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The quasiparticle spectra of atomically thin semiconducting transition metaldichalcogenides (TMDCs) and their response to an ultrafast optical excitationcritically depend on interactions with the underlying substrate. Here, wepresent a comparative time- and angle-resolved photoemission spectroscopy(TR-ARPES) study of the transient electronic structure and ultrafast carrierdynamics in the single- and bilayer TMDCs MoS$_2$ and WS$_2$ on three differentsubstrates: Au(111), Ag(111) and graphene/SiC. The photoexcited quasiparticlebandgaps are observed to vary over the range of 1.9-2.3 eV between our systems.The transient conduction band signals decay on a sub-100 fs timescale on themetals, signifying an efficient removal of photoinduced carriers into the bulkmetallic states. On graphene, we instead observe two timescales on the order of200 fs and 50 ps, respectively, for the conduction band decay in MoS$_2$. Thesemultiple timescales are explained by Auger recombination involving MoS$_2$ andin-gap defect states. In bilayer TMDCs on metals we observe a complexredistribution of excited holes along the valence band that is substantiallyaffected by interactions with the continuum of bulk metallic states.

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