Probing the Different Stages in Contacting a Single Molecular Wire

A crucial problem in molecular electronics is the control of the electronic contact between a molecule and its electrodes. As a model system, we investigated the contact between the molecular wire group of a ${\mathrm{C}}_{90}{\mathrm{H}}_{98}$ (Lander) molecule and the edge of a Cu(111) monatomic step. The reproducible contact and decontact of the wire was obtained by manipulating the Lander with a low temperature scanning tunneling microscope. The electronic standing wave patterns on the Cu(111) surface serve to monitor the local electronic perturbation caused by the interaction of the wire end with the step edge, giving information on the quality of the contact.

Figure 1

Top view of the chemical structure of the manipulated and contacted Lander molecule. The cross indicates the carbon atom selected as a reference for all the $z$ and $\Delta x$ positions discussed in the text.

Figure 2

Lander molecules on a free Cu(111) surface (column A) and contacted to a (100) step of an edge dislocation. The molecule can be contacted with the molecular board parallel (column B) or orthogonal (column C) to the step. Only in the latter case an influence, due to the molecular board, on the upper terrace becomes visible. In (C2) and (C3), an additional bump corresponding to the contact point of the wire to the step appears and in (C4) a modification of the upper terrace standing wave patterns is visible. Row 1: Sphere models of optimized molecular structures. Row 2: Calculated STM images, corresponding to the models above. Row 3: STM measurements. Black to white distance $3.5\AA$, $V=0.8\mathrm{V}$, $I=0.2\mathrm{n}\mathrm{A}$, $T=8\mathrm{K}$. Line 4: Pseudo-three-dimensional STM measurements visualizing the standing wave patterns. $V=-0.1\mathrm{V}$, $I=0.2\mathrm{n}\mathrm{A}$, $T=8\mathrm{K}$.

Figure 3

Electron density patterns in the vicinity of a contacted Lander molecule. (A) Measured image, size $(9\times 11){\mathrm{n}\mathrm{m}}^2$. $V=-0.1\mathrm{V}$, $I=0.2\mathrm{n}\mathrm{A}$, $T=8\mathrm{K}$. (B): Calculated electron density pattern for a scattering geometry as sketched in (D). Image size $(9\times 11){\mathrm{n}\mathrm{m}}^2$, height in arbitrary units. (C) Line profiles, taken from the measurement (solid lines) and calculation (dashed lines). The line scans are taken from positions as indicated in (A) and (B). The height scale of the calculation has been adjusted to match the experiment. (D) Model of the geometry of scatterers (black dots) used for the calculation; the model derived by the molecular mechanics calculation is shown faintly as background for comparison.

Figure 4

Calculated variation of the contact height $\sigma$ as a function of the lateral position of the molecular wire relative to the step edge for $z=0.35\mathrm{n}\mathrm{m}$ relative to the upper terrace. The arrow indicates the experimental jump to contact. The four experimental points are indicated with their error bars. The experimentally identified conformations (A), (B), (C), (D), and (E) are also presented with their corresponding STM experimental scans and the extracted molecular conformations. The origin chosen for the $\Delta x$ axis is reported. The experimental scan over a free step edge is provided with the conformation (C) for comparison. The top view of the conformation (C) is given in Fig. 2c2. The large apparent contact bump height in (D) is due to the imaging of the legs ${\mathrm{C}\mathrm{H}}_3$ groups and not to the contact bump itself.