Carrier density in monolayer MoS$_2$ govern by hBN encapsulation -- unveiling of negative trion's fine structure.

Atomically thin materials, like semiconducting transition metal dichalcogenides, are highly sensitive to the environment. This opens up an opportunity to externally control their properties by changing their surroundings. The optical response of monolayer MoS$_2$ encapsulated in hexagonal BN (hBN) is studied with the aid of photoluminescence and reflectance contrast experiments. We demonstrate that carrier concentration in MoS$_2$ monolayers can be significantly tuned from $n$-type doping with free-electron concentration up to about 2 $\times$ 10$^{12}$ cm$^{-2}$, through the neutrality point, and ending with the natural $p$-type doping of MLs achieved by the modification of the bottom hBN flake thickness from 4 nm to 134 nm. The fine structure of negatively charged excitons is also resolved due to the high quality of investigated structures. We propose that the observed components of negative trion originate from the intravalley singlet, intervalley singlet, and intervalley triplet states.