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

Abstract

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.

Electronic structure of colloidal 2H-MoS2 mono- and bilayers determined by spectroelectrochemistry

“…We stress that bleaches of the A, B and C excitons are instantaneously formed in the reductive EMAS scans upon departing from the OCP (Figure 2) even for potentials far below the conduction band edge. The bleaching below -1.2 V is therefore not caused by band filling but rather a sign of bandgap renormalization, previously observed in TA measurements [10,31,32] , by chemical doping [33] and by spectroelectrochemistry of 3-layer, 150 nm large 2H-MoS2 nanosheets produced by liquid based exfoliation. [25] Briefly, upon electron injection into 2H-MoS2 nanosheets the self-energy of the electron and hole states decreases due to many-body effects, and this decrease is typically larger for the electron states.…”

“…We synthesized colloidal 2H-MoS2 nanosheets based on the previously reported protocol [10,11] to obtain mono-and bilayers with a lateral dimension of approximately 20-25 nm. The Moprecursor was prepared by dissolving molybdenum(V)-chloride in a mixture of oleylamine and oleic acid of 10:1 by stirring inside a glovebox for 2 days.…”

“…[2] Within the three known polytypes (1T, 2H, and 3R) [3] of this material, 2H-MoS2 is of particular interest due to its semiconducting nature and thickness-dependent band gap. [4] Apart from the established synthetic pathways toward 2H-MoS2 with defined layer thickness, including mechanical [5] or liquid [6] based exfoliation and chemical [7] as well as physical vapor deposition, [8] wet-chemical techniques affording colloidally stable TMDCs have recently emerged, [9][10][11][12] which are based on principles originally developed for the synthesis of colloidal quantum dots. [13] Some advantages of wet-chemically synthesized TMDCs are their scalability, relative ease of purification and solution-processability, which render them particularly suitable for application in LEDs [14] or solar cells.…”