Two dimensional (2D) materials with a monolayer of atoms represent an ultimate control of material dimension in the vertical direction. Molybdenum sulfide (MoS₂) monolayers, with a direct bandgap of 1.8 eV, offer an unprecedented prospect of miniaturizing semiconductor science and technology down to a truly atomic scale. Recent studies have indeed demonstrated the promise of 2D MoS₂ in fields including field effect transistors, low power switches, optoelectronics, and spintronics. However, device development with 2D MoS₂ has been delayed by the lack of capabilities to produce large-area, uniform, and high-quality MoS₂ monolayers. Here we present a self-limiting approach that can grow high quality monolayer and few-layer MoS₂ films over an area of centimeters with unprecedented uniformity and controllability. This approach is compatible with the standard fabrication process in semiconductor industry. It paves the way for the development of practical devices with 2D MoS₂ and opens up new avenues for fundamental research.

Skin-mountable chemical sensors using flexible chemically sensitive nanomaterials are of great interest for electronic skin (e-skin) application. To build these sensors, the emerging atomically thin two-dimensional (2D) layered semiconductors could be a good material candidate. Herein, we show that a large-area WS₂ film synthesized by sulfurization of a tungsten film exhibits high humidity sensing performance both in natural flat and high mechanical flexible states (bending curvature down to 5 mm). The conductivity of as-synthesized WS₂ increases sensitively over a wide relative humidity range (up to 90%) with fast response and recovery times in a few seconds. By using graphene as electrodes and thin polydimethylsiloxane (PDMS) as substrate, a transparent, flexible, and stretchable humidity sensor was fabricated. This senor can be well laminated onto skin and shows stable water moisture sensing behaviors in the undeformed relaxed state as well as under compressive and tensile loadings. Furthermore, its high sensing performance enables real-time monitoring of human breath, indicating a potential mask-free breath monitoring for healthcare application. We believe that such a skin-activity compatible WS₂ humidity sensor may shed light on developing low power consumption wearable chemical sensors based on 2D semiconductors.

We demonstrate an erbium-doped fiber laser passively mode-locked by a multilayer molybdenum disulfide (MoS₂) saturable absorber (SA). The multilayer MoS₂ is prepared by the chemical vapor deposition (CVD) method and transferred onto the end-face of a fiber connector. Taking advantage of the excellent saturable absorption of the fabricated MoS₂-based SA, stable mode locking is obtained at a pump threshold of 31 mW. Resultant output soliton pulses have central wavelength, spectral width, pulse duration, and repetition rate of 1568.9 nm, 2.6 nm, 1.28 ps, and 8.288 MHz, respectively. The experimental results show that multilayer MoS₂ is a promising material for ultrafast laser systems.

We demonstrate the synthesis of large-area monolayer WS₂ films by chemical vapor deposition (CVD) and investigate their photoresponse properties by fabricating n-type field effect transistors (FETs) with Al as the ohmic contact. Our CVD-grown monolayer WS₂ shows an electron mobility of 0.91 cm² V⁻¹ s⁻¹ and an ON/OFF ratio of 10⁶, indicating its comparable electronic properties to the mechanically exfoliated flake sample. In a vacuum, by applying a gate bias (60 V), the responsivity of the monolayer WS₂ phototransistor can increase up to 18.8 mA W⁻¹ and a decent sub-second level response time can be maintained. In contrast, in air, it shows a very fast response time of less than 4.5 ms, but at the cost of responsivity reduction to 0.2 μA W⁻¹. Such a distinctive ambient-sensitive photo-detecting performance can be well-explained by the pronounced effect of charge-acceptor-like O₂/H₂O molecule adsorption/desorption on the photocarrier transport. Our CVD-grown high quality monolayer WS₂ may pave the way for developing industrial-scale optoelectronic devices for photo-detecting and chemical sensing applications.