Stacking-induced direct band gap in CVD-grown 1H MoS2 bilayers.

Bilayer transition metal dichalcogenides offer superior electronic properties over their monolayer counterparts, such as higher carrier mobility and electrically switchable polarity1-4, positioning them as attractive candidates for next-generation transistors1,5 and sliding ferroelectric devices6,7. However, their inherent indirect band gap severely limits light emission, hindering their integration into optoelectronic systems1,2,5,8-10. Here we show bilayer MoS2 with 1H stacking obtained by a two-step chemical vapour deposition process, yielding atomically precise vertical alignment. The 1H bilayers exhibit a direct band gap, as confirmed by angle-resolved photoemission spectroscopy and density functional theory calculations. Photoluminescence measurements show intensified excitonic emission and the absence of indirect bandgap features. Furthermore, these bilayers exhibit stronger valley polarization than monolayers under both resonant and nonresonant excitation, primarily because of the suppressed intervalley scattering in the top layer. Together, these results establish 1H MoS2 as a model system for stacking-engineered quantum materials and underscore its potential for valleytronic and optoelectronic applications.