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 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.
Highlights
Metal phthalocyanines (MePc) represent a well-known class of organic semiconductors with good thermal and chemical stability
We show that: (i) in comparison with experimental literature data, quantitatively accurate adsorption geometries can be obtained by applying the PBE+vdWsurf method; (ii) with these geometries at hand, we can apply a standard GGA functional to calculate work-function changes in excellent agreement with experimental values determined by our ultraviolet photoelectron spectroscopy (UPS) experiments; (iii) comparing our GGA-density functional theory (DFT) calculations with the UPS measurements in more detail, we find semi-quantitative agreement between the calculated Kohn-Sham eigenvalue spectrum and some of the features in the UPS spectrum, with the agreement strongly improving upon applying a hybrid functional
ZnPc on noble metal surfaces typically obtained after annealing, which have been measured by scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED) [50,51,52]
Summary
Metal phthalocyanines (MePc) represent a well-known class of organic semiconductors with good thermal and chemical stability. A particular challenge when calculating the structure of such MePc/metal interfaces is that conventional DFT methods (using exchange-correlation functionals in the generalized gradient approximation (GGA) or hybrid functionals including a fraction of Fock exchange) do not properly capture long-range van der Waals (vdW) interactions [30,31]. These are, absolutely crucial for describing bonding in weakly interacting systems and can dominate metal-molecule interactions even for interfaces at which massive charge rearrangements between substrate and adsorbate occur. We speculate that this observation is due to a fortuitous cancellation of errors in the simulations, as will be discussed in more detail below
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