Theoretical study of the charge transport through C${}_{60}$-based single-molecule junctions

We present a theoretical study of the conductance and thermopower of single-molecule junctions based on C${}_{60}$ and C${}_{60}$-terminated molecules. We first analyze the transport properties of gold-C${}_{60}$-gold junctions and show that these junctions can be highly conductive (with conductances above $0.1G_0$, where $G_0=2e^2/h$ is the quantum of conductance). Moreover, we find that the thermopower in these junctions is negative due to the fact that the lowest unoccupied molecular orbital dominates the charge transport, and its magnitude can reach several tens of microvolts per kelvin, depending on the contact geometry. On the other hand, we study the suitability of C${}_{60}$ as an anchoring group in single-molecule junctions. For this purpose, we analyze the transport through several dumbbell derivatives using C${}_{60}$ as anchors, and we compare the results with those obtained with thiol and amine groups. Our results show that the conductance of C${}_{60}$-terminated molecules is rather sensitive to the binding geometry. Moreover, the conductance of the molecules is typically reduced by the presence of the C${}_{60}$ anchors, which in turn makes the junctions more sensitive to the functionalization of the molecular core with appropriate side groups.

Figure 1

(a) Two ideal Au-C${}_{60}$-Au junctions with top (left) and hollow (right) binding geometries. (b) The transmission as a function of energy for the two geometries in (a). (c, d) The solid line corresponds to the total transmission, while the others correspond to the contribution of the individual transmission coefficients as a function of energy.

Figure 2

(a) Some representative geometries of the stretching simulation of a Au-C${}_{60}$-Au junction. They correspond to Au-Au distances (distance between the Au tips) of 4.2, 10.2, and 17.5 Å. (b) Binding energy of the junction, (c) charge on the C${}_{60}$ molecule, (d) conductance of the Au-C${}_{60}$-Au junction (circles) and, for comparison, conductance of a Au-Au junction with the same distance separation (squares), and (e) thermopower at room temperature during the stretching process.

Figure 3

Transmission as a function of energy for the Au-C${}_{60}$-Au junctions shown in Fig. 2a. The different curves correspond to different elongation stages, as indicated in the legend ($\Delta z$ refers to the Au-Au distance).

Figure 4

(a) HOMO and (b, c) twofold degenerate LUMO of BDC60 in the gas phase.

Figure 5

Total transmission and individual transmission coefficients as a function of energy for the two Au-BDC60-Au junction geometries shown in the upper part (top and hollow positions).

Figure 6

Geometries of the studied molecular junctions with a phenyl ring functionalized with chlorine and attached to the gold electrodes via (a) thiol, (b) amine, and (c) C${}_{60}$ groups.

Figure 7

From top to bottom: Transmission curves for molecules anchored via thiol, amine, and C${}_{60}$ groups, and conductance values for all the junctions.