Sheets Of Transition Metal Dichalcogenides Research Articles

Synthesis and ambipolar transistor properties of tungsten diselenide nanotubes

Recently, the electrical properties of inorganic nanotubes made by rolling up transition metal dichalcogenide sheets have attracted much attention. There are various types of inorganic nanotubes with different metal and chalcogen atoms, which can provide a variety of electrical properties; however, the inorganic nanotubes whose electrical properties were revealed have been mostly limited to WS2 nanotubes. Here, we investigated the electrical properties of another type of inorganic nanotube, namely, WSe2 nanotubes, and revealed their ambipolar transistor properties through the development of a facile synthesis method. WSe2 nanotubes were synthesized by the selenization of solution-synthesized tungsten oxide nanowires. We found that the introduction of a specific concentration of H2 during selenization was crucial for obtaining WSe2 nanotubes with relatively small diameters. The obtained WSe2 nanotubes exhibited ambipolar transistor operation with higher performance than WS2 nanotubes.

(Invited) Multi-Level Non-Volatile Memory in Au/Monolayer MoS2/Au Structures

The wide range of electronic and optical properties exhibited by two dimensional semiconductors, and their variation physical thickness, opens up new possibilities to create ultra-short length transistors [1], steep-slope switches [2, 3], light detecting/emitting devices [4, 5] and chemical/physical sensors. Recently, it was demonstrated that monolayer 2D semiconductors can also display resistive switching [6]. In this work we demonstrate that a single layer of large area CVD-grown moly disulphide (MoS2) sandwiched between two metal electrodes can be tuned to exhibit multi-level non-volatile resistive memory, with resistance values separated by 5 orders of magnitude. The switching process is unipolar and thermochemically driven (TCM memory) requiring significant Joule heating in the RESET process. Temperature dependent electrical measurements will be presented which demonstrate the charge transport mechanism varies between the initial state (IS), the high resistive state (HRS) and low resistive state (LRS). The temperature dependent behavior of the SET, RESET process is consistent with the creation and passivation of sulfur vacancies in the MoS2 monolayer. Conductive AFM measurements confirm that the multi-filamentary resistive memory effects are inherent to a single crystalline MoS2 triangle and not necessarily dependent of grain boundaries. This work demonstrates that potential for 2 terminal, multi-state, resistive memories for next generation digital memory and neuromorphic applications.[1] S. B. Desai et al., "MoS2 transistors with 1-nanometer gate lengths," Science, vol. 354, no. 6308, pp. 99-102, 2016.[2] S. Bhattacharjee, K. L. Ganapathi, S. Mohan, and N. Bhat, "A sub-thermionic MoS2 FET with tunable transport," Applied Physics Letters, vol. 111, no. 16, p. 163501, 2017.[3] D. Sarkar et al., "A subthermionic tunnel field-effect transistor with an atomically thin channel," Nature, vol. 526, no. 7571, p. 91, 2015.[4] K. Roy et al., "Graphene–MoS 2 hybrid structures for multifunctional photoresponsive memory devices," Nature nanotechnology, vol. 8, no. 11, p. 826, 2013.[5] X. Zhou et al., "2D Layered Material‐Based van der Waals Heterostructures for Optoelectronics," Advanced Functional Materials, vol. 28, no. 14, p. 1706587, 2018.[6] R. Ge et al., "Atomristor: Nonvolatile Resistance Switching in Atomic Sheets of Transition Metal Dichalcogenides," Nano letters, vol. 18, no. 1, pp. 434-441, 2017.

Multivalley Free Energy Landscape and the Origin of Stripe and Quasi-Stripe CDW Structures in Monolayer MX2 Compounds

Ultrathin sheets of transition metal dichalcogenides (MX2) with charge density waves (CDWs) is increasingly gaining interest as a promising candidate for graphene-like devices. Although experimental data including stripe/quasi-stripe structure and hidden states have been reported, the ground state of ultrathin MX2 compounds and, in particular, the origin of anisotropic (stripe and quasi-stripe) CDW phases is a long-standing problem. Anisotropic CDW phases have been explained by Coulomb interaction between domain walls and inter-layer interaction. However, these models assume that anisotropic domain walls can exist in the first place. Here, we report that anisotropic CDW domain walls can appear naturally without assuming anisotropic interactions: We explain the origin of these phases by topological defect theory (line defects in a two-dimensional plane) and interference between harmonics of macroscopic CDW wave functions. We revisit the McMillan-Nakanishi-Shiba model for monolayer 1T-TaS2 and 2H-TaSe2 and show that CDWs with wave vectors that are separated by 120° (i.e. the three-fold rotation symmetry of the underlying lattice) contain a free-energy landscape with many local minima. Then, we remove this 120° constraint and show that free energy local minima corresponding to the stripe and quasi-stripe phases appear. Our results imply that Coulomb interaction between domain walls and inter-layer interaction may be secondary factors for the appearance of stripe and quasi-stripe CDW phases. Furthermore, this model explains our recent experimental result (appearance of the quasi-stripe structure in monolayer 1T-TaS2) and can predict new CDW phases, hence it may become the basis to study CDW further. We anticipate our results to be a starting point for further study in two-dimensional physics, such as explanation of “Hidden CDW states”, study the interplay between supersolid symmetry and lattice symmetry, and application to other van der Waals structures.

Exfoliated transition metal dichalcogenide (MX2; M = Mo, W; X = S, Se, Te) nanosheets and their composites with polyaniline nanofibers for electrochemical capacitors

Ultrathin sheets of transition metal dichalcogenides have received a great deal of interest in an attempt to develop advanced electrode nanomaterials for electrochemical capacitors, with most of the attention focused on pure transition metal dichalcogenides. We systematically investigated and compared the charge storage capability of tert-butyllithium-exfoliated group 6 transition metal dichalcogenide materials (MX2; M=Mo, W; X=S, Se, Te). The group 6 MX2 2D materials exhibited distinctive charge storage performance as a result of their different chemical compositions and morphological features. MoTe2 nanosheets demonstrated the highest gravimetric capacitance of all materials. Furthermore, we hybridized these 2D materials with nanofibrous polyaniline (PANI) to study the impact of MX2 materials on its capacitance performance. All the fabricated nanocomposites exhibited a greatly enhanced capacitance compared with the pure PANI, with the highest increase observed for MoTe2/PANI (499Fg−1), which represents a 76% increase. This enhancement arises from the promotion of available active surface area of PANI and MoTe2 for electrolyte ions.

Ultrahigh-Sensitive Broadband Photodetectors Based on Dielectric Shielded MoTe2 /Graphene/SnS2 p-g-n Junctions.

2D atomic sheets of transition metal dichalcogenides (TMDs) have a tremendous potential for next-generation optoelectronics since they can be stacked layer-by-layer to form van der Waals (vdW) heterostructures. This allows not only bypassing difficulties in heteroepitaxy of lattice-mismatched semiconductors of desired functionalities but also providing a scheme to design new optoelectronics that can surpass the fundamental limitations on their conventional semiconductor counterparts. Herein, a novel 2D h-BN/p-MoTe2 /graphene/n-SnS2 /h-BN p-g-n junction, fabricated by a layer-by-layer dry transfer, demonstrates high-sensitivity, broadband photodetection at room temperature. The combination of the MoTe2 and SnS2 of complementary bandgaps, and the graphene interlayer provides a unique vdW heterostructure with a vertical built-in electric field for high-efficiency broadband light absorption, exciton dissociation, and carrier transfer. The graphene interlayer plays a critical role in enhancing sensitivity and broadening the spectral range. An optimized device containing 5-7-layer graphene has been achieved and shows an extraordinary responsivity exceeding 2600 A W-1 with fast photoresponse and specific detectivity up to ≈1013 Jones in the ultraviolet-visible-near-infrared spectrum. This result suggests that the vdW p-g-n junctions containing multiple photoactive TMDs can provide a viable approach toward future ultrahigh-sensitivity and broadband photonic detectors.

Epoxy composites with functionalized molybdenum disulfide nanoplatelet additives.

As a typical layered inorganic analogue of graphene, molybdenum disulfide (MoS2) has gained intensive attention and become a research hotspot due to its unique two dimensional nanostructure and excellent properties. The enhancement in the thermal and mechanical properties of polymer/inorganic nanosheet composites depends strongly on their interfacial interaction. In this study, we exfoliated bulk MoS2 into nanosheets which were subsequently functionalized using 3-mercaptopropyltriethoxysilane. The functionalized MoS2 (f-MoS2) were dispersed in epoxy polymers at loading fractions of up to 1% by weight via ultrasonication and three roll mills. We characterized the tensile, fracture and adhesive properties of the composite and show that f-MoS2 nanoplatelets are highly effective at enhancing the mechanical properties of the epoxy at very low nanofiller loading fractions (0.1–0.7% by weight). Our results show the potential of functionalized 2D sheets of transition metal dichalcogenides as reinforcing additives in polymeric composites. The results indicate that the glass transition temperature increases significantly for the lower weight fraction composites, from 135 °C for the baseline (unfilled) epoxy to 146 °C at 0.7% f-MoS2 loading. The apparent shear strength at 120 °C increases significantly for the lower weight fraction composites, from 13.8 MPa for the baseline (unfilled) epoxy to 24.9 MPa at 0.7% f-MoS2 loading.

High-yield exfoliation of tungsten disulphide nanosheets by rational mixing of low-boiling-point solvents

Developing high-throughput, reliable, and facile approaches for producing atomically thin sheets of transition metal dichalcogenides is of great importance to pave the way for their use in real applications. Here, we report a highly promising route for exfoliating two-dimensional tungsten disulphide sheets by using binary combination of low-boiling-point solvents. Experimental results show significant dependence of exfoliation yield on the type of solvents as well as relative volume fraction of each solvent. The highest yield was found for appropriate combination of isopropanol/water (20 vol% isopropanol and 80 vol% water) which is approximately 7 times higher than that in pure isopropanol and 4 times higher than that in pure water. The dramatic increase in exfoliation yield can be attributed to perfect match between the surface tension of tungsten disulphide and binary solvent system. Furthermore, solvent molecular size also has a profound impact on the exfoliation efficiency, due to the steric repulsion.

Engineering Chemically Active Defects in Monolayer MoS2 Transistors via Ion‐Beam Irradiation and Their Healing via Vapor Deposition of Alkanethiols

Irradiation of 2D sheets of transition metal dichalcogenides with ion beams has emerged as an effective approach to engineer chemically active defects in 2D materials. In this context, argon-ion bombardment has been utilized to introduce sulfur vacancies in monolayer molybdenum disulfide (MoS2 ). However, a detailed understanding of the effects of generated defects on the functional properties of 2D MoS2 is still lacking. In this work, the correlation between critical electronic device parameters and the density of sulfur vacancies is systematically investigated through the fabrication and characterization of back-gated monolayer MoS2 field-effect transistors (FETs) exposed to a variable fluence of low-energy argon ions. The electrical properties of pristine and ion-irradiated FETs can be largely improved/recovered by exposing the devices to vapors of short linear thiolated molecules. Such a solvent-free chemical treatment-carried out strictly under inert atmosphere-rules out secondary healing effects induced by oxygen or oxygen-containing molecules. The results provide a guideline to design monolayer MoS2 optoelectronic devices with a controlled density of sulfur vacancies, which can be further exploited to introduce ad hoc molecular functionalities by means of thiol chemistry approaches.

Substantial improvements of long-term stability in encapsulation-free WS2 using highly interacting graphene substrate

We report the novel role of graphene substrates in obstructing the aging propagation in both the basal planes and edges of two-dimensitional sheets of transition metal dichalcogenides (TMDs). Even after 300 d in ambient air conditions, the epitaxially grown WS2/graphene samples have a clean, uniform surface without any encapsulation. We show that high crystallinity is an effective factor that determines the excellent air stability of WS2/graphene, and we present impressive experimental evidence of the relation between defects and the aging phenomena. Moreover, we reveal the strong interlayer charge interaction as an additional factor for the enhanced air stability as a result of charge transfer-induced doping. This work not only proposes a simple method to create highly stable TMDs by the selection of a suitable substrate but also paves the way for the realization of practical TMDs-based applications.

Aging of Transition Metal Dichalcogenide Monolayers.

Two-dimensional sheets of transition metal dichalcogenides are an emerging class of atomically thin semiconductors that are considered to be "air-stable", similar to graphene. Here we report that, contrary to current understanding, chemical vapor deposited transition metal dichalcogenide monolayers exhibit poor long-term stability in air. After room-temperature exposure to the environment for several months, monolayers of molybdenum disulfide and tungsten disulfide undergo dramatic aging effects including extensive cracking, changes in morphology, and severe quenching of the direct gap photoluminescence. X-ray photoelectron and Auger electron spectroscopy reveal that this effect is related to gradual oxidation along the grain boundaries and the adsorption of organic contaminants. These results highlight important challenges associated with the utilization of transition metal dichalcogenide monolayers in electronic and optoelectronic devices. We also demonstrate a potential solution to this problem, featuring encapsulation of the monolayer sheet by a 10-20 nm thick optically transparent polymer (parylene C). This strategy is shown to successfully prevent the degradation of the monolayer material under accelerated aging (i.e., high-temperature, oxygen-rich) conditions.

Epoxy nanocomposites with two-dimensional transition metal dichalcogenide additives.

Emerging two-dimensional (2D) materials such as transition metal dichalcogenides offer unique and hitherto unavailable opportunities to tailor the mechanical, thermal, electronic, and optical properties of polymer nanocomposites. In this study, we exfoliated bulk molybdenum disulfide (MoS2) into nanoplatelets, which were then dispersed in epoxy polymers at loading fractions of up to 1% by weight. We characterized the tensile and fracture properties of the composite and show that MoS2 nanoplatelets are highly effective at enhancing the mechanical properties of the epoxy at very low nanofiller loading fractions (below 0.2% by weight). Our results show the potential of 2D sheets of transition metal dichalcogenides as reinforcing additives in polymeric composites. Unlike graphene, transition metal dichalcogenides such as MoS2 are high band gap semiconductors and do not impart significant electrical conductivity to the epoxy matrix. For many applications, it is essential to enhance mechanical properties while also maintaining the electrical insulation properties and the high dielectric constant of the polymer material. In such applications, conductive carbon based fillers such as graphene cannot be utilized. This study demonstrates that 2D transition metal dichalcogenide additives offer an elegant solution to such class of problems.