Fabrication of Devices: To fabricated the Gr/MoSe2/Si heterojunction photodetectors, a circular photoresist window (d = 3 mm) was first defined on the SiO2 (300 nm)/p‐type (100) Si (resistivity 1–10 Ω cm−1) substrate by photolithography (SUSS, MicroTec‐MJB4), followed by 5% HF etching for 300 s to remove the SiO2 in the window. After removing the residual photoresist by acetone, 200 nm MoSe2 film was deposited onto the window by sputtering. Afterwards, 50 nm Ag were deposited by e‐beam evaporation (Kurt J Lesker, PVD 75) around the circular window on the MoSe2 film using a shadow mask. Three‐layer graphene film was transferred onto the top of the MoSe2 film as transparent electrode. 50 nm Au was deposited by e‐beam evaporation onto the rear side of SiO2/Si substrate as ohmic contact to Si.