Van der Waals heterostructures composed of two-dimensional transition-metal dichalcogenides layers have recently emerged as a new family of materials, with great potential for atomically thin opto-electronic and photovoltaic applications. It is puzzling, however, that the photocurrent is yielded so efficiently in these structures, despite the apparent momentum mismatch between the intralayer/interlayer excitons during the charge transfer, as well as the tightly bound nature of the excitons in 2D geometry. Using the energy-state-resolved ultrafast visible/infrared microspectroscopy, we herein obtain unambiguous experimental evidence of the charge transfer intermediate state with excess energy, during the transition from an intralayer exciton to an interlayer exciton at the interface of a WS2/MoS2 heterostructure, and free carriers moving across the interface much faster than recombining into the intralayer excitons. The observations therefore explain how the remarkable charge transfer rate and photocurrent generation are achieved even with the aforementioned momentum mismatch and excitonic localization in 2D heterostructures and devices.

The recently discovered monolayer transition metal dichalcogenides (TMDCs) provide a fertile playground to explore new coupled spin-valley physics 1-3 . Although robust spin and valley degrees of freedom are inferred from polarized photoluminescence (PL) experiments 4-8 , PL timescales are necessarily constrained by short-lived (3-100 ps) electron-hole recombination 9,10 . Direct probes of spin/valley polarization dynamics of resident carriers in electron (or hole)-doped TMDCs, which may persist long after recombination ceases, are at an early stage 11-13 . Here we directly measure the coupled spin-valley dynamics in electron-doped MoS 2 and WS 2 monolayers using optical Kerr spectroscopy, and reveal very long electron spin lifetimes, exceeding 3 ns at 5 K (two to three orders of magnitude longer than typical exciton recombination times). In contrast with conventional III-V or II-VI semiconductors, spin relaxation accelerates rapidly in small transverse magnetic fields. Supported by a model of coupled spin-valley dynamics, these results indicate a novel mechanism of itinerant electron spin dephasing in the rapidly fluctuating internal spin-orbit field in TMDCs, driven by fast inter-valley scattering. Additionally, a long-lived spin coherence is observed at lower energies, commensurate with localized states. These studies provide insight into the physics underpinning spin and valley dynamics of resident electrons in atomically thin TMDCs.Studies of optical spin orientation and spin relaxation using polarized light have a long and exciting history in conventional III-V and II-VI semiconductors 14,15 . Early seminal works focused on magneto-optical studies of polarized PL from recombining excitons 14 , from which spin lifetimes could be indirectly inferred. However, it was the direct observation of very long-lived spin coherence of resident electrons in materials such as GaAs and ZnSe (refs 15,16)-revealed unambiguously by time-resolved Faraday and Kerr rotation studies-that captured widespread interest and helped to launch the burgeoning field of 'semiconductor spintronics' in the late 1990s (ref. 15). With a view towards exploring coupled spin/valley physics of resident electrons in the new atomically thin and direct-bandgap TMDC semiconductors, here we apply related experimental methods and directly reveal surprisingly long-lived and coherent spin dynamics in monolayer MoS 2 and WS 2 . Figure 1a depicts the experimental set-up. High-quality monolayer crystals of n-type MoS 2 and WS 2 , grown by chemical vapour deposition on SiO 2 /Si substrates 17 , were selected on the basis of low-temperature reflectance and PL studies (see Methods).Transverse magnetic fields (B y ) were applied using external coils. A weak pump laser illuminates individual crystals with right-or left-circularly polarized light (RCP or LCP) using wavelengths near the lowest-energy A exciton transition, which primarily photoexcites spin-polarized electrons and holes into the K or K valley, respectively 1-10

A facile chemical vapor deposition method to prepare single-crystalline VS2 nanosheets for the hydrogen evolution reaction is reported. The electrocatalytic hydrogen evolution reaction (HER) activities of VS2 show an extremely low overpotential of -68 mV at 10 mA cm(-2), small Tafel slopes of ≈34 mV decade(-1), as well as high stability, demonstrating its potential as a candidate non-noble-metal catalyst for the HER.

Here, the hydrogen evolution reaction (HER) activities at the edge and basal-plane sites of monolayer molybdenum disulfide (MoS ) synthesized by chemical vapor deposition (CVD) are studied using a local probe method enabled by selected-area lithography. Reaction windows are opened by e-beam lithography at sites of interest on poly(methyl methacrylate) (PMMA)-covered monolayer MoS triangles. The HER properties of MoS edge sites are obtained by subtraction of the activity of the basal-plane sites from results containing both basal-plane and edge sites. The catalytic performances in terms of turnover frequencies (TOFs) are calculated based on the estimated number of active sites on the selected areas. The TOFs follow a descending order of 3.8 ± 1.6, 1.6 ± 1.2, 0.008 ± 0.002, and 1.9 ± 0.8 × 10 s , found for 1T'-, 2H-MoS edges, and 1T'-, 2H-MoS basal planes, respectively. Edge sites of both 2H- and 1T'-MoS are proved to have comparable activities to platinum (≈1-10 s ). When fitted into the HER volcano plot, the MoS active sites follow a trend distinct from conventional metals, implying a possible difference in the reaction mechanism between transition-metal dichalcogenides (TMDs) and metal catalysts.

Transition metal dichalcogenides monolayers and black phosphorus thin crystals are emerging two-dimensional materials that demonstrated extraordinary optoelectronic properties. Exotic properties and physics may arise when atomic layers of different materials are stacked together to form van der Waals solids. Understanding the important interlayer couplings in such heterostructures could provide avenues for control and creation of characteristics in these artificial stacks. Here we systematically investigate the optical and optoelectronic properties of artificial stacks of molybdenum disulfide, tungsten disulfide, and black phosphorus atomic layers. An anomalous photoluminescence quenching was observed in tungsten disulfide-molybdenum disulfide stacks. This was attributed to a direct to indirect band gap transition of tungsten disulfide in such stacks while molybdenum disulfide maintains its monolayer properties by first-principles calculations. On the other hand, due to the strong build-in electric fields in tungsten disulfide-black phosphorus or molybdenum disulfide-black phosphorus stacks, the excitons can be efficiently splitted despite both the component layers having a direct band gap in these stacks. We further examine optoelectronic properties of tungsten disulfide-molybdenum disulfide artificial stacks and demonstrate their great potentials in future optoelectronic applications.