MS-2-P-2934 Probing band structures of atomically thin molybdenum disulfide by EELS

In recent years, semiconducting MoS2 has attracted much public attention because of its hexagonal structure, proper bandgap (1.3~1.8eV) and potential application in nanoelectronics and valleytronics. Currently a clear and complete picture of bandgap transition (<2eV), higher interband transition (~5eV) and plasmon resonance (~23eV) associated with the thickness-dependent electronic structure is still lacking. In this talk, we will present the EELS study on the electronic structures of atomically thin MoS2.
        We use a spherical aberration corrected TEM (FEI Titan Cube) to conduct angle resolved EELS measurement. This microscope is equipped with a monochromator providing an energy resolution of 0.14eV which help us resolve fine structures of low loss EEL spectrum and obtain band structure of MoS2. A transition from indirect to direct gap is illustrated as the thickness decreases down to monolayer. Other strong interband transition peaked at 3.1eV and 4.5eV and high-energy π+σ Plasmon excitation at 23eV are also presented as a function of thickness and momentum transfer q. These excitations (not easily accessible by conventional optical characterization) in atomically thin MoS2 are reported for the first time. Their energy redshift with the decreasing thickness and monotonically-increasing linewidth dispersion with q indicate the spilling-out effect and Landau damping, respectively, in this low dimensional electron gas system. Our investigation provides a successful paradigm to depict the electronic structures of any other novel transition metal dichalcogenides (TMDs).