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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 MoS 2 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 ( $${\mathrm{X}}_{1{\mathrm{s}}}^{\mathrm{A}}$$ X 1 s A and $${\mathrm{X}}_{1{\mathrm{s}}}^{\mathrm{B}}$$ X 1 s B ) and charged trions (T A and T B ) as well as up to eight additional spectral lines at energies above the $${\mathrm{X}}_{1{\mathrm{s}}}^{\mathrm{B}}$$ X 1 s 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. The unique optical and optoelectronic properties of transition metal dichalcogenides are dominated by excitonic effects, whose structure is not fully characterised. Here, exciton and trion transitions in a monolayer MoS 2 phototransistor are investigated by low temperature photocurrent spectroscopy.

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