“Dead” Exciton Layer and Exciton Anisotropy of Bulk MoS₂ Extracted from Optical Measurements

Abstract

Excitons (electron−hole pairs bound by the Coulomb potential) play an important role in optical and electronic properties of layered materials. They can be used to modulate light with high frequencies due to the optical Pauli blocking. The properties of excitons in 2D materials are extremely anisotropic. However, due to nanometre sizes of excitons and their short life times, reliable tools to study this anisotropy are lacking. Here, we show how direct optical reflection measurements can be used to evaluate anisotropy of excitons in transition metal dichalcogenides MoS 2 . Using focused beam spectroscopic ellipsometry, we have measured the polarized optical reflection of bulk MoS 2 for two crystal orientations: c-axis being perpendicular to the surface from which reflection is measured and c-axis being parallel to the surface from which reflection is measured. We found that for the parallel configuration the optical reflection near excitonic transitions is strongly affected by the presence of the exciton "dead" layer such that the excitonic reflection peaks become the excitonic dips due to light interference. At the same time, the optical reflection for the perpendicular orientation is not significantly altered by the exciton "dead" layer due to large anisotropy of exciton properties. Performing simultaneous Fresnel fitting for both geometries, we were able to evaluate exciton anisotropy in layered materials from simple optical measurements.

Tuning the Optoelectronic Properties of Pulsed Laser Deposited “3D”‐MoS2 Films via the Degree of Vertical Alignment of Their Constituting Layers

“…Indeed, it is well known that 2D layered materials (including MoS 2 ) present a highly anisotropic response for exciton excitation due to their strong in‐plane covalent bonding versus weak out‐of‐plane interactions. [ 71–73 ] Moreover, the A and B excitons in ultrathin MoS 2 have been confirmed to be of an in‐plane nature. [ 71,72 ] While the MoS 2 films have relatively comparable thicknesses, their optical absorbance and particularly their absorption onset are influenced by T d (Figure 5a).…”

“…[ 71–73 ] Moreover, the A and B excitons in ultrathin MoS 2 have been confirmed to be of an in‐plane nature. [ 71,72 ] While the MoS 2 films have relatively comparable thicknesses, their optical absorbance and particularly their absorption onset are influenced by T d (Figure 5a). To estimate the optical bandgap ( E g ) of the MoS 2 films, we first calculated their absorption coefficient (α) using the equation $${\mathrm{\alpha \;}}( {{\lambda}} ){\mathrm{\;}} = {\mathrm{\;\;}}\frac{1}{t}{\mathrm{ln}}[ {\frac{1}{{T( {{\lambda}} )}}} ],$$ [ 74 ] where t is the MoS 2 film thickness and T (λ) is the transmittance at a given wavelength λ.…”

Broadband Optical Properties of Bi2Se3

“…One of the most promising is a family of transition metal dichalcogenides (TMDCs), which includes MoS 2 , WS 2 , MoSe 2 , and WSe 2 [12,[15][16][17][18]. Due to their pronounced in-plane excitonic effects, they offer previously unimaginable optical constants with in-plane refractive indices exceeding 4 [12,19,20]. As a result, numerous studies consider vdW materials to be essential building blocks for next-generation nanophotonics [1,4,12,21].…”

Characterization of 2D Transition Metal Dichalcogenides Through Anisotropic Exciton Behaviors

“…The nonresonant Raman mode, which is close to the significantly resonant absorption band of the C exciton, is sensitive to the thickness and growth method because of the electronic structural change. Kravets et al [39] used optical spectroscopy and ellipsometry measurements to analyze the anisotropic excitons in a bulk MoS 2 crystal. They further confirmed that layered materials exhibit varying optical responses to excitons with different crystal orientations.…”

“…Kravets et al. [ 39 ] used optical spectroscopy and ellipsometry measurements to analyze the anisotropic excitons in a bulk MoS 2 crystal. They further confirmed that layered materials exhibit varying optical responses to excitons with different crystal orientations.…”