Layer dependent direct optical transitions at Г-point in synthesized large area MoSe2

Two-dimensional transition metal dichalcogenides (TMDs) have attracted great interest for applications in optoelectronic device. We studied the optical properties of the large area MoSe2 thin film on c-axis sapphire (0001)... [ view full abstract ]

Two-dimensional transition metal dichalcogenides (TMDs) have attracted great interest for applications in optoelectronic device. We studied the optical properties of the large area MoSe2 thin film on c-axis sapphire (0001) substrate grown by molecular beam epitaxy (MBE). Photoluminescence (PL), Raman, X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) were measured to evaluate the film quality. We obtained the optical absorption spectra of mono-, bi- and tri-layer MoSe2 and observed optical critical point which is correspond to the direct transition at Г-point of the Brillouin zone (BZ) at ~3 eV. Increasing the number of layer, the intensity of the absorption peak at Г-point was increased and its position was shifted. The changes in peak at Г-point are caused by the band structure depending on the number of layer, resulting in the modulation of the joint density of state (JDOS). Since the geometrical band shape of conduction band and valance band are similar around Г-point for few layer MoSe2, both van Hove singularities (VHS) and band nesting contribute to strong absorption peak. Using 3eV optical pump, we can selectively excite the MoSe2 by choosing suitable number of layer. The modulation of band structure and strong photon-electron interaction of 2D material would be widely applied to photonic and optoelectronic devices.