Extended Data Fig. 2: Voltage dependence of dark-current images over a single molecule.

Current images over the FBPc molecule at −2.0 and 0.0 V (image size, 3.0 nm × 3.0 nm). While the tip was scanning, the laser was turned off, and the STM feedback loop was open. The tip–molecule distance was maintained to be the same as the laser-on image in Fig. 2b. Shades of blue and red indicate current flow from the substrate to the tip and vice versa, respectively. The line profiles along the dashed lines are shown below the dark-current images. The red dots show the raw data at each pixel and the black line shows a smoothed profile by a moving average of five adjacent data points. At −2.0 V, the dark current slightly exceeds the margin of error around the centre of the molecule, whereas it does not at 0.0 V. The maximum absolute value of the detected current at −2.0 V was about 0.03 pA. Because this value was only 0.3% of the maximum detected current value under illumination (Fig. 2b), the contribution of the dark current to the photocurrent image in Fig. 2b is negligibly small.