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 (MoS(2)) was recently found to exhibit indirect-to-direct gap transition when the thickness is reduced to a single monolayer. Emerging photoluminescence (PL) from monolayer MoS(2) 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 WS(2) and WSe(2) 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 WS(2) and WSe(2), 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 WSe(2).

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 cm(2) 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.

Graphical AbstractHighlights: We demonstrate chemical vapor deposition of WSe2 and WS2 monolayers, promoted by alkali metal halides, MX (M= Na or K; X=Cl, Br or I).

Due to their layered structure, two-dimensional Ruddlesden-Popper perovskites (RPPs), composed of multiple organic/inorganic quantum wells, can in principle be exfoliated down to few and single layers. These molecularly thin layers are expected to present unique properties with respect to the bulk counterpart, due to increased lattice deformations caused by interface strain. Here, we have synthesized centimetre-sized, pure-phase single-crystal RPP perovskites (CH(CH)NH)(CHNH)PbI (n = 1-4) from which single quantum well layers have been exfoliated. We observed a reversible shift in excitonic energies induced by laser annealing on exfoliated layers encapsulated by hexagonal boron nitride. Moreover, a highly efficient photodetector was fabricated using a molecularly thin n = 4 RPP crystal, showing a photogain of 10 and an internal quantum efficiency of ~34%. Our results suggest that, thanks to their dynamic structure, atomically thin perovskites enable an additional degree of control for the bandgap engineering of these materials.

Chemical vapour deposition of two-dimensional materials typically involves the conversion of vapour precursors to solid products in a vapour-solid-solid mode. Here, we report the vapour-liquid-solid growth of monolayer MoS, yielding highly crystalline ribbons with a width of few tens to thousands of nanometres. This vapour-liquid-solid growth is triggered by the reaction between MoO and NaCl, which results in the formation of molten Na-Mo-O droplets. These droplets mediate the growth of MoS ribbons in the 'crawling mode' when saturated with sulfur. The locally well-defined orientations of the ribbons reveal the regular horizontal motion of the droplets during growth. Using atomic-resolution scanning transmission electron microscopy and second harmonic generation microscopy, we show that the ribbons are grown homoepitaxially on monolayer MoS with predominantly 2H- or 3R-type stacking. Our findings highlight the prospects for the controlled growth of atomically thin nanostructure arrays for nanoelectronic devices and the development of unique mixed-dimensional structures.