Robust enhancement of valley polarization and quantum yield in composition grading lateral heterostructure of MoS2-WS2 monolayer

Valley degeneracy can be broken owing to the strong spin-orbit coupling in two-dimensional transition metal dichalcogenides (2D-TMDCs). Valley-dependent interaction of carriers in TMDCs with different circular polarizations of light offers valley degree-of-freedom besides charge and spin to carry information. Thus, bandgap engineering of 2D-TMDCs plays a critical role in developing practical valleytronic devices. Hereby, we demonstrate a great enhancement in quantum yield as well as polarization of monolayer MoS2 achieved by gradually alloying W atoms in MoS2. By appropriately setting a time offset between the evaporation of MoO3 and WO3 precursors during chemical vapor deposition, a compositionally graded heterostructure of MoS2-WS2 monolayer can be readily grown at large scale. Raman and transmission electron microscopy measurements demonstrate that the interface possesses a steep gradient in composition, spanning from MoS2 to WS2 over a length ∼2 ​μm. Compared to pure monolayer MoS2, the photoluminescence intensity at the compositionally graded interface of Mo1-xWxS2 was observed to increase by a factor of 16 owing to the effective separation of photogenerated carriers by the built-in electric field. Particularly, a remarkably high polarization of 70% at 16 ​K is demonstrated for the compositionally graded interface of Mo1-xWxS2, which is ∼1.4 times larger than that in MoS2 and is attributed to the combined effect of the alloyed structure and graded bandgap. Such an engineering scheme with a graded bandgap offers new approach for the development of high-efficiency valleytronics devices.