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 (\({\mathrm{X}}_{1{\mathrm{s}}}^{\mathrm{A}}\) and \({\mathrm{X}}_{1{\mathrm{s}}}^{\mathrm{B}}\)) and charged trions (T^A and T^B) as well as up to eight additional spectral lines at energies above the \({\mathrm{X}}_{1{\mathrm{s}}}^{\mathrm{B}}\) 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.

通常通过光致发光光谱学研究二维半导体中的激子物理学。但是，该技术不允许直接观察非辐射的激子跃迁。在这里，我们使用低温光电流谱作为一种替代技术来研究高质量单层MoS ₂光电晶体管中的激子跃迁。产生的光谱呈现出激子峰，其线宽低至8 meV。我们确定了与中性激子（\（{\ mathrm {X}} _ {1 {\ mathrm {s}}} ^ {\ mathrm {A}} \}和\（{\ mathrm { X}} _ {1 {\ mathrm {s}}} ^ {\ mathrm {B}} \））和带电三重子（T ^A和T ^B）以及在\（{\ mathrm {X}} _ {1 {\ mathrm {s}}} ^ {\ mathrm {B}} \}以上的能量处最多增加8条谱线，我们将其归因于X ^A和X ^B的激发态的Rydberg级数。光谱特征的强度可以通过栅极和漏极-源极电压来调节。使用二维系统中激子的有效质量理论，我们能够准确地拟合测得的谱线，并将它们明确地与各自的里德堡态相关联。