Unusually large exciton binding energy in multilayered 2H-MoTe2

Abstract

Although large exciton binding energies of typically 0.6-1.0 eV are observed for monolayer transition metal dichalcogenides (TMDs) owing to strong Coulomb interaction, multilayered TMDs yield relatively low exciton binding energies owing to increased dielectric screening. Recently, the ideal carrier-multiplication threshold energy of twice the bandgap has been realized in multilayered semiconducting 2H-MoTe₂ with a conversion efficiency of 99%, which suggests strong Coulomb interaction. However, the origin of strong Coulomb interaction in multilayered 2H-MoTe₂, including the exciton binding energy, has not been elucidated to date. In this study, unusually large exciton binding energy is observed through optical spectroscopy conducted on CVD-grown 2H-MoTe₂. To extract exciton binding energy, the optical conductivity is fitted using the Lorentz model to describe the exciton peaks and the Tauc-Lorentz model to describe the indirect and direct bandgaps. The exciton binding energy of 4 nm thick multilayered 2H-MoTe₂ is approximately 300 meV, which is unusually large by one order of magnitude when compared with other multilayered TMD semiconductors such as 2H-MoS₂ or 2H-MoSe₂. This finding is interpreted in terms of small exciton radius based on the 2D Rydberg model. The exciton radius of multilayered 2H-MoTe₂ resembles that of monolayer 2H-MoTe₂, whereas those of multilayered 2H-MoS₂ and 2H-MoSe₂ are large when compared with monolayer 2H-MoS₂ and 2H-MoSe₂. From the large exciton binding energy in multilayered 2H-MoTe₂, it is expected to realize the future applications such as room-temperature and high-temperature polariton lasing.

Figure 1

Synthesis of 2H-MoTe₂ film and measurement transmittance spectra. (a) Schematic of chemical vapor deposition system used to synthesize 2H-MoTe₂ film on SiO₂/Si substrate and illustration of growth process. The oxidized Mo thin film transformed gradually to the 2H-MoTe₂ film as the processing time goes. (b) Thickness-dependent Raman spectra with three different thicknesses (2, 4, and 10 nm). (c) Measured transmittance spectra for 2H-MoTe₂ films at various temperatures between 8 and 350 K. (d) Measured transmittance spectra of the three samples and their simulated spectra by using the transfer matrix method at 8 K. The transmittance and fit of quartz substrate are also displayed.

Figure 2

Analysis of real part of optical conductivity and optical designation in band structure of TMDs. (a) Real part of optical conductivities for 2, 4, and 10 nm thick 2H-MoTe₂ at various temperatures between 8 and 350 K. (b) Schematic of band structure of multilayered TMDs with optical designations. (c) Real part of optical conductivities fitted using Lorentz and TL models at 8 K.

Figure 3

Fitting parameters extracted using Lorentz and TL models. (a) Δ_SO at K- and Γ-point for 2, 4, and 10 nm thick 2H-MoTe₂ at various temperatures between 8 and 350 K. (b) Temperature- and thickness-dependent indirect and direct bandgaps of 2H-MoTe₂ obtained using Lorentz and TL models. The fitting lines are calculated using temperature-dependent bandgap equation. (c) Exciton binding energy at K- and Γ-point for 2, 4, and 10 nm thick 2H-MoTe₂ at various temperatures between 8 and 350 K. (d) Color maps (${\sigma }_1$) of the A exciton and the direct bandgap edge at K-point as functions of photon energy and temperature.

Figure 4

Unusually large exciton binding energy in multilayered 2H-MoTe₂. (a) Comparison of exciton binding energy with diverse semiconducting materials as function of thickness. Exciton binding energies were taken from previous reports ^,,,,–,,,,. (b) Comparing exciton binding energy of MoX₂ as a function of Δ_SO. Calculated exciton binding energies of monolayer and multilayered MoX₂ were taken from previous reports ^,. (c) Schematics of possible scenarios for the exciton radii in screening effect and large exciton binding energy in multilayered 2H-MoTe₂.