Neutral and charged dark excitons in monolayer WS₂

Abstract

Low temperature and polarization resolved magneto-photoluminescence experiments are used to investigate the properties of dark excitons and dark trions in a monolayer of WS2 encapsulated in hexagonal BN (hBN). We...

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

“…It needs to be checked if it is the case for the dark exciton in MoS2 as well. Finally, phonon replicas of dark excitons, for instance in Figure 9 are observed energetically below the dark excitons in monolayer WS2 and WSe2 on the basis of their observed g factors (either about 9 or 12), energetic locations, and valley polarization 177,179,180,209 . Therefore, magnetooptical spectroscopy has been crucial in the identification of PL peaks previously under debate 87,166,203 .…”

“…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)).…”

“…Magnetic-field-brightened dark and gray excitons are polarized along and perpendicular to the direction of the inplane magnetic field, respectively 181,204 . The lifetime of dark excitons is longer than the bright excitons by about two to three orders of magnitude 170 .…”

“…First, a large numerical aperture (NA) objective lens to collect PL emitted parallel to the sample plane can be used and magneto PL is performed in Faraday geometry 170 . Alternatively, if no PL from dark excitons using high NA objective is observed, one can apply tilted magnetic fields to the sample 181,182 . In this case, a component of the magnetic field within the plane of the sample brightens the dark excitons, and the out-ofplane component splits them.…”

“…Trions are three-particle bound states with either two positive charges and a negative charge (positive trion), or two negative charges and a positive charge (negative trion). These quasiparticles have been extensively studied in doped quantum wells 57,58,218 and TMDCs 87,88,[223][224][225][226]146,165,186,209,[219][220][221][222] using optical spectroscopy and from theoretical perspectives. The binding energies of the trions in TMDCs are of the order of a few 10s of meVs, which is one to two orders of magnitude larger than the traditional quantum wells (typically 1 meV to few meVs) 218 .…”