Scattering of Surface State Electrons at Large Organic Molecules

The scattering of surface state electrons at Lander-type molecules on Cu(111) is investigated by means of scanning tunneling microscope (STM) experiments at low temperature and model calculations. Specific information concerning the electronic interaction of the different internal groups of the molecule with the surface is obtained. Remarkably, the central molecular wire of the molecule, although decoupled from the surface by spacer groups and therefore not visible in STM images, is the main one responsible for scattering of surface state electrons.

Figure 1

Models of the different types of Lander molecules. (a) Single Lander (SL), (b) reactive Lander (RL), (c) violet Lander (VL). The aromatic board of the RL is extended at the ends with respect to the SL, while the board of the VL is extended in the middle part. The four di-tert-butyl-phenyl sidegroups are identical for all three molecules.

Figure 2

RL molecule on a Cu(111) terrace. All measurements (a)–(d) show the same surface area ($100\times 100{\AA }^2$). From the constant current STM measurement in (a) ($U=700\mathrm{m}\mathrm{V}$, $I=0.2\mathrm{n}\mathrm{A}$) the orientation and configuration of the molecule can be deduced as shown in the model (e). Standing wave patterns of this molecule are measured by means of $dI/dV$ maps shown in (b)–(d) recorded at $V_{\mathrm{b}\mathrm{i}\mathrm{a}\mathrm{s}}=-300$, 300, and 700 meV, respectively. The same voltage was used both to set the height of the tip (with $I=0.2\mathrm{n}\mathrm{A}$) and to measure $dI/dV$ at $V_{\mathrm{b}\mathrm{i}\mathrm{a}\mathrm{s}}$. (f)–(h) are multiple scattering calculations, corresponding to the bias in (b)–(d), respectively, using the position of the C atoms of the molecular board as scattering centers.

Figure 3

Model calculations for the standing wave patterns of a RL molecule. Models (a)–(c) show the position of scattering centers (left) and corresponding multiple scattering calculations at $E=0$ meV (right). Model (a) is taking into account scattering of only the molecular board, (b) scattering of board and legs, and (c) scattering of legs only. In graphs (d), [(e)] the distance $x$ ($y$) of the second maximum from the molecular center parallel (perpendicular) to the molecular board is plotted as a function of energy.