Significant Enhancement in THz Emission and Piezoelectricity in Atomically Thin Nb-Doped MoS₂

A significantly enhanced THz radiation generation from femtosecond photoexcited MoS₂ layers due to Nb-doping is reported here. Different microscopic mechanisms involved in the THz photocurrent generation vary in their relative contributions in the two cases of photoexcitation, i.e., above and below the electronic bandgap of the layers. For a moderate Nb-doping level of just ∼0.05%, we have observed a multifold enhancement in the THz emission for the case of the above bandgap excitation, which is, though, nearly 1.5 times for the case of the below bandgap excitation of the monolayer MoS₂. Alongside the difference in THz generation efficiency, the THz pulse polarity is also reversed at the above bandgap excitation of the Nb-doped layers, consequent to the reversed surface depletion field. Except for a slightly smaller difference in the THz enhancement factor, all the observations are reproducible in the bilayers as well to imply a weaker inversion symmetry and reduced screening of the surface depletion field due to Nb-doping. Furthermore, we employed pristine MoS₂ and Nb-doped MoS₂ monolayers to fabricate piezoelectric nanogenerator devices. Like enhancement in the ultrafast THz emission, the piezoelectric performance of the nanogenerator, fabricated with the Nb-doped MoS₂ monolayer is also increased by a similar factor.