiency, proliferated density of exposed active sites, and excellent affinity to reactant molecules, reaching a superior hydrogen evolution rate of 73.1 μmol/h with an apparent quantum yield of 8.2% at 420 nm. Furthermore, density functional theory calculations show that 1T/2H-MoS2@CeO2 possesses the advantages of intensive electronic interaction from the built-in electric field (negative MoS2 and positive charged CeO2) and reduced H2O adsorption/dissociation energies. This work sheds light on the design of on-demand noble-metal-free Z-scheme heterostructures for solar energy conversion.

Molecular Functionalization of 2H-Phase MoS2 Nanosheets via an Electrolytic Route for Enhanced Catalytic Performance.

The molecular functionalization of two-dimensional MoS2 is of practical relevance with a view to, for example, facilitating its liquid-phase processing or enhancing its performance in target applications. While derivatization of metallic 1T-phase MoS2 nanosheets has been relatively well studied, progress involving their thermodynamically stable, 2H-phase counterpart has been more limited due to the lower chemical reactivity of the latter. Here, we report a simple electrolytic strategy to functionalize 2H-phase MoS2 nanosheets with molecular groups derived from organoiodides. Upon cathodic treatment of a pre-expanded MoS2 crystal in an electrolyte containing the organoiodide, water-dispersible nanosheets derivatized with acetic acid or aniline moieties (∼0.10 molecular groups inserted per surface sulfur atom) were obtained. Analysis of the functionalization process indicated it to be enabled by the external supply of electrons from the cathodic potential, although they could also be sourced from a proper reducing agent, as well as by the presence of intrinsic defects in the 2H-phase MoS2 lattice (e.g., sulfur vacancies), where the molecular groups can bind. The acetic acid-functionalized nanosheets were tested as a non-noble metal-based catalyst for nitroarene and organic dye reduction, which is of practical utility in environmental remediation and chemical synthesis, and exhibited a markedly enhanced activity, surpassing that of other (1T- or 2H-phase) MoS2 materials and most non-noble metal catalysts previously reported for this application. The reduction kinetics (reaction order) was seen to correlate with the net electric charge of the nitroarene/dye molecules, which was ascribed to the distinct abilities of the latter to diffuse to the catalyst surface. The functionalized MoS2 catalyst also worked efficiently at realistic (i.e., high) reactant concentrations, as well as with binary and ternary mixtures of the reactants, and could be immobilized on a polymeric scaffold to expedite its manipulation and reuse.

Doubling the cyclic stability of 3D hierarchically structured composites of 1T-MoS2/polyaniline/graphene through the formation of LiF-rich solid electrolyte interphase

MoS2 has been viewed as a unique anode material for lithium-ion batteries; however, its electrochemical performance is usually restrained by poor conductivity of 2H phase and an uneven solid-electrolyte interphase (SEI). To tackle the above issues, the ternary composite is synthesized by self-assembling MoS2 2D sheets with higher percentage of metallic 1T-phase on polyaniline functionalized reduced graphene oxide (PANI-rGO) through an electrostatic interaction. The flower-like aggregate of 1T-MoS2 having increased interlayer distances is beneficial for diffusion and intercalation of lithium ions. Furthermore, the graphene substrate facilitates the electron transportation. At the current density of 0.1 and 2 A g−1, the ternary composite has the specific capacities of 812 and 333 mAh g−1 respectively. On the other hand, the cyclic stability of ternary composite can be further improved when 10 wt% fluoroethylene carbonate (FEC) is added into the traditional carbonate-based electrolyte. The capacity retention of ternary composite cycled in a FEC-mixed electrolyte is two times higher than that in a traditional electrolyte after 400 charge–discharge cycles. This is attributed to the fact that the use of FEC enables the formation of active LiF-rich SEI.

Hydrazine hydrate-assisted adjustment of sulfur-rich MoS2 as hydrogen evolution electrocatalyst

The activity of MoS2 toward hydrogen evolution reaction (HER) originated from different sulfur configurations and phases. Metallic 1T phase of MoS2 with good electronic conductivity and abundant electrocatalytic active sites is favorable for electrocatalysis. In addition, bridging S22- has been verified as the major active sites for HER. However, there is a lack of in-depth understanding for the coexistence of 1T phase and bridging S22- of MoS2. Herein, a facile hydrazine hydrate-assisted method is reported to synthesize sulfur-rich MoS2. Specifically, when higher ratio of 1T to 2H phase was obtained, a higher proportion of bridging S22- to terminal S22- was produced simultaneously. With an optimum amount of hydrazine hydrate, MoS2 exhibits a low overpotential of 173 mV at a current density of 10 mA cm−2, a small Tafel slope of 56.6 mV dec−1, and long-term stability in 0.5 M H2SO4 solution. This work provides a new insight for improving the electrocatalytic activity of MoS2 via a simple method.

Performance improvement in p-Type WS2 field-effect transistors with 1T phase contacts

The non-ideal contact between the metal electrode and semiconducting channel has become one of the major bottlenecks degrading the performance of field-effect transistors (FETs) based on two-dimensional (2D) materials. The formation of the Schottky barrier prohibiting the carrier injection as well as the Fermi level pinning effect have a strong impact on the device performance. In this work, we fabricated a 2D WS2 FET device with engineered metallic 1T phase at the source/drain region by using Li intercalation method. As compared to the device with conventional 2H-WS2 channel, the engineered FET with a 2H-WS2/1T-WS2 junction has exhibited greatly improved performance such as over 10 times higher carrier mobility and steeper subthreshold slope. Such results have demonstrated the boosted carrier injection into the channel over the engineered metal contact as well as the reduced tunnel barrier width between the 2H/1T-WS2 junction. This can be attractive for the large-scale integration of the 2D devices towards high-performance and high-reproducibility nanoelectronics applications.

Diverse electronic and magnetic properties of CrS2 enabling strain-controlled 2D lateral heterostructure spintronic devices

Lateral heterostructures of two-dimensional (2D) materials, integrating different phases or materials into a single piece of nanosheet, have attracted intensive research interests for electronic devices. Extending the 2D lateral heterostructures to spintronics demands more diverse electromagnetic properties of 2D materials. In this paper, using density functional theory calculations, we survey all IV, V, and VI group transition metal dichalcogenides (TMDs) and discover that CrS2 has the most diverse electronic and magnetic properties: antiferromagnetic (AFM) metallic 1T phase, non-magnetic (NM) semiconductor 2H phase, and ferromagnetic (FM) semiconductor 1T′ phase with a Curie temperature of ~1000 K. Interestingly, we find that a tensile or compressive strain can turn the 1T′ phase into a spin-up or spin-down half-metal. Such strain tunability can be attributed to the lattice deformation under tensile/compressive strain that selectively promotes the spin-up/spin-down VBM (valence band bottom) orbital interactions. The diverse electromagnetic properties and the strain tunability enable strain-controlled spintronic devices using a single piece of CrS2 nanosheet with improved energy efficiency. As a demo, a prototypical design of the spin-valve logic device is presented. It offers a promising solution to address the challenge of high energy consumption in miniaturized spintronic devices.

Nanoflower-like 1T/2H mixed-phase MoSe2 as an efficient electrocatalyst for hydrogen evolution

Semiconducting 2H-phase MoSe2 usually has poor conductivity and does not exhibit satisfactory properties in the hydrogen evolution reaction (HER). The metallic 1T phase can effectively boost the electrocatalytic activity of MoSe2 by adjusting its electronic structure to improve conductivity and intrinsic activity. Here, we report a unique structural 1T/2H MoSe2 with defects, which is successfully prepared by controlling the reaction temperature, achieving synergistic modulation between active sites and conductivity. Defects can afford more electrocatalytic active sites, and the 1T phase can effectively enhance the conductivity and intrinsic activity. Profiting from the coexistence of the 1T phase and defects, 1T/2H mixed-phase MoSe2 shows superior HER electrocatalytic performance, a low Tafel slope (48 mV dec−1) and a small overpotential at 10 mA cm−2 (−192 mV). This work offers additional insight into enhancing the electrochemical activity of MoSe2 and generally layered dichalcogenide-based catalysts by a simple and direct method.

1T and 2H mixed phase MoS2 nanobelts coupled with Ti3+ self-doped TiO2 nanosheets for enhanced photocatalytic degradation of RhB under visible light

A novel 1T-2H MoS2/TiO2−x composite was fabricated via a facial hydrothermal and in-situ chemical reduction process, in which 1T and 2H mixed phase MoS2 (1T-2H MoS2) nanobelts were deposited on Ti3+ self-doped TiO2 (TiO2−x) nanosheets. This 2D-2D 1T-2H MoS2/TiO2−x composite exhibits excellent photocatalytic degradation performance toward RhB under visible light, which is much higher than that of pristine TiO2. The remarkably enhanced photocatalytic activity of 1T-2H MoS2/TiO2−x composite can be ascribed to the 2D heterostructure, Ti3+ and oxygen vacancy defects, which enhances the adsorption for pollutants, provides abundant reaction active sites, extends the photoresponse to visible light region, benefits the effective charge separation and transport. Furthermore, the experiment results showed that the metallic 1T phase MoS2 in 1T-2H MoS2/TiO2−x composite could effectively accelerate charge transfer between 2H phase MoS2 and TiO2−x, resulting in enhanced photocatalytic degradation ability. Moreover, a feasible photocatalytic reaction mechanism of 1T-2H MoS2/TiO2−x photocatalytic system was proposed in view of the active species trapping experiments, electron spin resonance tests and energy band analysis.

Facile preparation of metallic 1T phase molybdenum selenide as cocatalyst coupled with graphitic carbon nitride for enhanced photocatalytic H2 production

Low-cost, highly active and efficient alternative co-catalysts that can replace precious metals such as Au and Pt are urgently needed for photocatalytic hydrogen evolution reaction (HER). Herein, we show that 1T phase MoSe2 can act as the co-catalyst in the 1T-MoSe2/g-C3N4 composites and we synthesize this composite by a one-step hydrothermal method to promote photocatalytic H2 generation. Our prepared 1T-MoSe2/g-C3N4 composite exhibits highly enhanced photocatalytic H2 production compared to that of g-C3N4 nanosheets (NSs) only. The 7 wt%-1T-MoSe2/g-C3N4 composite presents a considerably improved photocatalytic HER rate (6.95 mmol·h−1·g−1), approximately 90 times greater than that of pure g-C3N4 (0.07 mmol·h−1 g−1). Moreover, under illumination at λ = 370 nm, the apparent quantum efficiency (AQE) of the 7 wt%-1T-MoSe2/g-C3N4 composite reaches 14.0%. Furthermore, the 1T-MoSe2/g-C3N4 composites still maintain outstanding photocatalytic HER stability.

Oxygen incorporated in 1T/2H hybrid MoS2 nanoflowers prepared from molybdenum blue solution for asymmetric supercapacitor applications

Molybdenum disulfide (MoS2) has shown potential as a supercapacitor electrode material, owing to its high theoretical specific capacitance and wide negative potential window, especially in neutral electrolytes. However, its practical application is hindered by low inherent electrical conductivity and narrow interlayer spacing. In this study, oxygen-incorporated MoS2 nanoflowers with 1T/2H hybrid phase were fabricated on a graphite foil substrate (1T/2H-O-MoS2@GF) from molybdenum blue solution via a facile hydrothermal process. Dispersible Mo-blue clusters, composed of polymerized structures containing numerous MoOx-type building units combined with NH4+ cations, were formed by introducing ammonium persulfate before hydrothermal synthesis; they were subsequently used as precursors of MoS2. Mo-blue clusters lead to the formation of metallic 1T-phase dominant heterostructure and incorporation of oxygens in MoS2 during the hydrothermal reaction. The 1T/2H hybrid phase formed by using Mo-blue clusters provided MoS2 with excellent stability in the 2H-phase and high electrical conductivity in the 1T-phase, simultaneously. The incorporated oxygens induced a wide interlayer spacing (9.8 Å) and improved the surface hydrophilicity of MoS2, thereby facilitating the fast diffusion of electrolyte ions. As a result, the 1T/2H-O-MoS2@GF electrode exhibited a high specific capacitance of 280 F g−1 at 1.0 A g−1 and 88% capacitance retention after 10,000 cycles at 10 A g−1. The asymmetric supercapacitor device, assembled using a 1T/2H-O-MoS2@GF negative electrode and MnO2@GF positive electrode, demonstrated superior power and energy densities of 450–18000 W kg−1 and 39.7–5.46 Wh kg−1 in a working window of 1.8 V.

Cobalt-Doping of Molybdenum Disulfide for Enhanced Catalytic Polysulfide Conversion in Lithium-Sulfur Batteries.

Metal sulfides, such as MoS2, are widely investigated in lithium-sulfur (Li-S) batteries to suppress the shuttling of lithium polysulfides (LiPSs) due to their chemical adsorption ability and catalytic activity. However, their relatively low conductivity and activity limit the LiPS conversion kinetics. Herein, the Co-doped MoS2 is proposed to accelerate the catalytic conversion of LiPS as the Co doping can promote the transition from semiconducting 2H phase to metallic 1T phase and introduce the sulfur vacancies in MoS2. A one-step hydrothermal process is used to prepare such a Co-doped MoS2 with more 1T phase and rich sulfur vacancies, which enhances the electron transfer and catalytic activity, thus effectively improving the LiPS adsorption and conversion kinetics. The cathode using the three-dimensional graphene monolith loaded with Co-doped MoS2 catalyst as the sulfur host shows a high rate capability and long cycling stability. A high capacity of 941 mAh g-1 at 2 C and a low capacity fading of 0.029% per cycle at 1 C over 1000 cycles are achieved, suggesting the effectively suppressed LiPS shuttling and improved sulfur utilization. Good cyclic stability is also maintained under a high sulfur loading indicating the doping is an effective way to optimize the metal sulfide catalysts in Li-S batteries.

High Stability of 1T-Phase MoS2xSe2(1–x) Monolayers Under Ambient Conditions

The phase stability in transition metal dichalcogenides (TMDs) is very significant for their utility in various applications. In particular, the 1T phase of group VI TMDs is of high technological importance owing to its electrically conducting properties and phase stability via strain engineering and chemical functionalization has been reported in the literature. In this regard, alloying with atoms of different sizes is an effective approach to introduce inherent strain in the lattice by modifying bond parameters. In the present study, it has been shown that the 1T polymorph of layered chalcogenides gets stabilized in strained, mixed chalcogen alloys, MoS2xSe2(1–x). The 2H phases of MoS2xSe2(1–x) are amenable for efficient chemical exfoliation to monolayers and successful conversion to metallic 1T phase. The metallic 1T phase of MoS2xSe2(1–x) with inherent strain is found to be very stable for a long duration under ambient conditions. Owing to different Mo–S and Mo–Mo bond lengths in mixed chalcogenides, effective tuning of stability is achieved by changing the S/Se ratio in the compound. The differential scanning calorimetry data reveals a shift in the thermal phase transition with a peak maximum at ∼160 °C in the case of 1T MoSSe which is different from its pristine counterparts. Ex situ XRD and Raman spectroscopic studies suggest the role of inherent strain in retaining the octahedral structure at high temperatures. The alloying approach may be extended to other transition metal chalcogenides to fully explore the potential of metastable polytypes.

Interlayer Coupling and Ultrafast Hot Electron Transfer Dynamics in Metallic VSe2/Graphene van der Waals Heterostructures.

Atomically thin vanadium diselenide (VSe2) is a two-dimensional transition metal dichalcogenide exhibiting attractive properties due to its metallic 1T phase. With the recent development of methods to manufacture high-quality monolayer VSe2 on van der Waals materials, the outstanding properties of VSe2-based heterostructures have been widely studied for diverse applications. Dimensional reduction and interlayer coupling with a van der Waals substrate lead to its distinguishable characteristics from its bulk counterparts. However, only a few fundamental studies have investigated the interlayer coupling effects and hot electron transfer dynamics in VSe2 heterostructures. In this work, we reveal ultrafast and efficient interlayer hot electron transfer and interlayer coupling effects in VSe2/graphene heterostructures. Femtosecond time-resolved reflectivity measurements showed that hot electrons in VSe2 were transferred to graphene within a 100 fs time scale with high efficiency. Besides, coherent acoustic phonon dynamics indicated interlayer coupling in VSe2/graphene heterostructures and efficient thermal energy transfer to three-dimensional substrates. Our results provide valuable insights into the intriguing properties of metallic transition metal dichalcogenide heterostructures and motivate designing optoelectronic and photonic devices with tailored properties.