“Strain engineering” in functional materials has been widely explored to tailor the physical properties of electronic materials and improve their electrical and/or optical properties. Here, we exploit both in plane and out of plane uniaxial tensile strains in MoS₂ to modulate its band gap and engineer its optical properties. We utilize X-ray diffraction and cross-sectional transmission electron microscopy to quantify the strains in the as-synthesized MoS₂ nanosheets and apply measured shifts of Raman-active modes to confirm lattice strain modification of both the out-of-plane and in-plane phonon vibrations of the MoS₂ nanosheets. The induced band gap evolution due to in-plane and out-of-plane tensile stresses is validated by photoluminescence (PL) measurements, promising a potential route for unprecedented manipulation of the physical, electrical and optical properties of MoS₂.

The enhanced photocatalytic performance of as-synthesized MoS₂ nanosheet–TiO₂ hybrid nanostructures.

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 cm² V⁻¹ s⁻¹ and a carrier concentration of 1.11 × 10¹⁸ 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 MoS_2(1−x)Se_2x 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 MoS_2(1−x)Se_2x, 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.