Phase Transition Of MoS2 Research Articles

Nitrogen plasma-induced phase engineering and titanium carbide/carbon nanotubes dual conductive skeletons endow molybdenum disulfide with significantly improved lithium storage performance

MoS2/Ti3C2 MXene composite has emerged as a promising anode material for lithium storage due to the synergistic combination of high specific capacity offered by MoS2 and conductive skeleton provided by Ti3C2 MXene. However, its two-dimensional/two-dimensional (2D/2D) structure is susceptible to collapse after long cycles, while the inherent low conductivity of MoS2 limits its rate performance. In this study, we developed a novel approach combining plasma-induced phase engineering with dual skeleton structure design to fabricate a unique P-MoS2/Ti3C2/CNTs anode material featuring highly conductive 1T phase MoS2 and a stable one-dimensional/two-dimensional (1D/2D) architecture. Within this architecture, growth of MoS2 nanosheets on the surface of Ti3C2 cross-linked by carbon nanotubes (CNTs) was achieved. The resulting Ti3C2/CNTs dual skeleton not only provides robust mechanical support to prevent structural collapse during long cycles but also offers increased specific surface area and additional Li+ storage space, thereby enhancing the lithium storage capacity of the composite. Subsequent N2 plasma treatment induced a phase transition in MoS2 from 2H to 1T configuration. Density functional theory (DFT) calculations confirmed that the induced 1T-MoS2 exhibits higher conductivity and lower Li+ diffusion barrier compared to 2H-MoS2. Benefiting from these synergistic effects, our P-MoS2/Ti3C2/CNTs anode demonstrated remarkable electrochemical performance including a high reversible specific capacity of 1120 mAh g−1 at 0.1 A g−1, excellent cycling stability with a specific capacity retention of 670 mAh g−1 after 600 cycles at 1 A/g, and superior rate performance with a specific capacity of 614 mAh g−1 at 2 A g−1. This combined modification strategy will serve as guidance for designing other energy storage materials.

Charge Mobility and Strain Engineering in Two-Step MS-Grown MoS2/Seed Layer Heterointerface and Photo-Excitation Mechanism.

Two-dimensional (2D) materials have potential application and wide development prospects in photoelectron and spintronic devices. However, the properties of different growth conditions are challenging to study in the future. This, in turn, hinders further research into 2D materials and the manufacture of high-quality devices. A comprehensive understanding of the ultrafast laser spectroscopy and dynamics that take into account the substrate-transition metal dichalcogenide (TMD) interaction is lacking. Here, the strain effect is elucidated by systematically investigating the interfacial interaction between different substrates and MoS2. The strain and interface engineering of MoS2/seeds layer heterointerface and light-matter coupling are discussed in the Raman and photoluminescence spectra. The dramatic enhanced PL originates from the phase transition of MoS2 on different substrates and electron-hole pairs dissociated by exciton screening effect. Finite-difference time-domain simulation confirmed that the electric field, magnetic field, and polarization field of the heterojunction system changed after the strain was applied. In addition, based on the dependence of physical parameters of MoS2, the relative numerical changes of physical parameters of MoS2 films on different substrates as well as the photoelectric transfer, strain, and charge doping levels on the surface or interface will provide a direction for optimizing the selection of various devices.

Anomalous electrical conductivity change in MoS2 during the transition from the amorphous to crystalline phase

Transition metal dichalcogenides exhibit unique properties, which make them interesting for fundamental studies and for applications in many devices. Here, we report on the anomalous electrical behavior during the amorphous-to-crystal phase transition of MoS2. While crystallization typically results in an increase in conductivity, the situation in MoS2 is opposite. Amorphous MoS2 shows a sheet resistance of 3.2 × 103Ω□ with the value remaining nearly constant until 200 °C, MoS2 samples annealed above 200 °C exhibit an unexpected two-step increase in sheet resistance. The first abrupt increase takes place after annealing at temperatures between 300 and 400 °C while the second increase occurs after heating to 700 °C with typical sheet resistance values of 1.2 × 105 and 8 × 106Ω□ after heating to 500 °C and 900 °C, respectively. Using a combination of X-ray photoelectron spectroscopy and X-ray diffraction studies and ab-initio modeling, we argue that the large electrical conductivity observed in amorphous MoS2 is associated with the existence of a large quantity of Mo–Mo homopolar bonds that exceed the percolation threshold. The dramatic increase in the sheet resistivity in the first step is accompanied by the formation of the Mo–S bonds upon the disassociation of homopolar Mo–Mo bonds for a further increase in the sheet resistance upon annealing temperatures of 700 °C that can be attributed to the grain boundary formation during crystallization of MoS2 into the 2H layered structure.

Heterointerface Control over Lithium-Induced Phase Transitions in MoS2 Nanosheets: Implications for Nanoscaled Energy Materials

Phase transitions of two-dimensional materials and their heterostructures enable many applications including electrochemical energy storage, catalysis, and memory; however, the nucleation pathways by which these transitions proceed remain underexplored, prohibiting engineering control for these applications. Here, we demonstrate that the lithium intercalation-induced 2H-1T' phase transition in MoS2 proceeds via nucleation of the 1T' phase at a heterointerface by monitoring the phase transition of MoS2/graphene and MoS2/hexagonal boron nitride (hBN) heterostructures with Raman spectroscopy in situ during intercalation. We observe that graphene-MoS2 heterointerfaces require an increase of 0.8 V in applied electrochemical potential to nucleate the 1T' phase in MoS2 compared to hBN-MoS2 heterointerfaces. The increased nucleation barrier at graphene-MoS2 heterointerfaces is due to the reduced charge transfer from lithium to MoS2 at the heterointerface as lithium also dopes graphene based on ab initio calculations. Further, we show that the growth of the 1T' domain propagates along the heterointerface, rather than through the interior of MoS2. Our results provide the first experimental observations of the heterogeneous nucleation and growth of intercalation-induced phase transitions in two-dimensional materials and heterointerface effects on their phase transition.

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.

Doping‐Assisted Phase Changing Effect on MoS2 Towards Hydrogen Evolution Reaction in Acidic and Alkaline pH

AbstractHydrogen evolution reaction (HER) was improved through nitrogen (N) doping in molybdenum disulfide (MoS2) due to the formation of 1T‐metallic phase as compared to the thermodynamically stable 2H‐semiconducting phase. Generally, the phase transition of MoS2 from semiconducting 2H to metallic 1T was carried out by chemical intercalation method. A facile solvothermal synthetic procedure is used to organize 1T@2H MoS2 nanoflower by incorporating N in MoS2 crystal lattice which improved the catalytic activity with the generation of metallic property of MoS2. Optimized N doping is an effective strategy for the development of mixed phase MoS2. Physicochemical characterization techniques confirmed the formation of hybrid phase (1T@2H) MoS2 by N incorporation. A tuned dopant concentration in MoS2 crystal lattice effectively enhanced the catalytic performance by modifying the physical and chemical properties. Moreover, optimal N doped MoS2 offered a very low overpotential of ∼108 and ∼141 mV to reach the benchmarking current density of 10 mA cm−2 for HER in acidic and basic medium, respectively. This work elucidated a rational implantation of phase engineering, which is an efficient strategy to develop efficient electrocatalysts, shedding light on the improvement of transition metal‐based electrocatalyst in renewable energy technologies.

Mechanically interlocked 1T/2H phases of MoS2 nanosheets for solar thermal water purification

Solar steam generation through photothermal conversion and heat localization is an emerging technology that can potentially alleviate shortages of clean water. As a two-dimensional transition-metal dichalcogenide, molybdenum disulfide (MoS2) has been employed for harvesting solar energy in hydrogen gas production, disinfection, and solar cells. Here, for the first time, we demonstrate that chemically exfoliated (ce) MoS2 can be a highly efficient, scalable, and environmentally benign (low toxicity) photothermal material for solar evaporators to create fresh water. Notably, the phase transition of MoS2 from 2H (trigonal prismatic coordination) to 1T (octahedral coordination) during the exfoliation process enhances the light absorption of the ce-MoS2, generating heat more effectively. Owing to the efficient photothermal conversion of ce-MoS2 nanosheets and heat localization from using bacterial nanocellulose foam as support, high solar evaporation efficiencies of ~ 76% and ~ 81% are achieved under 0.76 kW/m2 and 5.35 kW/m2 light intensities, respectively. In addition, the cytotoxicity of ce-MoS2 nanosheets was lower than that of graphene oxide (GO) nanosheets with a similar size, a commonly suggested material for solar steam generation, which can alleviate the potential environmental risk. These findings not only establish ce-MoS2 as a highly attractive material for solar steam generation, but also broaden the uses of ce-MoS2 to include solar harvesting applications.

MoS2 heterostructure with tunable phase stability: strain induced interlayer covalent bond formation.

The structural phase transition in MoS2 promises applications in novel nanoelectronic devices. Elastic strain engineering can not only serve as a potential route for phase transition engineering, but also reveal potential ferroelastic behavior of MoS2 nanostructures. However, the elastic strain required for phase transition in monolayer MoS2 is far beyond its elastic limit, thus inhibiting the experimental realization. In this study, employing density functional theory calculations, we uncover that by forming heterostructure with buckled 2D materials, such as silicene, germanene and stanene, the critical phase transition strain required in monolayer MoS2 can be drastically reduced. Particularly when MoS2 forms sandwiched structures with silicene or stanene, the uniaxial and biaxial critical strain can be reduced to ∼0.06 and ∼0.03, respectively, which is well below the experimental elastic limit. This theoretical study not only proposes an experimental achievable strategy for flexible phase transition design in MoS2 nanostructure, but also identifies those MoS2 heterostructures as 2D candidates for potential shape memory devices and pseudoelasticity applications.

MoS2 Field-Effect Transistor with Sub-10 nm Channel Length.

Atomically thin molybdenum disulfide (MoS2) is an ideal semiconductor material for field-effect transistors (FETs) with sub-10 nm channel lengths. The high effective mass and large bandgap of MoS2 minimize direct source-drain tunneling, while its atomically thin body maximizes the gate modulation efficiency in ultrashort-channel transistors. However, no experimental study to date has approached the sub-10 nm scale due to the multiple challenges related to nanofabrication at this length scale and the high contact resistance traditionally observed in MoS2 transistors. Here, using the semiconducting-to-metallic phase transition of MoS2, we demonstrate sub-10 nm channel-length transistor fabrication by directed self-assembly patterning of mono- and trilayer MoS2. This is done in a 7.5 nm half-pitch periodic chain of transistors where semiconducting (2H) MoS2 channel regions are seamlessly connected to metallic-phase (1T') MoS2 access and contact regions. The resulting 7.5 nm channel-length MoS2 FET has a low off-current of 10 pA/μm, an on/off current ratio of >107, and a subthreshold swing of 120 mV/dec. The experimental results presented in this work, combined with device transport modeling, reveal the remarkable potential of 2D MoS2 for future sub-10 nm technology nodes.

Phase Transition of MoS2 Bilayer Structures

In the present study, using density functional calculations we have investigated a possible mechanism for the structural phase transition of the semiconducting bilayer 2H-MoS2 via lithiation. The results indicate that the addition of lithium to the bilayer 2H-MoS2 transforms the bilayer to a heterostructure of the 2H and 1T structures instead of a complete conversion to the 1T bilayer structure. Therefore, we propose that the desired synthesis of the 1T-MoS2 from the bulk 2H-MoS2 takes place through the hybrid 2H–1T structure. Our finding gives physical insight into the experimentally described microscopic mechanism of the phase transition in MoS2 and enriches the atomic scale understanding of the interaction of MoS2 with the alkali ions and other transition-metal dichalcogenides manifesting a similar phase transition.

Enhancement of magnetism by structural phase transition in MoS2

The magnetic properties of 2H phase of MoS2 (2H-MoS2) and 1T phase of MoS2 (1T-MoS2) were investigated both experimentally and theoretically. Lithium (Li) intercalation method was used to prepare single-layer MoS2 sheets. It was found that pristine MoS2 (2H-MoS2) exhibited weak diamagnetism. After exfoliating by Li intercalation, the crystal structure transformed from 2H to 1T phase, and the magnetism was significantly enhanced from diamagnetism to paramagnetism accordingly. With further annealing in argon atmosphere, the 2H phase recovered gradually from 1T phase, and the magnetism decreased correspondingly. Using crystal field theory and combining the results of first principle calculation, we conclude that the enhanced magnetism can be attributed to the Mo atoms of 1T-MoS2.

Sulfur-catalyzed phase transition in MoS2 under high pressure and temperature

We report phase transition and stability of MoS2 with and without the presence of sulfur melt under high-pressure and high-temperature conditions. Rhombohedral (3R) phase is found to be a high-temperature phase of MoS2 at high pressures. Excess sulfur melt catalyzes the hexagonal (2H) to rhombohedral (3R) phase transformation and lowers the conversion temperature by more than 280K. Boundary between 2H and 3R phases has been delineated with a negative slope. Based on experimental observations, sulfur-catalyzed 2H→3R transformation mechanisms are proposed involving atomic exchange between MoS2 and sulfur, which is different from the case of without excess sulfur that proceeds through rotation and translation of the S–Mo–S sandwich layers.