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. 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.

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‐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.…”

“…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.…”

“…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.…”

“…[ 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 ] wet‐chemical techniques affording colloidally stable TMDCs have recently emerged, [ 9–12 ] 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.…”