Browsing by Author "Liu, Shiyuan"

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Complex Optical Conductivity of Two-Dimensional MoS2 : A Striking Layer Dependency
(2019-10-17)
Song, Baokun
;
Gu, Honggang
;
Fang, Mingsheng
;
Ho, Yen-Teng
;
Chen, Xiuguo
The complex optical conductivities of two-dimensaionl (2D) materials are fundamental for extended applications of related optoelectronic devices. Here, we systematically investigate the layer-dependent evolutions in the complex optical conductivities of 1-6 layer 2D MoS2 over an ultrawide spectral range (0.73-6.42 eV) by spectroscopic ellipsometry. We identify five feature peaks (A-E) in the optical conductivity spectra, which present interesting layer dependencies due to the scaling effect. Results suggest that the center energies of peaks A and B are nearly layer-independent, while those of peaks C and D exhibit redshifts as the layer increases. We interpret these layer-dependent evolutions as the competition between the decreasing exciton effect and the prominent band shrinkage with the increasing layer number. Additionally, the applicability of the classical slab model and the surface current model in evaluating the optical conductivities of 2D MoS2 with different layers is discussed from an experimental perspective.
Layer-Dependent Dielectric Function of Wafer-Scale 2D MoS2
(2019-01-18)
Song, Baokun
;
Gu, Honggang
;
Fang, Mingsheng
;
Chen, Xiuguo
;
Jiang, Hao
Wafer-scale, high-quality, and layer-controlled 2D MoS2 films on c-sapphire are synthesized by an innovative two-step method. The dielectric functions of MoS2 ranging from the monolayer to the bulk are investigated by spectroscopic ellipsometry over an ultra-broadband (0.73-6.42 eV). Up to five critical points (CPs) in the dielectric function spectra are precisely distinguished by CP analysis, and their physical origins are identified in the band structures with the help of first-principles calculations. Results demonstrate that the center energies of these CPs exhibit intriguing layer dependency, which are interpreted by the intrinsic layer-dependent transitions in MoS2. Specially, the change in the imaginary part of the dielectric functions versus the thickness exhibits a "W" like curve, and the two valley bottoms appear at about four-layer and 10-layer respectively. These complex fluctuations are attributed to the alternating domination of the decreasing excitonic effect, the increasing joint density of states, and the mass density increase in relative thick MoS2 samples.