2D TMDC Materials

About the 2D TMDC Materials

Two-dimensional (2-D) transition-metal dichalcogenides (TMDs); MX₂ (M = Mo, W; X = S, Se) are vibrantly studied since the variable physical and chemical properties based on their dimension. In particular, molybdenum disulfide (MoS₂) and molybdenum diselenide (MoSe₂) are potential substitutes for traditional semiconductors at the cutting-edge technologies and researches such as photodetecting, waveguiding, and superconducting devices [1]. To understand device functioning and to properly design devices, the optical properties of 2-D MoS₂ and MoSe₂ at the working temperature range are needed.

Figure 1. Atomic structure of TMDs & Band structure of MoS₂ [1].

Spectroscopic ellipsometry (SE) is one of solutions for obtaining intrinsic dielectric response. Especially, the sensitivity of SE detecting differences between few-angstroms-scale films is highly suitable for the studies on 2-D materials. The flat large-area monolayer MoS₂ samples were grown on SiO₂/Si substrates via chemical vapor deposition (CVD) in hot-wall two-zone furnace [2]. The MoSe₂ film was prepared by pulsed laser deposition (PLD) followed by selenization.

Figure 2. (a) Optical image of CVD-grown MoS₂ on SiO₂/Si and (b) the corresponding FE-SEM image. AFM image with cross-section thickness profile of grain boundary between (c) two MoS₂ monolayer domains and (d) isolated MoS₂ flake toward to edge of wafer [2].

For MoS₂, the analysis was proceeded for energies from 1.4 to 6.42 eV and for temperatures from 35 to 350 K. The ten (A^-, A⁰, B₁, B₂, C, and E_I-V) CPs are observed from ε and the numerically calculated second derivatives of ε spectra [3].

In case of MoSe₂, the submonolayer sample was measured for energies from 0.74 to 6.42 eV and for temperatures from 31 to 300 K. To obtain of monolayer MoSe₂ from the submonolayer MoSe₂ film, we developed non-uniformity model using Bruggeman EMA. The six (A⁰, B₂, C_a, C_b, E, and E_I) CPs were obtained at 300 K and the six other (A^-, B₁, F, and E_II-IV) CPs were additionally resolved by using the second derivative approach at cryogenic temperatures [3].

A clear separation of A^- and A⁰ CPs appeared at all measured temperatures for MoS₂ and at temperature less than 250 K for MoSe₂ as reported previously. There are a few indications about the split in the region of the highly asymmetric B-excitonic peak, which theoretically anticipated as B-trionic peak for MoS₂ and as transitions originated from the first excited state of the A exciton for MoSe₂. Most CP energies were blue-shifted and all CP structures are sharpened at the lowered temperature, which occurred by the shrunk lattice constants and the reduced electron-phonon interactions. Temperature dependences of CP energies are determined by using a phenomenological expression containing the Bose-Einstein statistical factor and the linear equation, which allow to approach the information of the CP energies of monolayer MoS₂ and MoSe₂ at any temperature. We anticipate this result to serve new insights in many studies: for example, growing atomic-scale-thin film, designing dimension-dependent-optoelectric devices, and comprehending the dielectric response of monolayer molybdenum dichalcogenides.

Figure 3. (a) of monolayer MoS₂ at various temperatures and (b) of monolayer MoSe₂ at 31K and 300K on the expanded scales [2,3].

[1] Wang, Q. H. et al. Nat. Nanotech., 7, 699 (2012).

[2] Park, H. G. et al. Appl. Spectrosc. Rev. 51, 621 (2016).

[3] Park, H. G. et al. Sci. Rep. 8, 3173 (2018).