Cooperative Modulation of Electronic Structures of Aromatic Molecules Coupled to Multiple Metal Contacts

We use cryogenic scanning tunneling microscopy and spectroscopy and density-functional theory calculations to inspect the modulation of electronic states of aromatic molecules. The molecules are self-assembled on a Cu(111) surface forming molecular networks in which the molecules are in different contact configurations, including laterally coupled to different numbers of coordination bonds and vertically adsorbed at different heights above the substrate. We quantitatively analyze the molecular states and find that a delocalized empty molecular state is modulated by these multiple contacts in a cooperative manner: its energy is down shifted by $\sim 0.16\mathrm{eV}$ for each additional lateral contact and by $\sim 0.1\mathrm{eV}$ as the vertical molecule-surface distance is reduced by 0.1 Å in the physisorption regime. We also report that in a molecule-metal-molecule system the bridging metal can mediate the electronic states of the two molecules.

Figure 1

(a) STM topograph of TPyB-Cu coordination networks on the Cu(111) surface ($26\times 15{\mathrm{nm}}^2$, $-0.1\mathrm{V}$ and 0.2 nA). The unit cells of the networks are shown as dashed rhombuses. (b)–(c) Top and side views of the DFT optimized structures of phase I and phase II, respectively.

Figure 2

Experimentally measured typical STS (a) and DFT calculated PDOS (Fermi level is aligned with respect to the vacuum level of a six-layer slab system) (b) at the central benzene of $M$ (noncoordinated), $A_2$, $A_3$, and $B_3$ molecules, respectively. The curves are shifted vertically for clarity. (c) STS and DFT simulated PDOS maps of $A_3$ at indicated bias voltages and energies ($4\times 4{\mathrm{nm}}^2$, 0.4 nA); the dashed outline marks one unit cell of phase I.

Figure 3

(a) Upper panel: STM topograph showing a $C_3{}^H$ molecule within a phase-II network ($8\times 8{\mathrm{nm}}^2$, $-0.1\mathrm{V}$ and 0.2 nA); Lower panel: Profile along line shown in upper panel. (b) Top and side views of the DFT calculated metastable structure of phase II. (c) Experimentally measured STS at the central benzene of $C_3{}^H$ and $C_3$ molecules. The curves are shifted vertically for clarity. (d) Experimentally measured energy levels of the delocalized molecular state for molecules of different configurations (square for $M$, circle for $A$, triangle for $B$, and diamond for $C$). (Note that the measured peak positions of different molecules of the same type fall in a range as indicated by the multiple data points.) The sloping lines are plotted to guide the eye.

Figure 4

(a) STS acquired at the centers of a $B_3$ molecule and its surrounding three $C_3$ molecules two normal $C_3$ (I&II) molecules and one $C_3{}^H$ (III) molecule, shown and identified in the inset STM topograph ($4\times 4{\mathrm{nm}}^2$, 1.0 V, and 0.2 nA). (b) As (a) but molecule $C_3$ (II) has converted to a high geometry: $C_3{}^H$ (II). The curves are shifted vertically for clarity.