Controlled Contact to a ${\mathrm{C}}_{60}$ Molecule

The tip of a low-temperature scanning tunneling microscope is approached towards a ${\mathrm{C}}_{60}$ molecule adsorbed at a pentagon-hexagon bond on Cu(100) to form a tip-molecule contact. The conductance rapidly increases to $\approx 0.25$ conductance quanta in the transition region from tunneling to contact. Ab-initio calculations within density functional theory and nonequilibrium Green’s function techniques explain the experimental data in terms of the conductance of an essentially undeformed ${\mathrm{C}}_{60}$. The conductance in the transition region is affected by structural fluctuations which modulate the tip-molecule distance.

Figure 1

Pseudo-three-dimensional representation of a constant-current STM image of $\mathrm{Cu}(100)\mathrm{\text{−}}{\mathrm{C}}_{60}$ at 8 K. (Sample voltage $V=1.7\mathrm{V}$, tunneling current $I=1\mathrm{nA}$, scan size $49\AA \times 49\AA$). A dashed circle indicates the ${\mathrm{C}}_{60}$ orientation on which we performed the conductance measurements.

Figure 2

Conductance $G$ in units of $G_0$ vs tip displacement $\Delta z$. Data are an average of 500 measurements. Zero displacement corresponds to the tip position before freezing the feedback loop at $V=300\mathrm{mV}$ and $I=3\mathrm{nA}$. Experimental data appear as a line due to the high data point density, calculated data are depicted as squares. Upper inset: setup for calculations. Lower inset: single conductance curve revealing a discontinuity at $\Delta z\approx -3.3\AA$.

Figure 3

(a) Calculated total energy differences vs tip displacement $\Delta z$ in the transition region from tunneling to contact. The data points (○) fall on one of two straight lines corresponding to either a tunneling (smaller slope) or a contact (larger slope) configuration. (b) Experimental (●) and theoretical (squares and dashed lines) conductance data in the tunneling-contact transition regime. Thin and thick dashed lines represent the theoretical conductance corresponding to a thermal average for a fluctuation between tunneling and contact configurations with $T=8\mathrm{K}$ and $T=400\mathrm{K}$, respectively (see text). (c) Ratio of the standard deviation $\Delta G$ over the mean conductance $⟨G⟩$ evaluated over 500 conductance curves within the tunneling-contact transition regime. Full line: calculated data for an effective temperature of 400 K (divided by 200 to be plotted on the same axis as experimental data).