Understanding the Adsorption of CuPc and ZnPc on Noble Metal Surfaces by Combining Quantum-Mechanical Modelling and Photoelectron Spectroscopy

Phthalocyanines are an important class of organic semiconductors and, thus, their interfaces with metals are both of fundamental and practical relevance. In the present contribution we provide a combined theoretical and experimental study, in which we show that state-of-the-art quantum-mechanical simulations are nowadays capable of treating most properties of such interfaces in a quantitatively reliable manner. This is shown for Cu-phthalocyanine (CuPc) and Zn-phthalocyanine (ZnPc) on Au(111) and Ag(111) surfaces. Using a recently developed approach for efficiently treating van der Waals (vdW) interactions at metal/organic interfaces, we calculate adsorption geometries in excellent agreement with experiments. With these geometries available, we are then able to accurately describe the interfacial electronic structure arising from molecular adsorption. We find that bonding is dominated by vdW forces for all studied interfaces. Concomitantly, charge rearrangements on Au(111) are exclusively due to Pauli pushback. On Ag(111), we additionally observe charge transfer from the metal to one of the spin-channels associated with the lowest unoccupied π-states of the molecules. Comparing the interfacial density of states with our ultraviolet photoelectron spectroscopy (UPS) experiments, we find that the use of a hybrid functionals is necessary to obtain the correct order of the electronic states.


Used vdW Coefficients
PBE-vdW surf needs reference C 6 , R and α coefficients for every atomic species (where due to the different screenings one needs to distinguish between atoms that are part of the molecules and part of the substrate. Correspondingly, the necessary coefficients are taken from [1] or [2]).

Used PAW Potentials
In the calculations we applied Projector Augmented Wave (PAW) potentials [3,4]. For the calculations using VASP 5.3.3 a new set of potentials released in 2012 (PBE 5.2) was used. Note that for the organic part, soft PAW potentials were applied in all calculations:

Tests Regarding the Convergence of the HSE Calculations for CuPc on Ag(111)
As discussed in the main manuscript, we find for CuPc on Ag(111) that the obtained magnetic moment per unit cell is significantly larger than the expected µ B . This is a consequence of states from only one spin channel being occupied upon electron transfer from the Ag substrate to the adsorbate Molecules 2014, 19 S2 layer (cf., Figure 6 of the main manuscript). To ensure that this is not an artifact of the convergence process, we performed several tests. Unfortunately, the first set of tests was done with a slightly too large unit cell (with 33 instead of 30 Ag surface atoms per unit cell). Bearing in mind the considerable cost of the HSE calculations, considering that consistent results were obtained for both unit cells, we refrained from redoing all tests with the reduced size unit cell.
1st test: starting the HSE calculation by reading in the WAVECAR (wave functions) and CHGCAR (charge density) files obtained from a PBE-calculation on the same system.  Figure S1. HSE calculated spin-up and spin-down densities of states for the CuPc/Ag(111) interface projected onto the CuPc monolayer calculated with the settings described in above. Note that in all these tests a slightly different unit cell was used compared to the calculations reported in the main manuscript (vide supra). The simulation denoted as b) is the one performed with settings identical to the ones used for the smaller unit cell in the main paper.

S4
We also performed tests using different broadenings for the DOS, respectively, occupation function, as we found an unexpected dependence of μ B on the chosen "smearing-parameter" SIGMA. We applied a Methfessel-Paxton order 1 smearing [5] with SIGMA determining the width of the smearing in eV. This second set of tests was done using the correct unit cell containing 30 surface atoms (i.e., the unit cell chosen also for all calculations reported in the main manuscript). We found that the asymmetric occupation of the spin channels prevails independent of the value of SIGMA. The z-component of the magnetic moment per unit cell, µ z , however, decreased with decreasing SIGMA. A more detailed analysis showed that this decrease in µ z had nothing to do with a different magnetization of the adsorbate layer, but was a consequence of a magnetization of the Ag substrate at small values of SIGMA that counteracted the extra moment of the adsorbate layer. This observation can have two origins. In principle, when decreasing the smearing, the number of k-points in the calculations ought to be increased. Bearing in mind the significant extent of the unit cell and the use of hybrid functionals, this, however turned out to be not feasible in the present case. This being said, we have, however, seen for Au 13 clusters with highly degenerate frontier orbitals that a non-zero spin of the cluster consistent with Hund's rule could only be obtained when considering energetically "sharp" states and very low Fermi-level smearing [6]. I.e., a spin-polarization of the Ag-slab might be missed for large values of SIGMA. Independent of which of these explanations applies, the main result of the calculations in the context of "magnetic effects", namely that the charge transfer to the CuPc monolayer from the Ag substrate is spin-polarized, prevails independent of the choice of SIGMA. Interestingly, the plane-integrated charge rearrangements are virtually identical for HSE and PBE functionals (cf., Figure 5 from main manuscript). Also the situations for CuPc and ZnPc are very similar.

Local Densities of States for ZnPc on Au(111) and Ag(111)
To identify the nature of the peaks in the projected density of states reported for ZnPc on Ag(111) and Au(111), we calculated the local densities of states (LDOS) for the lowest occupied maxima closest to the Fermi energy. This was necessary to clarify, which of the features is associated with a metal, respectively, ligand centered state. For CuPc, where it is known that the metal-centered states are spin-polarized, a calculation of the LDOS was not necessary (cf., main text).