Importance of Van Der Waals Interaction for Organic Molecule-Metal Junctions: Adsorption of Thiophene on Cu(110) as a Prototype

We report ab initio calculations for the interface energetics of a weakly adsorbed organic molecule on a metal surface, which serves as a model interface relevant for organic electronics. The studied thiophene ring is found to be physisorbed on the Cu(110) surface with an adsorption energy of $-0.50\mathrm{eV}$. Nonlocal correlations, i.e., van der Waals interactions, are solely responsible for the binding in this weakly interacting system, and the choice of the proper exchange-correlation function is crucially important. The adsorption of thiophene lowers the metal work function due to the formation of surface dipoles while no sizable charge transfer is found.

Figure 1

Side view (left) and top view (right) of the adsorption geometry of a thiophene ring on a Cu(110) surface.

Figure 2

Top: 1D electrostatic potential, total electron density, and charge density difference (dashed line) across the slab. The difference density is multiplied by a factor of 100, and the total electron density is divided by 2. The Fermi level ($E_F$), the work functions on both sides of the slab, as well as the position of the highest molecular orbital are indicated. The density of states is shown in the right panel, where the highest occupied molecular orbital and the lowest unoccupied molecular orbital are indicated by the dashed lines. Bottom: Charge density difference ${\rho }_{\mathrm{diff}}$ with an isovalue of $\pm 0.0015\mathrm{a}.\mathrm{u}.$ at the equilibrium adsorption position. Dark (light) regions denote charge accumulation (depletion) compared to the noninteracting system.

Figure 3

Calculated adsorption energy of a thiophene ring on the Cu(110) surface as a function of the shortest Cu-S distance for various exchange-correlation potentials (see text).