## 3 Results
In the vertical heterostructure, the bottom and top graphene layers act as electrodes, and the MoTe\({}_{2}\) sheet in between serves as light absorption layer; (**f**) the graphene–MoTe\({}_{2}\)–graphene vertical vdWs heterostructure array, which demonstrates the feasibility of our large-scale fabrication method. The scale bar in (**a**–**e**) is 100 \(\upmu\)m. The scale bar in (**f**) is 400 \(\upmu\)m.  
of weak graphene Raman scattering signal. Figure 4b shows the atomic force microscope (AFM) image of the MoTe\({}_{2}\) and the surface height profile along the white dashed line. The thickness of the MoTe\({}_{2}\) was about 12 nm. Figure 5a shows the photocurrent (\(I_{\text{ph}}\))-voltage (\(V\)) curves of a representative heterostructure device (device 1) under 1064 nm laser illumination with different illumination power (\(P\)) from 3 nW to 531 nW. The inset in Figure 5a shows the current (\(I\))-\(V\) curve of device 1 under dark conditions. Here, the photocurrent is defined as \(I_{\text{ph}}=I_{\text{light}}-I_{\text{dark}}\), where \(I_{\text{light}}\) and \(I_{\text{dark}}\) refer to the currents under light illumination and dark conditions, respectively. The illumination power is calculated by multiplying the power density by the device's effective area, i.e., the overlapping area (\(S\), 1600 \(\upmu\)m\({}^{2}\)) of the bottom and top graphene electrodes. It is shown that \(I_{\text{ph}}\) increases linearly with \(V\) regardless of illumination  
Figure 3: The optical and electrical characterizations of the graphene and MoTe\({}_{2}\) field-effect transistors (FETs): (**a**) the source-drain current-voltage (\(I_{\text{ds}}\)–\(V_{\text{ds}}\)) curve of a graphene FET. Inset: the optical image of the graphene FET. The scale bar is 100 \(\upmu\)m. The \(I_{\text{ds}}\)–\(V_{\text{ds}}\) curve shows linear behavior, indicating the ohmic contact between the graphene and the Pd/Au electrodes; (**b**) the transfer curve of the graphene FET, which exhibits \(p\)-type behavior; (**c**) the \(I_{\text{ds}}\)–\(V_{\text{ds}}\) curve of a MoTe\({}_{2}\) FET. Inset: the optical image of the MoTe\({}_{2}\) FET. The scale bar is 100 \(\upmu\)m. The \(I_{\text{ds}}\)–\(V_{\text{ds}}\) curve shows linear behavior, indicating the ohmic contact between the MoTe\({}_{2}\) and the Pd/Au electrodes; (**d**) the transfer curve of the MoTe\({}_{2}\) FET, which exhibits \(p\)-type behavior. power. In addition, for identical bias, \(I_{\text{ph}}\) increases monotonically with illumination power. The \(I_{\text{ph}}\) and illumination power relation measured at \(V\) = 1 V is shown in Figure 5b (the black line). We can see that \(I_{\text{ph}}\) increases almost linearly in the lower power range but shows a saturated trend when illumination power is higher. Similar phenomena have also been reported before [1; 3; 4; 8; 16]. The possible reasons are the increased recombination rate of photogenerated carriers [16], the reduced number of photogenerated carriers contributing to photoconduction under higher photon flux [16], and the existence of trap states in the device [4; 16]. As shown by the fitted curve in Figure 5b, the photocurrent and illumination power present a sublinear relation, which can be expressed as \(I_{\text{ph}}\) = \(aP^{b}\), where \(a\) and \(b\) are fitted constants. According to the fitting results, we acquired \(b\) = 0.91 \(\pm\) 0.07 (\(R^{2}\) = 0.997) in lower power range. The \(b\) value that is smaller than 1 suggests that the photoconduction gain is influenced by the charge traps in the materials and/or their interfaces [2; 4; 16; 17; 18; 19]. The device's responsivity (\(R\)) and illumination power relation measured at \(V\) = 1 V is shown in Figure 5b (the green line). The responsivity is defined as \(R\) = \(I_{\text{ph}}\)/\(P\)[4; 20]. As illumination power increases, the responsivity decreases monotonically, which corresponds to the saturated trend of the photocurrent. A maximum responsivity of up to 635 A/W is acquired with 3 nW illumination power.