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

Molybdenum disulfide nanodots (MoS2 NDs) exhibit exotic optical, electronic, and catalytic properties, but so far limited understanding of their nonlinear optical properties has restricted wide applications in multiphoton bioimaging and nonlinear optoelectronics.In this work, we combine the two-photon luminescence (TPL) excitation and Z-scan spectroscopies to study the second order response of chemically prepared MoS2 NDs and reveal, for the first time, that the efficient TPL occurs for the two-photon absorption (TPA) populated electrons from 1Sh to 1Pe through phonon-mediated relaxation to 1Se excitonic state, followed by fast transition to the surface defect states.The strong excitation-wavelength dependence of both one-photon luminescence and TPL in MoS2 NDs arises from the reabsorption effect and structural size inhomogeneity, distinctively different from their nanosheets counterpart that exhibits strong excitonic resonance enhanced second harmonic emission.Finally we demonstrate that the optimized excitation wavelength with a cross-section of 1.02×10 4 GM produces the highest-contrast multiphoton imaging of HeLa cells incubated with the MoS2 NDs.The present nonlinear results have not only determined the precise amount of energy splitting between 1Se and 1Pe but also offered critical criterion for optimizing the TPL-based multicolor cellular imaging performance with MoS2 NDs probes.