Exploring optical properties of 2H- and 1 T′- MoTe2 single crystals by spectroscopic ellipsometry

Molybdenum ditelluride (MoTe2) has received considerable attention as a two-dimensional material due to its intriguing physical properties and potential for applications in a variety of devices. The optical properties of 2H- and 1T′-MoTe2 single crystals are investigated in this study using spectroscopic ellipsometry over the spectra range 0.73-6.42 eV and at temperatures of 4.2-500 K. According to the optical absorption spectra at 4.2 K, 2H-MoTe2 has an indirect band gap of 1.06±0.01 eV, whereas 1T′-MoTe2 exhibits semimetal behavior. Furthermore, 2H-MoTe2 has three distinct intralayer A1s,A2s,andB1s excitons, and one interlayer AIL exciton at 1.136±0.002, 1.181±0.001, 1.469±0.006, and 1.21±0.01 eV, respectively. The spin-orbit coupling energy of 2H-MoTe2 is 333 meV and is independent of temperature. By contrast, 1T′-MoTe2 has two low-energy optical transitions at 0.78±0.01 and 1.45±0.01 eV, which are redshifted and broadened upon an increase in temperature, indicating the increased electron-phonon interaction. We characterize the optical transitions of both phases of MoTe2 by comparing experimental data with the results of first-principles calculations.