A novel crystal configuration of sandwiched S-Mo-Se structure (Janus SMoSe) at the monolayer limit has been synthesized and carefully characterized in this work. By controlled sulfurization of monolayer MoSe 2 the top layer of selenium atoms are substituted by sulfur atoms while the bottom selenium layer remains intact. The peculiar structure of this new material is systematically investigated by Raman, photoluminescence and X-ray photoelectron spectroscopy and confirmed by transmission-electron microscopy and time-of-flight secondary ion mass spectrometry.Density-functional theory calculations are performed to better understand the Raman vibration modes and electronic structures of the Janus SMoSe monolayer, which are found to correlate well with corresponding experimental results. Finally, high basal plane hydrogen evolution reaction (HER) activity is discovered for the Janus monolayer and DFT calculation implies that the activity originates from the synergistic effect of the intrinsic defects and structural strain inherent in the Janus structure.Keywords: Janus SMoSe, sulfurization, HER Since the discovery of graphene in 2004 1 , two-dimensional (2D) materials have been attracting increasing attention due to the many novel properties originating from the bulk to monolayer transition. Among the 2D family, transition metal dichalcogenides (TMDs)

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

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.

Precise control of the electronic surface states of two-dimensional (2D) materials could improve their versatility and widen their applicability in electronics and sensing. To this end, chemical surface functionalization has been used to adjust the electronic properties of 2D materials. So far, however, chemical functionalization has relied on lattice defects and physisorption methods that inevitably modify the topological characteristics of the atomic layers. Here we make use of the lone pair electrons found in most of 2D metal chalcogenides and report a functionalization method via a Lewis acid-base reaction that does not alter the host structure. Atomic layers of n-type InSe react with Ti(4+) to form planar p-type [Ti(4+)n(InSe)] coordination complexes. Using this strategy, we fabricate planar p-n junctions on 2D InSe with improved rectification and photovoltaic properties, without requiring heterostructure growth procedures or device fabrication processes. We also show that this functionalization approach works with other Lewis acids (such as B(3+), Al(3+) and Sn(4+)) and can be applied to other 2D materials (for example MoS2, MoSe2). Finally, we show that it is possible to use Lewis acid-base chemistry as a bridge to connect molecules to 2D atomic layers and fabricate a proof-of-principle dye-sensitized photosensing device.

Two dimensional transition metal dichalcogenides (2D TMDs) offer promise as opto-electronic materials due to their direct band gap and reasonably good mobility values. However, most metals form high resistance contacts on semiconducting TMDs such as MoS2. The large contact resistance limits the performance of devices. Unlike bulk materials, low contact resistance cannot be stably achieved in 2D materials by doping. Here we build on our previous work in which we demonstrated that it is possible to achieve low contact resistance electrodes by phase transformation. We show that similar to the previously demonstrated mechanically exfoliated samples, it is possible to decrease the contact resistance and enhance the FET performance by locally inducing and patterning the metallic 1T phase of MoS2 on chemically vapor deposited material. The device properties are substantially improved with 1T phase source/drain electrodes.