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 ...

The excitonic effect and JDOS dominated evolution in layer-dependent dielectric and optical properties of 2D WSe2 were investigated by spectroscopic ellipsometry.

The complex optical conductivities of two-dimensaionl (2D) materials are fundamental for extended applications of related optoelectronic devices. Here, we systematically investigate the layer-dependent evolutions in the complex optical conductivities of 1–6 layer 2D MoS2 over an ultrawide spectral range (0.73–6.42 eV) by spectroscopic ellipsometry. We identify five feature peaks (A–E) in the optical conductivity spectra, which present interesting layer dependencies due to the scaling effect. Results suggest that the center energies of peaks A and B are nearly layer-independent, while those of peaks C and D exhibit redshifts as the layer increases. We interpret these layer-dependent evolutions as the competition between the decreasing exciton effect and the prominent band shrinkage with the increasing layer number. Additionally, the applicability of the classical slab model and the surface current model in evaluating the optical conductivities of 2D MoS2 with different layers is discussed from an experimental perspective.

Anisotropy in low-dimensional materials offers an extra degree of freedom to understand and tune the peculiar or potential properties to design novel electronic, optical, thermal, and optoelectronic devices. However, most research attention has been paid to qualitatively observing anisotropic phenomena, lacking quantitatively revealing optical anisotropy, in particular, and deeply digging for the physical mechanism. In this work, for the first time to our knowledge, the complete dielectric tensor of quasi-one-dimensional ZrTe5 is determined by Mueller matrix spectroscopic ellipsometry (MMSE) to quantitatively and comprehensively investigate the giant optical anisotropy, and the underlying physical mechanism is revealed by combination with the first-principle calculations. The ZrTe5 demonstrates giant dispersive birefringence and dichroism, and the birefringence (Δn = 0.58) outperforms many other low-dimensional and conventional birefringent materials, showing great potential and advantages in fabricating miniature and integrated polarization-sensitive devices. By combining the critical point (CP) analysis and first-principle calculations, the specific interband transitions related to the CPs in dielectric function spectra along three crystal axes of ZrTe5 are identified, revealing the physical essence of the optical anisotropy from the perspective of quantum mechanics. The proposed method is general and can be easily extended to quantitatively investigate the optical anisotropies in other novel low-symmetry materials.

No abstract