Tuning the electrical conductance of metalloporphyrin supramolecular wires

Abstract

In contrast with conventional single-molecule junctions, in which the current flows parallel to the long axis or plane of a molecule, we investigate the transport properties of M(II)-5,15-diphenylporphyrin (M-DPP) single-molecule junctions (M=Co, Ni, Cu, or Zn divalent metal ions), in which the current flows perpendicular to the plane of the porphyrin. Novel STM-based conductance measurements combined with quantum transport calculations demonstrate that current-perpendicular-to-the-plane (CPP) junctions have three-orders-of-magnitude higher electrical conductances than their current-in-plane (CIP) counterparts, ranging from 2.10^-2 G₀ for Ni-DPP up to 8.10^-2 G₀ for Zn-DPP. The metal ion in the center of the DPP skeletons is strongly coordinated with the nitrogens of the pyridyl coated electrodes, with a binding energy that is sensitive to the choice of metal ion. We find that the binding energies of Zn-DPP and Co-DPP are significantly higher than those of Ni-DPP and Cu-DPP. Therefore when combined with its higher conductance, we identify Zn-DPP as the favoured candidate for high-conductance CPP single-molecule devices.

Figure 1

(a) Porphyrin skeleton aligned parallel to the direction of charge transport “current in plane” (CIP) up-right configuration and (b) the optimised sandwich configuration of DPP junction with the current perpendicular to the plane (CPP).

Figure 2

(a,b,c and d) show the semi-log conductance histograms for the experimental STM single-molecule transport experiment for the Co-DPP, Zn-DPP, Cu-DPP and Ni-DPP systems, respectively. The inset shows representative single current decay curves used to build the conductance histograms. The applied BIAS was set to +25 mV. The sharp increase in counts in both left and right sides of the histograms correspond to the current amplifier baseline and saturation respectively.

Figure 3. Scheme of contact of pyridine anchor above the porphyrin molecule.

The lower PY nitrogen is a distance d from the metal atoms, while the the upper PY nitrogen is placed a distance 4.6 Å above the lower PY nitrogen.

Figure 4

The total transmission coefficient as a function of energy for (a) Zn-DPP, (b) Cu-DDP, (c) Co-DPP and (d) Ni-DDP. Each PY-porphyrin is in its relaxed configuration, with the metal atom a distance d from the N of the lower PY. The upper PY-functionalised gold electrode was then positioned such that distance between the upper and lower PY nitrogens was fixed at 4.6 Å.

Figure 5. The calculated room-temperature electrical conductances for Zn-DPP, Co-DPP, Cu-DPP and Ni-DPP, obtained from Fig. 4.

(b) Comparison between experimental (orange circles) and theoretical conductances (blue circles) obtained by choosing an optimum values of E_F − E_F^DFT = −0.03 eV. The error bars in the experimental points (orange circles) represent the full width at half maximum from the corresponding conductance histogram peak in Fig. 2, which were obtained from the accumulation of hundreds of individual traces for every system.