Broadband optical properties of monolayer and bulk MoS2

Abstract

Layered semiconductors such as transition metal dichalcogenides (TMDs) offer endless possibilities for designing modern photonic and optoelectronic components. 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. 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. Using this method, we conduct a detailed study of monolayer MoS 2 and its bulk crystal in the broad spectral range (290-3300 nm). In the near-and midinfrared ranges, both configurations appear to have no optical absorption and possess an extremely high dielectric permittivity making them favorable for lossless subwavelength photonics. 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. Therefore, this technique as a whole offers a state-of-the-art metrological tool for next-generation TMD-based devices.

Giant optical anisotropy in transition metal dichalcogenides for next-generation photonics

“…In contrast, the confinement of electron and holes within the layer results in enormous binding energy (~ 50 meV) for intralayer A-and B-excitons at the visible range similar to its monolayer counterpart. 27 At the same time, it supports C and C' exciton complexes at ultraviolet wavelengths due to the nest banding effects and complex atomic orbital contributions. 28 The Tauc-Lorentz oscillator model best describes this excitonic behavior for abplane (see Supporting Information) because it captures two the most essential physical features: 29 (i) at low photon energies, excitons cannot be excited, as a consequence, absorption, or equivalently the imaginary part of refractive index (k), is equal to zero in this wavelength range and (ii) excitonic peaks exhibit an asymmetric shape due to phonon coupling of bright (excited by light) and dark (not excited by light) excitons.…”

Giant optical anisotropy in transition metal dichalcogenides for next-generation photonics

“…Therefore, along the c-axis, the material is transparent, and a Cauchy model describes its dielectric response (see Supplementary Note 2), which is an evident consequence of the Kramers-Kronig relation between real (n) and imaginary (k) parts of the refractive index and material transparency. In contrast, the confinement of electrons and holes within the layer results in enormous binding energy (~50 meV) for intralayer A-and B-excitons at the visible range similar to its monolayer counterpart 27 . At the same time, it supports C and C' exciton complexes at ultraviolet wavelengths due to the nest banding effects and complex atomic orbital contributions 28 .…”

The band-edge excitons observed in few-layer NiPS3

“…Two-dimensional (2D) semiconductors exhibit immense functionalities and practicalities of large area 1,2 , ultra-thin 3,4 , smooth surface 5 , high carrier mobility [6][7][8] , flexible layers [9][10][11] , and thickness-tunable band-gap modulation 12,13 that have gradually received blooming attention in semiconductor technological studies and development in the post-silicon era. Among the 2D semiconductors, transition-metal dichalcogenides (TMDCs) MX 2 (M = W, Mo, Re and X = S, Se) [14][15][16][17][18] comprising a monolayer structure with X-M-X and layered III-VI compounds NX (N = Ga, In and X = S, Se, Te) [19][20][21][22][23] that are composed of fundamental units of X-N-N-X might be two of the important braches of 2D materials that require further research and development in electronics and optoelectronics devices applications. The TMDC of ReSe 2 has been proven to be a bipolar channel material for application in analog and digital integrated circuits 24 .…”