These differences are caused by the sensitivity of the physical properties of TMDs to their dielectric surroundings 30, optically active defects 29, and the synthesis method 31. Although the results obtained show a similar trend of a dielectric function ( ε), the absolute values of its real ( ε 1) and imaginary ( ε 2) parts differ by up to 50% between different measurements. SE has already been successfully applied to characterize TMDs 19, 27, 28, 29. Nevertheless, the most convenient way for precise determination of optical constant is spectroscopic ellipsometry (SE) because it allows extracting the dielectric function in a broad wavelength range directly from the raw data 27. Nowadays, atomically thin TMDs are characterized by a variety of optical techniques such as photoluminescence 21, 22, absorbance 23, 24, and micro-reflectance spectroscopy 25, 26. Nevertheless, the accurate determination of the optical constants of these materials is nontrivial owing to their complicated excitonic structure. With such a broad range of optoelectronic applications, it is vitally important to precisely know optical constants of TMDs, i.e., real and imaginary parts of the dielectric permittivity, ε 1 and ε 2. More importantly, excitons play the dominant role in optical absorption, photoluminescence, and spin–valley dynamics in TMDs even at room temperatures allowing the room-temperature excitonic devices 19, 20. Interestingly, not only monolayer but also bulk configurations of TMDs have an excitonic response, and, as a result, the high refractive index in the near- and mid-infrared spectral intervals following from large excitonic absorption and the Kramers–Kronig relations 18. Therefore, upon light illumination, TMDs support tightly bound electron–hole pairs, named excitons 17, with the enormous binding energy of about 500 meV. Their low dimensionality confines electrons movement perpendicular to the layer and provides weak dielectric screening for the electric field outside the material, giving rise to strong Coulomb interaction. These materials have already been successfully implemented in solar cells 4, 5, ultrasensitive photodetectors 6, 7, sensors 8, 9, optical modulators 10, 11, light emitters 12, 13, and lasers 14, 15, demonstrating even better performance than that of the devices based on graphene 16. In this regard, one of the most promising is a family of transition metal dichalcogenides (TMDs) 3. Two-dimensional (2D) materials possess unique electrical and optical properties 1, 2, which make them suitable for a variety of practical applications in photonics and optoelectronics 1. Therefore, this technique as a whole offers a state-of-the-art metrological tool for next-generation TMD-based devices. In addition, the proposed approach opens a possibility to observe a previously unreported peak in the dielectric function of monolayer MoS 2 induced by the use of perylene-3,4,9,10-tetracarboxylic acid tetrapotassium salt (PTAS) seeding promoters for MoS 2 synthesis and thus enables its applications in chemical and biological sensing. In the near- and mid-infrared ranges, both configurations appear to have no optical absorption and possess an extremely high dielectric permittivity making them favorable for lossless subwavelength photonics. Using this method, we conduct a detailed study of monolayer MoS 2 and its bulk crystal in the broad spectral range (290–3300 nm). Here, we present an advanced approach based on ellipsometry measurements for retrieval of dielectric functions and the excitonic properties of both monolayer and bulk TMDs. However, their optical engineering is still a challenging task owing to multiple obstacles, including the absence of a rapid, contactless, and the reliable method to obtain their dielectric function as well as to evaluate in situ the changes in optical constants and exciton binding energies. Layered semiconductors such as transition metal dichalcogenides (TMDs) offer endless possibilities for designing modern photonic and optoelectronic components.
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