Assembly of Ferrocene Molecules on Metal Surfaces Revisited

Metallocene (MCp2) wires have recently attracted considerable interest in relation to molecular spintronics due to predictions concerning their half-metallic nature. This exciting prospect is however hampered by the little and often-contradictory knowledge we have concerning the metallocene self-assembly and interaction with a metal. Here, we elucidate these aspects by focusing on the adsorption of ferrocene on Cu(111) and Cu(100). Combining low-temperature scanning tunneling microscopy and density functional theory calculations, we demonstrate that the two-dimensional molecular arrangement consists of verticaland horizontal-lying molecules. The non-covalent Tshape interactions between Cp rings of vertical and horizontal molecules are essential for the stability of the physisorbed molecular layer. These results provide a fresh insight into ferrocene adsorption on surfaces, and may serve as an archetypal reference for future work with this important variety of organometallic molecules.

Metallocenes were discovered in the fifties and have boosted the development of organometallic chemistry, earning, in part, to Fischer and Wilkinson the Nobel Prize in chemistry in 1973. They are organometallic sandwich compounds of simplified architecture as build on two cyclopentadienyl rings (C 5 H − 5 , Cp) bound through a metal center, for example by Fe 2+ in the case of ferrocene (see Figure 1). In the past decade, metallocenes were again in the spotlight in relation to molecular spintronics. This emerging technology exploits the spin to convey information in hybrid metal-molecule devices. 1 Molecules are extremely appealing, as they ensure high-device efficiency and offer the unique possibility of having build-in spin functionalities. Numerous theoretical investigations have demonstrated that metallocene wires can play a prominent role in molecular spintronics as they can produce a nearly ideal spin-polarized current, in other words, a current with 100% spin polarization. [2][3][4][5][6][7][8] Despite this exciting prospect, the design of devices incorporating metallocenes is hold up by the limited knowledge we have concerning their interaction with a metal. After all, the device performance will be dictated by the adsorption geometry, molecular self-assembly and spin-state at a metal surface. 9 There is little consensus on the way these molecules adsorb onto surfaces. Several studies have reported an associative adsorption at low temperature of ferrocene (FeCp 2 ) on Ag, [10][11][12] Cu, 11 graphite 13,14 and Mo 15 by means of photoemission (PES) and electron energy loss spectroscopy (EELS), claiming a preferential orientation of the molecular axis perpendicular or parallel to the surface depending on the substrate (Figure 1a). On the other hand, based on STM measurements, K. F. Braun et al. 16 proposed a dissociative adsorption model of The adsorption of metallocenes remains elusive up to now. In this work we aim to clarify the self-assembly, adsorption geometry and interactions of ferrocene molecules deposited on a metallic surface. By means of scanning tunneling microscopy (STM) and density functional theory (DFT), we show how associatively adsorbed FeCp 2 molecules self-assemble on a Cu surface adopting a configuration which is independent of the surface orientation, dismissing the idea of a unique preferential orientation of the molecular axis with respect to the substrate suggested by previous works. In fact, the stability of the two-dimensional structure depends crucially on a combination of vertical (molecular axis perpendicular to the surface, Figure 1a) and horizontal (axis parallel to the surface) molecules having their Cp rings in a so-called eclipsed configuration (D 5h symmetry, Figure 1b). Interestingly, this arrangement bears similarities with both the gas phase and the bulk structure of ferrocene. Similar to the ferrocene crystal (see Figure 4e), the presence of the two molecules, vertical and horizontal, ensures the cohesion of the crystal through T-shape interactions, but as in the gas phase-and unlike bulk ferrocene-the eclipsed configuration is favored over the staggered configuration (D 5d symmetry, Figure 1b).
The adsorption of FeCp 2 on a cold Cu(111) surface gives rise to long-range well-ordered molecular layers with almost no defects and exhibiting an apparent height of 3.1Å ( Figure   2a). Two different molecular configurations, which we label "compact" and "zigzag" (see  θ (see Table 1 for values), contains four molecules, two horizontal and two vertical. As shown, consecutive vertical molecules in the a 2 direction do not lie exactly in the same axis (see the orange axis and the molecules marked with circles in Figure 2b), they are shifted by 1.5Å. Moreover, vertical molecules present an asymmetry in the a 1 direction, being brighter on one side of the ring; vertical molecules in the center and in the corner of the unit cell show the asymmetry at different sides of the ring. This asymmetry can be clearly observed in Figure 2c, and, as we show later, indicates that the molecules are tilted.  Table 1), as shown in white in Figure 2d. A similar ferrocene pairing has been also observed for ferrocene-derived molecules (FcCOOH and Fc-(CH 2 ) n -Fc) deposited on metallic surfaces. 18,20 Although the identification of vertical ferrocene molecules is straightforward, determining the presence of the horizontal molecules is experimentally challenging. Therefore, to confirm the proposed adsorption model for ferrocene we rotated some molecules from a vertical (horizontal) to a horizontal (vertical) position by using the STM tip, 21 as shown in the before-and-after images in Figure 3. The molecular structure of FeCp 2 is superimposed in the images below (the structure used has a random orientation). Upon application of a bias pulse of 2 V, molecules rotate out-of-plane from horizontal to vertical (molecules noted 1 and  Notice that a molecular tip has been employed in order to enhance the observed features. Additionally, a laplacian filter has been applied. Image sizes: (a-b) 4.5 × 4.5nm 2 .
In order to study the stability and electronic properties of the structures proposed for the molecular layer on Cu(111) we have performed extensive DFT calculations and simulated the corresponding STM images. First, we started by studying the intra-molecular structure of ferrocene molecules. As shown in Figure 1b, two different configurations can be found for a ferrocene molecule: eclipsed and staggered. The former one corresponds to the most stable structure in the gas phase, [22][23][24] while the latter is the most common one in the solid state. 25 We found that the eclipsed configuration is more stable by 45 meV on the surface.
According to our calculations, no matter what initial configuration is chosen, the eclipsed  Table 1). The stability of the proposed structure has been tested using this unit cell.
Comparing the simulated STM image (Figure 4a) with the experimental one ( Figure   2b), we observe that the main features of the image are reproduced. The structure shown in Figure 4b, in which the minimum distance from the surface to the molecule is 2.3Å,  Figure 4b). This apparent contradiction can be easily lifted by recalling that the tunneling transmission probability between the STM tip and the molecule depends on the tip and molecular orbitals. Orbitals that extend into the vacuum, such as s, p z or d 2 z , will be favored in the tunneling process. In the present case, for vertical molecules, the d-manifold Our calculations also reproduce subtle effects revealed by STM images. We observe a lateral displacement of 1.49Å between consecutive vertical molecules in the a 2 direction (marked in Figure 4b), measured as the distance between Fe atoms, which is consistent with the experimentally found shift of 1.5Å (marked in Figure 2b). Most importantly, we find that the slight asymmetry observed for the ring of the vertical molecules (Figure 2c To conclude, we consider now the zigzag structure. Due to the size of the parallelogram unit cell (see parameters in Table 1), we have just performed a few calculations to confirm the stability of the proposed structure. Figure 4c shows the simulated topographic STM image corresponding to the most stable structure that has been found (Figure 4d), which reproduces with high accuracy the main features of the experimental image (Figure 2d).
This structure shares some common features with the compact structure. First, the same reversal of the apparent height is observed, in which horizontal molecules always appear dimmer in the images, although the highest atom of horizontal molecules in this case is placed 0.4Å further from the surface than the topmost atom of the vertical ones. Second, we also find that four H atoms of the horizontal molecules have to point towards the surface in order to get a stable structure. Third, a 15 • tilt in two opposite directions is also observed for the vertical molecules (side view of Figure 4d), explaining the experimentally observed asymmetry between vertical molecules shown in Figure 2e.
According to an analysis of the energetics of the zigzag configuration, the adsorption energy per molecule is 1.26 eV, of which 1.02 eV comes from the contribution of vdW forces.
The interaction between the molecules and the copper surface is mainly related to vdW forces, as may be concluded after comparing these adsorption values with the cohesion energy of the free-standing monolayer (0.43 eV). There is a moderate charge transfer per unit cell, 0.18 e − , which is also indicating a weak molecule-substrate interaction. Table 1 summarizes the energetics of the adsorption of both, compact and zizag, configurations, showing that they are very similar, pointing in both cases to the same conclusion of a physisorbed layer.
We have presented a complete experimental and theoretical study of ferrocene molecules adsorbed onto copper surfaces, which solves the long-standing dilemma concerning the selfassembly of ferrocene on a metal surface. We demonstrate that ferrocene molecules adsorb associatively forming two different self-assemblies, compact and zigzag, which appear equally distributed on the surface. These arrangements, which include the presence of vertical and horizontal molecules, are lead by intermolecular interactions, in particular T-shape interactions, which are essential to stabilize the structures and to explain subtle submolecular features resolved in the STM images. As in the gas-phase ferrocene, the eclipsed configuration is found to be the most stable one for molecules adsorbed on a copper surface, while the presence of vertical and horizontal molecules on the surface is reminiscent of the crystallographic structure of bulk ferrocene. The fact that the same configurations, compact and zigzag, are found in differently oriented copper surfaces, together with the small charge transfer and the vdW interaction between the molecular monolayer and the Cu surface, confirms the physisorption of the molecules. We believe that these results may provide a solid guideline for future investigations on ferrocene, and eventually, related metallocenes on surfaces.

Methods Experiments
Samples have been prepared in a ultrahigh vacuum (UHV) chamber at a base pressure of 10 −10 mbar equipped with a LT-STM operating at 4.5 K. The Cu(111) and Cu(100) metallic surfaces were cleaned by repeated Ar + ion bombardments and annealing treatments at 860 K until clean large terraces were obtained. FeCp 2 molecules were sublimated at room-temperature in vacuo on the metallic cold surfaces (≤100 K) with a deposition rate of approximately 1.2 ML/min. After the molecular deposition, the samples were immediately cooled to 4.5 K for STM measurements. In situ cleaned W tips were used for measurements.
WSxM software has been used for data analysis. 26

Theory
We have studied the geometric and electronic structures of the adsorbed molecules by performing ab initio density functional theory calculations as implemented in the VASP code. 27 We have used a plane wave basis set to expand the wave functions with a cut-off energy of 400 eV. Core electrons have been treated using the projector augmented wave method, 28,29 and we have used the generalized gradient approximation in the PBE form for the exchange and correlation functional. 30 The long-range dispersion corrections have been treated using the DFT+D2 approach proposed by Grimme. 31 Although this empirical vdW correction remains an approximation in light of the more recent self-consistent approaches, 32 it readily corrects the major lack of GGA energies and enables a qualitative description of the interactions at play in the self-assembly process. Charge transfer has been obtained by performing a Bader analysis. 33 The Cu(111) surface has been modeled using a slab geometry with five Cu layers and a Supporting Information Figure 1 shows STM images acquired after deposition of ferrocene on Cu(100). In the large scale image of Figure 1(a) we see how for a ferrocene coverage below the monolayer, Cu (100) areas remain clean in between long-range ordered molecular layers. In Figure 1(b-d) we can distinguish the presence of the same compact and zigzag assemblies as on Cu(111). Table 1 presents the unit cell parameters for both arrangements as well as the apparent height of the molecular layer, which are almost identical to the parameters obtained for Cu(111). The fact that the substrate orientation does not influence the adsorption configuration of ferrocene suggests that the molecules have a weak coupling with the substrate, meaning that they are physisorbed. The low associative desorption temperature observed for this system, around This implies that the rotational domains are only related to the symmetry of the molecular layer itself and not to the one of the substrates. Table 1: Apparent height of the molecular layer with respect to Cu(100) (h Cu−F eCp 2 ) and unit cell parameters (a 1 , a 2 , b 1 , b 2 , θ) of both compact and zigzag arrangements.