Figure 2: Energy-band diagrams for STM imaging of DBs.

(a,b) Filled- and empty-state conditions for a single DB. The DB is negatively charged under filled-state imaging conditions, but can be either negative or positive under empty-state conditions, depending on whether it is in equilibrium with the substrate or not (see text). (c,d) Band diagrams for empty-state imaging of the double DB; its three bound states are indicated as calculated in Fig. 3j. In panel c, two alignments of the tip Fermi level with respect to the surface bands are shown (solid and dashed line), corresponding to two different bias conditions (note: the expected small increase in the band bending in the substrate is not shown in this diagram for clarity). Changing the tunnelling current magnitude and, hence, the tip-sample separation also changes the alignment of the tip and substrate bands; this is illustrated in panel d where two different tip-sample separations are shown by solid and dashed lines. The equivalence of decreasing the tunnelling current and increasing the sample bias is further highlighted in the STM data shown in Supplementary Fig. S3 and discussed in Supplementary Note S3.