High-quality single-layer nanosheets of MS₂ (M = Mo, Nb, Ta, Ti) directly exfoliated from AMS₂ (A = Li, Na, K) crystals

Layered transition metal dichalcogenides (TMDs) such as MoS₂ have attracted considerable interest as two-dimensional materials because of their unique physical and chemical properties. Single or few-layer TMD nanosheets can be achieved by conventional Li intercalation methods through organolithium chemistry or electrochemistry. However, these methods are hampered by the low yield, mixed phases and a lot of defects inside the nanosheets. Here we develop a novel general strategy to prepare single-layer TMD nanosheets by using AMS₂ (A = Li, Na, K; M = Mo, Nb, Ta, Ti) crystals as ideal precursors. The crystal structure of these compounds ensures the robust S-M-S layers and fully filled alkali atoms between them, which lead to a high-yield production of high-quality single-layer nanosheets by following chemical exfoliation. Surprisingly, MoS₂ nanosheets obtained by LiMoS₂ crystals show a high-quality 1T′ phase, while the widely used n-butyl lithium method can only prepare phase-mixed (2H, 1T, 1T′) nanosheets with abundant defects. The as-prepared 1T′ MoS₂ nanosheets exhibit a remarkable electrical conductivity (618 S cm^-1), which is much higher than that of MoS₂ nanosheets (35.4 S cm^-1) obtained using the n-butyl lithium method, also the highest value in MoS₂ related materials and superior to the best value of the reported 2D films of graphene (550 S cm^-1). © 2017 The Royal Society of Chemistry.