Layer‐Dependent Dielectric Function of Wafer‐Scale 2D MoS₂

Abstract

layer-dependent bandgap, [10] valley selective circular dichroism, [11] high on/off ratio, [12] and high thermostability [12] of the 2D MoS 2 . The performance of these novel devices significantly depends on the intrinsic optical properties (especially the dielectric function) of the 2D MoS 2 , which exhibit an intriguing layer dependency due to the enhanced quantum confinement effect and the absence of inversion symmetry. Therefore, the effective characterization of the layer-dependent optical properties of MoS 2 is critical for the performance improvement and the optimal design of those photoelectric devices based on MoS 2 .Layer-dependent optical properties of MoS 2 , including absorbance, [1] photoluminescence spectra, [10] Raman spectra, [13] and second-harmonic generation effect, [14] have been measured and discussed previously. However, these studies can hardly gain the basic optical constants, such as dielectric function, complex refraction index, etc., which play an important role in the quantitative design and optimization of those MoS 2 -based photoelectric devices. [15] Beyond these researches, there are also some reports on the dielectric function of the 2D MoS 2 , where the techniques they used can be roughly divided into three types: ellipsometry, [16][17][18][19][20][21][22][23][24][25][26] reflection (or absorption) spectrum method, [27,28] and contrast spectrum or differential reflection (or transmission) spectrum method. [29,30] By using of ellipsometry, Li et al. investigated the optical properties of the monolayer and bulk MoS 2 , and identified some critical points (CPs) in their dielectric function spectra. [22] The frequency-dependent reflection (transmission) spectra and corresponding differential spectra of the monolayer MoS 2 were simultaneously measured by Morozov et al., then the dielectric function was extracted. [27] Li et al. measured the reflection spectra of the mechanical exfoliation monolayer MoS 2 flake and the bulk MoS 2 , then their dielectric functions were calculated combined with a Kramers-Kronig constrained variational analysis. [28] Nevertheless, these published experimental reports on the dielectric function of MoS 2 mainly focus on the monolayer and bulk counterpart and the spectral range is relatively narrow. Apart from these experimental studies, some researchers have also devoted to predict the dielectric properties of the 2D MoS 2 with the help of theoretical calculations. [31,32] For example, Johari et al. studied the dielectric properties of monolayer, bilayer, and bulk MoS 2 by computing the electron Wafer-scale, high-quality, and layer-controlled 2D MoS 2 films on c-sapphire are synthesized by an innovative two-step method. The dielectric functions of MoS 2 ranging from the monolayer to the bulk are investigated by spectroscopic ellipsometry over an ultra-broadband (0.73-6.42 eV). Up to five critical points (CPs) in the dielectric function spectra are precisely distinguished by CP analysis, and their physical origins are identified in the band structures with ...

Surface susceptibility and conductivity of MoS2 and WSe2 monolayers: A first-principles and ellipsometry characterization

“…These are the excited excitonic states, marked as B 2s and B 3s . These peaks were not observed in previously reported ellipsometric data analysis of single-layer MoS 2 [52][53][54][55][56][57][58][59], but they have been identified in photoluminescence excitation spectroscopy [48]. We have been able to extract the B 2s peak from ellipsometry measurements by numerically Table II and in Table III.…”

In‐Plane and Out‐of‐Plane Optical Properties of Monolayer, Few‐Layer, and Thin‐Film MoS2 from 190 to 1700 nm and Their Application in Photonic Device Design

“…The n and k values are consistent with previously reported literature data for the in‐plane complex refractive index. The thickness dependence of peak size and location is thoroughly investigated by Yu et al [ 16 ] and Song et al [ 17 ] and attributed to decreasing excitonic binding, increasing joint density of states, and increasing mass density with the relative increase in film thickness. Unlike the previous published reports, the birefringence effect and corresponding out‐of‐plane optical properties of thin‐film MoS 2 are also reported here.…”

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