Evolution of Electronic Structure in Atomically Thin Sheets of WS₂ and WSe₂

Geometrical confinement effect in exfoliated sheets of layered materials leads to significant evolution of energy dispersion in mono- to few-layer thickness regime. Molybdenum disulfide (MoS₂) was recently found to exhibit indirect-to-direct gap transition when the thickness is reduced to a single monolayer. Emerging photoluminescence (PL) from monolayer MoS₂ opens up opportunities for a range of novel optoelectronic applications of the material. Here we report differential reflectance and PL spectra of mono- to few-layer WS₂ and WSe₂ that indicate that the band structure of these materials undergoes similar indirect-to-direct gap transition when thinned to a single monolayer. The transition is evidenced by distinctly enhanced PL peak centered at 630 and 750 nm in monolayer WS₂ and WSe₂, respectively. Few-layer flakes are found to exhibit comparatively strong indirect gap emission along with direct gap hot electron emission, suggesting high quality of synthetic crystals prepared by a chemical vapor transport method. Fine absorption and emission features and their thickness dependence suggest a strong effect of Se p-orbitals on the d electron band structure as well as interlayer coupling in WSe₂

Efficient Carrier-to-Exciton Conversion in Field Emission Tunnel Diodes Based on MIS-Type van der Waals Heterostack

We report on efficient carrier-to-exciton conversion and planar electroluminescence from tunnel diodes based on a metal–insulator–semiconductor (MIS) van der Waals heterostack consisting of few-layer graphene (FLG), hexagonal boron nitride (hBN), and monolayer tungsten disulfide (WS₂). These devices exhibit excitonic electroluminescence with extremely low threshold current density of a few pA·μm^–2, which is several orders of magnitude lower compared to the previously reported values for the best planar EL devices. Using a reference dye, we estimate the EL quantum efficiency to be ∼1% at low current density limit, which is of the same order of magnitude as photoluminescence quantum yield at the equivalent excitation rate. Our observations reveal that the efficiency of our devices is not limited by carrier-to-exciton conversion efficiency but by the inherent exciton-to-photon yield of the material. The device characteristics indicate that the light emission is triggered by injection of hot minority carriers (holes) to n-doped WS₂ by Fowler–Nordheim tunneling and that hBN serves as an efficient hole-transport and electron-blocking layer. Our findings offer insight into the intelligent design of van der Waals heterostructures and avenues for realizing efficient excitonic devices

Transport Properties of Monolayer MoS₂ Grown by Chemical Vapor Deposition

Recent success in the growth of monolayer MoS₂ via chemical vapor deposition (CVD) has opened up prospects for the implementation of these materials into thin film electronic and optoelectronic devices. Here, we investigate the electronic transport properties of individual crystallites of high quality CVD-grown monolayer MoS₂. The devices show low temperature mobilities up to 500 cm² V^–1 s^–1 and a clear signature of metallic conduction at high doping densities. These characteristics are comparable to the electronic properties of the best mechanically exfoliated monolayers in literature, verifying the high electronic quality of the CVD-grown materials. We analyze the different scattering mechanisms and show that the short-range scattering plays a dominant role in the highly conducting regime at low temperatures. Additionally, the influence of optical phonons as a limiting factor is discussed

Molecular Beam Epitaxy of Highly Crystalline MoSe₂ on Hexagonal Boron Nitride

Molybdenum diselenide (MoSe₂) is a promising two-dimensional material for next-generation electronics and optoelectronics. However, its application has been hindered by a lack of large-scale synthesis. Although chemical vapor deposition (CVD) using laboratory furnaces has been applied to grow two-dimensional (2D) MoSe₂ cystals, no continuous film over macroscopically large area has been produced due to the lack of uniform control in these systems. Here, we investigate the molecular beam epitaxy (MBE) of 2D MoSe₂ on hexagonal boron nitride (hBN) substrate, where highly crystalline MoSe₂ film can be grown with electron mobility ∼15 cm²/(V s). Scanning transmission electron microscopy (STEM) shows that MoSe₂ grains grown at an optimum temperature of 500 °C are highly oriented and coalesced to form continuous film with predominantly mirror twin boundaries. Our work suggests that van der Waals epitaxy of 2D materials is tolerant of lattice mismatch but is facilitated by substrates with similar symmetry