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.

Recently two-dimensional layered semiconductors with promising electronic and optical properties have opened up a new way for applications in atomically thin electronics and optoelectronics. Here we report a large-area growth of monolayer WSe2 directly on SiO2/Si substrates by a chemical vapor deposition (CVD) method under atmospheric pressure. A sub-cooling step was demonstrated to have a key role in achieving this large-area growth. The monolayer configuration of the as-grown WSe2 was proven by spherical-aberration-corrected high resolution scanning transmission electron microscopy (HRSTEM), atomic force microscopy (AFM), Raman spectroscopy and photoluminescence (PL) spectroscopy. P-type behavior of as-grown monolayer WSe2 with a mobility of ∼0.2 cm(2) V(-1) s(-1) and a carrier concentration of 1.11 × 10(18) cm(-3) was confirmed using back-gated field effect transistor (FET) devices. This large-area growth directly on a SiO2/Si substrate provides a new way to meet the industrial manufacturing requirements.

"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).

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Long-term stability is a great challenge for commercializing VO 2 film, but corresponding research is lacking. This work demonstrated that the cross-section of VO 2 films is important to the corresponding deterioration process. An encapsulation structure is designed to protect VO 2 films from deterioration by utilizing stable and naturally hydrophobic HfO 2 layers. The proposed structure shows greatly extended service life, which could be equal to 16 years in the real environment. It provides a new strategy for stable VO 2 films in commercialization of applications.

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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