Ellipsometry and optical spectroscopy of low-dimensional family TMDs

Abstract

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: MoS 2 , MoSe 2 , WS 2 , and WSe 2 . 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 MoS 2 , MoSe 2 , WS 2 , and WSe 2 , 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.

Trion Binding Energy Variation on Photoluminescence Excitation Energy and Power during Direct to Indirect Bandgap Crossover in Monolayer and Few-Layer MoS2

“…The direct cause of the excitation energy impact is clarified if we consider the optical absorption spectra of 1L MoS 2 studied by other authors. 58−60 In particular, Kravets et al 58 reported the absorption coefficient α ≈ 25 μm −1 at 2.33 eV, while it exceeds 100 μm −1 for 3.06 eV, that is, above 4 times stronger (Figure S7). According to the Beer−Lambert law, the excitation light intensity after passing the flake I = I 0 •exp(−αd), where I 0 is the incident intensity.…”

Atypical Defect-Mediated Photoluminescence and Resonance Raman Spectroscopy of Monolayer WS2

“…The large photon energies of the lasers at 514.5 and 532 nm, 2.41 and 2.33 eV, indicate that these two lasers can only be in resonance with the B exciton. Ramasubramaniam et al have reported that the spin–orbit splitting energy (Δ SO ) of monolayer 2D TMDs increases as the fourth power of the average atomic number of the constituent elements. , Further studies have shown that Δ SO is independent of temperature and defect levels . Thus, Δ SO can be set to 0.39 eV using the data measured by Hanbicki et al The B excitonic energy E B = E A + Δ SO can be calculated to be 2.377, 2.321, 2.289, and 2.263 eV for T1–T4, respectively.…”

Exfoliation methods for compositional and electronic characterization of interfacial Mo(Sex,Sy) in Cu(In,Ga)(Se,S)2 solar cells by X‐ray and UV photoelectron spectroscopy

“…Besides graphene, the two‐dimensional (2D) layered transition metal dichalcogenides MX 2 (where an atomic layer of transition metal M is sandwiched between two atomic layers of chalcogen X) receive considerable attention due to their unique properties, like atomic‐scale thickness, direct bandgap, 1,2 strong spin–orbit coupling, and suitable mechanical and optoelectronic properties for use in (opto)electronics, spintronics, or energy harvesting 3,4 . For instance, MoSe 2 is treated as a promising material for energy storage, catalysis, and optoelectronics 5 .…”