Qingliang Feng

Exploring atomic defects in molybdenum disulphide monolayers

Defects usually play an important role in tailoring various properties of two-dimensional materials. Defects in two-dimensional monolayer molybdenum disulphide may be responsible for large variation of electric and optical properties. Here we present a comprehensive joint experiment–theory investigation of point defects in monolayer molybdenum disulphide prepared by mechanical exfoliation, physical and chemical vapour deposition. Defect species are systematically identified and their concentrations determined by aberration-corrected scanning transmission electron microscopy, and also studied by ab-initio calculation. Defect density up to 3.5 × 1013 cm−2 is found and the dominant category of defects changes from sulphur vacancy in mechanical exfoliation and chemical vapour deposition samples to molybdenum antisite in physical vapour deposition samples. Influence of defects on electronic structure and charge-carrier mobility are predicted by calculation and observed by electric transport measurement. In light of these results, the growth of ultra-high-quality monolayer molybdenum disulphide appears a primary task for the community pursuing high-performance electronic devices.

Growth of Large-Area 2D MoS2(l_,)Se2, Semiconductor

Semiconducting MoS₂(₁-x) Se₂x mono-layers where x = 0-0.40 are successfully grown over large areas. A random arrangement of the S and Se atoms and a tunable bandgap photoluminescence are observed. Atomically thin, 2D semiconductor alloys with tunable bandgaps have potential applications in nano- and opto-electronics. Field-effect transistors fabricated with the monolayers exhibit high on/off ratios of >10(5).

High Responsivity and Gate Tunable Graphene-MoS2 Hybrid Phototransistor

A 2D atomic-layer-thickness phototransistor based on a graphene-MoS2 bybrid device is constructed with a photoresponse much larger than that of individual graphene or MoS2 based phototransistors. Strong and selective light absorption in the MoS2 layer creates electric charges that are transferred to graphene layers derived by a build-in electrical field, where they recirculate many times due to the high carrier mobility of graphene. Gate tunable Fermi level in graphene layer allows the responsivity of this hybrid phototransistor to be continuously tuned from 0 to about 10(4) mA/W by the gate voltage. Furthermore, large scale, flexible, and transparent 2D phototransistors with high responsivity are constructed from the CVD-grown graphene and MoS2 flakes. The high responsivity, gate-tunable sensitivity, wavelength selectivity, and compatibility with current circuit technologies of this type device give it great potential for future application in integrated nano-optoelectronic systems.

Growth of MoS2(1–x)Se2x (x = 0.41–1.00) Monolayer Alloys with Controlled Morphology by Physical Vapor Deposition

Transition-metal dichalcogenide (TMD) monolayer alloys are a branch of two-dimensional (2D) materials which can have large-range band gap tuning as the composition changes. Synthesis of 2D TMD monolayer alloys with controlled composition as well as controlled domain size and edge structure is of great challenge. In the present work, we report growth of MoS2(1-x)Se2x monolayer alloys (x = 0.41-1.00) with controlled morphology and large domain size using physical vapor deposition (PVD). MoS2(1-x)Se2x monolayer alloys with different edge orientations (Mo-zigzag and S/Se-zigzag edge orientations) have been obtained by controlling the deposition temperature. Large domain size of MoS2(1-x)Se2x monolayer alloys (x = 0.41-1.00) up to 20 μm have been obtained by tuning the temperature gradient in the deposition zone. Together with previously obtained MoS2(1-x)Se2x monolayer alloys (x = 0-0.40), the band gap photoluminescence (PL) is continuously tuned from 1.86 eV (i.e., 665 nm, reached at x = 0.00) to 1.55 eV (i.e., 800 nm, reached at x = 1.00). Additionally, Raman peak splitting was observed in MoS2(1-x)Se2x monolayer alloys. This work provides a way to synthesize MoS2(1-x)Se2x monolayer alloys with different edge orientations, which could be benefit to controlled growth of other 2D materials.

Microscopic Dimensions Engineering: Stepwise Manipulation of the Surface Wettability on 3D Substrates for Oil/Water Separation.

Microscopic dimensions engineering is proposed to devise a series of 3D superhydrophobic substrates with microstructures of different dimensions. Combined theoretical modeling and experiments give the relationship of surface roughness and superhydrophobic properties, important for guiding the design of superior superwettable materials for water remediation and other uses.

Tellurium-Assisted Epitaxial Growth of Large-Area, Highly Crystalline ReS2 Atomic Layers on Mica Substrate.

Anisotropic 2D layered material rhenium disulfide (ReS2 ) with high crystal quality and uniform monolayer thickness is synthesized by using tellurium-assisted epitaxial growth on mica substrate. Benefit from the lower eutectic temperature of rhenium-tellurium binary eutectic, ReS2 can grow from rhenium (melting point at 3180 °C) and sulfur precursors in the temperature range of 460-900 °C with high efficiency.

Controlled growth of large-area anisotropic ReS2 atomic layer and its photodetector application

As an anisotropic 2D layered material, rhenium disulfide (ReS2) has attracted much attention because of its unusual properties and promising applications in electronic and optoelectronic devices. However, the low lattice symmetry and interlayer decoupling of ReS2 make asymmetric growth and out-of-plane growth occur quite easily; therefore, thick flake, dendritic and flower-like structures of ReS2 have mostly been obtained previously. Here, we report on an approach based on space-confined epitaxial growth for the controlled synthesis of ReS2 films. Using this approach, large-area and high-quality ReS2 films with uniform monolayer thickness can grow on a mica substrate. Furthermore, the weak van der Waals interaction between the surface of mica and ReS2 clusters, which favors surface-confined growth while avoiding out-of-plane growth, is critical for growing ReS2 with uniform monolayer thickness. The morphological evolution of ReS2 with the growth temperature reveals that asymmetric growth can be suppressed at relatively low temperatures. A ReS2 field-effect transistor displayed a current on/off ratio of 106 and an electron mobility of up to 40 cm2 V-1 s-1, with outstanding photoresponsivity of 12 A W-1. This work not only promotes the large-scale employment of ReS2 in high-performance optoelectronic devices, but also provides a means of controlling the unusual growth behavior of low-lattice-symmetry 2D layered materials.

Hybrid-dimensional magnetic microstructure based 3D substrates for remote controllable and ultrafast water remediation

In the field of water remediation, a 3D hydrophobic material with both remote controllability and high oil adsorption performance is highly desirable. To achieve it, magnetic components and microstructures are most likely involved. However, the simple enrolling of magnetic materials always results in quite low adsorption capacity. Additionally, the control of microstructures on 3D materials is immature, which limits the improvement of water/oil selectivity and oil adsorption speed. Herein, we devised 0D/2D hybrid dimensional magnetic microstructures with a well-defined morphology on melamine foams, which provided magnetism for remote controllability and highly rough surfaces for substantially enhanced water/oil selectivity. Hence, the resultant materials acquired magnetic-driven properties and superhydrophobicity/superoleophilicity simultaneously. Thus, they possess controllable, ultrafast, and high throughput oil uptake ability and high oil/water separation performance. The present strategy may open a new avenue to devise high-performance magnetic 3D assemblies for water remediation.

Synthesis of Platinum Nanoparticles by using Molybdenum Disulfide as a Template and its Application to Enzyme‐like Catalysis

Platinum nanoparticles were synthesized with molybdenum disulfide (MoS2) as a template through a facile hydrothermal method. The as‐prepared nanocomposites (Pt‐MoS2) were characterized by TEM, HRTEM, electrochemical impedance spectroscopy, and X‐ray photoelectron spectroscopy, and they were then used to fabricate a biosensor for enzyme‐like catalysis of hydrogen peroxide (H2O2). The electrochemical activity for the reduction reactions of H2O2 was evaluated in N2‐saturated phosphate buffer solution. The cyclic voltammetry and amperometry results demonstrated that the biosensor modified by the nanocomposites exhibited a fast amperometric response and excellent electrocatalytic activity for reduction of H2O2 with a wide linear range from 0.004 to 48.5 mM and a low detection limit of 0.001 mM at 3σ. Thus, the present work indicates that Pt nanoparticles can be synthesized on the surface of few‐layer MoS2 owing to interfacial PtS bonds and that the composites show a clear enhancement in the catalytic activity relative to that of the platinum nanoparticles alone. This method provides a new way to prepare metal nanoparticles for extensive applications in the field of catalysis.

Epitaxial growth of large-area and highly crystalline anisotropic ReSe2 atomic layer

The anisotropic two-dimensional (2D) layered material rhenium disulfide (ReSe2) has attracted considerable attention because of its unusual properties and promising applications in electronic and optoelectronic devices. However, because of its low lattice symmetry and interlayer decoupling, anisotropic growth and out-of-plane growth occur easily, yielding thick flakes, dendritic structure, or flower-like structure. In this study, we demonstrated a bottom-up method for the controlled and scalable synthesis of ReSe2 by van der Waals epitaxy. To achieve controllable growth, a micro-reactor with a confined reaction space was constructed by stacking two mica substrates in the chemical vapor deposition system. Within the confined reaction space, the nucleation density and growth rate of ReSe2 were significantly reduced, favoring the large-area synthesis of ReSe2 with a uniform monolayer thickness. The morphological evolution of ReSe2 with growth temperature indicated that the anisotropic growth was suppressed at a low growth temperature (<600 °C). Field-effect transistors employing the grown ReSe2 exhibited p-type conduction with a current ON/OFF ratio up to 105 and a hole carrier mobility of 0.98 cm2/(V·s). Furthermore, the ReSe2 device exhibited an outstanding photoresponse to near-infrared light, with responsivity up to 8.4 and 5.1 A/W for 850- and 940-nm light, respectively. This work not only promotes the large-scale application of ReSe2 in high-performance electronic devices but also clarifies the growth mechanism of low-lattice symmetry 2D materials.