Qiyi Fang

Temperature-Mediated Selective Growth of MoS2/WS2 and WS2/MoS2 Vertical Stacks on Au Foils for Direct Photocatalytic Applications

A growth-temperature-mediated two-step chemical vapor deposition strategy is designed to synthesize MoS2 /WS2 and WS2 /MoS2 stacks on Au foils. Predominantly A-A stacked MoS2 /WS2 and A-B stacked WS2 /MoS2 are selectively achieved and confirmed. Relative enhancements or reductions in photocatalytic activities of MoS2 /WS2 or WS2 /MoS2 are observed under illumination, because the type-II band alignment enables directional electron flow from electrode to active site.

Metallic Vanadium Disulfide Nanosheets as a Platform Material for Multifunctional Electrode Applications

Nanothick metallic transition metal dichalcogenides such as VS2 are essential building blocks for constructing next-generation electronic and energy-storage applications, as well as for exploring unique physical issues associated with the dimensionality effect. However, such two-dimensional (2D) layered materials have yet to be achieved through either mechanical exfoliation or bottom-up synthesis. Herein, we report a facile chemical vapor deposition route for direct production of crystalline VS2 nanosheets with sub-10 nm thicknesses and domain sizes of tens of micrometers. The obtained nanosheets feature spontaneous superlattice periodicities and excellent electrical conductivities (∼3 × 103 S cm-1), which has enabled a variety of applications such as contact electrodes for monolayer MoS2 with contact resistances of ∼1/4 to that of Ni/Au metals, and as supercapacitor electrodes in aqueous electrolytes showing specific capacitances as high as 8.6 × 102 F g-1. This work provides fresh insights into the delicate structure-property relationship and the broad application prospects of such metallic 2D materials.

Direct Chemical Vapor Deposition Growth and Band-Gap Characterization of MoS2/h-BN van der Waals Heterostructures on Au Foils

Stacked transition-metal dichalcogenides on hexagonal boron nitride (h-BN) are platforms for high-performance electronic devices. However, such vertical stacks are usually constructed by the layer-by-layer polymer-assisted transfer of mechanically exfoliated layers. This inevitably causes interfacial contamination and device performance degradation. Herein, we develop a two-step, low-pressure chemical vapor deposition synthetic strategy incorporating the direct growth of monolayer h-BN on Au foil with the subsequent growth of MoS2. In such vertical stacks, the interactions between MoS2 and Au are diminished by the intervening h-BN layer, as evidenced by the appearance of photoluminescence in MoS2. The weakened interfacial interactions facilitate the transfer of the MoS2/h-BN stacks from Au to arbitrary substrates by an electrochemical bubbling method. Scanning tunneling microscope/spectroscopy characterization shows that the central h-BN layer partially blocks the metal-induced gap states in MoS2/h-BN/Au foils. The work offers insight into the synthesis, transfer, and device performance optimization of such vertically stacked heterostructures.

Two-dimensional metallic tantalum disulfide as a hydrogen evolution catalyst

Two-dimensional metallic transition metal dichalcogenides are emerging as prototypes for uncovering fundamental physical phenomena, such as superconductivity and charge-density waves, as well as for engineering-related applications. However, the batch production of such envisioned transition metal dichalcogenides remains challenging, which has hindered the aforementioned explorations. Herein, we fabricate thickness-tunable tantalum disulfide flakes and centimetre-sized ultrathin films on an electrode material of gold foil via a facile chemical vapour deposition route. Through temperature-dependent Raman characterization, we observe the transition from nearly commensurate to commensurate charge-density wave phases with our ultrathin tantalum disulfide flakes. We have obtained high hydrogen evolution reaction efficiency with the as-grown tantalum disulfide flakes directly synthesized on gold foils comparable to traditional platinum catalysts. This work could promote further efforts for exploring new efficient catalysts in the large materials family of metallic transition metal dichalcogenides, as well as exploiting their applications towards more versatile applications.Metallic transition metal dichalcogenides are important materials for catalysis, but scalable and controllable preparation methods are scarce. Here, the authors synthesize 2H-TaS2 as centimetre-scale films of tunable thickness and show they are an efficient catalyst for hydrogen evolution.

Application of chemical vapor–deposited monolayer ReSe2 in the electrocatalytic hydrogen evolution reaction

Controlled synthesis of structurally anisotropic rhenium diselenide (ReSe2) with macroscopically uniform and strictly monolayer thickness as well as tunable domain shape/size is of great interest for electronics-, optoelectronics-, and electrocatalysis-related applications. Herein, we describe the controlled synthesis of uniform monolayer ReSe2 flakes with variable morphology (sunflower- or truncated-triangle-shaped) on SiO2/Si substrates using different ambient-pressure chemical vapor deposition (CVD) setups. The prepared polycrystalline ReSe2 flakes were transferred intact onto Au foil electrodes and tested for activity in the hydrogen evolution reaction (HER). Interestingly, compared to the compact truncated-triangle-shaped ReSe2 flakes, their edge-abundant sunflower-shaped counterparts exhibited superior electrocatalytic HER activity, featuring a relatively low Tafel slope of ∼76 mV/dec and an exchange current density of 10.5 μA/cm2. Thus, our work demonstrates that CVD-grown ReSe2 is a promising two-dimensional anisotropic material for applications in the electrocatalytic HER.

Substrate effect on the growth of monolayer dendritic MoS2 on LaAlO3 (100) and its electrocatalytic applications

In accommodating the rapid development of two-dimensional (2D) nanomaterials, chemical vapor deposition (CVD) has become a powerful tool for their batch production with desirable characteristics, such as high crystal quality, large domain size, and tunable domain shape. The crystallinity and morphology of the growth substrates usually play a crucial role in the CVD synthesis of high-quality monolayer MoS2, a kind of 2D layered material which has ignited huge interest in nanoelectronics, optoelectronics and energy harvesting, etc. Herein, by utilizing a low-pressure chemical vapor deposition (LPCVD) system, we demonstrate a regioselective synthesis of monolayer MoS2 on the corrugated single-crystal LaAlO3 (100) with twin crystal domains induced by the second-order phase transition. Unique dendritic morphologies with tunable nucleation densities were obtained in different regions of the undulated substrate, presenting a strong substrate modulation effect. Interestingly, the exposure of abundant active edge sites along with the rather high nucleation density makes the monolayer dendritic MoS2 a good electrocatalyst for hydrogen evolution reaction (HER), particularly featured by a rather high exchange current density (70.4 μA cm−2). Furthermore, uniform monolayer MoS2 films can also be obtained and transferred to arbitrary substrates. We believe that this work provides a new growth system for the controllable synthesis of 2D layered materials with unique dendritic morphologies, as well as its great application potential in energy conversion and harvesting.

Ultrafast Charge Transfer in Perovskite Nanowire/2D Transition Metal Dichalcogenide Heterostructures

Mixed-dimensional van der Waals (vdW) heterostructures between one-dimensional (1D) perovskite nanowires and two-dimensional (2D) transition metal dichalcogenides (TMDCs) hold great potential for novel optoelectronics and light-harvesting applications. However, the ultrafast carrier dynamics between the 1D perovskite nanowires and 2D TMDCs are currently not well understood, which is critical for related optoelectronic applications. Here we demonstrate vdW heterostructures of CsPbBr3 nanowire/monolayer MoS2 and CsPbBr3 nanowire/monolayer WSe2 and further present systematic investigations on their charge transfer dynamics. We show that CsPbBr3/MoS2 and CsPbBr3/WSe2 are type-I and type-II heterostructures, respectively. Both electrons and holes transfer from CsPbBr3 to MoS2 with an efficiency of 71%. As a contrast, holes transfer from CsPbBr3 to WSe2 with a carrier transfer efficiency of 70% and electrons transfer inversely within 7 ps. The ultrafast and efficient charge transfer in the 1D/2D perovskite-TMDC heterostructures suggest great promise in light emission, photodetector, and photovoltaic devices.

All-Inorganic Perovskite Nanowires–InGaZnO Heterojunction for High-Performance Ultraviolet–Visible Photodetectors

Inorganic cesium lead halide perovskites have attracted intense interest in optoelectronic applications due to the relatively stable performance in air. However, most reported inorganic perovskite-based optoelectronic devices exhibit low photosensitivity, which greatly hinders their further applications. Here, we first demonstrate a hybrid optical structure, which combines the n-type thin-film InGaZnO and the all-inorganic perovskite nanowires of CsPbBr3 together. By this way, excellent optical and electrical properties such as a low dark current of 10-10 A (at -5 V), a high Iph/Idark of 1.2 × 104, a response time of 2 ms and photoresponsivity of 3.794 A/W have been obtained. It is also found that the photodetector shows good stability in air ambient for 2 months with little reduction in performance. Moreover, such hybrid photodetectors exhibit enhanced photocurrent and Iph/Idark in high-temperature environment. This work paves a new way for high-performance photodetectors and points out the possible application of the inorganic cesium lead halide perovskites in harsh environment.

Transformation of monolayer MoS2 into multiphasic MoTe2: Chalcogen atom-exchange synthesis route

Molybdenum ditelluride (MoTe2), which is an important transition-metal dichalcogenide, has attracted considerable interest owing to its unique properties, such as its small bandgap and large Seebeck coefficient. However, the batch production of monolayer MoTe2 has been rarely reported. In this study, we demonstrate the synthesis of large-domain (edge length exceeding 30 μm), monolayer MoTe2 from chemical vapor deposition-grown monolayer MoS2 using a chalcogen atom-exchange synthesis route. An in-depth investigation of the tellurization process reveals that the substitution of S atoms by Te is prevalently initiated at the edges and grain boundaries of the monolayer MoS2, which differs from the homogeneous selenization of MoS2 flakes with the formation of alloyed Mo−S−Se hybrids. Moreover, we detect a large compressive strain (approximately −10%) in the transformed MoTe2 lattice, which possibly drives the phase transition from 2H to 1T’ at the reaction temperature of 500 °C. This phase change is substantiated by experimental facts and first-principles calculations. This work introduces a novel route for the templated synthesis of two-dimensional layered materials through atom substitutional chemistry and provides a new pathway for engineering the strain and thus the intriguing physics and chemistry.

Heterostructured graphene quantum dot/WSe2/Si photodetector with suppressed dark current and improved detectivity

A high-performance heterojunction photodetector is formed by combining an n-type Si substrate with p-type monolayer WSe2 obtained using physical vapor deposition. The high quality of the WSe2/Si heterojunction is demonstrated by the suppressed dark current of 1 nA and the extremely high rectification ratio of 107. Under illumination, the heterojunction exhibits a wide photoresponse range from ultraviolet to near-infrared radiation. The introduction of graphene quantum dots (GQDs) greatly elevates the photodetective capabilities of the heterojunction with strong light absorption and long carrier lifetimes. The GQDs/WSe2/Si heterojunction exhibits a high responsivity of ∼ 707 mA·W–1, short response time of 0.2 ms, and good specific detectivity of ∼ 4.51 × 109 Jones. These properties suggest that the GQDs/WSe2/Si heterojunction holds great potential for application in future high-performance photodetectors.