Transient absorption measurements of excitonic dynamics in $3R\text{−}{\mathrm{MoS}}_2$

The excitonic dynamics in ${\mathrm{MoS}}_2$ monolayer, bilayer, and bulk flakes with different stacking orders, namely $3R$ and $2H$, are investigated through transient absorption spectroscopy at room temperature. Samples are obtained by the mechanical exfoliation of bulk ${\mathrm{MoS}}_2$ crystals with $3R$ and $2H$ stacking orders. Photoluminescence spectroscopy is performed to characterize and compare the samples. Time-resolved differential reflection measurements reveal that monolayer flakes exfoliated from the $3R$ and $2H$ crystals exhibit similar exciton lifetimes of a few picoseconds. However, the exciton lifetime in the $3R$ bilayer flake is approximately 85 ps, about twice as long as that in the $2H$ bilayer. This prolonged exciton lifetime is attributed to the reduction of the electron-hole wavefunction overlap in the $3R$ bilayer due to the built-in polarization. Similarly, in bulk $3R$ ${\mathrm{MoS}}_2$, the exciton lifetime is around 220 ps, approximately twice as long as those in bulk $2H$ ${\mathrm{MoS}}_2$. The extension of the exciton lifetime in $3R$ transition metal dichalcogenides could be advantageous for their applications in optoelectronic devices.