Structural Phases of Ordered FePc-Nanochains Self-Assembled on Au

Aug 29, 2012 - Sara Fortuna,. ◇,#. Xavier Torrelles,. +. Manvendra Kumar,. ⊥ and Maddalena Pedio*. ,⊥. †. Dipartimento di Fisica, Università ...
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Structural Phases of Ordered FePc-Nanochains Self-Assembled on Au(110) Maria Grazia Betti,† Pierluigi Gargiani,† Carlo Mariani,‡ Roberto Biagi,§,∥ Jun Fujii,⊥ Giorgio Rossi,⊥,∇ Andrea Resta,○ Stefano Fabris,◆,# Sara Fortuna,◆,# Xavier Torrelles,+ Manvendra Kumar,⊥ and Maddalena Pedio*,⊥ †

Dipartimento di Fisica, Università di Roma La Sapienza, Piazzale Aldo Moro 2, I - 00185 Roma, Italy Dipartimento di Fisica, CNISM, CNIS, Università di Roma La Sapienza, Piazzale Aldo Moro 2, I - 00185 Roma, Italy § Dipartimento di Fisica, Università di Modena e Reggio Emilia, via G. Campi 213/A, I-41100 Modena, Italy ∥ CNR-NANO, Institute of Nanoscience, S3 Center, I-41125 Modena, Italy ⊥ CNR-IOM TASC, Area Science Park S.S. 14, km 163.5, I-34149 Trieste, Italy ∇ Dipartimento di Fisica, Università di Milano, via Celoria 16, I-20133 Milano, Italy ○ European Synchrotron Radiation Facility (ESRF), 38043 Grenoble, France ◆ CNR-IOM DEMOCRITOS, Theory@Elettra group, S.S. 14, km 163.5, I-34149 Trieste, Italy # SISSA, Via Bonomea 265, I-34136, Trieste, Italy + Institut de Ciència de Materials de Barcelona, ICMAB-CSIC, 08193 Barcelona, Spain ‡

S Supporting Information *

ABSTRACT: Iron-phthalocyanine molecules deposited on the Au(110) reconstructed channels assemble into one-dimensional molecular chains, whose spatial distribution evolves into different structural phases at increasing molecular density. The plasticity of the Au channels first induces an ordered phase with a 5×5 symmetry, followed by a second long-range ordered structure composed by denser chains with a 5×7 periodicity with respect to the bare Au surface, as observed in the low-energy electron-diffraction (LEED) and grazing incidence X-ray diffraction (GIXRD) patterns. The geometry of the FePc molecular assemblies in the Au nanorails is determined by scanning tunneling microscopy (STM). For the 5×7 phases, the GIXRD analysis identifies a “4-3” rows profile along the [001] direction in the Au surface and an on-top FePc adsorption site, further confirmed by density functional theory (DFT) calculations. The latter also reveals the electronic mixing of the interface states. The chain assembly is driven by the molecule−molecule interaction and the chains interact with the Au nanorails via the central metal atom, while the chain−chain distance in the different structural phases is primarily driven by the plasticity of the Au surface.

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INTRODUCTION Understanding the kinetics and energetics of long-rangeordered molecular adlayers on solid templates is fundamental to control the formation of seed layers and the subsequent growth of epitaxial single-domain organic films aiming at the production of functional low-dimensional molecular architectures.1,2 Planar molecules with extended π-conjugated macrocycles are reference systems, as their simple bonding scheme favors a flat-lying adsorption geometry onto metallic surfaces.3−7 The inclusion of metallic centers in these planar molecules allows for the design of ordered metallorganic networks.8 For instance, metallic phthalocyanines exist with most of the transition elements coordinated in the center of their cross-shaped π-conjugated system and they can act as single-atom magnets, with potential applications for the © 2012 American Chemical Society

production of self-assembled data storage devices. The control of the relative strengths between intermolecular and substrate− molecule interactions allows for the design of molecular architectures with the desired structure and magnetic properties.9 Metal phthalocyanines (MPc's) on metallic templates selfassemble flat-lying with different ordered structures, thus forming a large variety of patterns all characterized by a regular grid of central metal atoms.10,11 The spatial distribution of the supramolecular architectures is mainly guided by the surface patterning, and suitable templates can favor long-range ordering and large domains.12−16 The local order of molecular layers on Received: May 30, 2012 Revised: August 21, 2012 Published: August 29, 2012 13232

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Figure 1. Self-assembly of FePc molecular chains on the Au(110) surface (vertical axes, [11̅0] direction, and horizontal axes [001] direction). STM topography collected in constant current mode at T = 300 K on FePc/Au(110) as a function of FePc coverage: (a) (5×5) symmetry structure at about 0.5 SL FePc (−0.20 V, 0.10 nA); (b) FePc ad-chains with (5×7) symmetry, at about 0.7 SL FePc (−0.20 V, 0.10 nA); (c) (5×7) symmetry structure at about 0.8 SL FePc (−0.05 V, 0.05 nA); (d) compact SL of FePc chains (−0.67 V, 0.03 nA). Above each image, a line-profile perpendicular to the chains is plotted. Below each image, a sketch of the molecular chains is depicted. Au(110) substrate. The same protocol for preparing the atomically clean Au(110)-1×2 surface was replicated in all apparatuses used for the present research. It consisted of sputter-annealing cycles (1000 eV, 720 K followed by 500 eV, 520 K) and monitoring the surface cleanness. FePc was evaporated from resistively heated quartz crucibles in UHV, and the nominal thickness was measured by oscillating quartz microbalances. The FePc molecules were deposited on the clean Au(110) substrate kept at 410 K, by using a deposition rate of about 0.5 Å/min. One single layer (SL) is defined as the nominal density of the FePc molecules arranged in the 5×7 structure. The SL coverage is experimentally well characterized both by LEED (5×7 pattern and/or 5×3.5 with extinction) and by STM. STM experiments have been performed in ultrahigh-vacuum (UHV) systems at the beamline ID08 of the European Synchrotron Radiation Facility (ESRF) in Grenoble, also equipped with ancillary equipment for sample preparation and cleaning. The STM apparatus (Omicron VT-STM) was operated at room temperature with a W tip. All the images shown in the article were taken in the constant current mode. The applied tip bias voltage and the tunneling current of each image are given in the figure caption. The high precision LEED patterns were taken with a low-current density apparatus at the surface physics laboratory SESAMO in Modena. They were obtained at room temperature with a sample current of 1 nA to prevent damage of the MPc film. The impinging beam diameter was about 0.5 mm. The amplification of the very weak current of diffracted electrons performed with a microchannel plate (MCP) enables observation of the patterns, on a flat screen. Due to this geometry, the acquired images have been corrected in order to get the usual view. In all the sample replicas grown in the different apparatuses of this multitechnique study, the diffraction patterns were checked by standard LEED optics to verify the symmetry of the FePc layers deposited on the Au(110) surface and consistency with the reference data from SESAMO. The grazing incidence X-ray diffraction data were collected at the surface beamline ID03 of ESRF26 in Grenoble. The samples for GIXRD experiments were prepared in two different UHV chambers (base pressure 3 × 10−10 mbar) available at the ID03 beamline: the surface diffraction chamber, coupled with a six circle horizontal diffractometer, and the off-line support-chamber equipped with standard surface science preparation tools. The surface preparation was performed either in situ or in the off line chamber where the FePc

noble metal surfaces can present multiple rotational domains.17,18 The presence of rotational domains hampers the growth of long-range-ordered architectures, but suitable anisotropic templates can drive the formation of single-domainordered seed layers, and this is a prerequisite for the fabrication of epitaxial thin films. Patterned metallic surfaces or vicinal substrates can drive the assembly of one-dimensional structures in π-systems.5,14,19−22 A long-range ordering, commensurate with the substrate lattice, has been observed on the Au(110) surface, where the reconstructed substrate channels drive ordered molecular assemblies.15,23,24 Recently, it has been demonstrated that MPc's with magnetic Fe metal centers can arrange into chains.12,13 Their interaction with the underlying substrate generates electronic states at the interface12,25 and modifies the spin and orbital configuration. In this paper, we report a systematic study of the formation of well-ordered and stable FePc chains grown on the Au(110) surface at increasing FePc density, up to completion of a compact single layer, the latter obtained at 410 K substrate temperature. STM images for coverages implying side-by-side FePc chains decorating the Au nanorails give evidence that the long-range order extends in the direction perpendicular to the chains giving rise to different patterns as a function of molecular density, as brought to light by the LEED and GIXRD diffraction techniques. The Au surface missing row reconstruction drives the formation of the chemisorbed molecular chains. Density functional theory calculations reveal the adsorption energetics and the electronic mixing at the Au−FePc interface. The basic architectural motif of the chains is dictated by the molecule−molecule interaction, while the chain spatial distribution is driven by the channel formation in the plastic Au(110) substrate.

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METHODS

A multitechnique investigation (STM, LEED, GIXRD) and a robust protocol to replicate the sample preparation in different apparatuses has been adopted to investigate the FePc chain formation on the 13233

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Figure 2. Fast Fourier transform (FT) of FePc STM topographies. The FT have been suitably rescaled to be compared. (a) FT from the structure shown in Figure 1a; (b) FT from the structure shown in Figure 1c; (c) FT from the structure shown in Figure 1d. A sketch of the spot profiles obtained by FT of panels a−c is reported in panels d, e, and f. Red circles indicate the ×1 Au unit cell along the [001] symmetry direction.

with the FePc plane lying parallel to the surface,10 even at the highest dilution explored (∼10−2 SL). The arrangement of these molecular chains is initially dictated by the gold surface reconstructions. When the chains first assemble into large domains, the FePc arrays are separated by one row of Au atoms and present a 5×5-symmetry (Figure 1a). The low energy barrier for the Au atoms on the Au(110) surface30 allows for their diffusion across the surface. In addition, a little energy cost allows the conversion of the pristine ×2 surface into ×5 reconstructed channels along the [001] direction.10 The ×5 spacing along the [11̅0] direction is compatible with the molecular size (1.38 nm), while the ×5 molecular chain packing along the [001] direction is consistent with the rearrangement of the underlying Au rows (5·a2 = 2.04 nm), as recently shown by high-resolution STM measurements at low coverage, and by ab initio theoretical calculations.10 Each molecule within the chain results rotated by an angle α ≃ ±(13 ± 2°) independent from the chain packing density, in agreement with theoretical predictions.10 Furthermore, the FePc molecules present a bright center in correspondence to the Fe atoms and the organic cycles appear as 4-fold lobes. The bright central spot represents the density of charge in the FePc molecule that is most affected by the chemisorption, i.e., by the coupling of the electronic states of the underlying Au and the Fe-related molecular orbitals.12,25,31 Increasing the molecular density beyond the completion of the 5×5 phase determines the self-assembly of further molecular chains, parallel to the 5×5 ones and registered on top of the underlying Au rows. In the intermediate coverage phase imaged in Figure 1b, the ad-chains induce a further rearrangement of both the Au substrate and the lower-lying chains, the latter being pushed apart from each other. The STM image in Figure 1b shows the detail of two single ad-chains whose distance matches with seven underlying Au lattice units along the [001] direction. When larger ordered domains are formed (Figure 1c), the ad-chains coalesce, forming a well-ordered molecular structure, where the interchain distance is exactly 7 times the Au unit cell along the [001] direction (0.408 ± 0.005 nm), while the

grown layers were characterized by UV photoemission (overall resolution 0.3 eV) and LEED. The sample could then be transferred to the diffraction chamber via an UHV suitcase (base pressure 2 × 10−9 mbar). Two Au(110) single crystals were used for the GIXRD experiments. Both samples presented the surface plane parallel to the crystallographic (110) planes within 0.2°, as determined by X-ray diffraction measurements. The GIXRD measurements were carried out using the Au(110) surface lattice cell described by the vectors (a1, a2, a3) parallel to the [11̅0], [001], and [110] directions, respectively, with a1 = a3 = b0/√2 and a2 = b0 (b0 = 0.408 nm is the Au bulk lattice constant). The coordinates of the corresponding reciprocal lattice vectors were denoted by H, K, and L, respectively. The Au(110) single crystal was mounted in the UHV diffraction chamber (base pressure 1 × 10−10 mbar). The photon beam energy was 17.8 keV, and the grazing angle adopted in the measurements was 0.25°, corresponding to the critical angle on gold. A second set of experimental crystal truncation rods have been measured at higher photon energy, 24 keV, using a grazing angle of 0.5° in order to extend the maximum vertical transfer momentum Lmax. Density functional theory (DFT) calculations were run with the Perdew−Burke−Ernzerhof generalized gradient corrected approximation (PBE-GGA) for the exchange and correlation energy functional.27 The spin-polarized Kohn−Sham equations were solved in the planewaves pseudopotential framework, as implemented in the PWscf code of the Quantum ESPRESSO distribution.28,29 The valence wave functions were described by a plane-wave basis limited to 30 Ry, with a charge density Fourier representation of 300 Ry. The system was described with periodic supercells, with the Au(110) surfaces separated in the z direction by more than 1.4 nm of vacuum, and the lattice parameter set to the calculated equilibrium (0.418 nm). Simulations were run following the same protocol as that in ref 10. During the structural relaxations, the lowermost two layers were kept fixed at their bulk-like coordinates. Integrals in the Brillouin zone were performed on a regular (4×4×1) k-point mesh together with a Marzari−Vanderbilt smearing of 0.02 Ry.

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RESULTS Local Structure of Self-Assembled Molecular Chains: STM. The evolution of the assembling structure of FePc molecular rows as a function of density on the Au(110) substrate was imaged by STM, as reported in Figure 1. The FePc molecules deposited on the Au(110)-(1×2) reconstructed surface self-assemble in planar molecular chains 13234

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Figure 3. LEED diffraction patterns of FePc/Au(110) as a function of the molecular deposition, taken at low current (1.0 nA). (a) (1×2) clean Au(110); (b) (5×5) FePc; (c) (5×7) FePc; (d) ×3.5-fold, (5×7) FePc chains with extinction (see text). Parts a, b, and c) were taken at 60 eV electron primary energy, and part d was taken at 80 eV electron beam energy. The panel d size is slightly magnified to be comparable with the other panels.

molecular periodicity is maintained 5-fold with respect to the Au lattice constant along [11̅0] (0.282 ± 0.005 nm). Each single molecule in the chain is in contact with the underlying gold rows, interacting via the central metal atom, as will be later shown by GIXRD and DFT calculations. The flat geometry of these additional chains allows the ligand to engage a molecule− molecule interaction, resulting in this configuration with the same rotation angle α with respect to the chain direction. At the SL coverage, a compact structure is observed in different areas (Figure 1d), with an interchain distance close to 3.5 times the underlying Au unit cell vectors along the [001] direction, compatible with a ×7 substrate symmetry. This compact structure presents domains coexisting with the 5×7 phase. The molecular network is compact and results less corrugated, with an interchain distance of 1.4 ± 0.1 nm along the [001] direction. When the spatial distribution of the chain is compact, the underlying metal substrate preserves a ×7 symmetry. The interactions at the molecular periphery dominate the FePc assembly. Thus, the molecular selforganization into chains is mainly determined by the molecule−molecule interaction in all the observed phases, while the chain−chain distance is determined by a concerted arrangement of molecular layer and Au surface. This picture is consistent with the observed chain structures of CuPc deposited on the Au(110) surface, where a similar sequence of (5×5) and (5×7) structures and compact molecular chain layer with a “disordered (1×3)” phase is observed,15 compatible with the present FePc compressed chain phase. Long-Range Chain Ordering: Diffraction Experiments. The local structures of the FePc chains observed in direct space by STM imaging have their long-range ordered counterpart, as detected by LEED and GIXRD in reciprocal space. By comparing the Fourier transform (FT) of the structures observed by STM (Figure 2) with the experimental electron diffraction patterns shown in Figure 3, we recognize the same behavior. All phases of FePc deposition on Au(110) kept at 410 K present long-range order, and the observed LEED reconstructions map one-to-one the FT-STM images. There are clear ×5, ×7, and ×3.5 (×7 with extinction) reconstructions along the [001] direction at the three coverages, respectively, as visible by the sharp diffraction spots. The first two highly ordered phases in the LEED pattern (Figure 3) present ×5 and ×7 reconstructions, respectively, characterized by sharp integer-order spots and slightly diffused non-integer-order peaks with stripes in the diffraction pattern along [001] stacked along the [11̅0] direction. These stripes are caused by the low correlation among the relative position of the parallel chains, as recently discussed considering the first adsorption phase (×5).10

Long-range ordering and correlation among molecular chains appear stronger for the ×7 structure than for the 5-fold phase, as shown by the more defined non-integer-order spots superimposed to the stripes. We can also observe that, compared to the FT of the STM images, the ×5 reconstruction LEED pattern displays definitely sharper integer-order spots. This is probably due to the efficient diffraction coming from the gold rows separating the molecular chains. Indeed, the gold rows are in close registry with each other and form a welldefined 1×5 superstructure, leading to sharp features in the diffraction pattern, at variance with the phase among adjacent molecular rows which is not so well-defined. The same holds for the 7-fold reconstruction, where the integer-order spots are sharper in the LEED pattern than in the corresponding FT image (Figures 3c and 2b). At variance with STM images, which essentially depend on the topmost layer, a LEED electron beam with some tens of eV kinetic energy has a probing depth of about 1−2 nm; thus, the LEED pattern has contributions from both molecules and the underlying substrate. The sharp integer-order spots of the 7-fold reconstruction are thus an indication of the efficient diffraction coming from the 1×7 superstructure of the gold substrate along the [001] direction induced by the FePc molecules. The GIXRD experiment presented in the following section will further confirm these conclusions. At about 80 eV electron kinetic energy, the ×7 LEED pattern switches to a ×3.5 pattern, and shows an apparent phase shift between the upper and lower parts (Figure 3d). This behavior is attributed to an energy-dependent spot extinction. However, a contribution from the native ×3.5 “compact structure” areas, observed by STM at these coverages, cannot be excluded. A similar diffraction pattern has been previously observed also for CoPc, NiPc, ZnPc,12 and CuPc15 on the same surface, and labeled as 5×3 12 or “disordered 5×3” 15 phases. Our combination of low-current-density LEED, GIXRD, and STM data allows the compact-SL structure to be understood as being due to compressed parallel FePc chains on top of a 7-fold reconstructed Au layer. Increasing deposition leads to coherence loss in the secondorder rows which get back to stripes, whereas the first-order ones, related to the gold substrate, still show spots superimposed to stripes. At the same time, the background gradually increases, up to the LEED pattern dissolution. All ordered phases present a reconstruction close to ×5 along [11̅0]: 4.79 ± 0.03 r.l.u. for the lower coverage phases and 4.91 ± 0.05 r.l.u. for the compact SL phase. This slight misfit compared to the Au periodicity justifies the low correlation between adjacent rows and the striped patterns along [001]. 10 These observations confirm the plasticity of the Au(110) surface, with the concerted arrangement of the missing Au rows and 13235

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reconstructed domains of the clean missing-row ×2 Au surface (∼80 nm). The 7-fold symmetry is detected for exposure times well beyond the SL coverage. All the curves obtained after molecular deposition are dominated by strong fractional diffraction peaks that are mainly sensitive to the atomic rearrangement of the Au topmost layers. This is due to the larger Z atomic number of Au atoms with respect to the elements composing the FePc molecules, and to concerted gold atom reconfiguration occurring in the structural phases. This behavior of the Au(110) substrate is common to other organic overlayers, as fullerene,33,34 pentacene,22 and CuPc.15,32 The FePc deposition has been made in the same scattering chamber of the GIXRD experiment, so as to have a straightforward continuous control of the molecule-induced substrate reconstructions, with subsequent plastic variation of the Au missing-rows. The kinetics of the surface reconstruction has been followed by measuring the relevant fractional order diffraction spots along the [001] direction, namely, the ×2, ×5, and ×7, corresponding to the substrate (0,1/2,0.3), (0,1/5,0.3), and (0,1/7,0.3) fractional peak intensities. The kinetics of the ×2, ×5, and ×7 in-plane reflections along the [001] direction as a function of FePc deposition time is shown in Figure 5.

added MPc chains being adjusted in different sequences (×2, ×3), in order to optimally host the chemisorbed molecules in a low corrugated highly interacting interface layer. The present data complete previous observations of FePc adsorption on Au(110)10,12,13 and indicate a common phenomenology with the CuPc15,24,32 adsorption on the same surface. The effects of the FePc deposition on the gold substrate reconstruction have been determined by grazing incidence Xray diffraction. The higher probing depth of GIXRD (few nanometers in our experimental conditions) and the scattering power of heavier atoms provide a direct measurement of the Au substrate contribution to the observed structural phases. The evolution of the FePc induced structures onto the Au(110) surface has been measured by K and H scans along [001] and [11̅0], respectively. This allows one to relate the GIXRD data with the previously discussed LEED patterns and STM images. The K-scan diffraction patterns measured along the [001] direction at H = 0 for the different FePc/Au(110) phases as a function of FePc coverage are shown in Figure 4. The topmost

Figure 5. GIXRD I×2, I×5, and I×7 fractional-order diffraction reflection intensity for the FePc/Au(110) structures, measured on the ×2 (blue dots), ×5 (red open squares), and ×7 (green triangles) measured during FePc deposition, as a function of molecular deposition time.

During deposition, the 1/2 clean-substrate fractional peak gradually decreases as the 1/5 fractional reflection takes place. Once the clean Au(110) reconstruction disappears, a clear 5fold structure emerges, while, upon further deposition, the ×5 peak slowly vanishes and the ×7 increases in intensity until saturation. This result indicates a strong rearrangement of the Au atoms on the gold surface driven by the FePc chain formation. We underline that the saturation coverage of compact FePc nanochains with the ×3.5 LEED structure, taken on the same FePc/Au(110) system in a different UHV chamber, corresponds to an underlying Au reconstruction with 7-fold symmetry. Modeling the Substrate Reconstruction and Adsorption Geometry. The structure shown in Figure 3c, presenting large ×7 reconstructed domains, has been further characterized by GIXRD measuring several points in 8 crystal truncation rods (CTRs) and in 48 fractional order reflections (FORs). The difficulties encountered in building a structural description of this system are due to the disorder along the [11̅0] direction, proved by STM, and partly to the multidomain nature of the

Figure 4. GIXRD K-scan at H = 0 of the FePc/Au(110) system, as a function of FePc molecular coverage. Intensity plotted in a logarithmic scale. From top to bottom (data stacked vertically for convenience): ×2 clean Au(110), ∼0.1 SL FePc, ×5-FePc phase, ×7-FePc phase. The structures labeled I×2, I×5, and I×7 correspond to the (0, 0.5, 0.3), (0, 0.2, 0.3), and (0, 0.143, 0) reflections, respectively.

curve related to the Au(110)×2 missing-row substrate shows a peak at K = 1/2, as expected. Upon depositing ∼0.1 SL of FePc, a ×n structure emerges with n = 12 (second K-scan from top, Figure 4), indicating a sensible mass transport of Au atoms. The ×n structure eventually evolves into different highly ordered FePc structural phases, namely, the ×5 and ×7 symmetry phases, in excellent agreement with the LEED and STM observations. From the full width at half-maximum (fwhm) of the fractional-order reconstruction peaks (of about 0.04 Å−1), we estimate a correlation length of the ordered domains of several tenths of nm, comparable to the size of the 13236

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system, discussed in the previous section. Nevertheless, our GIXRD analysis identifies the gold substrate profile along the [001] direction and the plausible adsorption site of the FePc molecules. The integrated intensities were recorded for a total of 455 independent reflections specific of the ×7 structure for H = 0, ±1, and ±2, and after averaging between equivalent reflections reduced to 300 nonequivalent reflections. A single fractional peak along the [11̅0] direction was detected in a position close to H = 1/5, reflecting the slight morphological misalignment among the molecular chains with respect to the surface lattice, found by STM (Figure 1a−c) and discussed in ref 10. The modeling of the Au reconstruction has been performed by a least-squares fitting method using the ROD35 code, taking into account the experimental CTRs and FORs as a function of vertical transfer momentum L. The analysis of the CTRs gives evidence of the corrugation induced by the ×7 structure. The fit procedure of the CTRs as a function of vertical transfer momentum L is based on the β-model to describe the surface roughness.36 The FePc molecules induce a gold surface arrangement in the topmost two layers, producing stripes and top rows that almost maintain the same number of gold vacancies as the (1×2) missing-row reconstruction. The resulting Au surface corrugation corresponds to 0.16 nm, indicating that the Au surface underneath the phthalocyanine molecules presents a rather flat profile. In the fitting of the whole GIXRD FOR data set, the positions and the occupancies of the gold atoms have been varied, taking into account the projection of the gold structure on the 1×7 cell along the [001] direction. Due to the lack of information along the [11̅0] direction, a rearrangement of the gold atoms along this direction in the form of an atomic reordering or vacancies cannot be excluded. As a first step of simulation, no molecular overlayer has been included, following the method described in refs 32 and 15 for the CuPc/Au(110) system. Different models for the Au reconstruction have been considered: a flat surface model (where six rows are missing to form the ×7 profile), the combination of two ×2 missing rows and one shallow ×3 subunit (“2-2-3” model, Figure 6a), and various combinations of subunits grouped to form a 1×7 Au cell. Both the “2-2-3” and the flat models resulted in a very poor fit compared to the experimental data. It is worth noting that the “2-2-3” profile roughness is twice the roughness of the flat model. Among the other models compatible with a ×7 reconstruction and a low roughness, the most plausible profiles display a gold surface arrangement alternating ×4 and ×3 flat subunits along the [001] direction. These two models, shown in Figure 6b and c, differ in the arrangement of the topmost gold atoms at the boundary between the subunits, as indicated in the figure. In both cases, the topmost subunits result higher than the lower plane by about 0.155 nm, confirming the low roughness value (0.16 nm) obtained by the β-model of the CTR fitting. Once the best plausible models for the Au ×7 profile are obtained, we simulate the presence of the molecules, by including two Fe atoms. The choice of mimicking the FePc molecule with the central metal atom is dictated by the minor contribution of the organic macrocycles to the GIXRD cross section. The 1×7 resulting models, visualized in Figure 6b and c, consist of 66 gold atoms distributed over 10 layers and 2 adsorbed Fe atoms. A total of 70 parameters were refined, 28 corresponding to x-coordinates and 41 to z-coordinates plus 1 scaling factor. The goodness-of-fit factors considered are both the χ2 and the unweighted residual or R-factor.37 The best

Figure 6. Model obtained from CTR analysis of the GIXRD data: (a) “2-2-3” model; (b) “4-3” model allowing Fe bridge/hollow adsorption configuration; (c) “4-3” model allowing on-top/short-bridge configuration of Fe after molecular adsorption. The molecular outlines are simple guides to the eyes and have not been included in the fitting. Central molecular Fe atoms are indicated in red.

simulations of the set of experimental data are obtained for the “4-3” in two possible arrangements: the bridge model (Figure 6b) where the inequivalent Fe atoms occupy the bridge adsorption sites (R = 29% and χ2 = 3.3) and the on-top model (Figure 6c) allowing Fe to adsorb in on-top configurations along the Au rows, corresponding to the best fit (R-factor = 26% and χ2 = 2.6). Finally, models based on the extended 5×7 symmetry of the gold substrate and the molecular layer have been simulated, including the whole of FePc molecules along each chain. The analysis has been performed by a modified version of ROD code, that takes into account the molecules as rigid bodies, reducing the number of independent parameters.38 The best fit expanded 5×7 model considers 295 gold atoms distributed over 10 layers and two inequivalent FePc molecules adsorbed on-top. It confirms the “4-3” profile along the [001] direction. This new model contains 18 more structural parameters than the (1×7) one as a consequence of the suppression of movement restrictions along the [11̅0] direction of the Au atoms in the three topmost surface layers. The resulting R-factor is achieved for the extended 5×7 model with a value of 16% and χ2 = 1.5. Some representative experimental reflections are shown in Figure 7 and compared with the fit related to the bridge and on-top 1×7 models and to the 5×7 structure. The curves reflect the improving of the two goodness-of-fit factors. In spite of the increased number of parameters in the extended model, the reduction of the R-factor is still relevant. It indicates that the gold atomic relaxations along the [11̅0] direction have an influence as expected. Further details on our GIXRD systematic analysis are reported in the Supporting Information. The GIXRD analysis, integrated with the information derived from the STM profiles, indicates that all the FePc molecules 13237

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Figure 8. End-simulation configurations and projected density of states for FePc adsorbed on the proposed ×7 unit cells of Figure 6. (a) The “4-3” on-top model allows for FePc to adsorb either on-top or in short-bridge configurations on the Au(110) reconstructed surface. (b) The “4-3” bridge model allows for hollow and long-bridge adsorption sites. In the lower parts of panels a and b, the corresponding p-DOS data are plotted: on the FePc (black dotted, filled yellow), Fe-d states (black solid), and Fe-d states with an “out-of-plane” component: dxz and dyz (filled blue/dark gray), dz2 (filled green/light gray). Figure 7. Six representative nonequivalent FOR experimental out-ofplane scans, compared with the fit obtained by the GIXRD analysis of the FePc/Au(110) models discussed in the text: on-top Fe atoms onto the 4-3 of the 1×7 profile (red dashed lines), long-bridge Fe atoms onto the “4-3” of the 1×7 profile (blue dashed lines), and two inequivalent on-top FePc molecules onto the 5×7 model of Au substrate (solid black line). L denotes the perpendicular momentum.

eV/molecule. A 50% lower interaction energy of about −0.46 ± 0.04 eV/molecule is obtained when FePc is adsorbed with the Fe either on the long-bridge or hollow sites of the “4-3” bridge model. Following these energetic considerations, the “4-3” ontop model is the most favorable one, thus supporting the GIXRD-derived configuration of Figure 6c. Looking at the STM images, the relative shift of the molecules in a chain with respect to the nearest chain is not random, but it is driven by the most favored adsorption site of the central metal atom, with a small energy difference (0.02 eV/molecule in the wider channels in the favored “4-3” on-top model). The interaction energy of this ×7 FePc adsorption phase is compatible with that calculated for the ×5 lower density phase (corresponding to −0.87 eV/molecule). Once the energetics of the FePc adsorption on Au are determined, a further important step is to determine how the substrate reconstruction and/or the adsorption site can influence the interaction process and consequently the electronic properties calculated by DFT. The calculated projected density of states (pDOS) for the “4-3” on-top model are shown in Figure 8, lower graphs of panel a. The sequence and the symmetry of the electronic states are only marginally affected by the Au surface reconstruction, while the major influence is given by the Fe adsorption site. We briefly remind you that the free FePc molecule has an experimentally observed 3Eg ground state with unpaired electrons in eg (dxz,yz) and a1g (dz2),39−42 as recently confirmed by DFT-GGA simulations.25,43 Recent experiments have clearly shown that the out-of-plane orbitals are strongly coupled to the Au metallic states, upon interaction.12,25 In Figure 8, lower graphs of panel a, we present the pDOS calculated for the top configuration. Both on-top and short-bridge adsorption sites share common distinctive pDOS features: there is an inversion between the a1g and eg orbitals with respect to those of the free molecule,25,43 leading to hybridized and redistributed states across the Fermi level, mixing the empty spin-down eg and filled a1g states with the gold metallic states. This electronic configuration is analogous to that observed for the (5×5)-symmetry structure, where molecules have been shown to be adsorbed on-top/ short-bridge Au sites, as well.10,25 This result is compatible with

remain in contact with Au atoms: the topmost Au rows provide a registry for the top lying molecular row of the ×7 structure (Figure 1b and c), while the molecular dark stripes in Figure 1c remain in contact with the lower Au plane. The proposed “4-3” on-top model unambiguously defines the Au reconstruction, while the analysis of the GIXRD “partial” data set cannot provide a full structural determination and is less sensitive to the precise FePc adsorption site. Thus, we perform density functional theory (DFT) calculations to identify the lowest energy FePc adsorption sites and their interfacial properties. Among the variety of 3D-metal-Pc adsorbed on Au(110), FePc and CoPc clearly revealed an interaction between the central metal atom and the underlying Au atoms, breaking the pure four-fold symmetry of the molecule, as deduced by photoemission and absorption spectroscopy.10,12 DFT calculations can rationalize this interaction depending on the specific adsorption site. Among the possible adsorption models, the adsorption on either flat or corrugated seven-fold surfaces is excluded on the basis of energetic arguments. These two most plausible “4-3” configurations have been modeled considering reduced systems, where each subunit represents a portion of the 5×7 unit cell, as shown in Figure 8a and b, respectively. Each reduced system presents two possible configurations for the FePc molecule, leading to four possible adsorption sites, namely: (i) for the “4-3” on-top model, Fe atom either on-top of Au atom in the row or short-bridged between two adjacent Au atoms along the [11̅0] chain direction; (ii) for the “4-3” bridge model, Fe atom in long-bridge position between two Au atoms along [001] or hollow (coordinated by 4 Au atoms in two dimensions). The adsorption sites for the FePc molecules in the “4-3” ontop model are either on-top or short-bridged, both leading to a comparable FePc−Au interaction energy of about −0.84 ± 0.04 13238

dx.doi.org/10.1021/la302192n | Langmuir 2012, 28, 13232−13240

Langmuir

Article

Project Nos. CSD2007-0004 and MAT2009-09308. This work was supported by the MIUR project PRIN contract N. 2008525SC7. S.F. acknowledges the CINECA Award N. HP10BJMJX0, 2012, for the availability of high performance computing resources and support. S.F. thanks the “Young SISSA Scientists & Research Projects” scheme 2011-2012, promoted by the International School for Advanced Studies (SISSA), Trieste, Italy.

the valence-band photoemission results whose spectral density of states is not dependent on the molecular density/coverage.12 The pDOS of both the hollow (Figure 8, lower graphs of panel b) and long-bridge sites (not shown) in the “4-3” bridge model differ significantly from that of the “4-3” on-top model. The sequence and the symmetry of the states resemble those of the free FePc molecule.25 In particular, in these cases, the inversion of the Fe-d orbitals and the consequent hybridization with the underlying gold states are not present.

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CONCLUSIONS The self-organization of FePc chains on the Au(110)reconstructed channels causes a sequence of n-fold reconstructions of the underlying gold surface, as a function of molecular density. The phase diagram for FePc/Au(110), dominated by the FePc−Au interaction, presents coverage-dependent phases involving FePc organization into chains, and is fully characterized by a multitechnique investigation and a robust protocol for replicating the sample preparation in different apparatuses. An effective “plasticity” of the Au surface as a function of FePc coverage allows one to optimize the interaction and lowers the total system energy by concerted substrate atomic reconfigurations. The molecular packing imaged by STM leads to a molecule-induced reorganization of the Au surface, as determined by diffraction techniques. We experimentally identify the most stable substrate reconstruction corresponding to the high-density 7-fold symmetry phase, in agreement with lowest energy configurations obtained by DFT. Furthermore, the calculated energetics and the subsequent pDOS analysis clearly reveal the role of the central metal atom in determining the adsorption site, resulting in the on-top/ short-bridge Fe position on the Au rows. The chain assembling is driven by the molecule−molecule interaction and the chains interact with the Au nanorails via the central metal atom, while the chain−chain distance in the different structural phases is basically driven by the plasticity of the Au surface. The architectural motif with the on-top/short-bridge Fe central atom in contact with the gold rows rationalizes the FePc−Au electronic mixing as confirmed by DFT calculations and previous experimental results. Finally, the robust pattern of the ordered supramolecular FePc assembly at different areal densities, induced by the Au(110) surface, may open new perspectives toward exploiting new functionalities associated with the central Fe ions.

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ASSOCIATED CONTENT

S Supporting Information *

Details on the STM results and GIXRD analysis. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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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 We thank the ID08 and ID03 beamline staff at the ESRF synchrotron radiation laboratory in Grenoble, in particular Dr. R. Felici, for useful discussions. The Sapienza Università di Roma is acknowledged for funding. X.T. thanks the Spanish MCINN agency for partially funding this project through 13239

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