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

Abstract

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 the monolayer MoSe2, we find that out-of-plane phonon mode A1g exhibit a strong softening and shifting towards lower wave numbers at a high electron doping level while in-plane phonon mode E12g remain unchanged. The softening and shifting of 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 the 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 valley 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.