Electronic charge rearrangement at metal/organic interfaces induced by weak van der Waals interactions

Electronic charge rearrangements at interfaces between organic molecules and solid surfaces play a key role in a wide range of applications in catalysis, light-emitting diodes, single-molecule junctions, molecular sensors and switches, and photovoltaics. It is common to utilize electrostatics and Pauli pushback to control the interface electronic properties, while the ubiquitous van der Waals (vdW) interactions are often considered to have a negligible direct contribution (beyond the obvious structural relaxation). Here, we apply a fully self-consistent Tkatchenko-Scheffler vdW density functional to demonstrate that the weak vdW interactions can induce sizable charge rearrangements at hybrid metal/organic systems (HMOS). The complex vdW correlation potential smears out the interfacial electronic density, thereby reducing the charge transfer in HMOS, changes the interface work functions by up to 0.2 eV, and increases the interface dipole moment by up to 0.3 Debye. Our results suggest that vdW interactions should be considered as an additional control parameter in the design of hybrid interfaces with the desired electronic properties.

Figure 1

2D slices of vdW-induced electron density $\left[\mathrm{\Delta }n{\left(\mathbf{r}\right)}_{\mathrm{vdW}}\right]$. The regions of electron density accumulation are in blue, while red indicates depletion. (a) Top view of CuPc on Ag(111). The limiting values are set to $\pm 5.0\times {10}^{-4}\mathrm{e}/{\AA }^3$. (b) Top view of CuPc on Ag(100). The limiting values are set to $\pm 4.0\times {10}^{-4}\mathrm{e}/{\AA }^3$. The CuPc molecule is sketched as a guide to the eye. In both figures the inclusion of vdW interactions produce large electron density rearrangements. The differences in the accumulation/depletion regions between the two isosurfaces stem from the fact that the molecule adsorbs with different configurations, depending on the type of surface employed.

Figure 2

DIP on Ag(111). Left: Top and side view of the unit cell, the red dashed line indicates the 2D plane used to cut a 2D slice of vdW-induced electron density $\left[\mathrm{\Delta }n{\left(\mathbf{r}\right)}_{\mathrm{vdW}}\right]$. Center: The 2D isosurface of $\mathrm{\Delta }n{\left(\mathbf{r}\right)}_{\mathrm{vdW}}$ displays the vdW effect on the electron density distribution, accumulation is in blue, depletion is in red. The limiting values are set to $\pm 2.0\times {10}^{-4}\mathrm{e}/{\AA }^3$. The profile of the DIP molecule is sketched as a guide to the eye. Right: The integral of $\mathrm{\Delta }n{\left(\mathbf{r}\right)}_{\mathrm{vdW}}$ is plotted as a function of $z$, the axis perpendicular to the surface. A dipolelike density redistribution emerges at the interface. The horizontal dotted lines indicate the monolayer plane and the four metal layers; this plot is aligned with the figure shown in the central panel.

Figure 3

DIP on Ag(111). Charge displaced during molecular adsorption $\left[Q\right(z\left)\right]$ computed with $\mathrm{PBE}+{\mathrm{vdW}}_{\mathrm{sc}}^{\mathrm{surf}}$ (solid line) and PBE (dashed line). The dotted vertical line refers to the DIP monolayer. The figure shows that the inclusion of vdW effects enhances the accumulation of $Q\left(z\right)$ at the interface. This contribution is reflected into a larger change of potential energy ($\mathrm{\Delta }{E}_{\mathrm{bond}}$), which ultimately determines the value of the work-function shift.

Figure 4

Adsorption-induced work-function modifications ($\mathrm{\Delta }\mathrm{\Phi }$) associated with CuPc, benzene, DIP, and PTCDA on Ag(111). The results of PBE and $\mathrm{PBE}+{\mathrm{vdW}}_{\mathrm{sc}}^{\mathrm{surf}}$ are compared with experiments. The reduction of $\mathrm{\Delta }\mathrm{\Phi }$ due to SC vdW effects is indicated with an arrow starting from the PBE value. The configurations of the four HMOS are drawn in the bottom half of the figure.