Figure 2 a illustrates the device structure of MoSe2/Si heterojunction photodetector. MoSe2 film was first deposited on p‐type Si substrate with a predefined SiO2 circular window (d = 3 mm) by magnetron sputtering, which determines the effective device area. Ag electrode (50 nm) was deposited around the window as the top ohmic contact to MoSe2 film (see Figure S3 in the Supporting Information). Then three‐layer graphene film with sheet resistance of ≈380 Ω sq–1 (conductivity ≈7740 S/cm) was transferred to the top of MoSe2 film as transparent electrode. The use of graphene electrode ensures the efficient light absorption of MoSe2/Si heterojunction, meanwhile facilitates the transport of photogenerated carriers from MoSe2 film to the Ag top electrode. Afterward, Au electrode (50 nm) was deposited at the rear side of p‐type Si as back ohmic contact. To gain more insight into the structure of the MoSe2/Si heterojunction, cross‐sectional transmission electron microscopy (TEM) investigation was performed, as shown in Figure 2b‐e. The MoSe2 film deposited by sputtering has a uniform thickness of ≈200 nm (Figure 2b). EDS elemental mappings on Si, Mo, and Se at the red square area in Figure 2b prove the formation of MoSe2/Si heterojunction. The uniform colour contrast for Mo and Se elements is an evidence of the high uniformity of the MoSe2 film. The energy dispersive X‐ray spectroscopy (EDS) line‐scanning analysis, Figure 2c, further confirms the compositions of MoSe2/Si heterojunction. From the high‐resolution TEM (HRTEM) image at the junction interface (Figure 2d), we can see that there is a thin interfacial oxide layer (≈5 nm) formed between the Si substrate and the MoSe2 film. Close investigation on the MoSe2 film, Figure 2e, discloses the distinct vertically standing layered structure of the film, i.e., the growth direction of the (001) molecule planes of MoSe2 are perpendicular to the Si substrate, in contrast the parallel growth of the MoSe2 nanosheets fabricated by conventional chemical vapour deposition (CVD) method.57, 64 The dimension of individual crystal grains in the MoSe2 film is 4–6 nm wide, corresponding to about 6–9 MoSe2 monolayers (Figure 2e). As one of the remarkable characteristics of 2D materials, they show pronounced anisotropic conduction in the in‐plane direction and out‐of‐plane direction. The weak van der Waals force and the large layer distance between adjacent layers lead to much lower out‐of‐plane electrical and thermal conductivities compared to those of their in‐plane analogs.65, 66 In this regards, the vertically standing layered structure endows the MoSe2 film distinct electrical properties with much efficient carrier transport from the junction interface to the top electrode.