Mechanistic Understanding of Excitation-Correlated Nonlinear Optical Properties in MoS₂ Nanosheets and Nanodots: The Role of Exciton Resonance

Low-dimensional molybdenum disulfide (MoS2) materials such as nanosheets and nanodots exhibit exotic optical, electronic, and catalytic properties, but so far the limited understanding of their nonlinear optical properties has restricted their potential use in nonlinear optoelectronics. In this work, we demonstrate that chemically prepared MoS2 nanosheets and nanodots have distinctive nonlinear emission properties: the former shows efficient second harmonic generation (SHG) with maximum intensity at its C-exciton resonance energy while the latter exhibits strong excitation-correlated two-photon luminescence (TPL). We combine two-photon photoluminescence excitation (TPPLE) and the Z-scan spectroscopies to study the second order response of the MoS2 nanodots and reveal, for the first time, that the most efficient and tunable TPL occurs through resonant two-photon absorption (TPA) induced transition from the 1Sh to 1Pe exciton state, followed by phonon-mediated exciton relaxation (1Pe → 1Se) and fast transition to surface states at different energies. With this novel nonlinear spectroscopy approach, we determine the energy splitting between the 1Se and 1Pe excitonic states to be 0.3 eV, which is slightly larger than that for MoS2 monolayers, probably due to the stronger quantum confinement effect in the nanodots. We also observe a clear TPA saturation behavior in the MoS2 NDs, and this is attributed to the state-filling effect in the 6-fold degenerate 1Pe state. Finally, we demonstrate that these new fundamental understandings of the nonlinear absorption and emission properties of the MoS2 NDs are critical for optimizing the performance of MoS2 TPL-based multicolor cellular imaging.