Untangling the Intertwined: Metallic to Semiconducting Phase Transition in Colloidal MoS2 Nanoplatelets and Nanosheets

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) have a thickness-tunable band gap in the semiconducting crystal phase and – in monolayer form – exhibit a direct band gap. TMDCs are highly promising for spin- and valleytronics and show an ultrafast response to external (optical) stimuli, an essential feature for optoelectronics. However, up to now, colloidal synthesis routes for ultrathin TMDCs typically yield different shapes and crystal phases and a thorough understanding of the product guiding reaction mechanism is missing. We investigate the colloidal synthesis of ultrathin MoS2 nanoplatelets (8 nm ± 4 nm) and nanosheets (22 nm ± 9 nm) in terms of the evolution of crystal phase and shape over the course of their formation. The reaction is followed by X-ray photoelectron spectroscopy, showing that a mixture of the semiconducting 2H and the metallic 1T crystal phase is formed initially, regardless of the molybdenum oleate precursor concentration used for the reaction. A low precursor concentration however leads to the formation of MoS2 nanoplatelets, while a high concentration yields laterally larger MoS2 nanosheets. Both structures have undergone a full transition to the semiconducting 2H crystal phase by the end of the reaction. Phase pure semiconducting MoS2 nanoplatelets with a lateral size approaching the MoS2 exciton Bohr radius exhibit strong additional lateral quantum confinement leading to a drastically shortened decay of the B-exciton, which we characterize by ultrafast transient absorption spectroscopy. Our results offer a straight-forward synthesis strategy to phase pure semiconducting 2D MoS2 and represent an important starting point for chemically exploring upcoming colloidal TMDC heterostructures for optical applications.