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

Abstract

Abstract: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 OPEN ACCESSMolecules 2014, 19 2970 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.

Energy level alignment at molecule-metal interfaces from an optimally tuned range-separated hybrid functional

“…However, PBE and OT-RSH yield essentially the same work function change of 0.3 eV and the same work function of 3.8 eV, suggesting that our new OT-RSH scheme leads to results on par with LDA/GGA for predicting accurate work functions and interface dipoles. [53][54][55][56][57][58][59] We note that the standard, non-self-consistent DFT + Σ yields a HOMO resonance within 0.1 eV difference from the OT-RSH result, as expected for such a weakly coupled interface. We also note that when the vdW-DF2 functional 117 is used to relax the coordinates of the molecule and the top layer of Al (111), the benzene molecule is found at about 3.5 Å above the Al(111) surface, which results in a level alignment of 4.2 eV based on the OT-RSH scheme.…”

“…In addition, since it is based on GKS and includes a fraction of short-range Fock exchange, it can also yield improved relative orbital spacing, as shown in the examples of AntSe/Au(111) and NTCDA/Ag(111), as well as improved orbital ordering, as was demonstrated previously for other systems. 58,85,92,[145][146][147] In spite of its success, we also would like to point out that the method proposed here is not a panacea. In particular, we focused here on the level alignment and PDOS of the molecule and did not discuss how the choice of α, β, and γ may affect specifics in the electronic structure of the metal substrate.…”

“…Results from calculations using the local density approximation (LDA) 40 and typical generalized gradient approximations (GGAs) in the KS DFT scheme, while often accurate for total energy or charge density related properties of moleculemetal interfaces such as work-function changes, [53][54][55][56][57][58][59] typically strongly underestimate the band gap of molecules and semiconductors. 21 We illustrate this with grey dashed lines in Fig.…”

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

“…Nevertheless, these systems are particularly challenging for standard DFT functionals. In fact, the metal-molecule interfaces are weakly bounded and the vdW dispersion interactions are of primary importance for a correct description of the geometries [14].…”

“…In this context, the long-range van der Waals (vdW) interactions-while absent in standard DFT exchangecorrelation (XC) functionals-have been proven of fundamental importance in determining the structure and the cohesive energies of layered systems, such as molecules adsorbed on metal surfaces [3,[7][8][9][10][11][12][13][14]. On the other hand, the long-range vdW energy represents only a small fraction (0.001%) of the total electronic energy for a wide range of systems [15], from small dimers (e.g., diatomic dimers, water dimer) to complex hybrid inorganic/organic systems, passing through large molecular systems and metal surfaces.…”