Metallic transition metal dichalcogenides (MTMDCs) have manifested a wealth of intriguing properties in their bulk states, such as magnetism, charge density waves, and superconductivity, attracting worldwide attention from condensed-matter physicists over several decades. Very recently, nano-thick MTMDCs have been reported to be essential building blocks for constructing next-generation electronic and energy-storage applications, as well as for exploring unique physical issues associated with the dimensionality effect. However, batch production of such envisioned few-layer MTMDCs remains challenging based on the existing physical or chemical exfoliation methods. Our group reported the direct synthesis of high-quality semiconducting and metallic TMDCs materials of MoS2, WS2, VS2, and TaS2, etc., on both on conducting Au foil substrates and insulating substrates towards different applications [1,2]. Particularly, we designed a facile chemical vapor deposition route (CVD) for the direct production of VS2 nanosheets with sub-10 nm thicknesses on SiO2/Si substrates. The obtained nanosheets represented spontaneous superlattice periodicities and excellent electrical conductivities (~3×103 S cm-1), enabling a variety of applications as contact electrodes for monolayer MoS2, and as supercapacitor electrodes in aqueous electrolytes. Subsequently, we also developed a van der Waals epitaxial strategy for the direct synthesis of thickness tunable metallic 1T-VSe2 monocrystalline nanosheets on mica substrate via a similar CVD method. In particular, our synthesized few-layer 1T-VSe2 nanosheets possess extremely high electrical conductivity of up to 106 S m-1, which is 1-4 orders of magnitude higher than that of state-of-the-art conductive 2D materials. The charge-density wave (CDW) phase transition is also detected with the transition temperature presenting the same dependence on thickness as that of exfoliated VSe2 nanosheets. More intriguingly, we also realized the first achievement of thickness-tunable 2H-TaS2 flakes and centimeter-size ultrathin films on an electrode material of Au foil via a facile CVD route. We also detected the transition from nearly commensurate to commensurate CDW phases with our ultrathin 2H-TaS2 flakes. Remarkably, we obtained extra high hydrogen evolution reaction efficiency on as-grown 2H-TaS2 flakes directly synthesized on Au foils with the efficiency even comparable to traditional Pt catalyst.  All these work therefore provides brand new insights into the direct synthesis and property investigations of nano-thick metallic 2D TMDs crystals.
1. Yanfeng Zhang*, Zhongfan Liu*, et al., Chem. Soc. Rev. 2015, 44, 2587; Adv. Mater. 2016, 28, 6207.
2. Yanfeng Zhang*, et al., Adv. Energy. Mater. 2016, 6, 1600459; Adv. Mater. 2016, 28, 10664.
3. Lin Gu*, Zhongfan Liu*, Yanfeng Zhang*, et al., Nano Lett. 2017, 17, 4908.
4. Yanfeng Zhang*, et al. Adv. Mater. 2017, 1702359.
5. Yanfeng Zhang*, et al. Nature Commun. 2017, 8, 958 .