Directional charge delocalization dynamics in semiconducting 2H-MoS 2 and metallic 1T-Li x MoS 2

Abstract

The layered dichalcogenide MoS2 is relevant for electrochemical Li adsorption/intercalation, in the course of which the material undergoes a concomitant structural phase transition from semiconducting 2H-MoS2 to metallic 1T-LixMoS2. With the core hole clock approach at the S L1 X-ray absorption edge we quantify the ultrafast directional charge transfer of excited S3p electrons in-plane (‖) and out-of-plane (⊥) for 2H-MoS2 as τ2H,‖= 0.38 ± 0.08 fs and τ2H,⊥= 0.33 ± 0.06 fs and for 1T-LixMoS2 as τ1T,‖= 0.32 ± 0.12 fs and τ1T,⊥= 0.09 ± 0.07 fs. The isotropic charge delocalization of S3p electrons in the semiconducting 2H phase within the S-Mo-S sheets is assigned to the specific symmetry of the Mo-S bonding arrangement. Formation of 1T-LixMoS2 by lithiation accelerates the in-plane charge transfer by a factor of ∼ 1.2 due to electron injection to the Mo-S covalent bonds and concomitant structural repositioning of S atoms within the S-Mo-S sheets. For excitation into out-of-plane orbitals, an accelerated charge transfer by a factor of ∼ 3.7 upon lithiation occurs due to S-Li coupling.