Fig. 1. (a) Optical microscope image of the phototransistor device array and the grayscale image of a device. (b) Raman spectrum of the graphene/MoS2 heterojunction under excitation by a 532 nm laser.

Fig. 2. Production process of the Au/graphene/MoS2 vdWHs bipolar phototransistor.

Fig. 3. (a) I–V characteristic curve of Au-graphene junction. The Au is the cathode and the graphene layer is the anode. (b) I–V characteristic curve of graphene-MoS2 junction. The graphene is the cathode and the MoS2 is the anode. Band diagrams of the Au/graphene/MoS2 vdWHs (c) in their original state, (d) with forward bias and irradiation.

Fig. 4. (a) Schematic of the Au/graphene/MoS2 bipolar phototransistor and its equivalent structure. (b) I–V characteristics in darkness and under different irradiance intensity values (VG=0). (c) ICE versus laser power density under different VCE at VG=0  V. (d) VG versus ICE at different VCE with 1.05  mW/cm2 irradiance intensity. The wavelength of the laser is 405 nm.

Fig. 5. (a) Responsivity of the device as a function of VCE under different VG. (b) Responsivity of the device as a function of laser power density at different VCE. (c) Relationship between photocurrent and dark current, normalized by the ratio Ilaser/Idark, and the detectivity of the bipolar phototransistor at different VCE values (irradiation under 405 nm 0.25  mW/cm2 irradiance intensity and VG=0  V). (d) Responsivity curves of Au/graphene/MoS2 vdWHs under different wavelengths of laser radiation with same laser power density 1.05  mW/cm2.

Fig. 6. (a) Transient response of the Au/graphene/MoS2 bipolar phototransistor. (b) A section between 80 s and 90 s of (a) with a rise time of 20 ms and a fall time of 92 ms. (c) I–V characteristic curves of graphene/MoS2 vdWHs under irradiation. (d) The photocurrent density of the Au/graphene/MoS2 bipolar phototransistor and graphene/MoS2 photodiode under the same laser power density and the amplification coefficient β depends on the bias voltages. (Irradiation under 405 nm 0.45  mW/cm2 irradiance intensity, VCE=17  V and VG=0  V.)

Fig. 7. Summary of comparison of the responsivity performance and generation speed of photocurrent of our Au/graphene/MoS2 vdWH with other 2D heterostructures based on MoS2, showing that our device achieves the highest generation speed of photocurrent.

Table 1. Summary of Comparison of the Au/graphene/MoS2 vdWHs with Other 2D Materials Heterostructures Based on Graphene or MoS2