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

Molybdenum ditelluride $(\mathrm{Mo}{\mathrm{Te}}_{2})$ 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 $2\text{H-}$ and $1{\mathrm{T}}^{\ensuremath{'}}\text{\ensuremath{-}}{\mathrm{MoTe}}_{2}$ 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, $2\text{H-MoT}{\mathrm{e}}_{2}$ has an indirect band gap of $1.06\ifmmode\pm\else\textpm\fi{}0.01$ eV, whereas $1{\mathrm{T}}^{\ensuremath{'}}\text{\ensuremath{-}}{\mathrm{MoTe}}_{2}$ exhibits semimetal behavior. Furthermore, $2\text{H-MoT}{\mathrm{e}}_{2}$ has three distinct intralayer ${A}_{1\mathrm{s}},\phantom{\rule{0.16em}{0ex}}{A}_{2\mathrm{s}},\phantom{\rule{0.16em}{0ex}}\mathrm{and}\phantom{\rule{0.16em}{0ex}}{B}_{1\mathrm{s}}$ excitons, and one interlayer ${A}_{\mathrm{IL}}$ exciton at $1.136\ifmmode\pm\else\textpm\fi{}0.002, 1.181\ifmmode\pm\else\textpm\fi{}0.001, 1.469\ifmmode\pm\else\textpm\fi{}0.006$, and $1.21\ifmmode\pm\else\textpm\fi{}0.01$ eV, respectively. The spin-orbit coupling energy of $2\text{H-MoT}{\mathrm{e}}_{2}$ is 333 meV and is independent of temperature. By contrast, $1{\mathrm{T}}^{\ensuremath{'}}\text{\ensuremath{-}}{\mathrm{MoTe}}_{2}$ has two low-energy optical transitions at $0.78\ifmmode\pm\else\textpm\fi{}0.01$ and $1.45\ifmmode\pm\else\textpm\fi{}0.01$ 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 $\mathrm{Mo}{\mathrm{Te}}_{2}$ by comparing experimental data with the results of first-principles calculations.