Untangling the intertwined: metallic to semiconducting phase transition of colloidal MoS₂ nanoplatelets and nanosheets

Abstract

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....

Electronic Structure of Colloidal 2H‐MoS2 Mono and Bilayers Determined by Spectroelectrochemistry

“…We stress that bleaches of the A, B, and C exci-tons 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-MoS 2 nanosheets produced by liquid-based exfoliation. [25] Briefly, upon electron injection into 2H-MoS 2 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.…”

“…Synthesis of 2H-MoS 2 Nanosheets: Colloidal 2H-MoS 2 nanosheets were synthesized based on the previously reported protocol [10,11] to obtain mono and bilayers with a lateral dimension of ≈20-25 nm. The Mo-precursor 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.…”

“…Further complementary characterization of the 2H-MoS 2 nanosheets is provided in ref. [10] (X-ray diffraction, electron diffraction, FT-IR spectroscopy, photoelectron spectroscopy, and transient absorption), as well as by Raman spectroscopy in the Supporting Information. The clearly visible E 1 2g and A 1g Raman peaks indicate the 2H-phase of MoS 2 , while their separation with a median of 21.7 cm −1 suggests a sample composed of few (mono-, bi-, or tri-) layer sheets of MoS 2 considering the 1 cm −1 energy resolution of this measurement.…”

“…[2] Within the three known polytypes (1T, 2H, and 3R) [3] of this material, 2H-MoS 2 is of particular interest due to its semiconducting nature and thickness-dependent band gap. [4] Apart from the established synthetic pathways toward 2H-MoS 2 with defined layer thickness, including mechanical [5] or liquid [6] based exfoliation and chemical [7] as well as physical vapor deposition, [8] wetchemical 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 wetchemically 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.…”

Charge Carrier Dynamics in Colloidally Synthesized Monolayer MoX2 Nanosheets

“…Because most of these c-TMDs show indirect band gap behavior for non-monolayer systems, there was a knowledge gap compared to those samples prepared by chemical vapor deposition (CVD) growth, where high-quality monolayers with bright PL are by now routinely obtained. , However, in the past years, several reports indicated the possibility to form monolayered c-TMDs through colloidal synthesis approaches. Frauendorf et al and Pippia et al reported on the room-temperature and cryogenic luminescence of colloidally synthesized WS 2 and MoX 2 (X = S and Se). − Given the still very limited luminescence efficiency, there is clearly still a need for further optimization. Unfortunately, there is little known about the fate of charge carriers in these monolayer c-TMD systems after photoexcitation.…”