Ultrastrong Coupling Leads to Slowed Cooling of Hot Excitons in Few-Layer Transition-Metal Dichalcogenides

Abstract

The excited-state dynamics of few-layer molybdenum disulfide (FL-MoS 2 ) are studied in conditions of strong light−matter coupling to plasmon polaritons. Hot carrier extraction in these systems has been proposed due to the observation of slow cooling of the high-energy C exciton relative to the bandgap A exciton. Here, we show that in conditions of ultrastrong coupling to plasmon polaritons, the lifetimes of the two slowest C exciton decay processes are extended by factors of 1.5 and 5.8 in FL-MoS 2 . We hypothesize that strong coupling delocalization suppresses multiple decay mechanisms throughout the visible spectrum in MoS 2 such as defect scattering, intervalley scattering of band-nested excitations to the Κ-valley band edges, and exciton−exciton annihilation. We also find that decay from the upper to the lower hybrid mode is not ultrafast as seen in organic systems but in fact introduces an additional delay of ∼20 ps. Our observations show that strong coupling can be used to extend the lifetimes of hot excitons in FL-MoS 2 and potentially in other two-dimensional transition-metal dichalcogenides, with potential for above-bandgap photophysics and photochemistry applications such as hot carrier extraction, which could lead to more efficient solar energy conversion.

Ultrastrong Coupling of Band‐Nested Excitons in Few‐Layer Molybdenum Disulphide

“…[21] The C exciton oscillator strength is large because of this parallel band region of k-space, and thus, the density of states at the C exciton transition energy is large.An unusual consequence of the nested bands is that C exciton carriers are expected to spontaneously self-separate in momentum space [22][23][24] and exhibit slowed hot carrier cooling relative to the A and B excitons. [23] This slowed cooling has been seen by Wang et al [23] and ourselves [25] in ultrafast spectroscopic studies. Typically, photoexcited hot carriers in semiconductor devices thermalize to the band edges on femtosecond timescales before reaching the contacts.…”

“…[ 23 ] This slowed cooling has been seen by Wang et al. [ 23 ] and ourselves [ 25 ] in ultrafast spectroscopic studies. Typically, photoexcited hot carriers in semiconductor devices thermalize to the band edges on femtosecond timescales before reaching the contacts.…”

“…[26] Strong coupling has also been used to enhance SHG in organics and TMDCs while also modifying the emission spectra via Rabi splitting. [27,28] Thus, as ultrastrong coupling at the C exciton in MoS 2 prolongs carrier lifetimes, [25] it may offer a way to further tune and enhance the SHG, as well as other non-linear properties in the TMDCs.…”

“…In our previous work analyzing the kinetics of the ultrastrong coupling described here, we showed increased lifetimes of the two slowest C exciton decay processes by factors of 1.5 and 5.8. [ 25 ] Ultrastrong coupling of the C exciton in the TMDCs, therefore, may enhance hot carrier utilization for more efficient photovoltaic or photoelectrochemical energy conversion.…”

Quantum Coherent Phenomena in Energy Harvesting and Storage

“…Newer directions in quantum coherent light harvesting processes include the creation of polaritonic states, which are formed as a result of strong coupling between electromagnetic fields and molecular orbitals, along with the use of entangled light to generate and probe coherences. The increasing interest in polaritons is represented in this VSI through work examining plasmon–polariton coupling in MoS 2 , predictions that polariton formation may modulate singlet fission rates through impacts on charge transfer states, and polaritonic impacts on exciton delocalization . Finally, the use of deliberately entangled light to drive absorption could be quite significant in the generation and lifetime of quantum coherences, which is explored in this VSI through work examining entanglement effects on perylenediimide samples …”