Crystal phase control in layered transition metal dichalcogenides is central for exploiting their different electronic properties. Access to metastable crystal phases is limited as their direct synthesis is challenging, restricting the spectrum of reachable materials. Here, we demonstrate the solution phase synthesis of the metastable distorted octahedrally coordinated structure (1T' phase) of WSe 2 nanosheets. We design a kinetically-controlled regime of colloidal synthesis to enable the formation of the metastable phase. 1T' WSe 2 branched fewlayered nanosheets are produced in high yield and in a reproducible and controlled manner. The 1T' phase is fully convertible into the semiconducting 2H phase upon thermal annealing at 400°C. The 1T' WSe 2 nanosheets demonstrate a metallic nature exhibited by an enhanced electrocatalytic activity for hydrogen evolution reaction as compared to the 2H WSe 2 nanosheets and comparable to other 1T' phases. This synthesis design can potentially be extended to different materials providing direct access of metastable phases.

The solar-assisted oxidation of water is an essential half reaction for achieving a complete cycle of water splitting. nanosheets results in a 10-fold increase in incident-photon-to-current-efficiency compared to the individual constituents. This proves that charge carrier lifetime is tailorable in atomically thin crystals by creating heterojunctions of different compositions and architectures. Our results suggest that the MoS 2 and WS 2 nanosheets and their bulk heterojunction blend are interesting photocatalytic systems for water oxidation, which can be coupled with different reduction processes for solar-fuel production.

We demonstrate strong anisotropic spin-orbit interaction (SOI) in graphene induced by monolayer WS_{2}. Direct comparison between graphene-monolayer WS_{2} and graphene-bulk WS_{2} systems in magnetotransport measurements reveals that monolayer transition metal dichalcogenide can induce much stronger SOI than bulk. Detailed theoretical analysis of the weak antilocalization curves gives an estimated spin-orbit energy (E_{so}) higher than 10 meV. The symmetry of the induced SOI is also discussed, and the dominant z→-z symmetric SOI can only explain the experimental results. Spin relaxation by the Elliot-Yafet mechanism and anomalous resistance increase with temperature close to the Dirac point indicates Kane-Mele SOI induced in graphene.

The rise of atomically thin materials has the potential to enable a paradigm shift in modern technologies by introducing multi-functional materials in the semiconductor industry. To date the growth of high quality atomically thin semiconductors (e.g. WS2) is one of the most pressing challenges to unleash the potential of these materials and the growth of mono- or bi-layers with high crystal quality is yet to see its full realization. Here, we show that the novel use of molecular precursors in the controlled synthesis of mono- and bi-layer WS2 leads to superior material quality compared to the widely used direct sulfidization of WO3-based precursors. Record high room temperature charge carrier mobility up to 52 cm2/Vs and ultra-sharp photoluminescence linewidth of just 36 meV over submillimeter areas demonstrate that the quality of this material supersedes also that of naturally occurring materials. By exploiting surface diffusion kinetics of W and S species adsorbed onto a substrate, a deterministic layer thickness control has also been achieved promoting the design of scalable synthesis routes.

We report a systematic study on strong enhancement of spin-orbit interaction (SOI) in graphene induced by transition-metal dichalcogenides (TMDs). Low temperature magnetotoransport measurements of graphene proximitized to different TMDs (monolayer and bulk WSe2, WS2 and monolayer MoS2) all exhibit weak antilocalization peaks, a signature of strong SOI induced in graphene. The amplitudes of the induced SOI are different for different materials and thickness, and we find that monolayer WSe2 and WS2 can induce much stronger SOI than bulk WSe2, WS2 and monolayer MoS2. The estimated spin-orbit (SO) scattering strength for graphene/monolayer WSe2 and graphene/monolayer WS2 reaches ∼ 10 meV whereas for graphene/bulk WSe2, graphene/bulk WS2 and graphene/monolayer MoS2 it is around 1 meV or less. We also discuss the symmetry and type of the induced SOI in detail, especially focusing on the identification of intrinsic (Kane-Mele) and valley-Zeeman (VZ) SOI by determining the dominant spin relaxation mechanism. Our findings pave the way for realizing the quantum spin Hall (QSH) state in graphene.