Typical Transition Metal Dichalcogenide Research Articles

Enhanced photoluminescence of WS2/WO3 heterostructural QDs

Abstract Tungsten disulfide (WS2), as one of the typical transition-metal dichalcogenides (TMDCs), possesses attractive physical properties and widely potential applications, especially in optoelectronic devices. However, the low photoluminescence (PL) quantum yield (QY) still limits the practical applications of WS2. In this paper, WS2/WO3 heterostructural (core/shell) quantum dots (QDs) are prepared by pulsed laser deposition (PLD) and rapid thermal annealing (RTA) techniques, which exhibit PL enhancement and higher PLQY. The improvement may originate from the WO3 shells, which act as passivation layer to relieve the surface defect-states and many-body effects. Our work may provide a new pathway to utilize the heterostructural QDs to design physical properties of TMDCs, and these heterostructural QDs are promising candidates in optoelectronic applications such as photodetectors and light-emitting devices.

Ultrathin exfoliated WS2 nanosheets in low-boiling-point solvents for high-efficiency hydrogen evolution reaction

Two-dimensional mono-or few-layer WS2, as a typical transition metal dichalcogenide, usually exhibits excellent electrocatalytic hydrogen evolution reaction (HER) activity. In this work, low-boiling-point solvents are adopted for the controllable preparation of ultra-thin WS2 nanosheets by an improved liquid phase exfoliation process directly from the raw WS2 powder. The thickness of as-obtained WS2 nanosheets is about ∼1.5 nm with lateral size of ∼1.2 μm. Their HER performance was investigated by loading them onto a glass carbon electrode in 0.5M H2SO4 solution. The resulting polarization and Tafel curves indicate that the as-obtained WS2 nanosheets are effective electrocatalysts for HER with a low onset potential of 65mV and a Tafel slope of 55mV/dec. The proposed liquid phase exfoliation technique is lucrative for exfoliating transition metal disulfide materials in various applications.

Pressure induced superconductive 10-fold coordinated TaS2: a first-principles study

By crystal structure prediction and first-principles calculations, we researched the structure transformation and electronic states for a typical transition metal dichalcogenides (TMDs): 1T-TaS2 under hydrostatic pressure. The layered 1T-TaS2 will first transform to an 8-fold monoclinic C2/m phase and then to a 10-fold coordinated tetragonal I4/mmm phase which has 3D covalent bond network linked in space. Our calculations suggest that the lone pair electrons of S in 1T-TaS2, which keep the stablity of the layered structure, will be activated by pressure and participate the chemical bonding with Ta, to form the high-pressure C2/m and I4/mmm phases. Additionally, collective electronic states of superconductivity also retains in the I4/mmm phase and the critical transition temperature of superconductivity is 9 K at 100 GPa.

Dimensionally driven crossover from semimetal to direct semiconductor in layered SbAs

Two-dimensional (2D) materials have attracted a great deal of attention because they exhibit intriguing physical and chemical properties with great potential applications in electronic and optoelectronic devices, and even energy conversion. Due to the high anisotropy and unique crystal structure of layered materials, the properties can be effectively tuned by simply reducing dimensions to 2D. In this work, a unique 2D semiconductor, namely, monolayered SbAs, with high stability and indirect band gap, is predicted on the basis of first-principles calculations together with cluster expansion and Monte Carlo simulations. Interestingly, although the bulk antimony arsenide compound SbAs is known to exhibit semimetallic behavior, our calculations find that it is transformed into a direct semiconductor with a band gap of 1.28 eV when thinned down to a single atomic layer. The monolayer with antisite defects is transformed from indirect into a direct band-gap semiconductor. Such dramatic changes in the electronic structure could pave the way for SbAs to play a role in electronic device applications. Moreover, we find that the interlayer interactions in SbAs lead to a higher exfoliation energy than typical transition metal dichalcogenides such as $\mathrm{Mo}{\mathrm{S}}_{2}$, and hence we suggest that chemical deposition methods might be better than mechanical exfoliation methods for obtaining monolayer samples.

Broadband high responsivity large-area plasmonic-enhanced multilayer MoS2 on p-type silicon photodetector using Au nanostructures

High responsivity, large-area plasmonic-enhanced nanostructure photodetector based on multilayer (ML) molybdenum disulfide (MoS2) deposited on p-type Silicon (Si) substrates is reported. A large area ML-MoS2 is deposited onto the Si photodetector (PD) using a modified membrane filtration method. This large area ML-MoS2 and Au NSs on the p-Si form a cavity-like structure that dramatically enhances the incident light path. The increase of incident light path due to light trapping effect enhances the electron-hole pair generation tremendously. The plasmonic-enhanced ML MoS2 on Si PD has achieved a stable and repeatable photoresponse up to 37 A W−1, whereas the detectivity is around 1012 Jones at the broad wavelengths (405–780 nm) with a modulation frequency of 1 kHz. The enhancement of photoresponsivity is 8, 5.3 and 11 times with 5 V bias at an incident wavelength of 405 nm, 650 nm and 780 nm respectively as compared to the bare p-Si PD. The experimental results also show that the plasmonic-enhanced ML-MoS2 on Si PD exhibited fast photoresponse (rise time of ∼1 μs and fall time of ∼18 μs), which is much higher compared to typical transition metal dichalcogenide PD or single layer MoS2 based PD. These excellent performances show that the plasmonic-enhanced MoS2 structure is highly potential to apply in Si photovoltaics, visible range photodetection, and visible bio/chemical sensing application.

All-optical applications for passive photonic devices of TaS2 nanosheets with strong Kerr nonlinearity

2D Tantalum disulphide (TaS2) has been widely studied as a typical transition metal dichalcogenides (TMDCs) with excellent optical, chemical and electrochemical properties. However, there is little research on the comprehensively characterizing the interaction between light and the 2D TaS2 material. In this work, we studied the nonlinear optical response of the high quality few-layer TaS2 nanosheets and its all-optical applications. Our research shows that 2D TaS2 nanosheet has a broadband characteristic and exhibits a strong Kerr nonlinear optical response based on the spatial self-phase modulation (SSPM). More importantly, by taking advantage of the highly nonlinear optical response of the2D TaS2, typical all-optical phenomena, such as all-optical switching and nonreciprocal light propagation, have been demonstrated experimentally. This work can be viewed as an important demonstration of a TMDCs-based application prototype in nonlinear photonics, which could potentially indicate an essential step toward miscellaneous TMDCs-based passive photonic devices (all-optical diodes, all-optical switches, etc.)

Controlled one step thinning and doping of two-dimensional transition metal dichalcogenides

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) have drawn intensive attention due to their ultrathin feature with excellent electrostatic gating capability, and unique thickness-dependent electronic and optical properties. Controlling the thickness and doping of 2D TMDCs are crucial toward their future applications. Here, we report an effective HAuCl4 treatment method and achieve simultaneous thinning and doping of various TMDCs in one step. We find that the HAuCl4 treatment not only thins thick MoS2 flakes into few layers or even monolayers, but also simultaneously tunes MoS2 into p-type. The effects of various parameters in the process have been studied systematically, and an Au intercalation assisted thinning and doping mechanism is proposed. Importantly, this method also works for other typical TMDCs, including WS2, MoSe2 and WSe2, showing good universality. Electrical transport measurements of field-effect transistors (FETs) based on MoS2 flakes show a big increase of On/Off current ratios (from 102 to 107) after the HAuCl4 treatment. Meanwhile, the subthreshold voltages of the MoS2 FETs shift from −60 to +27 V after the HAuCl4 treatment, with a p-type doping behavior. This study provides an effective and simple method to control the thickness and doping properties of 2D TMDCs, paving a way for their applications in high performance electronics and optoelectronics.

A Facile and Effective Method for Patching Sulfur Vacancies of WS2 via Nitrogen Plasma Treatment.

Although transition metal dichalcogenides (TMDs) are attractive for the next-generation nanoelectronic era due to their unique optoelectronic and electronic properties, carrier scattering during the transmission of electronic devices, and the distinct contact barrier between the metal and the semiconductors, which is caused by inevitable defects in TMDs, remain formidable challenges. To address these issues, a facile, effective, and universal patching defect approach that uses a nitrogen plasma doping protocol is developed, via which the intrinsic vacancies are repaired effectively. To reveal sulfur vacancies and the nature of the nitrogen doping effects, a high-resolution spherical aberration corrected scanning transmission electron microscopy is used, which confirms the N atoms doping in sulfur vacancies. In this study, a typical TMD material, namely tungsten disulfide, is employed to fabricate field-effect transistors (FETs) as a preliminary paradigm to demonstrate the patching defects method. This doping method endows FETs with high electrical performance and excellent contact interface properties. As a result, an electron mobility of up to 184.2 cm2 V-1 s-1 and a threshold voltage of as low as 3.8 V are realized. This study provides a valuable approach to improve the performance of electronic devices that are based on TMDs in practical electronic applications.

Two-step growth of VSe2 films and their photoelectric properties**Project supported by the National Natural Science Foundation of China (Grant Nos. 51572132, 61674082, and 61774089), the National Key R&D Program of China (Grant No. 2018YFB1500202), Tianjin Natural Science Foundation of Key Project, China (Grant Nos. 18JCZDJC31200 and 16JCZDJC30700), Yang Fan Innovative and Entrepreneurial Research Team Project, China (Grant No. 2014YT02N037), 111

We put forward a two-step route to synthesize vanadium diselenide (VSe2), a typical transition metal dichalcogenide (TMD). To obtain the VSe2 film, we first prepare a vanadium film by electron beam evaporation and we then perform selenization in a vacuum chamber. This method has the advantages of low temperature, is less time-consuming, has a large area, and has a stable performance. At 400 °C selenization temperature, we successfully prepare VSe2 films on both glass and Mo substrates. The prepared VSe2 has the characteristic of preferential growth along the c-axis, with low transmittance. It is found that the contact between Al and VSe2/Mo is ohmic contact. Compared to Mo substrate, lower square resistance and higher carrier concentration of the VSe2/Mo sample reveal that the VSe2 film may be a potential material for thin film solar cells or other semiconductor devices. The new synthetic strategy that is developed here paves a sustainable way to the application of VSe2 in photovoltaic devices.

Ultrathin transition-metal dichalcogenide nanosheet-based colorimetric sensor for sensitive and label-free detection of DNA

Two−dimensional transition−metal dichalcogenide (TMD) nanosheets have been regarded as a class of promising nanomaterials in various applications due to their outstanding optical, electronic, and catalytic properties. Here, we explored the metal ion-induced optical behavior of four typical TMD nanosheets (WS2, MoS2, WSe2, and MoSe2) prepared via a kitchen blender-assisted shear exfoliation method. Results show that upon the addition of certain amount of metal ions, layered 2H phase TMD nanosheets would aggregate rapidly, resulting in their growth in both planar size and thickness, which leads to the change of their characteristic absorption peak position. Inspired by this phenomenon, a sensitive and label-free colorimetric sensing strategy for DNA detection was develop combined with the discriminative affinity of TMD nanosheets towards single−strand DNA and double−strand DNA. Furthermore, hybridization chain reaction is introduced to achieve higher sensitivity towards DNA detection. We anticipate that this proposed colorimetric sensor would significantly expand the scope of 2D TMD nanosheets and hold great application prospects in DNA sensing strategies in various fields.

A metal-semiconductor transition triggered by atomically flat zigzag edge in monolayer transition-metal dichalcogenides

Due to the structure of three stacked layers, monolayer transition-metal dichalcogenides (TMDs) is different from graphene. Creating atomically flat graphene-like edges in them has long been expected, which is crucial to the modulation of electronic structures in two-dimensional systems. Recently, by thermal annealing, Chen et al. [21] successfully synthesized atomically flat Mo-terminated edge in monolayer MoS2. Inspired by this, through first-principles calculations, we studied the electronic and transport properties of typical TMD monolayers with transition atom-terminated flat zigzag edges, i.e., ScS2, VS2, CrS2, FeS2, NiS2, MoS2 and WS2. It is found that the nanoribbons with and without flat edges are both metallic. Interestingly, the vacancy in the flat edge could open a transmission gap at the Fermi level in the ScS2 ribbon, and trigger a metal-semiconductor transition. Further analysis shows that, the opening of bandgap around the Fermi level induced by the specific pattern of vacancies is the mechanism behind, which could be used as an modulating method for electronic structures. We believe our results are quite beneficial for the development of many other monolayer transition-metal dichalcogenides configurations, showing great application potential.

Atomic-registry-dependent electronic structures of sulfur vacancies in ReS2 studied by scanning tunneling microscopy/spectroscopy

Rhenium disulfide (ReS2) is regarded as a promising candidate for optoelectronic applications (e.g., infrared photodetector), as it maintains a direct bandgap regardless of the number of layers unlike other typical transition metal dichalcogenides. Therefore, it is very important to understand and control the defects of ReS2 for enhancing the performance of photodevices. In this work, we studied the electronic structures of ReS2 affected by sulfur vacancies of different atomic registries at the atomic scale. The atomic and electronic structures of the mechanically exfoliated ReS2 flakes were investigated using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS), and were confirmed using density functional theory (DFT) calculations. The atomic structural models indicate four distinguishable atomic registries of sulfur vacancies on one face of ReS2. Energetically, these atomic vacancies prefer to locate on the bottom side of the top monolayer of ReS2 flakes. Only two among four possible kinds of vacancies could be observed using STM and STS, and they were identified using additional DFT calculations. We believe that our results regarding the identification of the defects and understanding the corresponding effects for electronic structures will provide important insights to enhance the performances of ReS2-based optoelectronic devices in the future.

Doxorubicin-loaded Fe3O4@MoS2-PEG-2DG nanocubes as a theranostic platform for magnetic resonance imaging-guided chemo-photothermal therapy of breast cancer

Molybdenum disulfide (MoS2), a typical transition-metal dichalcogenide, has attracted increasing attention in the field of nanomedicine because of its preeminent properties. In this study, magnetic resonance imaging (MRI)-guided chemo-photothermal therapy of human breast cancer xenograft in nude mice was demonstrated using a novel core/shell structure of Fe3O4@MoS2 nanocubes (IOMS NCs) via the integration of MoS2 (MS) film onto iron oxide (IO) nanocubes through a facile hydrothermal method. After the necessary PEGylation modification of the NCs for long-circulation purposes, such PEGylated NCs were further capped by 2-deoxy-D-glucose (2-DG), a non-metabolizable glucose analogue to increase the accumulation of the as-prepared NCs at the tumor site, as 2-DG molecules could be particularly attractive to resource-hungry cancer cells. Such 2-DG-modified PEGylated NCs (IOMS-PEG-2DG NCs) acted as drug-carriers for doxorubicin (DOX), which could be easily loaded within the NCs. The obtained IOMS-PEG(DOX)-2DG NCs exhibited a T2 relaxivity coefficient of 48.86 (mM)−1·s−1 and excellent photothermal performance. 24 h after intravenous injection of IOMS-PEG(DOX)-2DG NCs, the tumor site was clearly detected by enhanced T2-weighted MRI signal. Upon exposure to an NIR 808-nm laser for 5 min at a low power density of 0.5 W·cm−2, a marked temperature increase was noticed within the tumor site, and the tumor growth was efficiently inhibited by the chemo-photothermal effect. Therefore, our study highlights an excellent theranostic platform with great potential for targeted MRI-guided precise chemo-photothermal therapy of breast cancer.

Two-Dimensional MoTe­2 PN Diode and CMOS Inverter By Atomic Layer Deposition-Induced Hydrogen Doping

Recently, 2D transition metal dichalcogenides(TMDs) MoTe2 has shown outstanding properties for future electronic devices. Such TMD structures without surface dangling bonds make the 2D MoTe2 more favorable candidate than conventional 3D Si in a few nanometer scale. The band gap of thin MoTe2 appears close to that of Si and quite smaller than those of other typical TMD semiconductors. Even though there have been a few attempts to control charge carrier polarity of MoTe2, functional devices such as complementary metal–oxide–semiconductor (CMOS) inverter have not been reported. Here we demonstrate a 2D CMOS inverter in a single MoTe2 nanosheet by straightforward selective doping technique. In a single MoTe2 flake, initially p-doped channel was selectively converted to n-doped region with high electron mobility of 18 cm2/Vs by atomic layer deposition-induced H-doping. Our ultrathin CMOS inverter exhibited a high DC voltage gain of 29, AC gain of 18 at 1 kHz, and a low static power consumption of a few nW. Our results show a great potential of MoTe2 for future electronic devices based on 2D semiconducting materials Figure 1

Temperature-driven evolution of critical points, interlayer coupling, and layer polarization in bilayer MoS2

The recently emerging two-dimensional (2D) transition-metal dichalcogenides (TMDCs) have been a fertile ground for exploring abundant exotic physical properties. Critical points, the extrema or saddle points of electronic bands, are the cornerstone of condensed-matter physics and fundamentally determine the optical and transport phenomena of the TMDCs. However, for bilayer $\mathrm{Mo}{\mathrm{S}}_{2}$, a typical TMDC and the unprecedented electrically tunable venue for valleytronics, there has been a considerable controversy on its intrinsic electronic structure, especially for the conduction band-edge locations. Moreover, interlayer hopping and layer polarization in bilayer $\mathrm{Mo}{\mathrm{S}}_{2}$ which play vital roles in valley-spintronic applications have remained experimentally elusive. Here, we report the experimental observation of intrinsic critical points locations, interlayer hopping, layer-spin polarization, and their evolution with temperature in bilayer $\mathrm{Mo}{\mathrm{S}}_{2}$ by performing temperature-dependent photoluminescence. Our measurements confirm that the conduction-band minimum locates at the ${K}_{\mathrm{c}}$ instead of ${Q}_{\mathrm{c}}$, and the energy splitting between ${Q}_{\mathrm{c}}$ and ${K}_{\mathrm{c}}$ redshifts with a descent of temperature. Furthermore, the interlayer hopping energy for holes and temperature-dependent layer polarization are quantitatively determined. Our observations are in good harmony with density-functional theory calculations.

Dimensional Crossover Transport Induced by Substitutional Atomic Doping in SnSe2

AbstractSubstitutional atomic doping is one of the most convenient and precise routes to modulate semiconducting material properties. Although two‐dimensional (2D) layered transition metal dichalcogenides (TMDs) are of great interest as a prominent semiconducting material due to their unique physical/chemical properties, such a practical atomic doping is still rare, possibly due to the intrinsic localization nature of conduction paths based on d‐band states. Here, using single‐crystalline Cl‐doped SnSe2, the dimensional crossover in carrier transport accompanied by semiconductor‐to‐metal transition is reported. Nondoped SnSe2 shows semiconducting transport behavior dominated by 2D variable range hopping conduction, exhibiting relatively strong localization of carriers at low‐temperature regions. Moderately electron‐doped SnSe2 by substitution on Se with higher valent Cl exhibits superior electrical conductivity even than the heavily doped one owing to the higher electron mobility of the former (167 cm2 V−1 s−1 at 2 K). Combined with Raman spectra, temperature dependence of mobility clearly evidences the effective screening of homopolar optical mode phonon compared to typical TMD materials. Detailed characterizations with magnetoresistance behaviors finally demonstrate that the suppression of both homopolar optical mode phonon and carrier localization as retaining low‐dimensionality is key for high mobility conduction in electron‐doped SnSe2.

Metal–insulator transition in a transition metal dichalcogenide: Dependence on metal contacts

Transition metal dichalcogenides are promising layered materials for realizing novel nanoelectronic and nano-optoelectronic devices. Molybdenum disulfide (MoS2), a typical transition metal dichalcogenide, has been extensively investigated due to the presence of a sizable band gap, which enables the use of MoS2 as a channel material in field-effect transistors (FET). The gate-voltage-tunable metal–insulator transition and superconductivity using MoS2 have been demonstrated in previous studies. These interesting phenomena can be considered as quantum phase transitions in two-dimensional systems. In this study, we observed that the transport properties of thin MoS2 flakes in FET geometry significantly depend on metal contacts. On comparing Ti/Au with Al contacts, it was found that the threshold voltages for FET switching and metal–insulator transition were considerably lower for the device with Al contacts. This result indicated the significant influence of the Al contacts on the properties of MoS2 devices.

Electrochemical Reaction Mechanism of the MoS2 Electrode in a Lithium-Ion Cell Revealed by in Situ and Operando X-ray Absorption Spectroscopy.

As a typical transition metal dichalcogenide, MoS2 offers numerous advantages for nanoelectronics and electrochemical energy storage due to its unique layered structure and tunable electronic properties. When used as the anode in lithium-ion cells, MoS2 undergoes intercalation and conversion reactions in sequence upon lithiation, and the reversibility of the conversion reaction is an important but still controversial topic. Here, we clarify unambiguously that the conversion reaction of MoS2 is not reversible, and the formed Li2S is converted to sulfur in the first charge process. Li2S/sulfur becomes the main redox couple in the subsequent cycles and the main contributor to the reversible capacity. In addition, due to the insulating nature of both Li2S and sulfur, a strong relaxation effect is observed during the cycling process. This study clearly reveals the electrochemical lithiation-delithiation mechanism of MoS2, which can facilitate further developments of high-performance MoS2-based electrodes.

Ultrahigh, Ultrafast, and Self-Powered Visible-Near-Infrared Optical Position-Sensitive Detector Based on a CVD-Prepared Vertically Standing Few-Layer MoS2/Si Heterojunction.

MoS2, as a typical transition metal dichalcogenide, has attracted great interest because of its distinctive electronic, optical, and catalytic properties. However, its advantages of strong light absorption and fast intralayer mobility cannot be well developed in the usual reported monolayer/few‐layer structures, which make the performances of MoS2‐based devices undesirable. Here, large‐area, high‐quality, and vertically oriented few‐layer MoS2 (V‐MoS2) nanosheets are prepared by chemical vapor deposition and successfully transferred onto an Si substrate to form the V‐MoS2/Si heterojunction. Because of the strong light absorption and the fast carrier transport speed of the V‐MoS2 nanosheets, as well as the strong built‐in electric field at the interface of V‐MoS2 and Si, lateral photovoltaic effect (LPE) measurements suggest that the V‐MoS2/Si heterojunction is a self‐powered, high‐performance position sensitive detector (PSD). The PSD demonstrates ultrahigh position sensitivity over a wide spectrum, ranging from 350 to 1100 nm, with position sensitivity up to 401.1 mV mm−1, and shows an ultrafast response speed of 16 ns with excellent stability and reproducibility. Moreover, considering the special carrier transport process in LPE, for the first time, the intralayer and the interlayer transport times in V‐MoS2 are obtained experimentally as 5 and 11 ns, respectively.

Synergistic Effect of MoS2 Nanosheets and VS2 for the Hydrogen Evolution Reaction with Enhanced Humidity-Sensing Performance.

As a typical transition-metal dichalcogenides, MoS2 has been a hotspot of research in many fields. In this work, the MoS2 nanosheets were compounded on 1T-VS2 nanoflowers (VS2@MoS2) successfully by a two-step hydrothermal method for the first time, and their hydrogen evolution properties were studied mainly. The higher charge-transfer efficiency benefiting from the metallicity of VS2 and the greater activity due to more exposed active edge sites of MoS2 improve the hydrogen evolution reaction performance of the nanocomposite electrocatalyst. Adsorption and transport of an intermediate hydrogen atom by VS2 also enhances the hydrogen evolution efficiency. The catalyst shows a low onset potential of 97 mV, a Tafel slope as low as 54.9 mV dec-1, and good stability. Combining the electric conductivity of VS2 with the physicochemical stability of MoS2, VS2@MoS2 also exhibits excellent humidity properties.