Long-Range Repulsive Interaction between Molecules on a Metal Surface Induced by Charge Transfer

The adsorption of a molecular electron donor on Au(111) is characterized by the spontaneous formation of a superlattice of monomers spaced several nanometers apart. The coverage-dependent molecular pair distributions obtained from scanning tunneling microscopy data reveal an intermolecular long-range repulsive potential, which decreases as the inverse of the molecular separation. Density functional theory calculations show a charge accumulation in the molecules due to electron donation into the metal. Our results suggest that electrostatic repulsion between molecules persists on the surface of a metal.

Figure 1

(a) STM image of an Au(111) region with 0.03 ML of TTF deposited at room temperature. (b) TTF molecules deposited on a cold sample (80 K). (c) After annealing to room temperature, the TTF arrays along fcc regions are formed. (d) STM image (inset; $Vs=-1\mathrm{V}$) and its Laplace filtered image [30] of a TTF molecule. The latter reveals that two of the sulfur atoms appear brighter, suggesting a small tilt of the molecular plane with respect to the surface.

Figure 2

(a)–(c) STM images of TTF on Au(111) at various coverages. (d) Pair distributions $f$ of the one-dimensional TTF arrays for the data shown in Fig. 1 (0.03 ML) and (a)–(c). For 0.08 and 0.16 ML, the distributions are performed on hcp regions. More than 500 pairs are analyzed in each plot. The molecular coverage is determined from STM images of large surface areas, assuming that 1 ML corresponds to $2\mathrm{\text{molecules}}/{\mathrm{nm}}^2$. From the lowest to the largest coverage, we obtain an average pair distance $\overline{r}$ of 3.5, 2.5, 3.3, and 1.7 nm in the one-dimensional arrays. The corresponding 1D distribution functions for noninteracting particles $f_{\mathrm{ran}}$ are included.

Figure 3

Results from DFT simulations. (a) Fully relaxed configuration of TTF on Au(111). The uppermost two gold layers as well as the molecular degrees of freedom are relaxed until atomic forces are lower than $0.01\mathrm{eV}/\AA$. (b) Tersoff-Hamman constant current image [25] of the molecule in (a) ($V=-0.5\mathrm{V}$). (c) Induced electronic density by the molecule-surface interaction. (d) Lateral ($x\mathrm{\text{−}}y$ planes) integration of the induced charge. The arrows show the vertical distance values at which the two topmost surface layers and the two binding S atoms lie. (e) Accumulated induced dipole. Together with (d), it reveals that the molecule becomes positively charged. (f) Projected density of states on molecular orbitals. The electronic states with HOMO character are partially empty, in agreement with the data of (c)–(e).

Figure 4

Mean interaction potentials $\omega (r)$ of one-dimensional TTF arrays obtained from the pair distributions shown in Fig. 2d. The dashed line represents the pair electrostatic interaction $E(r)$ between particles charged with $0.3e$ and a temperature ($T=160\mathrm{K}$) to fit the repulsive part of $\omega (r)$ for the most dilute case. Each curve has been shifted upwards an amount (8.4, 5.4, 4.1, 3.8, from top to bottom) representing the coverage-dependent zeroth order internal potential and approximated here as the electrostatic energy per molecule in a fully periodic lattice and using the fitted temperature, for consistency. Note that the range of the interaction energy obtained from this fitting (30–100 meV) is considerably larger than the energy of a long-range interaction mediated by surface electrons [8].