Bonding Mechanisms of Graphene on Metal Surfaces - The Journal

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Bonding Mechanisms of Graphene on Metal Surfaces Sergey M. Kozlov, Francesc Viñes,* and Andreas Görling Lehrstuhl für Theoretische Chemie, Universität Erlangen-Nürnberg, Egerlandstr. 3, 91058 Erlangen, Germany ABSTRACT: The basic bonding mechanisms of graphene on transition-metal surfaces leading to chemisorption and physisorption are identified and studied in the example of graphene adsorption on Ni(111) by means of density functional theory at the general gradient approximation level with semiempirical corrections for dispersive interactions. In the more stable chemisorbed graphene, relatively strong Pauli repulsion between graphene and the substrate is compensated by donation/back-donation bonding of the same magnitude. In this case, the electronic interactions with the substrate significantly perturb the electronic structure of graphene, but the adsorption energy is still dominated by van der Waals (vdW) interactions. In physisorbed graphene, weak Pauli repulsion equilibrates the vdW attraction without affecting the electronic structure of graphene. The relative stability of physisorbed and chemisorbed graphene is shown to be changed by carbidic C impurities in the subsurface region. band model,43 which predicts stronger binding of the adsorbates to metals with the d-band center closer to the Fermi level. However, the d-band model cannot explain the existence of two distinct groups of adsorption types that have drastically different properties without any cases in between and, furthermore, does not apply to interaction cases due mainly to dispersive interactions. The goal of the present study is to elucidate the differences between chemisorbed and physisorbed graphene and the nature of the graphene−metal interaction in these two cases by theoretical investigation of the two types of graphene adsorption on the same surface. The surface chosen as a representative example is the Ni(111) surface. Ni(111) as a substrate for graphene has two important features making it particularly interesting. First, among all possible CVD substrates, only for Ni(111)12,15 and Cu(111),42 there are known wet chemical etching techniques to detach graphene from the substrate. Second, conveniently for various experimental and theoretical investigations, graphene adopts a (1×1) structure22,23,25 on Ni(111) thanks to a very small mismatch between the lattice parameters of the Ni(111) surface249 pmand graphene246 pm. In contrast, graphene on other metal surfaceswith the exception of Co(0001), featuring a lattice parameter of 251 pmforms Moiré patterns with reported periodicities ranging from 0.5 nm on Pt(111)32,44,45 to 2.5 nm on Ir(111)46 and even 6.2 nm on Ru(0001).47 The latter feature of the Ni(111) surface makes possible the existence of six high-symmetry adsorption positions of graphene, that is, relative arrangements of C atoms with respect to the surface metal atoms in the (1×1) unit cell

1. INTRODUCTION Graphene,1 a novel, highly stable2 material with unique electronic3−5 and mechanical6 properties, has been the subject of numerous studies, which have recently been intensely reviewed. Graphene and its derivatives are suggested to have applications in a handful of different domains: electrochemistry and biosensors,7,8 energy applications (fuel cells, Li-ion batteries, supercapacitors),7,9−11 solar cells,6,9,11 transparent electrodes,6,7,12 electronics,7,13 and others. Among proposed strategies6,7,9,14 for graphene synthesis, some (exfoliation of graphite, chemical derivation of graphene through graphite oxide) are particularly suited for industrial-scale production, while other more costly techniques (chemical vapor deposition (CVD), segregation of bulk-dissolved carbon on the surface, epitaxial growth on SiC) allow one to obtain high-quality graphene sheets up to a few centimeters in size. The critical advantage of the latter techniques is that they are compatible with chip fabrication processes.15 To increase energy efficiency and throughput of the CVD, several techniques were proposed, such as microwave plasma-enhanced,16,17 radio frequency catalytic,18,19 and ambient pressure CVD.20 Transition metals that usually serve as a support for CVD synthesized graphene may be divided into two distinct groups.21 Supports, such as Ni(111) and Ru(0001), chemisorb graphene with graphene−metal distances22−24 of ∼210 pm, modify the graphene electronic structure, 25−27 transfer charge26,28 to graphene, and, consequently, soften phonons in the graphene vibrational spectra.29−31 Other metal surfaces, such as Pt(111) and Ir(111), physisorb graphene, keeping its electronic structure intact32−34 and exhibit graphene−metal distances34−36 of more than 300 pm. Among less-studied metal surfaces, chemisorption of graphene was reported on Pd(111)37 and Co(0001)38,39 surfaces, while there is no conclusive experimental evidence for Cu(111).28,40−42 The appearance of the metals in one or the other group goes along with the d© 2012 American Chemical Society

Received: November 7, 2011 Revised: March 3, 2012 Published: March 5, 2012 7360

dx.doi.org/10.1021/jp210667f | J. Phys. Chem. C 2012, 116, 7360−7366

The Journal of Physical Chemistry C

Article

with a GGA exchange-correlation functional augmented with semiempirical corrections for dispersive interactions,57 which yield adsorption energies of graphene on Ni and graphene−Ni distances in line with experimental results without relying on fortuitous cancelation of errors of LDA methods. This method was used successfully in a previous work, where the coexistence of graphene in top-fcc and bridge-top adsorption modes on Ni(111) was addressed.51 Thus, spin-polarized DFT calculations were carried out with the VASP58 software package, employing the Perdew−Burke− Ernzerhof (PBE)59 GGA-type exchange-correlation functional with semiempirical corrections for dispersive interactionsalso known as DFT+D approximationwith a damping function previously optimized for graphite.57 The projector augmented wave60 method was used to treat interactions between core and valence electrons. Total energies were found to be converged to 0.01 eV per unit cell with respect to the basis set size at a planewave energy cutoff of 415 eV. Reciprocal space was sampled with a Monkhorst−Pack61 grid of 17×17×1, and the electronic occupancies were calculated with a first-order Methfessel− Paxton62 smearing of 0.15 eV. A six-layer p(1×1) Ni(111) slab unit cell was used in the calculations. During the geometry optimizations, the three bottom layers were fixed to PBE bulk-optimized positions d(Ni−Ni) = 248.9 pmwhile the three top layers were relaxed, known as a 3+3 slab. The adjacent slabs were separated in the vacuum direction by more than 1 nm. During calculations of potential energy surface (PES) profiles, the coordinate perpendicular to the surface of one of the two C atoms in the unit cell was kept fixed, while all other coordinates of C atoms were allowed to relax. The geometry optimization was performed until all forces became less than 0.1 eV nm−1. Further relaxations without constraints of the minima found in this way were carried out, although no changes were found neither for the structure nor for its energy. The adsorption energy of graphene on Ni(111), Eads, was calculated by subtracting the energies of a pristine Ni(111) slab, Eslab, and of a free-standing graphene, Egraphene, from the energy of the adsorption complex, Ecomplex; that is, Eads = Ecomplex − (Egraphene + Eslab). With this definition, more negative adsorption energies indicate stronger bonding. The adsorption energy, Eads, may be then decomposed into the sum of electronic DFT contributions, EDFT, and the vdW contribution, EvdW, resulting from the DFT exchange-correlation functional and the

(Figure 1). Initially, the so-called hcp-fcc adsorption position was proposed on Ni(111),48,49 but later on, the top-fcc

Figure 1. Adsorption geometries of graphene on Ni(111): (a) top-fcc, (b) bridge-top, (c) top-hcp, (d) bridge-fcc, (e) hcp-fcc, and (f) bridgehcp. C atoms are displayed in black, Ni atoms of the first surface layer in dark blue, and Ni atoms of the second surface layer in cyan.

structure was found to be in better agreement with experiments.22,23 Density functional theory (DFT) studies,50,51 however, predict nearly the same stability for the bridge-top and the top-fcc adsorption modes, which were actually found to coexist on Ni(111) in recent X-ray photoelectron spectroscopy studies.51 In the present work, physisorption and chemisorption of graphene on Ni(111) in these three adsorption configurations are investigated in detail in order to gain insight in the bonding mechanisms of graphene on transition-metal surfaces.

2. COMPUTATIONAL DETAILS The relative stability of graphene on Ni(111) adsorbed in different configurations has been studied previously by DFT means within either the local density approximation (LDA)50 or the generalized gradient approximation (GGA),50,52−54 which do not account for nonlocal dispersive interactions. In general, LDA binds graphene to Ni(111) with adsorption energies of ∼−0.2 eV, while more sophisticated GGA methods yield very small or even positive adsorption energies. Studies employing exchange-correlation functionals containing recently developed van der Waals (vdW) terms show variations in the obtained results, depending on the employed functional form.55,56 In the present study, we perform DFT calculations

Figure 2. Potential energy surface profile (coordinate axis on the left side) for lateral motion of chemisorbed (black circles) and physisorbed (rhombuses) graphene on Ni(111). The distance of graphene to the topmost Ni layer is also shown (dashed lines, coordinate axis on the right side). Graphene chemisorbed in configurations close to the hcp-fcc adsorption mode spontaneously converts to physisorbed graphene during geometry optimization. 7361



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semiempirical corrections, respectively; that is, Eads = EDFT + EvdW.

3. RESULTS AND DISCUSSION 3.1. Chemisorption and Physisorption of Graphene on Ni(111). Incorporation of semiempirical corrections into DFT calculations revealed the existence of two types of graphene adsorption on Ni(111) that exhibit different properties. Figure 2 displays adsorption energies and graphene−Ni distances of chemisorbed and physisorbed graphene depending on the lateral position of C atoms with respect to the Ni atoms of the top layer, that is, the adsorption position. Chemisorbed graphene is stable only in nearly isoenergetic top-fcc and bridge-top adsorption geometries with adsorption energies of ∼−12 kJ mol−1 per C atom. Other adsorption geometries, such as hcp-fcc, top-hcp, bridge-hcp, and bridge-fcc, are not stable local minima. Moreover, it was not possible to optimize the chemisorbed adsorption complex in configurations close to the hcp-fcc adsorption position. Irrespective of the adsorption geometry, chemisorbed graphene exhibits graphene−substrate distances of ∼210 pm typical for graphene chemisorption.22−24 In contrast, the smaller adsorption energy of physisorbed graphene (∼−9 kJ mol−1 per C atom) is largely insensitive to the configuration of the Ni atoms below. This is due to the large graphene−substrate distance of 320−340 pm typical for other metal surfaces that physisorb graphene.32,35,36 The binding energy profiles of graphene adsorption in topfcc and bridge-top geometries calculated at different graphene− Ni distances clearly exhibit two local minima at the separations of ∼210 and ∼325 pm with a small barrier in between (Figure 3). A decomposition of the total adsorption energy into electronic and vdW contributions reveals that dispersive interactions yield the most significant contributions to the adsorption energy in both cases. The contribution of vdW attraction to the adsorption energy is around −14.5 kJ mol−1 per C atom for both chemisorbed graphene in top-fcc and bridge-top adsorption positions. As one may expect, it gradually decreases with growing graphene−Ni separation and reaches ∼−10.5 kJ mol−1 at the second stable minimum on the potential energy surface profile. In contrast, the electronic contribution to the graphene adsorption energy is always repulsive with a magnitude of only ∼1 kJ mol−1 for physisorbed graphene and only ∼2 kJ mol−1 for chemisorbed graphene. However, it grows rapidly as the graphene−Ni distance decreases below 200 pm and also forms an energy barrier of a few kJ mol−1 at the separation of 250−300 pm, making the physisorbed graphene locally stable. The existence of two local minima (at ∼210 pm and at infinite separation) in the energy profile of the electronic contribution and a barrier between them indicates that the electronic interaction contains an attractive part and an overcompensating repulsive part, which depend differently on the graphene−Ni distance. The adsorption energy of graphene in the hcp-fcc adsorption geometry, however, exhibits only one minimum at a separation of 353 pm typical for physisorption. The variation of the vdW adsorption energy with the graphene−Ni separation is similar to that observed for top-fcc and bridge-top adsorption geometries, although in the latter cases, a shoulder is found at small graphene−nickel separations. Its origin is the suppression of the dispersive interaction between the C atoms located atop of surface Ni atoms, technically due to the damping function in the vdW corrections, taking into account that, at small distances, electronic interactions are

Figure 3. Binding energy of graphene adsorbed on Ni(111) in different adsorption geometries versus distance between graphene and the Ni surface. Total binding energies are displayed as a solid line, DFT and vdW contributions to the binding energy as dashed and broken lines, respectively.

correctly described by the density functional, physically probably due to Pauli repulsion with the topmost Ni atomsthus reducing the vdW contributionand the growing of the dispersive interactions with other surface and subsurface Ni atomsthus increasing the vdW term at the smallest studied heights. However, the electronic repulsion in the hcp-fcc case is significant already at a separation of 300 pm and does not feature a minimum at lower separation values. This finding suggests that the attractive contribution of the electronic interaction between graphene and Ni is not present in the case of the hcp-fcc adsorption geometry. This is due to the lack of direct electronic interaction between C atoms located atop of surface Ni atoms, which originate a significant charge donation/ back-donation; see below. Next, we compare results of our DFT+D calculations with results calculated previously with other DFT-based methods. In line with the present results, studies performed with LDA and PBE exchange-correlation functionals revealed a slightly higher stability of the bridge-top compared to the top-fcc adsorption geometry.50 A graphene−Ni separation in top-fcc, bridge-top, and top-hcp geometries was also found to be around 210 pm by various LDA and GGA methods.50,53−55 In the hcp-fcc configuration, conversely to the present results, these methods yield very weakly bound graphene (Eads ≤ 3 kJ mol−1 per C 7362



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atom).50,53 In agreement with LDA results, bridge-top and topfcc adsorption modes were found to be the most stable in the present study, in line with experimental observations.22,23,51 In contrast, studies performed with the PBE functional without semiempirical corrections favor the hcp-fcc adsorption mode.50,52 One can see many similarities between adsorption of graphene on Ni(111) and on Co and Ru. GGA studies of graphene on the so-called fcc-Co(111) employing the Perdew− Wang (PW91) functional yielded negligible adsorption energies of graphene (Eads < 5 kJ mol−1), graphene−Co distances of 216−225 pm for top-fcc, top-hcp, and bridge-top positions, and a distance of more than 380 pm for the hcp-fcc geometry.63 In contrast, LDA studies reveal that, on Co(0001), the most stable adsorption mode is top-fccEads = 13 kJ mol−1, d(graphene− Co) = 207 pmwith the hcp-fcc adsorption geometry being clearly disfavoredEads = 3 kJ mol−1.39 These results were corroborated by an STM investigation, which revealed that half of the graphene C atoms reside directly above Co atoms, in line with the aforementioned commensurability.39 On Ru(0001), graphene forms Moiré patterns, which feature some regions with adjacent C atoms located above top and fcc sites and other regions with adjacent C atoms located above hcp and fcc sites. LEED studies found the graphene−Ru distances in top-fcc and hcp-fcc regions to be ∼215 and ∼370 pm, respectively, in agreement with DFT studies utilizing the PBE functional.24,64 Thus, on all three metals, graphene chemisorbs in top-fcc or top-hcp geometries with graphene−substrate distances of ∼210 pm and physisorbs in the hcp-fcc geometry with the distance of more than 300 pm. From this similarity, one can conclude that the binding mechanisms of graphene on Ni, Co, and Ru are essentially similar. The difference in height of physisorbed and chemisorbed cases is likely to result from a difference in the nature of graphene−metal bonding. This issue is addressed in the next section. 3.2. Electronic Interactions between Graphene and Ni(111). The difference between the types of binding in chemisorbed and physisorbed graphene becomes manifested in the band structures of the different adsorption geometries and modes (Figure 4). Graphene physisorbed on Ni(111) has a band structure very close to that of free-standing graphene irrespective of the adsorption configuration due to the large graphene−Ni distance. In contrast, chemisorbed graphene has a band gap of around 3 eV at the K point, the position of the Dirac point in free-standing graphene, and also features a rigid down shift of σ-bands by more than 1 eV in comparison with physisorbed graphene. Moreover, the location of C atoms in different chemical environments in the case of top-fcc leads to a band splitting in the vicinity of the Dirac point. A Bader analysis65 reveals that there is an electron transfer of ∼0.14 electrons per unit cell from Ni(111) to chemisorbed graphene. Charge density difference (CDD) plots (Figure 5) show that not only does Ni donate electrons to the π-band of graphene but also graphene back-donates electrons from its σband, thus forming a chemical bond via a donation/backdonation mechanism. One can see also that, in the case of bridge-top geometry, both C atoms in the unit cell attract electrons, while in the case of top-fcc, only the atoms located directly above surface Ni atoms accumulate electrons. The situation is different for graphene in a hcp-fcc geometry for a graphene−Ni separation fixed at 205 pm, characteristic for chemisorption. In this case, no C atoms are located directly above Ni atoms of the first surface layer. Consequently, there is

Figure 4. Band structure of graphene adsorbed on Ni(111) along the direction Γ → K → M. Sizes of yellow dots indicate the amount of graphene contribution to the bands. In the case of top-fcc, contributions of C atoms located above the top (fcc) site are displayed in yellow (red).

no electron donation/back-donation, but only charge redistribution and polarization of the graphene sheet. The complex is energetically unstable because electronic interaction in the form of Pauli repulsion (30 kJ mol−1 per C atom) is stronger than vdW attraction (−16 kJ mol−1 per C atom). Note that, when graphene in the hcp-fcc geometry is physisorbed, neither charge transfer nor charge redistribution or polarization is observed. In contrast, in graphene chemisorbed in a bridge-top or top-fcc adsorption geometry, the Pauli repulsion is nearly compensated by donation/back-donation interaction of similar 7363



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Figure 6. Adsorption geometries of graphene on Ni(111) with impurities of carbidic C in the subsurface region. Graphene atoms are displayed in black, subsurface C atoms in gray, surface Ni atoms in dark blue, and Ni atoms of the first subsurface layer in cyan.

Figure 5. CDD plots of graphene (a) chemisorbed in bridge-top geometry, (b) chemisorbed in top-fcc geometry, and (c) in hcp-fcc geometry at a fixed graphene−Ni separation of 205 pm typical for chemisorption. Red color indicates electronic density accumulation, blue color, depletion. The value of ±0.027 a.u. is used for the isosurfaces.

at lower concentrations.51,66,67 Their formation was also observed on other transition metals, such as Pd, where C atoms are able to occupy oss and tetrahedral subsurface (tss) sites and, thus, may affect the metal chemical activity.68−70 Indeed, our calculations show that subsurface C impurities do destabilize chemisorbed graphene (Table 1). Graphene

magnitude. Assuming that Pauli repulsion between Ni and graphene in top-fcc or bridge-top positions is not weaker than that in hcp-fcc geometry, one can roughly estimate the strength of the donation/back-donation interaction to be around 30 kJ mol−1 per C atom. The strength of the donation/back-donation interaction between metals and adsorbates according to the d-band model43 depends on the position and the width of the dband of a given metal. Thus, our calculations suggest that graphene chemisorbs on a given metal only when donation/ back-donation bonding formed between graphene and the substrate is stronger than the Pauli repulsion at a graphene− metal distance of ∼210 pm (tens of kJ mol−1). The existence of relatively strong attractive and repulsive electronic interactions explains several peculiarities of graphene on metals. First, the high magnitude of the electronic interactions explains the strong effect of chemisorbing substrates on the electronic and vibrational band structures of graphene. At the same time, the adsorption energy is dominated by the vdW contribution, because the two types of electronic interactions nearly cancel each other. Second, magnitudes of all three types of interactions have different dependencies on graphene−metal separation, which essentially do not seem to vary from metal to metal. Thus, different metals feature local minima on graphene PES profiles at similar graphene−metal distances. The first minimum at a separation of more than 300 pm is located at a point where forces due to Pauli repulsion are compensated by dispersive forces. The second minimum (if present) is located at the graphene−metal separation of ∼210 pm, where Pauli repulsion is equilibrated by donation/back-donation interaction. In the region in between, Pauli repulsion cannot be equilibrated by more long-range vdW attraction and by more short-range donation/back-donation. 3.3. Adsorption of Graphene on Ni(111) with Subsurface Carbidic C Impurities. As was shown before, despite the very different properties of chemisorbed and physisorbed graphene, the difference between the respective adsorption energies on Ni(111) is only ∼3 kJ mol−1 per C atom. Thus, certain modifications of the Ni(111) surface may be able to destabilize chemisorbed graphene and may make possible only physisorption on a modified Ni substrate. Here, we illustrate this concept by considering adsorption of graphene on p(1×1) Ni(111) with impurities of carbidic C in octahedral subsurface (oss) positions between the first and the second layers of the surface (Figure 6). One should note here that carbidic impurities are often observed in CVD experiments on Ni, but

Table 1. Adsorption Energetics and Geometries of Graphene on Ni(111) with Subsurface Impurities of Carbidic C

top-fcc top-hcpa

Eads, kJ mol−1

EDFT, kJ mol−1

EvdW, kJ mol−1

d(graphene−Ni), pm

−9.9 −10.9

13.9 14.7

−23.8 −25.0

309 295

a

On p(1×1) Ni(111) with C subsurface impurities, the bridge-top adsorption geometry is no longer a local minimum; graphene moves to the top-hcp position during geometry optimization.

initially separated by ∼200 pm from Ni(111) moves away from the substrate during geometry optimization and, in the case of the bridge-top adsorption position, also moves laterally to the top-hcp geometry. The destabilization of chemisorbed graphene by subsurface carbidic C may be rationalized by competition for electron transfer from Ni atoms and mutual repulsion of negatively charged species. Notably, subsurface C increases significantly the strength of the vdW attraction and electronic repulsion between physisorbed graphene and Ni(111), but only mildly affects the total adsorption energies (∼−10 kJ mol−1) and the graphene−Ni separation (∼300 pm). Thus, our calculations suggest that one can selectively obtain physisorbed or chemisorbed graphene on a given substrate by modifying the surface adsorptive properties with certain impurities, for example, carbidic C in the subsurface region.

4. CONCLUSIONS To summarize, we have studied chemisorption and physisorption of graphene on transition-metal surfaces by considering as a representative example graphene adsorption on the Ni(111) surface by means of DFT calculations using a GGA exchange-correlation functional augmented with semiempirical corrections. Graphene on Ni(111) chemisorbs in topfcc or bridge-top adsorption geometries with graphene−Ni distances of ∼210 pm and calculated adsorption energies of −12 kJ mol−1 per C atom. Physisorbed graphene on Ni(111), on the other hand, is not sensitive to the lateral positions of C atoms with respect to the surface Ni atoms. In this case, adsorption energies of −9 kJ mol−1 per C atom are accompanied by graphene−Ni distances of 325−355 pm. Stabilization of chemisorbed graphene in top-fcc and bridge-top 7364



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positions occurs due to donation/back-donation bonding with Ni atoms located directly below C atoms. As in the hcp-fcc geometry, no C atoms are located directly above surface Ni atoms, no bond can be formed, and the adsorption complex is strongly destabilized by Pauli repulsion. Different distance dependencies of Pauli repulsion, donation/back-donation bonding, and vdW attraction explain the existence of two energy minima at 325−355 pm and at ∼210 pm. In the case of physisorption, Pauli repulsion equilibrates vdW attraction with donation/back-donation being negligible. In the case of chemisorption, Pauli repulsion with an estimated extent of 30 kJ mol−1 is equilibrated by donation/back-donation bonding of similar magnitude. The relatively high magnitude of the two types of electronic interactions explains the strong influence of the support on the electronic and vibrational band structure of chemisorbed graphene, while the adsorption energy is still dominated by the vdW contribution. We also show that, due to the small difference of only 3 kJ mol−1 between the binding energies of chemisorbed and physisorbed graphene, it should be feasible to destabilize chemisorbed graphene by certain surface modifications. According to our calculations, carbidic impurities in the subsurface region make chemisorption of graphene on Ni(111) impossible, but do not affect significantly physisorption. Finally, numerous similarities in adsorption of graphene on Ni(111) and on other transition-metal surfaces suggest similar binding mechanisms on various transition metals. Thus, we attribute the ability of different metals to chemisorb graphene to donation/back-donation interaction strong enough to overcome Pauli repulsion at graphene−metal distances of ∼210 pm, characteristic for chemisorption. In analogy with Ni(111), the existence of two distinct types of adsorption types without any cases in between may be explained by the interplay of three interactionsVan der Waals interactions, Pauli repulsion, and donation/back-donation interactionthat depend differently on the graphene−metal distance.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS F.V. thanks the Alexander von Humboldt Foundation for financing his postdoctoral grant. The authors gratefully acknowledge the funding of the German Research Council (DFG), which supports the Collaborative Research Center 953, and which supports, within the framework of its “Excellence Initiative”, supports the Cluster of Excellence “Engineering of Advanced Materials” (www.eam.uni-erlangen.de) at the University of Erlangen-Nuremberg.

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