We have investigated strong optical nonlinearity of monolayer MoS2(1–x)Se2x across the exciton resonance, which is directly tunable by Se doping. The quality of monolayer alloys prepared by chemical vapor deposition is verified by atomic force microscopy, Raman spectroscopy, and photoluminescence analysis. The crystal symmetry of all of our alloys is essentially D 3h , as confirmed by polarization-dependent second-harmonic generation (SHG). The spectral structure of the exciton resonance is sampled by wavelength-dependent SHG (λ = 1000–1800 nm), where the SHG resonance red-shifts in accordance with the corresponding optical gap. Surprisingly, the effect of compositional variation turns out to be much more dramatic owing to the unexpected increase of B-exciton-induced SHG, which indeed dominates over the A-exciton resonance for x ≥ 0.3. The overall effect is therefore stronger and broader SHG resonance where the latter arises from different degrees of red-shift for the two exciton states. We report the corresponding absolute SHG dispersion of monolayer alloys, χ(2), as a function of Se doping. We believe that our finding is a critical step toward engineering highly efficient nonlinear optical van der Waals materials working in a broader performance range.

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In this study, we utilized picosecond pulses from an Nd:YAG laser to investigate the nonlinear optical characteristics of monolayer MoSe 2 . Two-step growth involving the selenization of pulsed-laser-deposited MoO 3 film was employed to yield the MoSe 2 monolayer on a SiO 2 /Si substrate. Raman scattering, photoluminescence (PL) spectroscopy, and atomic force microscopy verified the high optical quality of the monolayer. The second-order susceptibility χ (2) was calculated to be ß50 pm V −1 at the second harmonic wavelength λ SH G ß810 nm, which is near the optical gap of the monolayer. Interestingly, our wavelength-dependent second harmonic scan can identify the bound excitonic states including negatively charged excitons much more efficiently, compared with the PL method at room temperature. Additionally, the MoSe 2 monolayer exhibits a strong laser-induced damage threshold ß16 GW cm −2 under picosecond-pulse excitation . Our findings suggest that monolayer MoSe 2 can be considered as a promising candidate for high-power, thin-film-based nonlinear optical devices and applications.

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Abstract:In this work, the friction characteristics of single-layer MoS 2 prepared with chemical vapor deposition (CVD) at three different temperatures were quantitatively investigated and compared to those of single-layer MoS 2 prepared using mechanical exfoliation. The surface and crystalline qualities of the MoS 2 specimens were characterized using an optical microscope, atomic force microscope (AFM), and Raman spectroscopy. The surfaces of the MoS 2 specimens were generally flat and smooth. However, the Raman data showed that the crystalline qualities of CVD-grown single-layer MoS 2 at 800 °C and 850 °C were relatively similar to those of mechanically exfoliated MoS 2 whereas the crystalline quality of the CVD-grown single-layer MoS 2 at 900 °C was lower. The CVD-grown single-layer MoS 2 exhibited higher friction than mechanically exfoliated single-layer MoS 2 , which might be related to the crystalline imperfections in the CVD-grown MoS 2 . In addition, the friction of CVD-grown single-layer MoS 2 increased as the CVD growth temperature increased. In terms of tribological properties, 800 °C was the optimal temperature for the CVD process used in this work. Furthermore, it was observed that the friction at the grain boundary was significantly larger than that at the grain, potentially due to defects at the grain boundary. This result indicates that the temperature used during CVD should be optimized considering the grain size to achieve low friction characteristics. The outcomes of this work will be useful for understanding the intrinsic friction characteristics of single-layer MoS 2 and elucidating the feasibility of single-layer MoS 2 as protective or lubricant layers for micro-and nano-devices.