In Situ Formation of SnSe2/SnSe Vertical Heterostructures toward Polarization Selectable Band Alignments

2D van der Waals (vdW) layered semiconductor vertical heterostructures with controllable band alignment are highly desired for nanodevice applications including photodetection and photovoltaics. However, current 2D vdW heterostructures are mainly obtained via mechanical exfoliation and stacking process, intrinsically limiting the yield and reproducibility, hardly achieving large-area with specific orientation. Here, large-area vdW-epitaxial SnSe₂/SnSe heterostructures are obtained by annealing layered SnSe. These in situ Raman analyses reveal the optimized annealing conditions for the phase transition of SnSe to SnSe₂. The spherical aberration-corrected transmission electron microscopy investigations demonstrate that layered SnSe₂ epitaxially forms on SnSe surface with atomically sharp interface and specific orientation. Optical characterizations and theoretical calculations reveal valley polarization of the heterostructures that originate from SnSe, suggesting a naturally adjustable band alignment between type-II and type-III, only relying on the polarization angle of incident lights. This work not only offers a unique and accessible approach to obtaining large-area SnSe₂/SnSe heterostructures with new insight into the formation mechanism of vdW heterostructures, but also opens the intriguing optical applications based on valleytronic nanoheterostructures.