Molecular Symmetry Governs Surface Diffusion

In chemistry and physics symmetry principles are all important, for example, leading to the selection rules governing optical transitions. We have investigated the influence of the molecular symmetry on the surface potential landscape of molecules in the limit of weak molecule-substrate binding. For this purpose, the induced lateral motion of Cu(II)-tetraazaphthalocyanine molecules, for which four symmetry distinct isomers exist, on NaCl(100) was studied by scanning tunneling microscopy. This nonthermal diffusion induced by inelastic excitations is found to be qualitatively different for all four symmetry distinct isomers, demonstrating that symmetry governs the surface potential landscape.

Figure 1

STM images and schematics of the four different 4NCuPc isomers. (a) Constant-current STM image acquired at $V=0.1\mathrm{V}$ and $I=5\mathrm{pA}$, showing five molecules. The molecules appear crosslike, resembling their geometry ($140\AA \times 120\AA$). The inset shows a model of the $C_4$ isomer. Arrows indicate the two equivalent positions for the N atom. Gray, blue, white, and orange indicate C, N, H, and Cu atoms, respectively. (b) Image acquired at a voltage corresponding to tunneling through the LUMO ($140\AA \times 120\AA$, 5 pA, 1.6 V). The inset shows a model of the NaCl lattice, indicating the polar ($p$, solid line) and nonpolar ($n$, dashed lines) directions (green: Cl, purple: Na; not drawn to scale). (c) Orbital image (bottom) and schematic (top) of the $C_4$, $C_2$, $C_s$, and $C_1$ isomers (left to right), respectively. In the orbital images, the molecules are oriented in the same way as in the schematic. Blue squares indicate the position of the outer N atoms in the carbon framework. A red square (rectangle) indicates a fourfold (twofold) rotation axis. Red dotted lines indicate mirror planes perpendicular to the molecular plane. For $C_2$ isomers the mirror planes do not coincide with mirror planes of the NaCl film and are therefore lost (see Fig. 2).

Figure 2

(a) Exemplary images out of a series in which the current-induced motion of the $C_1$ isomer was studied. Magenta and yellow dots indicate the positions of current injection and the previous position of the molecule, respectively. Imaging parameters: $V=0.1\mathrm{V}$, $I=5\mathrm{pA}$. (b) Histograms of occurrences of azimuthal angles between a molecular arm and the nonpolar direction of the NaCl(100) lattice. The angles are defined as indicated by the insets shown in (c). In the right panel the same data are reduced to one quadrant taking into account the fourfold symmetry of the NaCl lattice. (c) Patterns of motion for the different isomers. Blue, green, orange, and red symbols indicate the relative positions of individual $C_4$, $C_2$, $C_s$, and $C_1$ isomers with respect to a marker molecule, respectively. Black crosses indicate Cl sites in the NaCl(100)-surface lattice. Light gray crosses indicate equivalent lattice sites. For the $C_4$ isomer, data acquired on two molecules are combined. The black dashed lines in the insets shown in (c) indicate one of the nonpolar directions of the NaCl lattice. The red dotted lines indicate the mirror planes of the isomers perpendicular to the molecular plane. For $C_2$ isomers the mirror planes do not coincide with mirror planes of the NaCl film and are therefore lost.