Terahertz (THz) radiation of layered molybdenum disulfide (MoS2) crystal under femtosecond laser irradiation was observed using THz surface emission spectroscopy under variable-angle transmission configuration. Although MoS2 demonstrates inversion symmetry, surface-symmetry breaking will introduce the resonant optical rectification, which is consistent with the incident polarization and azimuthal angle dependences of the THz radiation from MoS2. However, the surface depletion field induced THz radiation will make an important contribution under oblique incidence, which is consistent with the radiation saturation due to the electrostatic screening effect by photoexcited carriers. This pump-dependent saturable THz radiation can be fitted well by the calculation from Maxwell equations with electromagnetic boundary conditions. The maximum of the surface depletion field is estimated to be 1.45 × 104 V/cm with 130 nm in depth under −40° incidence. Interestingly, when the incident angle is tuned from −40° to 0°, the optical rectification contribution varies from 40% to 90%. In addition, MoS2 is diagnosed to be p-type from THz waveforms by comparison with GaAs(100). The results not only afford a comprehensive understanding of THz radiation from layered materials like MoS2 but also put forward THz emission spectroscopy for characterizing the surface and interface properties of two-dimensional materials.

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A scalable liquid exfoliation of layered BiS employing a mixed-solvent strategy was used for the fabrication of BiS nanosheets. We found that 10% deionized water in 90% isopropyl alcohol is the best mixed solvent for the efficient and effective exfoliation of layered BiS. These results are consistent with the absorbance spectra and enthalpy of mixing theory. The obtained BiS nanosheets had few-quintuple layers and were investigated by transmission electron microscopy, atomic force microscopy, and Raman spectroscopy. These BiS nanosheets can be used to fabricate large-scale thin films by filtration method; the films demonstrated sensitive photoelectric response with the rise and decay response of photocurrent on the sub-second scale under visible light excitation. The electronic structures of bulk and one-quintuple layer BiS are calculated by first-principle calculation for better understanding of the photoelectric response. A green mixed-solvent processing of BiS opens up the potential application of BiS optoelectric films to photoelectric detection and solar energy conversion devices.

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