¹School of Physics and Astronomy, University of Manchester, Manchester, M13 9PL, UK
²Taras Shevchenko National University of Kyiv, Department of Physics 4, Prospect Glushkova, 03187 Kyiv, Ukraine

Abstract. Here, we report a comprehensive study of fundamental optical properties of two-dimensional materials. These properties have been ascertained using spectroscopic ellipsometry, optical spectroscopy of Raman scattering, and photoluminescence. We have focused on optical properties of the chemically exfoliated layered TMDs: MoS2, MoSe2, WS2, and WSe2. The complex refractive index and optical conductivity within the region 1 to 4.5 eV were extracted, which lead to identification of many unique interband transitions at high symmetry points in the electron band structure. The positions of the so-called A and B excitons in monolayers are found to shift upwards in energy as compared with those of the bulk form and have smaller separation because of the decreased interactions between the layers. For monolayer TMDs, the valence-band spin-orbit splitting at the K point was estimated from the energy difference between the A and B exciton peaks. Our findings of the spin-orbit coupling of 0.16, 0.26, 0.37, and 0.55 eV in monolayers MoS2, MoSe2, WS2, and WSe2, respectively. All these findings not only extend our understanding the novel electronic structures of mono- and few-layers TMDs but also provide foundation for future technological applications of optoelectronic and spintronic device components.

Keywords: ellipsometry, optical spectroscopy, low-dimensional structures, transition metal dichalcogenides.

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