Unraveling biexciton and excitonic excited states from defect bound states in monolayer MoS2

Being direct band gap semiconductor, two-dimensional monolayer (ML) MoS2 has remarkable optical transitions arising from excitons, trions, biexciton and defects mediated bound states. However, experimental realization of biexciton in ML MoS2 has been challenging due to broad spectral feature of exciton. Here, we report on systematic observation of biexciton (A(XX) similar to 1.90 eV) along with A- trion similar to 1.92 eV and ground state exciton A(1s) similar to 1.96 eV in ML MoS2 at 4 K, by laser-power and temperature-dependent non-resonant photoluminescence (PL) spectroscopy. At low temperatures the excited state of A exciton has also been observed, A(2s) similar to 2.13 eV, which consequently merges in thermal broadening of B excitons, with rise in temperature. With excitation energy and power dependent PL, emission arising from exciton bound to sulfur vacancy (similar to 1.82 eV) have been distinguished from biexciton. Thus understanding of such excitonic states and biexcitons is useful for future quantum information processing, optoelectronic, photonics and THz applications.