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

High-quality van der Waals heterostructures assembled from hBN-encapsulated monolayer transition metal dichalcogenides enable observations of subtle optical and spin-valley properties whose identification was beyond the reach of structures exfoliated directly on standard SiO/Si substrates. Here, we describe different van der Waals heterostructures based on uncapped single-layer MoS stacked onto hBN layers of different thicknesses and hBN-encapsulated monolayers. Depending on the doping level, they reveal the fine structure of excitonic complexes, i.e. neutral and charged excitons. In the emission spectra of a particular MoS/hBN heterostructure without an hBN cap we resolve two trion peaks, T and T, energetically split by about 10 meV, resembling the pair of singlet and triplet trion peaks (T and T ) in tungsten-based materials. The existence of these trion features suggests that monolayer MoS has a dark excitonic ground state, despite having a 'bright' single-particle arrangement of spin-polarized conduction bands. In addition, we show that the effective excitonic g-factor significantly depends on the electron concentration and reaches the lowest value of -2.47 for hBN-encapsulated structures, which reveals a nearly neutral doping regime. In the uncapped MoS structures, the excitonic g-factor varies from -1.15 to -1.39 depending on the thickness of the bottom hBN layer and decreases as a function of rising temperature.