3.
Constant height experimental STM images (b–d) at different biases illustrating the sharp transition when the bias is increased from within the transport gap (at 1.60 and 1.65 V) to the LUMO (1.70 V). The leftmost and rightmost panels (a,e) show theoretical results for 1.50 and 1.70 eV, respectively. All panels show an area of 20 × 20 Å2. Panels d and e illustrate that the theory accurately describes the LUMO (1.70 V). As the bias is lowered below the LUMO the image changes very rapidly and dramatically. For 1.70 V the there is very little weight along the x- and y-axes while for 1.6 V most of the weight is along these axes. This is well described by theory (see panel a), only including propagation through MOs and neglecting propagation directly from the buffer to the tip. The overlaid PtPc molecule in panel a clearly shows that the apparent STM image when propagating through the gap is smaller than the molecule.