Slow cooling and efficient extraction of C-exciton hot carriers in MoS2 monolayer

Abstract

In emerging optoelectronic applications, such as water photolysis, exciton fission and novel photovoltaics involving low-dimensional nanomaterials, hot-carrier relaxation and extraction mechanisms play an indispensable and intriguing role in their photo-electron conversion processes. Two-dimensional transition metal dichalcogenides have attracted much attention in above fields recently; however, insight into the relaxation mechanism of hot electron-hole pairs in the band nesting region denoted as C-excitons, remains elusive. Using MoS2 monolayers as a model two-dimensional transition metal dichalcogenide system, here we report a slower hot-carrier cooling for C-excitons, in comparison with band-edge excitons. We deduce that this effect arises from the favourable band alignment and transient excited-state Coulomb environment, rather than solely on quantum confinement in two-dimension systems. We identify the screening-sensitive bandgap renormalization for MoS2 monolayer/graphene heterostructures, and confirm the initial hot-carrier extraction for the C-exciton state with an unprecedented efficiency of 80%, accompanied by a twofold reduction in the exciton binding energy.

Slow Cooling of High‐Energy C Excitons Is Limited by Intervalley‐Transfer in Monolayer MoS2

“…Different from previously reported timescales within hundreds of femtoseconds, the decay profiles of C exciton were fitted with a biexponential attenuation function (red solid line) with two different time‐components: τ 1 (1.4 ± 0.2 ps) and (13.5 ± 1 ps). Such results also differ from recent study in which a super‐slow cooling process in C exciton is reported but the cooling process keeps in a timescale with hundreds of picoseconds . The dynamics of exciton C remains controversial to date.…”

“…The difference in increased amplitude of the two time constants may be ascribed to the difference between different species of phonon modes in monolayer MoS 2 . Compared to the previous study reported by Wang et al, our results demonstrate a relatively fast C exciton cooling within tens of picoseconds than them with hundreds of picoseconds. The reason can be attributed to abundant defect states in CVD‐synthesized ML MoS 2 films, involving sulfur and molybdenum vacancies and interstitials, grain boundaries, and dislocations, could rapidly capture band‐edge excitons in sub‐picosecond timescale via nonradiative recombination channels (see Figure S2, Supporting Information).…”

“…Figure b exhibits the calculated results of energy band structure of ML MoS 2 based on density functional theory (DFT). The parallel band region could promote self‐separation of C excitons to form hot carriers in k ‐space, and then these hot carriers rapidly relax to their immediate band extremums, that is, Λ valley and Γ hill, respectively . For the unique parallel band region, it also could give rise to a strong optical response in absorption spectrum as with band‐edge excitons.…”

“…had demonstrated the fast relaxation pathways of C exciton (in comparison with exciton A and B) in 2D TMDs based on photoluminescence excitation spectroscopy . Moreover, Wang et al had observed a slow cooling process with hundreds of picoseconds in C exciton and deduced that the slow cooling process arises from the transient excited‐state Coulomb environment …”

Absorption and emission modulation in a MoS2–GaN (0001) heterostructure by interface phonon–exciton coupling

“…The transient absorption spectrum of MoS2 on quartz substrate with an optical excitation of 2.33 eV consists of A, B and C excitonic bands centered at 1.85 eV, 1.99 eV and 2.91 eV, respectively, saturated at a delay time of about 750 fs. The formation of C exciton band with 2.33 eV excitation is attributed to upconversion [40]. The transient absorption spectrum of MoS2 is strongly modified due to GaN substrate.…”