Excitonic Complexes in n-Doped WS₂ Monolayer

Abstract

We investigate the origin of emission lines apparent in the low-temperature photoluminescence spectra of n-doped WS 2 monolayer embedded in hexagonal BN layers using external magnetic fields and first-principles calculations. Apart from the neutral A exciton line, all observed emission lines are related to the negatively charged excitons. Consequently, we identify emissions due to both the bright (singlet and triplet) and dark (spin- and momentum-forbidden) negative trions as well as the phonon replicas of the latter optically inactive complexes. The semidark trions and negative biexcitons are distinguished. On the basis of their experimentally extracted and theoretically calculated g -factors, we identify three distinct families of emissions due to exciton complexes in WS 2 : bright, intravalley, and intervalley dark. The g -factors of the spin-split subbands in both the conduction and valence bands are also determined.

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

“…For instance, emission lines corresponding to the dark excitons 170,182,185,[204][205][206] , dark trions 176,178,181,205,207 , phonon replicas of spin-and momentum-forbidden dark excitons 179,180 , and phonon replicas of positive and negative dark trions 177,179,189,207,208 have been identified in monolayer WSe2 magneto PL, on the bases of their observed g factors (ranging from -6 to -13). Another recent work also identifies these lines in hBN-encapsulated monolayer WS2 209 . We discuss some of these features in the following.…”

“…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 .…”

“…In a recent work, spin-forbidden dark excitons are resonantly controlled in a waveguide coupled monolayer WSe2 device, providing access to the out-of-plane polarization 210 . Alternatively, these darkish states can be brightened by applying a magnetic field ∥ in the plane of the sample (Voigt geometry, see Figure 6) 181,185,[204][205][206]209,211 . The in-plane magnetic field mixes the bright and dark excitons through a Zeeman coupling of $ ∥ ∥ where $ ∥ is the transverse (inplane) g factor of the conduction electrons 204,205 .…”

“…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 .…”

Electrostatic control of the trion fine structure in transition metal dichalcogenide monolayers

“…Both the experiment and theoretical calculations demonstrated that the lowest energy optical excitations in a doped TMDC ML are often associated with trions [10][11][12][13][14][15][16][17][18][19] . A sophisticated band structure with two direct band gaps at the K and K = −K points of the Brillouin zone and the presence of the spin-valley locking effect [26][27][28][29] makes it possible to realize various trionic states in TMDC ML's [30][31][32][33][34][35][36][37][38][39][40][41][42]. Commonly, they are classified by spin and valley quantum numbers and can be dark and bright depending on their combination.…”