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Defects play a significant role in optical properties of semiconductingtwo-dimensional transition metal dichalcogenides (TMDCs). In ultra-thin MoSe2,a remarkable feature at ~250 cm-1 in Raman spectra is ascribed to be adefect-related mode. Recent attempts failed to explain the origin of this peak,leaving it being a mystery. Here in this work, we demonstrate that this peak isa Se vacancy induced defect mode. Heat effect and hydrogen etching are two mainfactors to introduce Se vacancies in CVD process of growing MoSe2. A phononconfinement model can well explain the behaviors of intrinsic Raman modes.Density functional theory (DFT) calculation reveals that single Se vacancy(VSe) is responsible for the appearance of Raman peak at ~250 cm-1 and thismode is an A1g-like localized mode which is also confirmed by polarized Ramanscattering experiment. The relative strength of this mode can be acharacterization of the quality of 2D MoSe2. This work may offer a simplemethod to tailor chalcogenide vacancies in 2D TMDCs and provide a way to studytheir vibrational properties.

A Se vacancy induced localized Raman mode in two-dimensional MoSe2 grown by CVD