Monolayer transition metal dichalcogenide semiconductors, with versatile experimentally accessible exciton species, offer an interesting platform for investigating the interaction between excitons and a Fermi sea of charges. Using hexagonal boron nitride encapsulated monolayer MoSe2, we study the impact of charge density tuning on the ground and excited Rydberg states in the atomic layer. Consistent excitonpolaron behavior is revealed in both photoluminescence and reflection spectra of the A exciton 1s (A:1s) Rydberg state, in contrast to previous studies. The A:2s Rydberg state provides an opportunity to understand such interactions with greatly reduced exciton binding energy. We found that the impact of the Fermi sea becomes much more dramatic. With a photoluminescence upconversion technique, we further verify the 2s polaron-like behavior for the repulsive branch of B:2s exciton whose energy is well above the bare bandgap. Our studies show that the polaron-like interaction features are quite generic and highly robust, offering key insights into the dressed manybody state in a Fermi sea.

Understanding the Coulomb interactions between two-dimensional (2D) materials and adjacent ions/impurities is essential to realizing 2D material-based hybrid devices. Electrostatic gating via ionic liquids (ILs) has been employed to study the properties of 2D materials. However, the intrinsic interactions between 2D materials and ILs are rarely addressed. This work studies the intersystem Coulomb interactions in IL-functionalized InSe field-effect transistors by displacement current measurements. We uncover a strong self-gating effect that yields a 50-fold enhancement in interfacial capacitance, reaching 550 nF/cm 2 in the maximum. Moreover, we reveal the IL-phase-dependent transport characteristics, including the channel current, carrier mobility, and density, substantiating the self-gating at the InSe/IL interface. The dominance of self-gating in the rubber phase is attributed to the correlation between the intra-and intersystem Coulomb interactions, further confirmed by Raman spectroscopy. This study provides insights into the capacitive coupling at the InSe/IL interface, paving the way to developing liquid/2D material hybrid devices.

In monolayer WSe2, interactions between the lower-energy momentum- and spin-indirect “dark” excitons and the bright exciton (X) are likely to be significant in determining the optical properties of X at high power, and limit the ultimate exciton densities that can be achieved, yet little is known about them. Here, by employing time-resolved photoluminescence measurements, we demonstrate an efficient population of dark excitons via inter-state conversion between X and the spin-indirect intravalley excitons (D) through spin-flip, and between D and the momentum-indirect intervalley excitons (XK) mediated by the exchange interaction (D+D ←→ XK +XK). Moreover, we observe a persistent redshift of the X exciton on sub-ns timescales due to strong excitonic screening by the long-lived dense XK exciton. Our results provide a new insight into the many-body interactions between bright and dark excitons, and point to a possibility to employ dark excitons for investigating exciton condensation and valleytronics.