Direct exciton emission from atomically thin transition metal dichalcogenide heterostructures near the lifetime limit

Abstract: We demonstrate the reduction of the inhomogeneous linewidth of the free excitons in atomically thin transition metal dichalcogenides (TMDCs) MoSe${2}$, WSe${2}$ and MoS${2}$ by encapsulation within few nanometer thick hBN. Encapsulation is shown to result in a significant reduction of the 10K excitonic linewidths down to $\sim3.5 \text{ meV}$ for n-MoSe${2}$, $\sim5.0 \text{ meV}$ for p-WSe${2}$ and $\sim4.8 \text{ meV}$ for n-MoS${2}$. Evidence is obtained that the hBN environment effectively lowers the Fermi level since the relative spectral weight shifts towards the neutral exciton emission in n-doped TMDCs and towards charged exciton emission in p-doped TMDCs. Moreover, we find that fully encapsulated MoS$_{2}$ shows resolvable exciton and trion emission even after high power density excitation in contrast to non-encapsulated materials. Our findings suggest that encapsulation of mechanically exfoliated few-monolayer TMDCs within nanometer thick hBN dramatically enhances optical quality, producing ultra-narrow linewidths that approach the homogeneous limit.