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Yanfeng Zhang1

1, Peking University, Beijing, , China

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.[3] 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.[4] 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. [5] All these work therefore provides brand new insights into the direct synthesis and property investigations of nano-thick metallic 2D TMDs crystals.
References
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 .

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