Valley and band structure engineering of folded MoS2 bilayers

Artificial structures made of stacked two-dimensional crystals have recently been the focus of intense research activity(1) As in twisted or stacked graphene layers(2-6), these structures can show unusual behaviours and new phenomena(1). Among the various layered compounds that can be exfoliated, transition-metal dichalcogenides(7) exhibit interesting properties governed by their structural symmetry(8,9) and interlayer coupling(10-13), which are highly susceptible to stacking. Here, we obtain-by folding exfoliated MoS2 monolayers-MoS2 bilayers with different stacking orders, as monitored by second harmonic generation and photoluminescence. Appropriate folding can break the inversion symmetry and suppress interlayer hopping, evoking strong valley(14-16) and spin(17-19) polarizations that are not achieved in natural MoS2 bilayers of Bernal stacking(20). It can also enlarge the indirect bandgap by more than 100 meV through a decrease in the interlayer coupling. Our work provides an effective and versatile means to engineer transition-metal dichalcogenide materials with desirable electronic and optical properties.