Intravalley Spin–Flip Relaxation Dynamics in Single-Layer WS₂

Abstract

In monolayer (1L) transition metal dichalcogenides (TMDs) the valence and conduction bands are spin-split because of the strong spin−orbit interaction. In tungsten-based TMDs the spin-ordering of the conduction band is such that the so-called dark excitons, consisting of electrons and holes with opposite spin orientation, have lower energy than A excitons. The transition from bright to dark excitons involves the scattering of electrons from the upper to the lower conduction band at the K point of the Brillouin zone, with detrimental effects for the optoelectronic response of 1L-TMDs, since this reduces their light emission efficiency. Here, we exploit the valley selective optical selection rules and use two-color helicity-resolved pump−probe spectroscopy to directly measure the intravalley spin−flip relaxation dynamics in 1L-WS 2 . This occurs on a sub-ps time scale, and it is significantly dependent on temperature, indicative of phonon-assisted relaxation. Time-dependent ab initio calculations show that intravalley spin−flip scattering occurs on significantly longer time scales only at the K point, while the occupation of states away from the minimum of the conduction band significantly reduces the scattering time. Our results shed light on the scattering processes determining the light emission efficiency in optoelectronic and photonic devices based on 1L-TMDs.

Enhanced electron–phonon scattering in Janus MoSSe

“…The local maximum at Γ point in VB for MoS 2 is just equal to the lower VB at K (both are 148 meV lower than VBM). More importantly, the conduction and the valence band edges hold opposite spin orientation as shown figure S1 is available online at stacks.iop.org/NJP/21/113040/mmedia, which means the electrons scatter from the upper to the lower conduction band and from the lower to the upper valence band at the K point would be a intravalley spin-flip process [29]. The significant SOC induced splittings for the conduction and the valence bands in both MoSSe and MoS 2 would bring about different scattering rates for electrons and holes.…”

“…The carrier's lifetime and the hot carrier cooling process in materials are dependent strongly on the electronphonon (e-ph) interaction [17][18][19][20][21][22][23][24][25]. The carrier relaxation dynamics of several TMDs are investigated experimentally and theoretically [26][27][28][29][30], which mainly focus on the spin dependent e-ph interaction. Although synthesized successfully, Janus MoSSe structures have been studied mostly on the electronic structures [15,16].…”

Magneto-optics of layered two-dimensional semiconductors and heterostructures: Progress and prospects

“…Δ is of the order of few meVs to a few 10s of meVs, while Δ ranges from about 150 meV to 500 meV [162][163][164][165] . Due to the specific sign of the conductionband splitting, the ground-state exciton in WS2, WSe2, and MoS2 is found to be optically "spin forbidden" or "dark" 87,162,[172][173][174][175][176][177][178][179][180][181]164,182,[165][166][167][168][169][170][171] (see Figure 2(b)). In MoSe2 and MoTe2, however, the ground-state exciton is an optically "bright" state due to a reversed conduction band splitting compared to the other three materials 165,[182][183][184][185] (Figure 2(b)).…”

Phonon-assisted exciton dissociation in transition metal dichalcogenides

“…A similar behaviour is observed when exploring temperature as a potential knob to tune the performance of a photodetector. As intra-and intervalley scattering of excitons is governed by phonons in the considered lowexcitation regime, the strength of these scattering channels strongly depends on temperature [11,13,61,62]. Here, the boost in the IQE (Fig.…”