Hot Carrier Cooling and Trapping in Atomically Thin WS2 Probed by Three-Pulse Femtosecond Spectroscopy

Transition metal dichalcogenides (TMDs) have 7 shown outstanding semiconducting properties which make 8 them promising materials for next-generation optoelectronic 9 and electronic devices. These properties are imparted by 10 fundamental carrier−carrier and carrier−phonon interactions 11 that are foundational to hot carrier cooling. Recent transient 12 absorption studies have reported ultrafast time scales for carrier 13 cooling in TMDs that can be slowed at high excitation densities 14 via a hot-phonon bottleneck (HPB) and discussed these 15 findings in the light of optoelectronic applications. However, 16 quantitative descriptions of the HPB in TMDs, including details 17 of the electron−lattice coupling and how cooling is affected by 18 the redistribution of energy between carriers, are still lacking. Here, we use femtosecond pump−push−probe spectroscopy as a 19 single approach to systematically characterize the scattering of hot carriers with optical phonons, cold carriers, and defects in a 20 benchmark TMD monolayer of polycrystalline WS2. By controlling the interband pump and intraband push excitations, we 21 observe, in real-time (i) an extremely rapid “intrinsic” cooling rate of ∼18 ± 2.7 eV/ps, which can be slowed with increasing 22 hot carrier density, (ii) the deprecation of this HPB at elevated cold carrier densities, exposing a previously undisclosed role of 23 the carrier−carrier interactions in mediating cooling, and (iii) the interception of high energy hot carriers on the 24 subpicosecond time scale by lattice defects, which may account for the lower photoluminescence yield of TMDs under the 25 above-band-gap excitation condition.