Ultrafast Energy Dissipation via Coupling with Internal and External Phonons in Two-Dimensional MoS₂

Abstract

Atomically thin two-dimensional materials have emerged as a promising system for optoelectronic applications; however, the low quantum yield, mainly caused by nonradiative energy dissipation, has greatly limited practical applications. To reveal the details for nonradiative energy channels, femtosecond pump-probe spectroscopy with a detection wavelength ranging from visible to near-infrared to mid-infrared is performed on few-layer MoS. With this method, the many-body effects, occupation effects, and phonon dynamics are clearly identified. In particular, thermalization of the MoS lattice via electron-phonon scattering is responsible for a redshift of the exciton resonance energy observed within tens to hundreds of picoseconds after photoexcitation, which provides a direct real-time sensor for measuring the change in lattice temperature. We find that the excess energy from the cooling of hot carriers and the formation of bound carriers is efficiently transferred to the internal phonon system within 2 ps, while that from Shockley-Read-Hall recombination (∼9 ps) is mainly dissipated from the MoS surfaces to external phonons.

How defects influence the photoluminescence of TMDCs

“…Similar to many traditional nanostructured semiconductors that possess large surface-to-volume ratios, the nonradiative energy dissipation pathways in few-layer and monolayer TMDCs are mainly on the surface [11,61,62]. Many studies have confirmed that these pathways are assisted by surface defects, including carrier cooling, capture and recombination processes as shown in Fig.…”

“…The competition between Auger-type and phonon-driven trapping processes is likely to be influenced by the layer number. With increasing thickness, the weakened dimensionality effect facilitates the phonon-driven processes, and most of the energy is dissipated to the lattice as heat [62,69]. In addition, defects can also trap electron-hole pairs (neutral excitons) on a very fast time scale.…”

Thermal Transport in 2D Semiconductors—Considerations for Device Applications

“…In monolayer or few‐layer TMDs energy dissipation during nonradiative emission plays a vital role in the ultrafast processes of the carriers. Generally, two pathways of nonradiative energy channels exist after photoexcitation of layered TMDs (see Figure ): 1) ultrafast cooling of hot carriers via electron–phonon scattering and subsequent formation of excitons, and 2) nonradiative recombination of excitons at the surface. While thermalization of the MoS 2 leads to a redshift of the exciton resonance energy due to electron–phonon scattering, the excess energy released from the cooling of hot carriers can be dissipated and transferred to the phonons within ≈2 ps for pathway 1), while process 2) is a Shockley–Read–Hall recombination responsible for energy dissipation from surfaces to external phonons within ≈9 ps.…”

“…b) Sketch of nonradiative energy channels that follow the ultrafast excitation. Reproduced with permission . Copyright 2018, American Chemical Society.…”

Phonon-assisted processes in the ultraviolet-transient optical response of graphene

“…In addition to these static modifications, it is of utter importance to comprehend transient electronic structure changes in time domain. Many pump-probe spectroscopy techniques (e.g., ultrafast photoemission and optical absorption) were performed on a variety of two-dimensional materials in order to gain such an insight [19][20][21][22][23][24][25][26][27][28][29][30]. In these time-resolved experiments, it is believed that subpicosecond regime is dominated by many-body Coulomb interactions between photoinduced electrons, which in turn renormalizes the electronic band structure [24].…”