2D semiconducting transition metal dichalcogenides (TMDCs) are highly promising materials for future spin- and valleytronic applications and exhibit an ultrafast response to external (optical) stimuli which is essential for optoelectronics....

The electronic structure of mono and bilayers of colloidal 2H‐MoS2 nanosheets synthesized by wet‐chemistry using potential‐modulated absorption spectroscopy (EMAS), differential pulse voltammetry, and electrochemical gating measurements is investigated. The energetic positions of the conduction and valence band edges of the direct and indirect bandgap are reported and observe strong bandgap renormalization effects, charge screening of the exciton, as well as intrinsic n‐doping of the as‐synthesized material. Two distinct transitions in the spectral regime associated with the C exciton are found, which overlap into a broad signal upon filling the conduction band. In contrast to oxidation, the reduction of the nanosheets is largely reversible, enabling potential applications for reductive electrocatalysis. This work demonstrates that EMAS is a highly sensitive tool for determining the electronic structure of thin films with a few nanometer thicknesses and that colloidal chemistry affords high‐quality transition metal dichalcogenide nanosheets with an electronic structure comparable to that of exfoliated samples.

2D semiconducting transition metal dichalcogenides (TMDCs) are highly promising materials for future spin- and valleytronic applications and exhibit an ultrafast response to external (optical) stimuli which is essential for optoelectronics. Colloidal nanochemistry on the other hand is an emerging alternative for the synthesis of 2D TMDC nanosheet (NS) ensembles, allowing for the control of the reaction via tunable precursor and ligand chemistry. Up to now, wet-chemical colloidal syntheses yielded intertwined/agglomerated NSs with a large lateral size. Here, we show a synthesis method for 2D mono- and bilayer MoS2 nanoplatelets with a particularly small lateral size (NPLs, 7.4 nm ± 2.2 nm) and MoS2 NSs (22 nm ± 9 nm) as a reference by adjusting the molybdenum precursor concentration in the reaction. We find that in colloidal 2D MoS2 syntheses initially a mixture of the stable semiconducting and the metastable metallic crystal phase is formed. 2D MoS2 NPLs and NSs then both undergo a full transformation to the semiconducting crystal phase by the end of the reaction, which we quantify by X-ray photoelectron spectroscopy. Phase pure semiconducting MoS2 NPLs with a lateral size approaching the MoS2 exciton Bohr radius exhibit strong additional lateral confinement, leading to a drastically shortened decay of the B exciton which is characterized by ultrafast transient absorption spectroscopy. Our findings represent an important step for utilizing colloidal TMDCs, for example small MoS2 NPLs represent an excellent starting point for the growth of heterostructures for future colloidal photonics.

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.