Excitons, trions and Rydberg states in monolayer MoS2 revealed by low-temperature photocurrent spectroscopy

Exciton physics in two-dimensional semiconductors are typically studied by photoluminescence spectroscopy. However, this technique does not allow for direct observation of non-radiating excitonic transitions. Here, we use low-temperature photocurrent spectroscopy as an alternative technique to investigate excitonic transitions in a high-quality monolayer MoS2 phototransistor. The resulting spectra presents excitonic peaks with linewidths as low as 8 meV. We identify spectral features corresponding to the ground states of neutral excitons (X-1s(A) and X-1s(B)) and charged trions (T-A and T-B) as well as up to eight additional spectral lines at energies above the X-1s(B) transition, which we attribute to the Rydberg series of excited states of X-A and X-B. The intensities of the spectral features can be tuned by the gate and drain-source voltages. Using an effective-mass theory for excitons in two-dimensional systems we are able to accurately fit the measured spectral lines and unambiguously associate them with their corresponding Rydberg states.