The first realization of a tunable band-gap in monolayer WS2(1-x) Se2x is demonstrated. The tuning of the bandgap exhibits a strong dependence of S and Se content, as proven by PL spectroscopy. Because of its remarkable electronic structure, monolayer WS2(1-x) Se2x exhibits novel electrochemical catalytic activity and offers long-term electrocatalytic stability for the hydrogen evolution reaction.

Bismuthene, the last and heaviest group-VA elemental two-dimensional material, has received tremendous interests owing to its advantages in electronic-transport, semimetallic bonding, and intrinsic spin-orbit coupling. However, light−bismuthene interaction is relatively less investigated. Herein, sonochemical exfoliation approach had been employed to deliver a successful synthesis of few-layer bismuthene with an average thickness of ∼3 nm and a lateral size of ∼0.2 μm. The corresponding band structure from monoto sextuple-layer had been therotically calculated and it was found that bismuthene possesses a thickness dependent energy gap from almost zero to 0.55 eV, suggesting that bismuthene may also find unique applications from terahertz, mid-infrared toward infrared regime. The nonlinear optical absorption and refraction parameters had been well characterized by laser Z-scan and spatial phase modulation measurement techniques, respectively. By taking advantage of its strong nonlinear refraction effect, all optical switching of two different laser beams based on spatial cross-phase modulation had been eventually realized. It is further found that a modulated signal light clearly observed as switch light is turned on. The achievement of all optical switching suggests that the bismuthene-based 2D material is indeed an excellent candidate for an all optical switcher. Particularly, the semimetallic and long-term stable property in few layer bismuthene make it as a promising nonlinear optical material for infrared and midinfrared optoelectronics. Our work demonstrates a large potential of this new material for nonlinear photonics and this contribution may provide new photonics avenue toward bismuthene-based devices (such as broadband detector, nonlinear optical switcher, phase modulator, etc.).

"Band gap engineering" in two-dimensional (2D) materials plays an important role in tailoring their physical and chemical properties. The tuning of the band gap is typically achieved by controlling the composition of the semiconductor alloys. However, large-area preparation of 2D alloys remains a major challenge. Here, we report the large-area synthesis of high-quality monolayered MoS2(1-x)Se2x with a size coverage of hundreds of microns using a chemical vapor deposition method. The photoluminescence (PL) spectroscopy results confirm the tunable band gap in MoS2(1-x)Se2x, which is modulated by varying the Se content. Atomic-scale analysis was performed and the chemical composition was characterized using high-resolution scanning transmission electron microscopy and X-ray photoemission spectroscopy. With the introduction of Se into monolayered MoS2, it leads to enhanced catalytic activity in an electrochemical reaction for hydrogen generation, compared to monolayered MoS2 and MoSe2. It is promising as a potential alternative to expensive noble metals.

Tungsten disulfide (WS 2 ), with its transformation from indirect to direct band transitions when scaled down to a monolayer, exhibits great potential for future micro-device applications. In this work, we report a controllable route for monolayer WS 2 synthesis. The high-quality of as-grown monolayer WS 2 was confirmed by optical microscopy, atomic force microscopy (AFM), high resolution scanning transmission electron microscopy (HRSTEM), Raman spectroscopy, and photoluminescence (PL). The impact of growth parameters (including gas flow rate and reaction temperature) on the morphology of the WS 2 domain was investigated. A growth mechanism is proposed based on the experimental analysis. Our results also provide some general guidelines for other two dimensional (2D) monolayer syntheses of transition metal dichalcogenides (TMD).

Recently, two-dimensional (2D) layered transition metal dichalcogenides (LTMDs) have attracted great scientific interest for ion battery applications. Because of its remarkable metallic property, vanadium disulfide (VS 2 ) as a typical family member of LTMDs, can be an alternative anode material for ion battery applications. In this paper, we systematically investigate the adsorption energy and diffusion coefficient of the lithium and sodium ions in monolayer and bulk VS 2 for lithium and sodium ion batteries by a density functional theory method. Our calculations show that the VS 2 capacity can reach up to 466 mA h g À1 . It exhibits a low output voltage of 0.72 and 0.48 V for lithium and sodium ion batteries in the monolayer VS 2 as well as an output voltage of 0.88 and 0.6 V in the bulk VS 2 , respectively. The calculated lithium and sodium ion diffusion coefficients in the bulk VS 2 are enhanced by five and seven orders of magnitude compared to the reported bulk MoS 2 , respectively. Our investigations also reveal that VS 2 exhibits better electrochemical performance as an anode in the sodium ion battery than in the lithium ion battery.

Two-dimensional (2D) semiconductor materials have attracted broad interest due to their unique structures and physical properties. The stability of the 2D-material-based devices plays a key role in their practical applications. Here, we report the promising stable electrical performance in the large-scale monolayer WSe2(1-x)S2x with a tunable band gap. Photoluminescence (PL) spectroscopy was utilized to verify the tunable band gap in the as-grown monolayer with a tuning capability of 120 meV. Gated field effect transistor (FET) performance confirmed the p-type transport behavior in monolayer WSe2(1-x)S2x with a high on/off ratio (>10(4)). Top-gated FET configuration improves the carrier mobility with two orders larger than that in the back-gated FET device. After exposure to air for three months, the device performance manifested excellent stability with no source-drain current drop observed. P-type WSe2(1-x)S2x with a tunable band gap is the ideal complement to n-type tunable monolayers in the application of pn junction-related flexible nanodevices.

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