Two-dimensional Metal Dichalcogenides Research Articles

Tailoring the electronic, mechanical, and carrier transport properties of 1T‐SnS2 monolayer via strain engineering: A first-principles study

Two-dimensional metal dichalcogenides have garnered significant interest in the realm of electronic applications on account of their tunable electronic structures and high carrier mobility. Utilizing first-principles calculations and deformation potential (DP) theory, a comprehensive investigation on the electronic, mechanical, and carrier transport properties of 1T-SnS2 monolayer under strain engineering is investigated in this work. The 1T-SnS2 monolayer emerges as energetically stable within an applied equal-biaxial strain interval of −8% to 8%, satisfying Born’s criterion for the elastic constants and thus manifesting remarkable mechanical stability. The lattice parameter, bond length, bond angle, and bandgap dynamically respond to the applied equal-biaxial strain. The band structure calculated using generalized gradient approximation (GGA) and spin-orbit coupling (SOC) of the 1 T-SnS2 monolayer demonstrates the characteristic of an indirect bandgap semiconductor, accompanied by tunable bandgap under strain engineering. Additionally, the carrier transport properties of 1T-SnS2 monolayer under strain engineering are also evaluated on the basis of DP theory, and a pronounced anisotropy is discovered in the effective mass, carrier relaxation time, and carrier mobility along the x- and y-direction. Notably, the application of a strain of −4% and 4% on the 1T-SnS2 monolayer has resulted in high electron mobility of 6233.21 cm2/V·s and hole mobility of 1195.62 cm2/V·s. Our present work not only underscores the substantial influence of strain engineering on the electronic, mechanical, and carrier transport properties of 1 T-SnS2 monolayer but also provides critical insights into the strain-based tuning of two-dimensional metal dichalcogenides for electronics and optoelectronics.

High performance broadband photodetector in two-dimensional metal dichalcogenides mediated by topologically protected surface state

Carrier scattering in photodetectors employing 2D dichalcogenide as an absorber impedes carrier transport. As scattering at the interface between the semiconductor and the electrode restricts carrier transport, it is essential to develop alternative electrode materials to achieve high photoresponsivity. A carrier can effectively move through a topologically protected Dirac surface state without scattering: i.e., alternatives such as topological insulators (TI) containing the surface state can effectively reduce scattering at the interface between the semiconductor and the electrode. In this stucy, TI Bi2Se3 with a well-ordered interfacial structure was used as a carrier transport layer (CTL) to effectively employ the topological surface state (TSS) for improving WSe2 photoresponsivity. At 520 nm, the Bi2Se3 TI-CTL photodetector has a photoresponsivity of up to 11.47 AW−1, 100 times higher than the photodevice using only Au electrodes. The photoresponsivity and time-resolved photoluminescence as a function of thickness indicate that TSS contributes to improved carrier separation with high carrier transfer efficiency, thereby affecting photocurrent generation. The fast carrier transfer through TSS helps efficiently separate electron-hole pairs and suppress recombination at the interface, generating ultra-fast and high-efficiency photocurrents in the broadband region (450–1550 nm). Therefore, TSS based on carrier dynamics can lead to further applications of next-generation optoelectronic devices.

1T/1H-SnS2 Sheets for Electrochemical CO2 Reduction to Formate.

Understanding the catalytic mechanism of highly active two-dimensional electrocatalysts is crucial to their rational design. Herein, we reveal the element dependence of the reactivity of two-dimensional metal dichalcogenide sheets for electrocatalytic CO2 reduction. We found that tin(IV) disulfide (SnS2) and molybdenum(IV) disulfide (MoS2) sheets exhibited Faradaic efficiencies of 63.3% and ∼0%, respectively, for formic acid. Scanning electrochemical cell microscopy and theoretical calculations were used to identify the catalytically active sites of SnS2 as terraces and edges. Owing to the effective utilization of the entire surface area, SnS2 can effectively accelerate catalytic reactions. This finding provides a direction for material research in two-dimensional electrocatalysts for energy-efficient chemical production from electrochemical CO2 reduction, as well as for other energy devices.

High-Performance NiS2 Hollow Nanosphere Cathodes in Magnesium-Ion Batteries Enabled by Tunable Redox Chemistry

Two-dimensional metal dichalcogenides have demonstrated outstanding potential as cathodes for magnesium-ion batteries. However, the limited capacity, poor cycling stability, and severe electrode pulverization, resulting from lack of void space for expansion, impede their further development. In this work, we report for the first time, nickel sulfide (NiS2) hollow nanospheres assembled with nanoparticles for use as cathode materials in magnesium-ion batteries. Notably, the nanospheres were prepared by a one-step solvothermal process in the absence of an additive. The results show that regulating the synergistic effect between the rich anions and hollow structure positively affects its electrochemical performance. Crystallographic and microstructural characterizations reveal the reversible anionic redox of S2-/(S2)2-, consistent with density functional theory results. Consequently, the optimized cathode (8-NiS2 hollow nanospheres) could deliver a large capacity of 301 mA h g-1 after 100 cycles at 50 mA g-1, supporting the promising practical application of NiS2 hollow nanospheres in magnesium-ion batteries.

Multifunctional PbS2 Monolayer with an In-Plane Negative Poisson Ratio and Photocatalytic Water Splitting Properties.

Designing novel multifunctional materials at the nanoscale is vitally important for flexible electronics. Here, we have uncovered a two-dimensional metal dichalcogenide PbS2 with intriguing negative Poisson ratio behavior and favorable optical and photocatalytic water splitting properties. The calculations indicate that the Poisson ratio of the PbS2 monolayer is -0.061 along both x and y lattice directions, which is attributed to its unique tetrahedral motif and the ligand field of the local PbS4 units in the PbS2 monolayer. The electronic band structures show that the narrow band gap (1.59 eV) of the PbS2 monolayer could be effectively modulated by strain engineering. Most importantly, the strain-induced tunability of optical absorbance and suitable band edge alignment make the PbS2 monolayer a promising catalyst for photocatalytic water splitting, which is further confirmed by the reaction free energies. These findings offer an effective avenue for the design and synthesis of a novel optoelectronic functional material.

Second harmonic scattering of redox exfoliated two-dimensional transition metal dichalcogenides

We report the second harmonic scattering experiments from acetonitrile liquid suspensions of two-dimensional metal dichalcogenide nanoflakes,: the semiconducting MoS2 and WS2, the metallic NbS2, and the semi-metallic ZrTe2. Because the study is directly performed in the liquid phase, no symmetry breaking can be attributed to a supporting substrate as it is often the case in the investigation of the second harmonic generation efficiency for these materials. A comparative look of the origin of the nonlinear response using polarization analysis of the hyper Rayleigh scattering intensity is performed. Retardation is clearly exhibited by all nanoflakes considering their average dimensions but remains weak. This is attributed to the non-negligible role played by the nanoflake edges or the presence of surface absorbed polyoxometalates (POMs), in all cases.

Out-of-plane longitudinal sound velocity in SnS2 determined via broadband time-domain Brillouin scattering

Here, we report time-resolved broadband transient reflectivity measurements performed in a single crystal of SnS2. We made use of time-domain Brillouin scattering and a broadband probe to measure the out-of-plane longitudinal sound velocity, υL=(2950±100)ms–1, in this semiconducting two-dimensional metal dichalcogenide. Our study illustrates the potential of this non-invasive all-optical pump–probe technique for the study of the elastic properties of transparent brittle materials and provides the value of the elastic constant c33=(39±3)GPa.

Towards the application of 2D metal dichalcogenides as hydrogen evolution electrocatalysts in proton exchange membrane electrolyzers

The electrolysis of water using renewable power inputs has tremendous potential for storing renewable energy in the form of hydrogen fuel. Proton exchange membrane electrolyzers are amongst the more promising classes of electrolyzer for renewables-driven hydrogen production, but these devices require expensive and scarce precious metal electrocatalysts (such as platinum) that add considerably to device costs and lifecycle carbon footprints. Replacing platinum in proton exchange membrane electrolyzers with cheaper and more abundant alternatives will thus make renewables-to-hydrogen devices more viable. Two-dimensional metal dichalcogenides have the required stability, electronic and catalytic properties to challenge platinum's position as the electrocatalyst of choice in proton exchange membrane electrolyzers. In this minireview, we give an overview of recent progress in the development of two dimensional metal dichalcogenides as hydrogen evolution electrocatalysts, with a particular focus on studies from the last two years.

Robust growth of two-dimensional metal dichalcogenides and their alloys by active chalcogen monomer supply

The precise precursor supply is a precondition for controllable growth of two-dimensional (2D) transition metal dichalcogenides (TMDs). Although great efforts have been devoted to modulating the transition metal supply, few effective methods of chalcogen feeding control were developed. Here we report a strategy of using active chalcogen monomer supply to grow high-quality TMDs in a robust and controllable manner, e.g., MoS2 monolayers perform representative photoluminescent circular helicity of ~92% and electronic mobility of ~42 cm2V−1s−1. Meanwhile, a uniform quaternary TMD alloy with three different anions, i.e., MoS2(1-x-y)Se2xTe2y, was accomplished. Our mechanism study revealed that the active chalcogen monomers can bind and diffuse freely on a TMD surface, which enables the effective nucleation, reaction, vacancy healing and alloy formation during the growth. Our work offers a degree of freedom for the controllable synthesis of 2D compounds and their alloys, benefiting the development of high-end devices with desired 2D materials.

Annealing induced phase transformation from amorphous to polycrystalline SnSe2 thin film photo detector with enhanced light-matter interaction

Two-dimensional metal dichalcogenides thin films have been the theme of concentrated exploration due to their tunable optoelectronic properties that upsurged photosensing device applications. Alteration in deposition/synthesis methods and pre/post treatment given to thin film by which its capability to respond against broad spectral range has received incredible consideration. Here, first time we report the influence of annealing on physical and photodetection properties of thermally evaporated SnSe2 thin films. EDAX confirmed stoichiometry of SnSe2 powder. Deposited thin films are slowly vacuum annealed at 50°, 100°, 150° and 200 °C for 2 h. XRD pattern demonstrated annealing induced phase transformation from amorphous to polycrystalline thin film. UV–Vis-NIR spectroscopy displayed red shift in characteristic peak due to annealing. Photodetection properties presented decent photosensing ability of thin film photodetectors. Among all thin films, 150 °C annealed thin film displayed superior photoresponsivity against switched illumination that makes it suitable for future optical switching device. [Display omitted]

Physio-chemical influence of high electron-phonon coupling induced by 120 MeV Ag9+ SHI irradiation on exfoliated MoS2 - PVA nanocomposite films for achieving remarkable electrical conductivity for potential application in organic electronics

Swift heavy ion (SHI) irradiation technology is known to enhance the optical, electronic, mechanical, and electrical properties in polymer nanocomposites by the virtue of electron-phonon coupling. In the present work, Molybdenum disulphide (MoS2), a two-dimensional metal dichalcogenide, has been exfoliated via liquid-phase exfoliation using N-methyl-2- pyrrolidone (NMP) as the solvent that yielded nanosheets of around 2–4 layers as depicted by HR-TEM images. MoS2 - PVA free-standing films were prepared by wet chemical technique i.e. solution casting method and irradiated by focussed high-energy Ag9+ ion beam at fluence range of 1E10 - 3E11 ions/cm2. As a consequence, the structural modification was observed by X-Ray diffraction studies that showed the shift of (002) plane of MoS2 while Raman studies indicated the decrease of degree of disorderness at fluence 1E10 ions/cm2. SHI irradiation has found to induce a two-order increase in the electrical conductivity yielding a 9.7 E-3 S/cm against that of the pristine films at 2.6E-5 S/cm. The enhanced conductivity is attributed to the induced dispersion and annealing of MoS2 nanosheets in the PVA matrix due to the interaction of 120 MeV Ag9+ ion beam irradiation as explained by Thermal spike model.

A polymer-assisted strategy for hierarchical SnS@N-doped carbon microspheres with enhanced lithium storage performance

Two-dimensional metal dichalcogenides show a promising potential in energy storage due to their high specific capacity. Different from the bulk SnS prepared by conventional solvothermal method, our strategy to prepare the three-dimensional (3D) hierarchical SnS@N-doped carbon (SnS@NC) microspheres assembled from ultrathin nanosheets is realized by the polymer-assisted solvothermal method. The key factor of SnS@NC microspheres is the use of polyvinylpyrrolidone, which not only acts as surfactant to obtain the hierarchical spherical structure but also is used as heteroatom sources to realize the N-doped carbon layer coating. Compared with the bulk SnS, the enhanced electrochemical performance of SnS@NC electrode can be attributed to its 3D hierarchical structure and composition advantages, including enhanced Li+ diffusion kinetics, faster charge transfer, enough void space to accommodate the volume variation, higher electronic conductivity, and more active sites.

Configurational electronic states in layered transition metal dichalcogenides

Mesoscopic irregularly ordered and even amorphous self-assembled electronic structures were recently reported in two-dimensional metallic dichalcogenides (TMDs), created and manipulated with short light pulses or by charge injection. Apart from promising new all-electronic memory devices, such states are of great fundamental importance, since such aperiodic states cannot be described in terms of conventional charge-density-wave (CDW) physics. In this paper, we address the problem of metastable mesoscopic configurational charge ordering in TMDs with a sparsely filled charged lattice gas model in which electrons are subject only to screened Coulomb repulsion. The model correctly predicts commensurate CDW states corresponding to different TMDs at magic filling fractions Doping away from results either in multiple near-degenerate configurational states, or an amorphous state at the correct density observed by scanning tunnelling microscopy. Quantum fluctuations between degenerate states predict a quantum charge liquid at low temperatures, revealing a new generalized viewpoint on both regular, irregular and amorphous charge ordering in transition metal dichalcogenides.

(Invited) what Limits the Intrinsic Carrier Mobility of Two Dimensional Metal Dichalcogenides?

Two-dimensional (2D) metal dichalcogenides (MX2) are the most common type of 2D semiconductors and have shown great potential for a wide range of chemical and physical applications. However, they are limited by a low electron/hole mobility, which has been recognized as one of the major challenges impeding their further developments, and urges efforts to understand the mobility-limiting factors and discovery of higher-mobility alternatives. Here using density functional perturbation theory and Wannier interpolation of the electron–phonon matrix to study a wide range of MX2, we find that the intrinsic carrier mobility, in contrast to common belief, neither correlates with the effective mass nor can be assessed by the widely used deformation potential theory; instead it is limited by the longitudinal optical (LO) phonon scattering for most MX2, while for MoS2 and WS2, the mobility is limited by the longitudinal acoustic (LA) phonon scattering. Furthermore, we find that the LO scattering strength is strongly correlated with the magnitude of the Born effective charge, suggesting that the carrier transport is greatly affected by the electric polarization change induced by the atomic vibration. This finding enables us to use the Born effective charge to rapidly screen the 2D MX2 database for high-mobility semiconductor candidates. Our work reveals the underlying factors governing the intrinsic carrier mobility of 2D MX2, and offers a practical descriptor for discovering high-mobility candidates. Ref: L. Cheng, Y. Liu, JACS, 2018, DOI: 10.1021/jacs.8b07871

(Invited) heterostructured “Binary Materials” for Photodetection from Mid-Infrared, Visible, to X-Ray

Light-matter interaction is a long-lasting theme in condensed matter physics and optoelectronics. Light detection in different wavelength regimes is the foundation of a wide range of sensing, imaging, medical and surveillance technologies. In this talk I will discuss the use of transition-metal oxides, hybrid organo-metal perovskites and other nanomaterials in "binary" mixed-dimensional heterostructures with proper bandgaps and architectures to detect photons with different wavelengths. First, for visible light detection, we demonstrate that combining 3D hybrid perovskites with high-mobility 1D carbon nanotubes or 2D two-dimensional metal dichalcogenides significantly enhances charge transport and device performance. Second, we report a mid-infrared (up to 10 um) hybrid graphene photodetector enabled via coupling graphene with a narrow bandgap semiconductor Ti2O3. Finally, using epitaxial ferroelectric/semiconductor oxide junctions with a current-perpendicular-to-plane geometry, we achieve a new X-ray detector with colossal persistent X-ray-induced photoconductivity.

Enhanced two-photon absorption and two-photon luminescence in monolayer MoS2 and WS2 by defect repairing.

In this work, we investigated the nonlinear optical properties of monolayer MoS2 and WS2 modulated by defect engineering via chemical treatment. The results demonstrate that the two-photon luminescence (TPL) and two-photon absorption (TPA) coefficient were remarkably improved after the repair of sulfur vacancies for both monolayer MoS2 and WS2. After the chemical treatment, the nonradiative relaxation path dominant in pristine monolayer MoS2 is significantly alleviated, resulting in enhanced TPL. Our work affords an effective way to tailor the nonlinear absorption, luminescence and relaxation properties of sulfur-based two-dimensional metal dichalcogenides by defect engineering.

What Limits the Intrinsic Mobility of Electrons and Holes in Two Dimensional Metal Dichalcogenides?

Two-dimensional (2D) metal dichalcogenides (MX2) are the most common type of 2D semiconductors and have shown great potential for a wide range of chemical and physical applications. However, they are limited by a low electron/hole mobility, which has been recognized as one of the major challenges impeding their further developments, and urges efforts to understand the mobility-limiting factors and discovery of higher-mobility alternatives. Here using density functional perturbation theory and Wannier interpolation of the electron-phonon matrix to study a wide range of MX2, we find that the intrinsic carrier mobility, in contrast to common belief, neither correlates with the effective mass nor can be assessed by the widely used deformation potential theory; instead it is limited by the longitudinal optical (LO) phonon scattering for most MX2, while for MoS2 and WS2, the mobility is limited by the longitudinal acoustic (LA) phonon scattering. Furthermore, we find that the LO scattering strength is strongly correlated with the magnitude of the Born effective charge, suggesting that the carrier transport is greatly affected by the electric polarization change induced by the atomic vibration. This finding enables us to use the Born effective charge to rapidly screen the 2D MX2 database for high-mobility semiconductor candidates. Our work reveals the underlying factors governing the intrinsic carrier mobility of 2D MX2, offers a practical descriptor for discovering high-mobility candidates, and serves as a paradigm to accurately assess the carrier mobility in 2D semiconductors, thereby paving critical steps toward the development of 2D materials.

Rapid synthesis of MoS2-PDA-Ag nanocomposites as heterogeneous catalysts and antimicrobial agents via microwave irradiation

Two dimensional nanomaterials have attracted great interest for diverse applications due to their unique structure feature, small size and high specific surface areas. Among them, the two-dimensional metal dichalcogenides have recently emerged as the outstanding ones for various applications ranged from biosensing, energy conversion and storage, and cancer photothermal treatment. In this work, a fast, facile and environmentally benign microwave irradiation method was developed to fabricate MoS2-PDA-Ag nanocomposites. To obtain MoS2-PDA-Ag nanocomposites, MoS2 nanosheets were first coated with polydopamine (PDA) films via self-polymerization of dopamine in alkaline aqueous solution. Then Ag nanoparticles were facilely deposed onto the surface of MoS2-PDA via in situ reduction by using microwave irradiation. Here, the PDA thin films were as the ad-layers for reduction and stabilizing the Ag nanoparticles simultaneously. The successful preparation of MoS2-PDA-Ag nanocomposites was characterized by means of transmission electron microscope, thermogravimetry analysis and X-ray photoelectron spectroscopy. We demonstrated that MoS2-PDA-Ag nanocomposites can be facilely fabricated via combination of mussel inspired chemistry and microwave irradiation. The resultant composites exhibited high catalytic activity towards 4-nitrophenol reduction and great antibacterial activity compared with pure MoS2 and MoS2-PDA.

(Invited) Photodetection from Mid-IR to UV Using Semiconductor Heterostructures

I will discuss the use of semiconductor heterostructures with proper bandgaps and architectures to detect photons with different wavelengths. First, for the most pursued visible light detection, hybrid perovskites have aroused lots of interest because of their exceptional properties, such as low-cost solution processing, tunable direct bandgap, high light absorption and long carrier diffusion length. Our experiments demonstrate that combining 3D perovskites with high-mobility 1D carbon nanotubes or 2D two-dimensional metal dichalcogenides significantly enhances charge transport and device performance. Second, we demonstrate a mid-infrared hybrid graphene photodetector enabled via coupling graphene with a narrow bandgap semiconductor Ti2O3. This hybrid photodetector achieves high responsivity of ~200 A/W in a broadband wavelength range up to 10 μm. Finally, using epitaxial ferroelectric/semiconductor junctions with a current-perpendicular-to-plane geometry, we achieve a colossal X-ray-induced photoconductivity with a magnitude of six orders. This X-ray-induced persistent photoconductivity persists for days with negligible decay. This new device scheme open a new route towards X-ray sensing and imaging applications.

Tuning the catalytic activity of heterogeneous two-dimensional transition metal dichalcogenides for hydrogen evolution

This study establishes big data for the catalytic properties of two-dimensional metal-dichalcogenides (2D-TMDs) toward the hydrogen evolution reaction (HER). In addition to conventionally known active sites of edges, it proposes that terrace sites (or the basal plane) can be substantially activated for the HER.