Passing Current through Touching Molecules

The charge flow from a single ${\mathrm{C}}_{60}$ molecule to another one has been probed. The conformation and electronic states of both molecules on the contacting electrodes have been characterized using a cryogenic scanning tunneling microscope. While the contact conductance of a single molecule between two Cu electrodes can vary up to a factor of 3 depending on electrode geometry, the conductance of the ${\mathrm{C}}_{60}\mathrm{\text{−}}{\mathrm{C}}_{60}$ contact is consistently lower by 2 orders of magnitude. First-principles transport calculations reproduce the experimental results, allow a determination of the actual ${\mathrm{C}}_{60}\mathrm{\text{−}}{\mathrm{C}}_{60}$ distances, and identify the essential role of the intermolecular link in bi- and trimolecular chains.

Figure 1

STM images ($I=10\mathrm{nA}$; $V=2.5\mathrm{V}$; $14\times 11{\mathrm{nm}}^2$) of Au(111) partially covered with ${\mathrm{C}}_{60}$ molecules (lower right) obtained with a (a) metal and (b) ${\mathrm{C}}_{60}$ tip over the same area. Gold adatoms ($\alpha$) and a small gold cluster ($\beta$) of two or three adatoms are discernible.

Figure 2

Differential conductance ($dI/dV$) spectra acquired over (a) a ${\mathrm{C}}_{60}$ with a metal tip and (b) the bare metal with a ${\mathrm{C}}_{60}$ tip. (c)–(e) are reverse STM images ($I=10\mathrm{nA}$; $5\times 4.2{\mathrm{nm}}^2$) acquired over $\alpha$ and $\beta$ type atomic clusters with a ${\mathrm{C}}_{60}$ tip at the voltages corresponding to peaks in the spectra of (b). (f)–(h) are $dI/dV$ maps acquired simultaneously with the images. Reverse STM images (i)–(l) of a Au adatom with a ${\mathrm{C}}_{60}$ tip ($V=2.5\mathrm{V}$; $1.5\times 1.4{\mathrm{nm}}^2$). Between image acquisitions, the tip was moved to the surface so as to reach $I\approx 1\mu \mathrm{A}$, where reorientation of the ${\mathrm{C}}_{60}$ at the tip occurred.

Figure 3

(a),(b) $dI/dV$ spectra acquired over (a) a ${\mathrm{C}}_{60}$ molecule on Cu(111) with a metal tip and (b) the bare metal with a ${\mathrm{C}}_{60}$ tip. Insets show (a) a STM image ($V=2\mathrm{V}$; $2\times 1.6{\mathrm{nm}}^2$) of a ${\mathrm{C}}_{60}$ array on Cu(111), where all molecules expose hexagons to vacuum, and (b) a reverse STM image ($V=-2\mathrm{V}$; $2.7\times 1.8{\mathrm{nm}}^2$) of the ${\mathrm{C}}_{60}$ tip used for the contact experiment (a $5:6$ bond is exposed to the surface). The inset to (c) displays sketches of the contact experiments performed by approaching (1) a sharp metallic tip to a ${\mathrm{C}}_{60}$ adsorbed on a hexagon on Cu(111), a $5:6$ oriented ${\mathrm{C}}_{60}$ tip (2) to the bare Cu(111) surface and (3) to a ${\mathrm{C}}_{60}$ adsorbed on a hexagon. (c) Experimental (solid lines) and calculated (symbols) conductances versus distance. In cases (1) and (2), $z$ is measured from the ${\mathrm{C}}_{60}$ center to the center of the outermost atom of the other electrode. In case (3), $z$ is the ${\mathrm{C}}_{60}$ center to ${\mathrm{C}}_{60}$ center distance. This corresponds to an offset of $\approx 0.36\mathrm{nm}$, i.e., half of the maximum atomic distance within the ${\mathrm{C}}_{60}$ cage. The calculated repulsive force (crosses) between two ${\mathrm{C}}_{60}$ molecules suggests an elastic deformation of the junction at small separations that maps real molecule-molecule distances (open triangles) with apparent distances (filled triangles). Sample voltages: (1) 100 mV; (2) $-100\mathrm{mV}$; (3) 200 mV.

Figure 4

(a) Visualizations of the most transmitting eigenchannel wave function (incoming from above) around $E_F$. Isosurfaces of the real and imaginary parts of the wave function (with sign). White and dark blue (black) corresponds to the real part and orange (light gray) and light blue (darker gray) to the imaginary part. (b) Calculated transmission of suspended chains of one, two, or three ${\mathrm{C}}_{60}$. The molecular orientations are (1) adatom vs hexagon, (2) $5:6$ bond vs flat surface, (3) hexagon vs $5:6$ bond, and (4) hexagon vs $5:6$ bond vs hexagon.