Demonstrating the Impact of the Adsorbate Orientation on the Charge Transfer at Organic–Metal Interfaces

Charge-transfer processes at molecule–metal interfaces play a key role in tuning the charge injection properties in organic-based devices and thus, ultimately, the device performance. Here, the metal’s work function and the adsorbate’s electron affinity are the key factors that govern the electron transfer at the organic/metal interface. In our combined experimental and theoretical work, we demonstrate that the adsorbate’s orientation may also be decisive for the charge transfer. By thermal cycloreversion of diheptacene isomers, we manage to produce highly oriented monolayers of the rodlike, electron-acceptor molecule heptacene on a Cu(110) surface with molecules oriented either along or perpendicular to the close-packed metal rows. This is confirmed by scanning tunneling microscopy (STM) images as well as by angle-resolved ultraviolet photoemission spectroscopy (ARUPS). By utilizing photoemission tomography momentum maps, we show that the lowest unoccupied molecular orbital (LUMO) is fully occupied and also, the LUMO + 1 gets significantly filled when heptacene is oriented along the Cu rows. Conversely, for perpendicularly aligned heptacene, the molecular energy levels are shifted significantly toward the Fermi energy, preventing charge transfer to the LUMO + 1. These findings are fully confirmed by our density functional calculations and demonstrate the possibility to tune the charge transfer and level alignment at organic–metal interfaces through the adjustable molecular alignment.


Supporting Information ARUPS polarization factor
Within the framework of photoemission tomography and the plane wave final state approximation, momentum maps can be understood as the product of the molecular orbital's Fourier transform, and a weakly angle-dependent polarization factor, | A · k| 2 . The latter depends on the direction and polarization of the incident light described by A and the electron's emission direction described by its wave vector k. In our experiments, the He discharge light is directed onto the surface at an angle of 22 • between light and surface parallel level. For the measurements in this work, the light is cast on the surfaces along the y axis in positive direction. It consists of s-polarized (69.4%) and p-polarized (30.6%) components leading to a polarization factor shown in Figure S1. 1 As a result of this | A · k| 2 term, features parallel to the [001] direction (horizontal direction in Figure S1) are slightly enhanced. It should that the | A · k| 2 reasonably well accounts for the directivity of emissions from molecules. Thus molecular emissions yield higher signal intensity on the upper half of momentum maps.
Due to the limitations of the plane wave final state approximation for describing emissions S2 from metallic states, however, metal emissions are typically higher on the lower half of the momentum maps.

Adsorption geometry
The strong interaction between heptacene and Cu leads to geometric distortions of the molecule. Figure S4 shows a high-resolution STM image of the heptacene monolayer together with the apparent height profile over the length of one molecule. This suggests a bend of heptacene of ∼0.4Å. A bend of this magnitude is also seen in the DFT calculations in Figure S4, where the carbon backbones for four adsorption configurations are shown. It should also be emphasized, that the most favoured calculated adsorption site, "[110] hollow", which exhibits the largest charge transfer, is significantly closer to the surface by ∼0.2Å than all other configurations.

Surface characterization of liquid Nitrogen cooled heptacene film
We have investigated the heptacene film prepared under liquid nitrogen temperatures with STM and LEED. Compared to Figure 1, the STM image in Figure S5a shows an increasing amount of molecules oriented along the [001] direction. However, we were not able to obtain  Table S1). In total, we have thus analyzed the MOPDOS for five overlayer structures and four different adsorption configurations characterized by the adsorption sites, bridge or hollow, and the molecular orientations, [110] or [001], respectively. The results are summarized in Figure S6.
Here, the full MOPDOS curves for the HOMO-1, HOMO, LUMO and LUMO+1, re-  To verify the applicability of our projection scheme, we compare the calculated partial charge densities for our heptacene/Cu-interfaces in the energy range of the MOPDOS peaks of Figure 4 in the main manuscript to the charge densities of the freestanding molecular monolayers ( Figure S7). With the characteristic lobes of the HOMO, LUMO and LUMO+1 visually distinguishable and recognizeable, we find our MOPDOS analysis well suited for the frontier orbitals.
S9 Figure S7: Partial charge densities of the interfaces integrated over the energy range of 0.3 eV around the MOPDOS peaks shown in Figure 4 of the main manuscript, compared to the charge densities of the freestanding molecular monolayer.

Analysis of work function changes
The work function changes upon adsorption of 7A can be partitioned into several contributions according to the following equation Here, ∆Φ bend denotes the work function change due to a molecular dipole induced by geometrical changes of the molecule upon adsorption, ∆Φ CT represents the increase of the work function due to charge transfer from the surface to the molecule, and ∆Φ push-back reduces the work function as electrons are pushed back towards the surface by Pauli repulsion. The latter two effects can be summarized as the charge rearrangements due to electronic interactions between molecules and substrate and are combined in ∆Φ bond .
The work function change due to charge rearrangements can be obtained from an analysis of the plane-averaged charge density difference between the molecule-substrate interface and its organic and inorganic subsystems. Figure S8 depicts the total plane-averaged charge density (gray line) as well as the charge densities, red (blue) denoting electron accumulation (depletion), for four adsorption configurations. By solving the one-dimensional Poisson equation, the corresponding change in the electro-static potential can be obtained (green dashed line) leading to the potential step ∆Φ bond . The results are summarized in Table S2.
It is interesting to note that despite significantly different work functions of the four investigated adsorption configurations (compare Table 1 of main manuscript), at first sight, there is only little difference in their plane-averaged density differences ( Figure S8). To substantiate this unexpected finding, we further analyze the different contributions to the work function change introduced in Eq. 1. It is clear that the separation between the influence of charge transfer and push-back is somewhat ambiguous, in particular for strongly interacting systems with chemisorption character like the present one. Nevertheless , we want to elaborate on the range of the total work function changes presented in Table S2 as where ∆Φ CT is in eV and CT is in C/m 2 . The average distance between the molecule and the surface is taken for d, 0 is the vacuum permittivity and r represents the estimated dielectric constant of heptacene. 6 We assume the remaining contribution to the total work function change to be ∆Φ push-back . Again, it has to be stressed that absolute numbers have to be taken with care considering the approximations in GGA/DFT and post-processing.
Note that the actual r of a heptacene monolayer will most likely be smaller than the bulk value and, thus, our approximated contributions of ∆Φ CT represent lower limits. However, the values in Table S2 follow an expected trend as adsorbates closer to the surface experience stronger charge transfer and at the same time are able to repel more electron density back to the surface. In comparison for the system Xe/Cu(110), a work function change of 0.6 eV was measured and, due to the noble gas character of Xe, solely attributed to the push-back effect. 7 There, the adsorption height is 1Å larger than for 7A/Cu(110). 8