Probing of negatively charged and neutral excitons in MoS2/hBN and hBN/MoS2/hBN van der Waals heterostructures.

We present results of optical experiments and theoretical analysis on the high-quality single-layer MoS2which reveal the fine structure of charged excitons, i.e., trions. In the emission spectra we resolve and identify two trion peaks, T1and T2, resembling the pair of singlet and triplet trion peaks (TSand TT) in tungsten-based materials. However, in polarization-dependent photoluminescence measurements we identify these peaks as intra- and inter-valley singlet trions due to the trion fine structure distinct from that already known in bright and dark 2D materials with large conduction-band splitting induced by the spin-orbit coupling. We show that the trion energy splitting in MoS2is a sensitive probe of inter- and intra-valley carrier interaction. With additional support from theory we claim that the existence of these singlet trions combined with an anomalous excitonic g-factor together suggest that monolayer MoS2has a dark excitonic ground state, despite having "bright" singleparticle arrangement of spin-polarized conduction bands.