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    nanoscience and nanotechnology: small is different

2D transition metal dichalcogenides: Production, characterization and applications

Dr. Víctor Vega Mayoral
School of Physics, Trinity College Dublin
Wednesday, 10 July 2019 15:30

IMDEA Nanociencia Conference Hall

 

Transition metal dichalcogenides (TMDCs) are two-dimensional layered semiconductors consisting of a hexagonally packed layer of a transition metal M (typically Mo or W) sandwiched between two layers of chalcogen atoms X (typically S or Se) and interacting through strong covalent bonds, forming a MX2 compound. In multilayer TMDCs, the single layers are loosely bound to each other by van-der-Waals interactions. The best studied members of the family are the earth abundant WS2 and MoS2, two semiconductors whose gap increases from the bulk to the monolayer.

Since the first report[1] of printed electronics from 2D TMDCs it is of particular interest to study the relaxation pathways of charges after photoexcitation. We present here a thickness-fluence dependence study of photoexcited states in MoS2. [2] We find bimolecular coalescence of charges into indirect excitons as the dominant relaxation process in two- to three-layer flakes while thicker flakes show a much higher density of defects, which  efficiently trap charges before they can coalesce.

The liquid nature of liquid phase exfoliated samples facilitates the processing of composites. By modifying the graphene content in a WS2-graphene composite the properties of a thin film transistor can be tuned. We have studied how some basic quantities such as carrier density or mobilities scale with volume fraction. We find that many important quantities follow percolation theory.[3]

1.    Kelly, A.G., et al., All-Printed Thin-Film Transistors from Networks of Liquid-Exfoliated Nanosheets. Science, 2017. 356: p. 69-72.
2.    Vega-Mayoral, V., et al., Charge Trapping and Coalescence Dynamics in Few Layer MoS2. 2D Mater, 2017. 5(1): p. 015011.
3.    O'Suilleabhain, D., et al., Percolation Effects in Electrolytically-Gated WS2/Graphene Nano:Nano Composites. ACS Appl. Mater. Interfaces, 2019.