Fig. 1. (Color online) (a) Structure schematic of BP/ReS₂ heterojunction device. The source electrode (the contact connected to ReS₂) is grounded. The drain electrode (the contact connected to BP) is applied a voltage V_d. (b) AFM image of p-BP/n-ReS₂ heterojunction device. The inset shows the height profile along the red solid line, indicating the thicknesses of BP (~10 nm) and ReS₂ (~12 nm). (c) The transfer curve of the ReS₂ FET with Au contact was measured at room temperature and V_ds = 0.5 V. Top inset: Optical microscope image of the ReS₂ FET. The scale bar is 10 μm. The thickness of the ReS₂ is about 6 nm. Bottom inset: Current–voltage (I_ds–V_ds) curves for different gate voltages (V_bg). (d) I_ds as a function of V_bg for BP FET at room temperature and V_ds = 0.5 V. Top inset: Optical microscope image of the BP FET. The thickness of the BP is about 10 nm. Bottom inset: I_ds–V_ds curve of the BP FET.

Fig. 2. (Color online) (a) Band alignment for isolated p-BP and n-ReS₂ layers. Electron affinities of BP and ReS₂ are around 4.2 and 4.4 eV, respectively. (b–d) schematic band diagrams at the interface of the BP/ReS₂ heterojunction at different applied voltages V_ds ((b)zero bias, (c) positive bias and (d) negative bias). The transportation of electrons and holes are indicated by blue arrows and red arrows, respectively.

Fig. 3. (Color online) (a) |I_ds|–V_ds curves of the BP/ReS₂ heterojunction device at room temperature in the dark and under illumination with a 1550 nm laser at various excitation intensities (P = 10, 25, 50, 100, 250, 500 W/cm²). (b) Photocurrent as a function of the illumination intensity at different V_ds (V_ds = 0, –0.4, –1 V). The solid lines are fits to the data. (c) Photoresponsivity (R) of the BP/ReS₂ heterojunction device as a function of the illumination intensity at different V_ds (V_ds = 0, –0.4, –1 V). (d) The external quantum efficiency, EQE, of the heterojunction device as a function of the illumination intensity P at different V_ds (V_ds = 0, –0.4, –1 V). The wavelength of incident light is 1550 nm.

Fig. 4. (Color online) (a) Optical microscope image of the BP/ReS₂ heterojunction device, component materials are outlined in different colors. (b) The normalized photocurrent as a function of the illumination wavelength at V_ds = –1 V and illumination intensity P = 100 W/cm². (c), (d) Scanning photocurrent microscope images of the BP/ReS₂ heterojunction at V_ds = 0 V (c) and –1 V (d) with illumination wavelength 1550 nm (illumination intensity P = 100 W/ cm²). The red dotted line outlines the BP flake and yellow dotted line outlines the ReS₂ flake, respectively. The laser beam is focused by microscope objective lens (50×) and the diameter of the spot size is about 5 μm.

Fig. 5. (Color online) (a) Time dependences of I_ds during incident light switched on/off at V_ds = –1 V with different wavelength (λ = 400, 532, 1064, 1320, 1550 nm). (b) Source–drain current I_ds as a function of time with photoswitching at different V_ds (V_ds = 0, –0.2, –0.4, –0.6, –0.8, –1 V). The light intensity P = 50 W/cm².

Fig. 6. (Color online) (a) Optical microscope image of the BP/ReS₂ heterojunction device. The angles of zero degree and 90 degree represents the rotation direction of the linearly polarized light. (b), (c) The anisotropic response in I_ph plotted in the polar coordination at visible light wavelength of 532 and 650 nm. The visible light intensity P = 25 W cm². (d–f) The evolution of the I_ph plotted in the polar coordination at near infrared light wavelength of (d) 1064 nm, (e) 1550 nm, and (f) 1750 nm. The near infrared light intensity P = 100 W/ cm². The V_ds is fixed in –1 V.