Electrostatically Enhanced Electron-Phonon Interaction in Monolayer 2H-MoSe₂ Grown by Molecular Beam Epitaxy

The enhancement of electron-phonon interaction provides a reasonable explanation for gate-tunable phonon properties in some semiconductors where multiple inequivalent valleys are simultaneously occupied upon charge doping, especially in few-layer transition metal dichalcogenides (TMDs). In this work, we report var der Waals epitaxy of 2H-MoSe₂ by molecular beam epitaxy (MBE) and gate-tunable phonon properties in monolayer and bilayer MoSe₂. In monolayer MoSe₂, we find that out-of-plane phonon mode A_1g exhibits a strong softening and shifting toward lower wavenumbers at a high electron doping level, while in-plane phonon mode E_2g¹ remains unchanged. The softening and shifting of the out-of-plane phonon mode could be attributed to the increase of electron-phonon interaction and the simultaneous occupation of electrons in multiple inequivalent valleys. In bilayer MoSe₂, no corresponding changes of phonon modes are detected at the same doping level, which could originate from the occupation of electrons only in single valleys upon high electron doping. This study demonstrates electrostatically enhanced electron-phonon interaction in monolayer MoSe₂ and clarifies the relevance between occupation of multiple valleys and phonon properties by comparing Raman spectra of monolayer and bilayer MoSe₂ at different doping levels.