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.

Monolayers of semiconducting transition metal dichalcogenides are two-dimensional direct-gap systems which host tightly-bound excitons with an internal degree of freedom corresponding to the valley of the constituting carriers. Strong spin-orbit interaction and the resulting ordering of the spin-split subbands in the valence and conduction bands makes the lowest-lying excitons in WX2 (X being S or Se) spin-forbidden and optically dark. With polarization-resolved photoluminescence experiments performed on a WSe2 monolayer encapsulated in a hexagonal boron nitride, we show how the intrinsic exchange interaction in combination with the applied in-plane and/or out-of-plane magnetic fields enables one to probe and manipulate the valley degree of freedom of the dark excitons.Monolayers of transition metal dichalcogenides (TMDs), such as MX 2 with M=Mo or W, and X=S, Se or Te, are two-dimensional direct-gap semiconductors 1 which attract a lot of interest due to their unique physical properties and potential applications in optoelectronics, photonics and the development of valleytronics 2-5,7,18 . The direct bandgap in semiconducting TMDs (S-TMDs) is located at the two inequivalent K ± points (valleys) of the first Brillouin zone, related by time reversal symmetry. In monolayers, the tightly bound and optically bright excitons 8-11 from K ± valley can efficiently couple to light with right/left circular polarization 12,13 , respectively.A unique feature of S-TMD monolayers is the so-called spin-valley locking 13 : strong spin-orbit interaction lifts the degeneracy between the two spin projections s =↑, ↓ in each valley, leaving only the Kramers degeneracy between the opposite valleys, K + , s ↔ K − , −s. While the valence band spin-orbit splitting ∆ v is very large (several hundred meV 13 ), its conduction band counterpart ∆ c is by an order of magnitude smaller 14-18 , thus allowing some degree of manipulation by an in-plane magnetic field. In tungsten-based S-TMDs, ∆ c has the same sign as ∆ v , leading to the spin subband ordering shown in Fig. 1(a). In each valley, the optically bright exciton has a higher energy than the dark exciton which is composed of the conduction and valence electronic states with opposite spin projections. This, among other things, results in a strong temperature dependence of the photoluminescence (PL) efficiency, as the bright states become more populated at higher temperatures 8,19,21 .Dark excitons can couple to light only via a residual spin-flip dipole matrix element d ⊥ , whose direction is perpendicular to the monolayer plane 22-24 . In consequence, the emission of dark excitons is directed predominantly along the monolayer plane, in contrast to the out-of-plane emission of bright excitons characterized by strong inplane optical dipoles, d , see Fig. 1(b). Notably, the valley degeneracy of dark excitons is lifted by the exchange interaction which mixes the valleys and produces two eigenstates with different energies. The higher energy component takes up the whole oscillator strength ...

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...

Monolayer transition-metal dichalcogenides (TMDs) manifest exceptional optical properties related to narrow excitonic resonances. However, these properties have been so far explored only for structures produced by techniques inducing considerable large-scale inhomogeneity. In contrast, techniques which are essentially free from this disadvantage, such as molecular beam epitaxy (MBE), have to date yielded only structures characterized by considerable spectral broadening, which hinders most of the interesting optical effects. Here, we report for the first time on the MBE-grown TMD exhibiting narrow and resolved spectral lines of neutral and charged exciton. Moreover, our material exhibits unprecedented high homogeneity of optical properties, with variation of the exciton energy as small as ±0.16 meV over a distance of tens of micrometers. Our recipe for MBE growth is presented for MoSe2 and includes the use of atomically flat hexagonal boron nitride substrate. This recipe opens a possibility of producing TMD heterostructures with optical quality, dimensions, and homogeneity required for optoelectronic applications.

We present a detailed spectroscopic study of the photoluminescence quenching in an epitaxial structures containing CdSe/ZnSe quantum dots doped with low concentration of Mn 2+ ions. Our time-resolved and time-integrated experiments reveal the origin of the quenching observed in macrophotoluminescence studies of ensembles of such dots. We show that incorporation of even a few ions to an individual dot does not quench its luminescence effectively, although some fingerprints of expected spin-dependent quenching are visible. At the same time, the presence of Mn 2+ ions in the sample significantly affects the luminescence intensity of the wetting layer, resulting in a quenching of the global luminescence from studied structure. On the other hand, the luminescence decay dynamics is found to be independent of the presence of Mn 2+ ions, which suggests that the observed quenching occurs for the excited excitonic states.