Conductance of a Freestanding Conjugated Molecular Wire

A freestanding molecular wire is placed vertically on Au(111) using a platform molecule and contacted by a scanning tunneling microscope. Despite the simplicity of the single-molecule junction, its conductance $G$ reproducibly varies in a complex manner with the electrode separation. Transport calculations show that $G$ is controlled by a deformation of the molecule, a symmetry mismatch between the tip and molecule orbitals, and the breaking of a $\mathrm{C}\equiv \mathrm{C}$ triple in favor of a $\mathrm{Au}─\mathrm{C}─\mathrm{C}$ bond. This tip-controlled reversible bond formation or rupture alters the electronic spectrum of the junction and the states accessible for transport, resulting in an order of magnitude variation of the conductance.

Figure 1

(a) Lewis structure of propynyl-trioxatriangulenium (P-TOTA). (b) Constant-current STM topograph of a single P-TOTA ($V=100\mathrm{mV}$, $I=30\mathrm{pA}$). (c) Experimental apparent height profile along the line indicated in (b) and the corresponding theoretical profile scaled by an arbitrary factor of 0.5 [16]. (d) Experimental and theoretical conductance curves $G(\mathrm{\Delta }z)$ of P-TOTA. The experimental conductance curve consists of 40 forward and 40 backward traces. Zero tip displacement is defined for the experimental curve by feedback loop parameters of 19 pA and 100 mV, whereas for the theoretical data points it corresponds to a distance $z$ of 13.1 Å between apex atom and Au surface. ($\mathbf{1}$–$\mathbf{5}$) Calculated junction geometries corresponding to $\mathrm{\Delta }z=-1.3$, $-2.1$, $-3.1$, $-4.1$, $-5.1\AA$ as indicated in panel (d).

Figure 2

(a) Calculated transmission functions vs energy for configurations $\mathbf{2}$–$\mathbf{4}$ (solid lines). Vertical gray bars indicate resonances related to the HOMO and LUMO states of an isolated P-TOTA. For $\mathbf{2}$ and $\mathbf{4}$ the transmission functions were also calculated by projection onto MPSH eigenstates (Figs. S4 and S5 [16]), that dominate at $E_F$, namely, the HOMO for $\mathbf{2}$ and LUMO for 4 (dashed lines) [21]. (b)–(d) Isosurface plots of the real part of the most transmitting eigenchannel scattering states of $\mathbf{2}$–$\mathbf{4}$ at $E_F$. Electron waves are incoming from the tip electrode at the $\overline{\mathrm{\Gamma }}$ point [25, 26, 27].

Figure 3

(a) Energy change of the tip-molecule subsystem versus tip displacement $\mathrm{\Delta }z$ with data highlighted corresponding to configurations $\mathbf{2}$–$\mathbf{5}$ from Fig. 1. (b)–(c) Electron density redistributions for the tip and the molecule as a result of the tip-molecule interaction before and after bond formation. Blue (red) colored isosurfaces correspond to depletion (accumulation) of electron density.