Interlayer and excited-state exciton transitions in bulk 2H−MoS2

Photoreflectance (PR) spectrum of bulk $2H\text{\ensuremath{-}}{\mathrm{MoS}}₂$ at energies around its direct band gap is shown to have at least three distinct spectral features over a certain temperature range, although only two are seen in absorption and reflectance spectra. These observations are compared with results from many-body perturbation theory band structure calculations that include exciton formation through the Bethe-Salpeter equation. Apart from the ground state $A\phantom{\rule{4pt}{0ex}}(n=1)$ exciton transition around 1.92 eV at 25 K, the next spectral feature around 1.96 eV, whose origin has been much debated, is identified here with a transition $I$ with strong interlayer contributions. Its greater sensitivity to electric fields is established through electroreflectance measurements. The third one around 1.99 eV, which is prominent only in PR, is shown to arise from the first excited state $A\phantom{\rule{4pt}{0ex}}(n=2)$ exciton transition. We discuss the previously unexplained observation as to why the $A\phantom{\rule{4pt}{0ex}}(n=2)$ feature dominates the PR spectrum at high temperatures. The $A$ exciton is shown to have a quasi-two-dimensional (2D) nature with binding energy of $77\ifmmode\pm\else\textpm\fi{}3$ meV under a 2D hydrogenic exciton model.