Spectroscopic view of ultrafast charge carrier dynamics in single- and bilayer transition metal dichalcogenide semiconductors

Paulina Majchrzak,Igor Marković,Dan Matselyukh,Charlotte E Sanders,Jeppe V Lauritsen,Signe S Grønborg,Philip Hofmann,Silvano Lizzit,Deepnarayan Biswas,Antonija Grubišić Čabo,Adam S Wyatt,Richard T Chapman,Luca Bignardi,Søren Ulstrup,Daniel Lizzit,Klara Volckaert,Federico Andreatta,Maciej Dendzik,Oliver Alexander,Jill A Miwa,Marco Bianchi,P D C King ,S K Mahatha ,Céphise Cacho ,E Springate ,J M Riley ,J Johannsen

doi:10.1016/j.elspec.2021.147093

Abstract

The quasiparticle spectra of atomically thin semiconducting transition metal dichalcogenides (TMDCs) and their response to an ultrafast optical excitation critically depend on interactions with the underlying substrate. Here, we present a comparative time- and angle-resolved photoemission spectroscopy (TR-ARPES) study of the transient electronic structure and ultrafast carrier dynamics in the single- and bilayer TMDCs MoS2 and WS2 on three different substrates: Au(111), Ag(111) and graphene/SiC. The photoexcited quasiparticle bandgaps are observed to vary over the range of 1.9–2.5 eV between our systems. The transient conduction band signals decay on a sub-50 fs timescale on the metals, signifying an efficient removal of photoinduced carriers into the bulk metallic states. On graphene, we instead observe a fast timescale on the order of 170 fs, followed by a slow dynamics for the conduction band decay in MoS2. These timescales are explained by Auger recombination involving MoS2 and in-gap defect states. In bilayer TMDCs on metals we observe a complex redistribution of excited holes along the valence band that is substantially affected by interactions with the continuum of bulk metallic states.

Highlights

Semiconducting transition metal dichalcogenides (TMDCs) in the 2H structural modification with the formula unit MX2 (M = {Mo, W}; X = {S, Se}) have attracted sustained attention due to their indirectto-direct bandgap crossover upon thinning to the single layer (SL) limit [1,2,3]
This suggests that the excitation leads to a significant population of in-gap states (IGS) that arise from defects in the sample
In the TMDCs supported on metals we found an ultrafast relaxation of the signal on the order of our 40 fs time-resolution

Summary

Introduction

Semiconducting transition metal dichalcogenides (TMDCs) in the 2H structural modification with the formula unit MX2 (M = {Mo, W}; X = {S, Se}) have attracted sustained attention due to their indirectto-direct bandgap crossover upon thinning to the single layer (SL) limit [1,2,3] The bandstructure in this class of materials can be externally tuned via strain [4,5], as well as doping via alkali adsorption [6,7,8,9], electrostatic gating [10,11,12,13] and substrate interactions [14,15,16,17,18]. Our work emphasizes the strong substrate dependence of carrier dynamics in SL and BL TMDCs from the perspective of TR-ARPES, providing an outset for expanding this methodology to advanced heterostructures and devices based on the TMDCs

Experimental

Snapshots of excited free carriers

Extraction of photoexcited quasiparticle bandgaps

Analysis of carrier relaxation timescales

Dynamics along Γ -Kin bilayers

Conclusion and outlook