Thickness-Dependent Differential Reflectance Spectra of Monolayer and Few-Layer MoS₂, MoSe₂, WS₂ and WSe₂

Abstract

The research field of two dimensional (2D) materials strongly relies on optical 21 microscopy characterization tools to identify atomically thin materials and to determine 22 their number of layers. Moreover, optical microscopy-based techniques also opened the 23 door to study the optical properties of these nanomaterials. We present a comprehensive 24 study of the differential reflectance spectra of 2D semiconducting transition metal 25 dichalcogenides (TMDCs), MoS2, MoSe2, WS2 and WSe2, with thickness ranging from 26 one layer up to six layers. We analyze the thickness-dependent energy of the different 27 excitonic features, indicating the change in the band structure of the different TMDC 28 materials with the number of layers. Our work provides a route to employ differential 29 reflectance spectroscopy for determining the number of layers of MoS2, MoSe2, WS2 and 30 WSe2. 31 32The isolation of atomically thin semiconducting TMDCs by mechanical exfoliation of 33 bulk layered crystals has aroused the interest of the nanoscience and nanotechnology transmittance with different number of layers in a wide spectral range, which is still 10 lacking. 11Here we systematically study the differential reflectance of single-and few-layer MoS2, 12MoSe2, WS2 and WSe2 from the near-infrared (1.4 eV) to the near-ultraviolet (3.0 eV). 13The differential reflectance spectra show prominent features due to excitons. The 14 thickness dependence of these excitonic features is analyzed. The optical spectra of the fabricated flakes are characterized by using a homebuilt micro- transmittance measurements acquired on the same sample. 9The differential reflectance spectrum is calculated as (R-R0)/R and it is related to the 10 absorption coefficient of the material α(λ) as [34,35] 11where R is the intensity reflected by the flake, R0 the intensity reflected by the substrate, 13n is the refractive index of the flake under study and n0 is the refractive index of the 14 substrate. Figure 2 shows the differential reflectance spectra measured on the single-and Brillouin zone. [18][19][20] This feature is the most studied one, as it is also the dominant one TMDCs, the origin of this higher energy transition at the K point is related to the splitting 5 of the valence band due to the spin-orbit interaction. For multilayer systems, the splitting 6 of the valence band is driven by a combination of spin-orbit-and interlayer interaction. 7Apart from the narrow A and B exciton peaks, the differential reflectance spectra of 8 MoS2, MoSe2 and WS2 also show other broader spectroscopic features in an energy range 9 from 2.5 eV to 2.9 eV (referred to as C exciton peak), which is due to singularities in the 10 joint density of states between the first valence and conduction bands in a circle around 18The differential reflectance spectra have been fit to a sum of Gaussian/Lorentzian peaks Interestingly, we also find that the C exciton shows a prominent shift with the thickness, Peer-reviewed version available at Nanomaterials 2018, 8, 725; doi:10.33...

Strain engineering in single-, bi- and tri-layer MoS2, MoSe2, WS2 and WSe2

“…The spectra show two prominent peak features at ~ 1.9 and ~ 2.0 eV that correspond to direct band gap transitions at the K point of the Brillouin zone that generates excitons labelled A and B in agreement with Refs. [19,24,25]. Both A and B excitons redshift upon increasing tensile strain.…”

“…Single-, bi-and tri-layer flakes are identified at first glance by inspecting the Gel-Film substrate with transmission mode optical microscopy (Motic BA 310 MET-T optical microscope). Quantitative analysis of the transmission mode optical images provides a first way to assess the number of layers [17,18] that can be further double-checked by micro-reflectance spectroscopy [19].…”

“…S4 in the ESM for a comparison between photoluminescence and differential reflection spectra acquired on the same flake upon uniaxial strain). Moreover, it has been shown that differential reflectance provides similar information as compared with transmission or absorption spectroscopy [19].…”

Characterization of Layer Number of Two-Dimensional Transition Metal Diselenide Semiconducting Devices Using Si-Peak Analysis

“…Especially, due to the infeasibility of extracting multiple Raman peaks of MoSe 2 , which has only one Raman peak, and WSe 2 , which has two Raman peaks that are not fully separated, the relative Raman peak position could not be characterized like what has been done with MoS 2 [15]. Recent studies have been using reectance spectroscopy [31,32] and absorbance spectroscopy [33] to characterize the layer number of 2-D transition metal dichalcogenides, but the limitation with some studies [31,33] is that these 2-D materials are fabricated on the transparent substrate, and the additional fabrication step is needed to transfer 2-D materials to the target substrate for semiconducting applications. Moreover, a clear trend of re ectance spectroscopy and absorbance spectroscopy could be observed in 2-D materials with di erent layer numbers [31][32][33], but there lacks a quantitative value to identify each layer number alone.…”

Multidimensional analysis of excitonic spectra of monolayers of tungsten disulphide: toward computer-aided identification of structural and environmental perturbations of 2D materials

“…Finally, the assignment of the domains of elevated absorption amplitudes to multilayers is based on the previous measurements of the dependence of the absorption spectra on the number of layers. 58,[71][72][73][74] We note the presence of the intermediate domain (blue) identified by the K-means learning algorithm, which is located in the phase-space in the middle of the torus (the torus' "hole") and connects all four major domains together.…”