Figure 2.

Adsorption geometry and electronic structure of isolated molecules. (a–d) Constant-current topographs of isolated SnPc complexes recorded at low sample voltages (10–20 meV) and elevated currents (600–800 pA). The Pb(100) substrate lattice is resolved around the molecules, which enables a determination of the adsorption sites and the azimuthal orientations. Dashed red lines and white arrows indicate ⟨110⟩ directions of the Pb lattice. The orientations of the molecules are defined by the angle of their lobes with respect to these substrate directions. The SnPc↑ molecules in panels (a) and (b) exhibit a central protrusion, are centered atop Pb atoms, and are oriented at 0 and 45°. The SnPc↓ molecules in panels (c) and (d) show minimum at their center, are located above 4-fold hollow sites, and enclose angles of ±25° with the substrate directions. The color range spans 290 and 200 pm in (a, b) and (c, d), respectively. (e) dI/dV spectra recorded above the center of each molecule. The approximate positions of the lowest-energy peaks, which are attributed to the lowest unoccupied molecular orbital, are marked with arrows. Parameters used prior to define the tip height for spectroscopy: V = 600 mV and I = 200 pA. All spectra are vertically shifted by 0.2 nS for clarity.