Based on the first-principles calculations, we studied the structural stability of PtSe_2 monolayer, Janus XPtY (A, Y=S, Se, Te, X≠Y) monolayers and h-BN monolayer and electronic and optical properties of PtSe_2 (XPtY)/BN van der Waals heterostructures (vdWHs). The results show SePtS/ BN vdWH exhibits unique type-Ⅱ band alignment. The band alignment of all vdWHs can be modulated into type-Ⅰ, type-Ⅱ, and type-Ⅲ by applying the external electric field (E-field). Interestingly, TePtS/BN vdWH is more sensitive to E-field and can achieve three types of band alignment by applying a small range of E-field (～0.10 V/A). Moreover, the PtSe_2 (XPtY) monolayers and their vdWHs with BN monolayer show a broad optical absorption range from ultraviolet, visible to infrared light, and the optical absorption peaks can reach the order of magnitude of 10s. Our results suggest PtSe_2 (XFtY)/BN vdWHs have a promising application in the fields of the photodetector and nanoelectronic devices.
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