## 3 Results
### The Photodetection Characteristics and Discussion  
To further verify the physical mechanisms of as-fabricated carbon-based photodiodes, the energy band diagrams at the graphene/s-SWNT junctions under various \(\mathrm{V_{BG}}\) are shown in Figure 4.  
Since the work function of graphene is about 4.5 eV [30; 31] compared with the work function of S-SWCNT, about 4.7-5.2 eV [4; 32], there should be a Schottky barrier between s-SWCNTs and graphene. When graphene was transferred on the s-SWCNT film substrate, the built-in electrical  
Figure 3: (**a**) Schematic of the graphene/s-SWCNT Schottky photodiode and the test perform circuit. (**b**) The output characteristics curve of the as-fabricated diode at different \(\mathrm{V_{BG}}\). (**c**) The diode current as a function of back-gate bias for the Schottky device before and after under illumination (\(\mathrm{V_{SD}}=-5\) V). The insert is the corresponding \(\mathrm{I_{photo}}-\mathrm{V_{BG}}\) curves. (**d**) The \(\mathrm{V_{SD}}\)-dependent diode current with and without illumination. The insert is the corresponding \(\mathrm{I_{photo}}-\mathrm{V_{SD}}\) curve. field formed in graphene/s-SWCNT Schottky interface (Figure 4a). As shown in Figure 4b, at zero gate-bias under illumination, ground-state electrons of SWNTs were excited into excited states. Then, electron-hole pairs generated in the s-SWCNT film and photoexcited electrons transported from s-SWNTs to graphene under the built-in field at the SWNT-graphene junction. There were existing traps acted as carrier capture centers in SWCNT film [7; 33] and some photo-generated holes were trapped with long lifetimes in the capture centers of s-SWNTs, which made the photogenerated electrons recycle several times in the circuit before they were recombined, yielding a photoconductive gain. The tunable charge carriers' transport characteristics with V\({}_{\text{BG}}\) was also demonstrated. When V\({}_{\text{BG}}\) < 0, the lowered Fermi level of graphene resulted in a steeper upward band bending (Figure 4c) and an enhanced built-in electric field at the s-SWNT-graphene Schottky junction, while the effective Schottky barrier height was increased. More photoelectrons transferred from s-SWNTs to graphene and contributed to an increasing photocurrent. When V\({}_{\text{BG}}\) > 0, the Fermi level of graphene was raised up, resulting in the reduced effective Schottky barrier height and weakened built-in electric field, so there were few photogenerated carriers contributing to the photocurrent (Figure 4d). Meanwhile, there was no diode current observed when V\({}_{\text{BG}}\) > 0, which is consistent with the p-type electric conductivity of the carbon-based device. For photodiodes, responsivity is a key figure of merit for the performance. In our carbon-based Schottky device, the responsivity can be described by the following equation [34; 35; 36]:  
\[R=\frac{I_{photo}}{P}=e\frac{P}{h\upsilon}\eta G\cdot\frac{1}{P}=\frac{e}{h \upsilon}\eta G. \tag{1}\]  
Here, \(h\) is Planck's constant, \(\upsilon\) is the excitation light frequency, \(\eta\) is the efficiency factor depended on the absorption efficiency and charge transfer efficiency in the as-fabricated graphene/s-SWCNTs Schottky junction, and \(G\) is the photoconductive gain. Firstly, to investigate the photoresponse characteristics of our device, the responsivity-wavelength excitation spectrum was generated and is shown in Figure 5a, where the device exhibits ultra-broadband photoresponse characteristics from ultraviolet to near infrared wavelengths (V\({}_{\text{SD}}\) = \(-\)5 V, V\({}_{\text{BG}}\) \(-\)5 V). The photoelectric testing was performed under the 0.1 mW irradiation at a temperature of 100 K to suppress the outside interference. The wavelength-dependent photoresponse was consistent with the absorption spectrum of the graphene/s-SWNT hybrid film.