Noble Metal Surfaces Research Articles

Microscopic origin of the relativistic splitting of surface states

Spin-orbit splitting of surface states is analyzed within and beyond the Rashba model using as examples the (111) surfaces of noble metals, Ag2Bi surface alloy, and topological insulator Bi2Se3. The ab initio analysis of relativistic velocity proves the Rashba model to be fundamentally inapplicable to real crystals. The splitting is found to be primarily due to a spin-orbit induced in-plane modification of the wave function, namely to its effect on the nonrelativistic Hamiltonian. The usual Rashba splitting -- given by charge distribution asymmetry -- is an order of magnitude smaller.

Van der Waals effects at molecule-metal interfaces

We present the results of plane-wave pseudopotential periodic density functional theory (DFT) calculations on the geometries, energetics, and electronic structure of small molecules on Au(111). The chosen molecules---benzene, ammonia and cytosine---are representative of different adsorption regimes and interaction strengths. The chosen substrate is a prototype noble-metal surface that is widely employed as a support for organic materials. We assess the relevance of van der Waals effects in the adsorption process and the accuracy of different first-principle density functionals that have been recently developed to embody such effects. We find that there is no unique functional that is optimal for any system. In particular, our results reveal that functionals designed to reduce the short-term repulsion between the adsorbate and the substrate usually overestimate the adsorption strength and may even predict the wrong adsorption orientation. We show that an accurate description of the substrate does not ensure an accurate evaluation of the adsorption energetics, while the electronic structure is less sensitive to the specific choice. We propose the best choice for DFT calculations of DNA bases on Au(111) and similar systems in which both short-range and long-range interactions exist.

Weak competing interactions control assembly of strongly bonded TCNQ ionic acceptor molecules on silver surfaces

The energy scales of interactions that control molecular adsorption and assembly on surfaces can vary by several orders of magnitude, yet the importance of each contributing interaction is not apparent a priori. Tetracyanoquinodimethane (TCNQ) is an archetypal electron acceptor molecule and it is a key component of organic metals. On metal surfaces, this molecule also acts as an electron acceptor, producing negatively charged adsorbates. It is therefore rather intriguing to observe attractive molecular interactions in this system that were reported previously for copper and silver surfaces. Our experiments compared TCNQ adsorption on noble metal surfaces of Ag(100) and Ag(111). In both cases we found net attractive interactions down to the lowest coverage. However, the morphology of the assemblies was strikingly different, with two-dimensional islands on Ag(100) and one-dimensional chains on Ag(111) surfaces. This observation suggests that the registry effect governed by the molecular interaction with the underlying lattice potential is critical in determining the dimensionality of the molecular assembly. Using first-principles density functional calculations with a van der Waals correction scheme, we revealed that the strengths of major interactions (i.e., lattice potential corrugation, intermolecular attraction, and charge-transfer-induced repulsion) are all similar in energy. The van der Waals interactions, in particular, almost double the strength of attractive interactions, making the intermolecular potential comparable in strength to the diffusion potential and promoting self-assembly. However, it is the anisotropy of local intermolecular interactions that is primarily responsible for the difference in the topology of the molecular islands on Ag(100) and Ag(111) surfaces. We anticipate that the intermolecular potential will become more attractive and dominant over the diffusion potential with increasing molecular size, providing new design strategies for the structure and charge transfer within molecular layers.

One-dimensional surface phonon polaritons in boron nitride nanotubes.

Surface polaritons, which are electromagnetic waves coupled to material charge oscillations, have enabled applications in concentrating, guiding and harvesting optical energy below the diffraction limit. Surface plasmon polaritons involve oscillations of electrons and are accessible in noble metals at visible and near-infrared wavelengths, whereas surface phonon polaritons (SPhPs) rely on phonon resonances in polar materials, and are active in the mid-infrared. Noble metal surface plasmon polaritons have limited applications in the mid-infrared. SPhPs at flat interfaces normally possess long polariton wavelengths and provide modest field confinement/enhancement. Here we demonstrate propagating SPhPs in a one-dimensional material consisting of a boron nitride nanotube at mid-infrared wavelengths. The observed SPhP exhibits high field confinement and enhancement, and a very high effective index (neff~70). We show that the modal and propagation length characteristics of the SPhPs may be controlled through the nanotube size and the supporting substrates, enabling mid-infrared applications.

Invited: Hydrogen Oxidation and Evolution Reaction (HOR/HER) on Pt Electrodes in Acid vs. Alkaline Electrolytes: Mechanism, Activity and Particle Size Effects

Fuel cells and electrolyzers based on proton-exchange membranes (PEMs), operating at low pH (pH≈0), offer high power densities, but require large amounts of noble metal for the oxygen reduction reaction (ORR) in fuel cells and the oxygen evolution reaction (OER) in electrolyzers. For the hydrogen oxidation/evolution reaction (HOR/HER) only very small amounts of Pt is required due to its high activity for the HOR/HER.1 In alkaline electrolyte, non-noble metal catalysts are very active for the ORR and for the OER. Therefore, a replacement of the noble-metal intensive PEM technology by alkaline membrane technology seems promising. Unfortunately, for yet unclear reasons, the HOR/HER kinetics on Pt are much slower in alkaline than in acid electrolytes, and more active catalysts are needed to catalyze the HOR/HER in alkaline environment.2 It is therefore critical to elucidate the reasons for the poor HOR/HER activity of Pt in alkaline electrolytes, and this will be possible only by having a clear idea about the reaction mechanism and the rate determining step.Historically, the HOR/HER equilibrium has been described by invoking over potential deposited hydrogen, H-OPD, as the reactive intermediate species. 3 In contrast with adsorbed H intermediate present on noble metal surfaces at potentials positive to the reversible H+/H2 potentials in the process of the so-called underpotential-deposition (UPD), the believed reacting H-OPD species would form at potentials negative to the reversible H+/H2 potentials. So far, no clear evidence of the difference in the physical nature of H-OPD and H-UPD species has ever been reported. Recently, computational studies have addressed the HOR/HER rates by considering one of the three microscopic steps (Tafel, Volmer, Heyrovsky) as rate determining and only one state of adsorbed hydrogen species as reaction intermediate. 4 However, the link between this reacting adsorbed hydrogen intermediate and the H-UPD has never been explicitly stated.By comparing HOR/HER exchange current densities of carbon supported nanoparticles (Fig. 1) with the H-UPD charge transfer resistance (Fig. 2) which was determined by electrochemical impedance spectroscopy on polycrystalline surfaces (following refs.5 , 6) and, for the first time, on carbon supported nanoparticles, we will show that the H-UPD charge transfer rates closely match the HOR/HER exchange current densities in acid and alkaline electrolytes. Therefore, no H-OPD species has to be invoked to describe the two decades HOR/HER activity decrease from acid to base. This fundamental result gives credence to a HOR/HER rate controlled by the Volmer reaction.Moving away from the platinum system, iridium being an oxophilic surface is an interesting electrocatalyst system as it probes the hypothesis advanced by Strmcnik et al. that a more oxophilic surface might be effective in catalyzing the interaction with H2O/OH-, which they proposed to be the governing reaction in base. 7 However, our findings demonstrate explicitly that Ir does not have a higher activity than Pt in base (nor in acid). This leads to the conclusion that water dissociation is not the rate determining step in base, and, therefore, points toward identical microscopic HOR/HER reaction steps in acid and base.In conclusion, these novel insights confirm the H-binding energy as a relevant HOR/HER activity descriptor.8 Future studies will have now to address the pH effect on this binding energy and the synthesis of electrocatalysts with tuned H-binding energies.1. K. C. Neyerlin, W. Gu, J. Jorne and H. A. Gasteiger, J. Electrochem. Soc., 2007, 154, B631-B635.2. W. Sheng, H. A. Gasteiger and Y. Shao-Horn, J. Electrochem. Soc., 2010, 157, B1529-B1536.3. B. Conway and B. Tilak, Electrochim. Acta, 2002, 47, 3571-3594.4. E. Skúlason, V. Tripkovic, M. E. Björketun, S. Gudmundsdottir, G. Karlberg, J. Rossmeisl, T. Bligaard, H. Jónsson and J. K. Nørskov, J. Phys. Chem. C, 2010, 114, 18182-18197.5. K. Schouten, M. van der Niet and M. Koper, Phys. Chem. Chem. Phys., 2010, 12, 15217-15224.6. B. Łosiewicz, R. Jurczakowski and A. Lasia, Electrochim. Acta, 2012, 80, 292-301.7. D. Strmcnik, M. Uchimura, C. Wang, R. Subbaraman, N. Danilovic, D. Van der Vilet, A. P. Paulikas, V. Stamenkovic and N. Markovic, Nat. Chem., 2013, 5, 300-306.8. W. Sheng, M. Myint, J. G. Chen and Y. Yan, Energy Environ. Sci., 2013, 6, 1509-1512.

Development and STM Study of Hydrogen based Deposition of Pt on Pd Thin Films

The key technological challenge for fuel-cells commercialization is design of nanomaterials with low-content of Pt and Pt-group noble-metals exhibiting high activity, improved selectivity and stability. New Pt-bi and tri-metallic catalysts of different shape, composition as well as geometrical configurations have been actively explored in recent years.1 Among different methods for design of Pt films, the surface limited redox replacement (SLRR) has an important place.2 This method is unique because it enables epitaxial deposition of: Pt films, monolayers and clusters, onto other noble metal surfaces with the atomic scale control. This has been achieved by utilization of underpotentially deposited (UPD) metal layer as sacrificial, in the galvanic replacement by more noble Pt.Several research groups demonstrated successful SLRR deposition of epitaxial Pt films using Cu UPD and Pb UPD for the design of nanopraticles as well as 2D-model surfaces.3-6 Our recent work demonstrated that using the SLRR protocol, a H-UPD layer can be exploited for homoepitaxial grow of Pt. 7 We extend further the concept of H mediated growth utilizing SLRR approach, onto Pd-thin film system. Pt thin films on Au are well studied and well known for their unique H-sorption behavior, and catalytic properties.8 Moreover, recent studies on the Pt-on-Pd9 and Pt-Pd-Au10 showed that this system is of interest for its electrocatalysis as well as H- storage potential.11 Here in this talk we will present the electrochemical and the in-situ STM study of Pt electrodeposition in the SLRR configuration. Our substrates were epitaxial Pd films (2-10 ML) electrodeposited on Au(111) films evaporated on mica or glass.8 High quality of the deposited epitaxial Pd films was confirmed electrochemically and by the in-situ STM. Surface morphology evolution of Pt films was studied in solution and conducted in the environmentally isolated STM system using custom designed set up for the solutions exchange. Furthermore, the Electrochemical Quartz Crystal Microbalance study of the growth kinetics showed that Pt structure and thickness can be controlled by the amount of absorbed hydrogen, solution pH, and Pt concentration. Electrochemical behavior of Pt films on 2ML and 10 ML Pd-Au show distinctly different H-sorption characteristics. Films on 2ML Pd/Au have characteristics of typical Pt surface, while films on the Pt-10ML Pd/Au showed uninhibited H absorption with more reversible behavior than on corresponding 10ML Pd/Au substrate.

Anisotropic Fabry-Pérot resonant states confined within nano-steps on the topological insulator surface.

The peculiar nature of topological surface states, such as absence of backscattering, weak anti-localization, and quantum anomalous Hall effect, has been demonstrated mainly in bulk and film of topological insulator (TI), using surface sensitive probes and bulk transport probes. However, it is equally important and experimentally challenging to confine massless Dirac fermions with nano-steps on TI surfaces. This potential structure has similar ground with linearly-dispersed photons in Fabry-Pérot resonators, while reserving fundamental differences from well-studied Fabry-Pérot resonators and quantum corrals on noble metal surfaces. In this paper, we study the massless Dirac fermions confined within steps along the x (Γ–K) or y (Γ–M) direction on the TI surface, and the Fabry-Pérot-like resonances in the electronic local density of states (LDOS) between the steps are found. Due to the remarkable warping effect in the topological surface states, the LDOS confined in the step-well running along Γ-M direction exhibit anisotropic resonance patterns as compared to those in the step-well along Γ-K direction. The transmittance properties and spin orientation of Dirac fermion in both cases are also anisotropic in the presence of warping effect.

Catalytic reduction of aqueous nitrates by metal supported catalysts on Al particles

Abstract To facilitate the reduction of nitrate to nitrogen in water, a class of mono- or dual-catalyst particles was prepared by loading mono-metals (Cu, Ni) or bimetals (Pd–Cu, Pt–Cu, Au–Cu, and Ru–Cu) on Al particles. The prepared catalyst-loaded Al samples were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, and transmission electron microscopy analysis. Batch kinetic experiments indicate that 0.4%Pd–4%Cu/Al at pH 4.0 resulted in the highest selectivity for nitrogen (N2). Kinetic studies reveal that nitrate can be first reduced to nitrite on the Al and Cu surface at a relatively slow rate and subsequently was rapidly reduced to nitrogen on the noble metal surface and to ammonia on the Al and Cu surface. The selectivity for nitrogen formation can be influenced by the type of metals deposited and the concentration of intermediate nitrite and atomic hydrogen adsorbed on the Pd surface.

Angstrom-resolved real-time dissection of electrochemically active noble metal interfaces.

Electrochemical solid|liquid interfaces are critically important for technological applications and materials for energy storage, harvesting, and conversion. Yet, a real-time Angstrom-resolved visualization of dynamic processes at electrified solid|liquid interfaces has not been feasible. Here we report a unique real-time atomistic view into dynamic processes at electrochemically active metal interfaces using white light interferometry in an electrochemical surface forces apparatus. This method allows simultaneous deciphering of both sides of an electrochemical interface-the solution and the metal side-with microsecond resolution under dynamically evolving reactive conditions that are inherent to technological systems in operando. Quantitative in situ analysis of the potentiodynamic electrochemical oxidation/reduction of noble metal surfaces shows that Angstrom thick oxides formed on Au and Pt are high-ik materials; that is, they are metallic or highly defect-rich semiconductors, while Pd forms a low-ik oxide. In contrast, under potentiostatic growth conditions, all noble metal oxides exhibit a low-ik behavior. On the solution side, we reveal hitherto unknown strong electrochemical reaction forces, which are due to temporary charge imbalance in the electric double layer caused by depletion/generation of charged species. The real-time capability of our approach reveals significant time lags between electron transfer, oxide reduction/oxidation, and solution side reaction during a progressing electrode process. Comparing the kinetics of solution and metal side responses provides evidence that noble metal oxide reduction proceeds via a hydrogen adsorption and subsequent dissolution/redeposition mechanism. The presented approach may have important implications for designing emerging materials utilizing electrified interfaces and may apply to bioelectrochemical processes and signal transmission.

Substrate-controlled linking of molecular building blocks: Au(111) vs. Cu(111)

The coupling of dibromohexabenzocoronene (Br2-HBC) as a precursor molecule is investigated by scanning tunneling microscopy (STM) on two noble metal surfaces: Au(111) and Cu(111). It is found that the on-surface polymerization of molecular building blocks equipped with halogen atoms is strongly influenced by the choice of the substrate. While on Au(111) a heating step of up to 520K is required to activate the molecules and form polymers, on Cu(111) the catalytic reactivity causes activation already below room temperature. Due to the different substrates, the intramolecular bonds in the polymers between the HBC units differ: The HBC molecules are covalently coupled on Au(111) while on Cu(111) a copper adatom mediates the bonding. This effect is proven by the comparison with gas phase calculations and by lateral manipulation with the STM tip. The choice of the substrate thus does not only define the activation temperature but also lead to different bonding strengths between the molecular building blocks.

Strong correlation between surface stress and mechanical strain in Cu, Ag and Au

The surface stress, in addition to the surface energy and elastic modulus, is a fundamental measure of the surface effects in a nanostructure. Here, we investigate the strong correlation between the surface stress and mechanical strain in three noble-metal (001) surfaces. For this investigation, we have developed a simple and efficient method to calculate the surface stress directly from the strained configurations in molecular statics simulations. While the surface stress of the copper (001) surface along the strained direction is almost constant, that of the gold (001) surface decreases drastically as the strain increases. We explain these different responses of the surface stress to strain in terms of the different surface relaxation occurring in different noble-metal nanoplates.

Nickel-iron oxyhydroxide oxygen-evolution electrocatalysts: the role of intentional and incidental iron incorporation.

Fe plays a critical, but not yet understood, role in enhancing the activity of the Ni-based oxygen evolution reaction (OER) electrocatalysts. We report electrochemical, in situ electrical, photoelectron spectroscopy, and X-ray diffraction measurements on Ni(1-x)Fe(x)(OH)2/Ni(1-x)Fe(x)OOH thin films to investigate the changes in electronic properties, OER activity, and structure as a result of Fe inclusion. We developed a simple method for purification of KOH electrolyte that uses precipitated bulk Ni(OH)2 to absorb Fe impurities. Cyclic voltammetry on rigorously Fe-free Ni(OH)2/NiOOH reveals new Ni redox features and no significant OER current until >400 mV overpotential, different from previous reports which were likely affected by Fe impurities. We show through controlled crystallization that β-NiOOH is less active for OER than the disordered γ-NiOOH starting material and that previous reports of increased activity for β-NiOOH are due to incorporation of Fe-impurities during the crystallization process. Through-film in situ conductivity measurements show a >30-fold increase in film conductivity with Fe addition, but this change in conductivity is not sufficient to explain the observed changes in activity. Measurements of activity as a function of film thickness on Au and glassy carbon substrates are consistent with the hypothesis that Fe exerts a partial-charge-transfer activation effect on Ni, similar to that observed for noble-metal electrode surfaces. These results have significant implications for the design and study of Ni(1-x)Fe(x)OOH OER electrocatalysts, which are the fastest measured OER catalysts under basic conditions.

Adsorption height alignment at heteromolecular hybrid interfaces

The formation of metalorganic hybrid interfaces is determined by the fine balance between molecule-substrate and molecule-molecule interactions at the interface. Here, we report on a systematic investigation of interfaces between a metal surface and organic monolayer films that consist of two different molecular species, one donor and one acceptor of electronic charge. Our x-ray standing wave data show that in heteromolecular structures, the molecules tend to align themselves to an adsorption height between those observed in the respective homomolecular structures. We attribute this alignment effect to a substrate-mediated charge transfer between the molecules, which causes a mutual enhancement of their respective donor and acceptor characters. We argue that this effect is of general validity for $\ensuremath{\pi}$-conjugated molecules adsorbing on noble metal surfaces.

Individual Template-Stripped Conductive Gold Pyramids for Tip-Enhanced Dielectrophoresis.

Gradient fields of optical, magnetic, or electrical origin are widely used for the manipulation of micro- and nanoscale objects. Among various device geometries to generate gradient forces, sharp metallic tips are one of the most effective. Surface roughness and asperities present on traditionally produced tips reduce trapping efficiencies and limit plasmonic applications. Template-stripped, noble metal surfaces and structures have sub-nm roughness and can overcome these limits. We have developed a process using a mix of conductive and dielectric epoxies to mount template-stripped gold pyramids on tungsten wires that can be integrated with a movable stage. When coupled with a transparent indium tin oxide (ITO) electrode, the conductive pyramidal tip functions as a movable three-dimensional dielectrophoretic trap which can be used to manipulate submicrometer-scale particles. We experimentally demonstrate the electrically conductive functionality of the pyramidal tip by dielectrophoretic manipulation of fluorescent beads and concentration of single-walled carbon nanotubes, detected with fluorescent microscopy and Raman spectroscopy.

Image Potential State Influence on Charge Exchange in Li+–Metal Surface Collisions

The energy dependence of the neutral fraction of ions scattered by metal surfaces is observed to be affected by the presence of energy gaps and surface states in their surface band structure. The effect is more dramatic when the ion energy level remains close to the surface Fermi level of the surface, as it occurs in the scattering of Li positive ions by Cu and noble metal surfaces. We have incorporated the interaction of the ion state with the bulk- and surface-like states within an extended Anderson model and by using an ab initio calculation of the Hamiltonian terms. In this form we found that the increase of the neutral fraction at low projectile energies is explained solely by the presence of energy gaps in the case of Cu(111), while the bonding interaction with the first image potential state is also important in the Cu(100), Ag(100), and Ag(111) surfaces.

Highly Efficient Surface-Enhanced Raman Scattering Substrate Formulation by Self-Assembled Gold Nanoparticles Physisorbed on Poly(N-isopropylacrylamide) Thermoresponsive Hydrogels

Employing thermoresponsive hydrogels as scaffolding material for noble metal surface loading might be useful for the fabrication of surface-enhanced Raman scattering (SERS) surfaces. Here, we report on a new, reproducible, and simple approach to engineer poly(N-isopropylacrylamide) (PNIPAAm) hydrogel surfaces optimized for physisorption of gold nanoparticles (AuNPs). The advantage of this approach consists of the simple mechanism by which AuNPs are adsorbed on hydrogel templates, without sophisticated chemical treatments for their conjugation with the hydrogel. The resulting PNIPAAm-40 nm AuNP modes demonstrate that this approach gives the capability to tune the interparticle distance and, therefore, to control and modulate SERS affinity upon temperature changing.

Synthesis of Extended Graphdiyne Wires by Vicinal Surface Templating

Surface-assisted covalent synthesis currently evolves into an important approach for the fabrication of functional nanostructures at interfaces. Here, we employ scanning tunneling microscopy to investigate the homocoupling reaction of linear, terminal alkyne-functionalized polyphenylene building-blocks on noble metal surfaces under ultrahigh vacuum. On the flat Ag(111) surface, thermal activation triggers a variety of side-reactions resulting in irregularly branched polymeric networks. Upon alignment along the step-edges of the Ag(877) vicinal surface drastically improves the chemoselectivity of the linking process permitting the controlled synthesis of extended-graphdiyne wires with lengths reaching 30 nm. The ideal hydrocarbon scaffold is characterized by density functional theory as a 1D, direct band gap semiconductor material with both HOMO and LUMO-derived bands promisingly isolated within the electronic structure. The templating approach should be applicable to related organic precursors and different reaction schemes thus bears general promise for the engineering of novel low-dimensional carbon-based materials.

Self-Assembly of Bicomponent Molecular Monolayers: Adsorption Height Changes and Their Consequences

Codeposition of two molecular species [copper phtalocyanine (CuPc, donor) and perfluoropentacene (PFP, acceptor)] on noble metal (111) surfaces leads to the self-assembly of an ordered mixed layer with a maximized donor-acceptor contact area. The main driving force behind this arrangement is assumed to be the intermolecular C-H ⋯ F hydrogen-bond interactions. Such interactions would be maximized for a coplanar molecular arrangement. However, precise measurement of molecule-substrate distances in the molecular mixture reveals significantly larger adsorption heights for PFP than for CuPc. Most surprisingly, instead of leveling to increase hydrogen-bond interactions, the height difference is enhanced in the blends as compared to the heights found in single-component CuPc and PFP layers. The increased height of PFP in mixed layers points to an overall reduced interaction with the underlying substrate, and its influence on electronic properties like the interface dipole is investigated through work function measurements.

Physical Adsorption: Theory of van der Waals Interactions between Particles and Clean Surfaces

van der Waals (vdW) interactions between particles and surfaces are critical for the study of physical adsorption. In this work, we develop a method to calculate the leading- and higher-order coefficients, describing the dependence of vdW interaction on height above the surface. We find that the proposed method can produce the vdW coefficients for atoms on surfaces of metals and semiconductors, with a mean absolute relative deviation of about 5%. As an important application, we study the adsorption energies for rare-gas atoms on noble-metal surfaces by combining the present method, which accounts for the long-range part, with semilocal density functional theory (DFT), which accounts for the short-range part. This combined DFT+vdW approach yields adsorption energies in excellent agreement (5%) with experiments. This suggests that the present method may serve as a useful dispersion correction to density functional approximations.

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.