Correlation between Charge Transfer and Adsorption Site in CoPc

Article pubs.acs.org/JPCC

Correlation between Charge Transfer and Adsorption Site in CoPc Overlayers Adsorbed on Ag(100) E. Salomon,*,† D. Beato-Medina,† A. Verdini,‡ A. Cossaro,‡ D. Cvetko,§ G. Kladnik,§ L. Floreano,‡ and T. Angot† †

Aix Marseille Université, CNRS, PIIM UMR 7345, 13397, Marseille, France CNR IOM Lab TASC, I-34149 Trieste, Italy § Faculty of Mathematics and Physics, University of Ljubljana, Jadranska 19, 1000 Ljubljana, Slovenia ‡

ABSTRACT: In this paper we investigate the local adsorption geometry of CoPc films adsorbed on a Ag(100) surface and its effect on the molecule−substrate interactions. Using high-resolution electron energy loss spectroscopy we demonstrate that the charge-transfer mechanism at the organic−metal interface depends on the structural properties of the CoPc film. We discuss these findings in terms of the molecular adsorption sites, as determined by X-ray photoelectron diffraction measurements and multiple scattering simulations. We show that the distance between the central Co atom and the topmost Ag layer is of the order of 0.3 nm. In addition, we demonstrate that the CoPc molecules adsorb on different atomic sites depending on the superlattice symmetry, as varied by changing the coverage. This is further confirmed by low-energy electron diffraction and scanning tunneling microscopy measurements, emphasizing that, depending on the molecular superstructure adopted by the molecules, there are one or two molecules per unit cell.

■

INTRODUCTION Organic−metal (O−M) interfaces are essential to organicbased devices and constitute a fascinating research topic, as the interaction between π-conjugated organic materials with metal surfaces is affected by the interplay of complex fundamental mechanisms.1−4 These effects, like dispersive interactions, Pauli repulsion, interface states, interface dipole, covalent, and/or ionic bonds, are still not completely understood on a fundamental level. Among the important parameters governing organic−metal interface energetics, the adsorption geometry and adsorption height of the organic material with respect to the underlying substrate appear as important parameters for the control of the charge transfer or band bending.5−11 Out of the various possible O−M interfaces, those involving metal−phthalocyanines (MPc) are among the most widely studied as MPcs can be used in a broad range of applications, including electronic, optical, or magnetic devices.12−14 The variety of possible central atoms offers the opportunity to tune their optoelectronic properties as well as their reactivity toward metal surfaces. In addition, MPcs are known to form wellordered films on various metal surfaces and thus can be used as model systems for both device-relevant and theoretical investigations. In the case of MPc films adsorbed onto Ag, it has been demonstrated that the charge-transfer mechanism between the substrate and the molecules depends on the nature of the central atom. For M = H2, Cu, and Zn, it has been shown that the charge transfer occurs between the substrate and the lowest-unoccupied molecular orbital (LUMO) of the mole© XXXX American Chemical Society

cules, leading to the appearance of a new electronic state in the gap that is attributed to the partial filling of the LUMO.15−18 However, for M = Co or Fe, because of the specific electronic configuration of these two elements, and more particularly the specific filling of the 3d orbitals, the charge-transfer mechanism can be different. Indeed, it has been shown that with such central atoms, the charge transfer does not necessarily involve the LUMO, but the unoccupied molecular orbitals which are highly localized on the central atom instead.4,19−22 Nevertheless, even if the experimental data were recorded for similar film thicknesses, i.e., close to the monolayer coverage, the orientations of the Ag substrate and/or the molecular superstructures are not necessarily similar. Hence, we can not attribute the difference in the molecule−substrate interaction solely to the nature of the central atom. In fact, to satisfactorily characterize the O−M interface, it is important to determine the correlation between the local environment of the molecules and their electronic properties. Indeed, site-specific effects for such a type of molecules were reported in the literature.23,24 With this aim, we carried out scanning tunneling microscopy (STM) and X-ray photoelectron diffraction (PED) measurements together with high-resolution electron energy loss spectroscopy (HREELS) experiments on two different molecular superstructures of CoPc/Ag(100), namely the (5 × Received: June 23, 2015 Revised: September 23, 2015

A

DOI: 10.1021/acs.jpcc.5b05999 J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

■

5) and the (7 × 7) phases, for coverages close to the monolayer completion. By studying the vibrational spectra, we show that there is a difference in the O−M interaction mechanisms. Combining these measurements with local probe techniques, i.e., STM and PED, we demonstrate that there are two different local adsorption sites and orientations for the CoPc molecules. Hence, we suggest this difference in the molecular environment to be the source of the different charge-transfer mechanisms observed at the O−M interface.

■

Article

RESULTS AND DISCUSSION

In the case of organic materials deposited on surfaces, different molecular arrangements can be obtained depending on the growth’s conditions and/or coverage. In this work we compare two different phases of CoPc/Ag(100), the (5 × 5) and the (7 × 7) phases. While the first one has already been observed and discussed in the literature,27,28 we report and describe here a new phase, the (7 × 7) phase. Figure 1 represents an STM image of the (5 × 5) phase of CoPc/Ag(100).

EXPERIMENTAL DETAILS

All experiments were performed at room temperature and under ultrahigh vacuum conditions. High-resolution electron energy loss spectroscopy was carried out using a VSI DELTA 0.5 spectrometer with a typical integration momentum and ultimate resolution of Δq// = 0.005 Å−1 and ΔE = 0.5 meV, measured in straight-through mode. The incoming electron beam energy was measured from the cutoff of the loss spectra, and the typical energetic resolution, estimated from the full width at half-maximum (fwhm) of the elastic peak, was better than 4 meV. The LEED optics used were a four-grids SpectaLeed Omicron system. STM images were acquired by using a commercial Omicron VT-STM. Home-made STM tips were fabricated from electrochemically etched tungsten wires in 2 M NaOH solution. STM images were recorded in constantcurrent mode and processed using the WSxM software.25 Linear electronic drift correction was systematically used to compensate for possible thermal and mechanical drift of the probe. The photoelectron diffraction (PED) experiments were performed at the ALOISA beamline of the Elettra Synchrotron Light Source in Trieste, Italy.26 The PED polar scans have been measured by collecting the photoemission signal as a function of the polar emission angle by rotating the electron analyzer in the scattering plane for different orientations of the surface azimuth. The incidence angle of the photon beam was kept fixed at 4° with the polarization in the transverse magnetic condition and the surface normal in the scattering plane. The Co 2p3/2 photoemission core line has been selected for the PED measurements with a kinetic energy of 160 eV in order to have a good compromise between photon flux, energy resolution, and PED signal-to-noise ratio. The angle-resolved intensity was measured for polar angles ranging from 0° up to 67° from the surface normal and for azimuthal angles over a range of 107°, covering the two nonequivalent symmetry directions ⟨011⟩ and ⟨010⟩ of the underlying Ag substrate. No difference in the XPS spectra of Co, C, and N was detected after several hours of illumination of ∼1 keV X-rays. The Ag(100) single-crystal surface was cleaned by repeated cycles of Ar+ sputtering with 1500 eV followed by annealing to 730 K. Subsequently, the CoPc molecules were deposited on the clean Ag(001) surface held at room temperature by thermal evaporation from a quartz crucible. Typical evaporation rates were 0.4 ML/min. The CoPc molecules, commercially supplied by Sigma-Aldrich, were thoroughly degassed prior to experiments to achieve a background pressure in the 10−10 mbar range during deposition. Under these conditions, the (5 × 5) and (7 × 7) phases were obtained after deposition of about 0.80 ± 0.05 and 0.95 ± 0.05 ML, respectively.

Figure 1. (a) 29 × 23 nm2 STM image of the (5 × 5) film of CoPc/ Ag(100). Vb = −0.32 V (filled states); It = 0.17 nA. The black arrows represent the Ag⟨011⟩ and the direction of the molecular axis (M.A.). (b) Corresponding LEED pattern, E0 = 32 eV. The circled spots correspond to the Ag(100)-(1 × 1). (c) Line profile along the Ag⟨011⟩.

On the micrograph, one CoPc molecule appears as a four-leaf clover with a bright center, and its molecular axis (M.A.) is oriented at 28° ± 1° from the Ag⟨011⟩. Along the latter direction, the film shows a periodicity of 14.6 ± 0.2 Å, as highlighted by the line profile displayed in Figure 1c. This distance is comparable to the expected 14.45 Å distance for a 5fold periodicity, and it is consistent with the van der Waals radius of the molecule. These measurements are actually consistent with the data from the literature, which demonstrate that isolated molecules are oriented with their molecular axis at ±30° from the highest symmetry directions of the Ag(100) surface, independent of the nature of the central atom, and that at the molecular density of the (5 × 5) phase, the molecules present similar adsorption geometry except from a slight azimuthal rotation of their molecular axis.22,27,28 The STM image recorded on the (7 × 7) surface is given in Figure 2a. The line profile recorded along the Ag⟨011⟩ (Figure 2c, bottom curve) shows a periodicity of 20.3 ± 0.2 Å, a value matching the 20.23 Å spacing expected for the 7-fold periodicity. Interestingly, the line profile recorded along the Ag⟨001⟩ (Figure 2c top curve), which is the direction corresponding to the nearest neighbor molecules, shows an intermolecular spacing of 14.5 ± 0.2 Å. However, adjacent molecules along the Ag⟨001⟩ direction display different azimuthal orientations. At variance with the (5 × 5) phase, the (7 × 7) unit cell thus contains two nonequivalent molecules, which are azimuthally rotated by 30° and 5° with respect to the substrate Ag⟨011⟩ direction. This point is further highlighted in the top-right inset in Figure 2 showing a 4.5 × 4.5 nm2 image of the (7 × 7) phase, which clearly depicts two different M.A. orientations, one along the green arrows and the B


Article


film of CoPc. This latter spectrum can be used as a reference for CoPc molecules not interacting with the substrate. In the cases of the spectra corresponding to the (5 × 5) and the (7 × 7) films, they both exhibit a similar overall aspect, but there are some subtle differences that should be noted and addressed. As depicted in Figure 3, the spectrum corresponding to the (7 × 7) phase presents six additional peaks, as compared to the spectrum of the (5 × 5) phase, located at 248, 321, 461, 940, 1134, and 1340 cm−1. The physical origins of these additional peaks, as well as of all the others, are described in Table 1. Table 1. Vibrational Modes Assignmenta peak position (cm−1)

Figure 2. (a) 28 × 14 nm2 STM image of the (7 × 7) film of CoPc/ Ag(100). Vb = −0.76 V (filled states); It = 0.23 nA. Top right inset, zoom-in: 7 × 7 Å2. (b) Corresponding LEED pattern, E0 = 31 eV. The circled spots correspond to the Ag(100)-(1 × 1). (c) Line profiles along Ag⟨001⟩ (top curve) line and Ag⟨011⟩ (bottom curve).

other one along the blue arrows. We also emphasize that while the molecules appear with a 4-fold symmetry on the (5 × 5) pahse, the molecules in the (7 × 7) phase exhibit a slightly distorted shape with a 2-fold symmetry. This is due to the pairing of two nonequivalent molecules in the (7 × 7) unit cell. The fact that there are two nonequivalent molecules per unit cell in the case of the (7 × 7) phase is particularly interesting as it may suggests two different adsorption sites, which is not the case for the (5 × 5) phase. Moreover, because different arrangements or different adsorption sites may involve different reactivity or charge-transfer mechanism, we performed HREELS measurement on both phases to emphasize these points. Figure 3 displays HREELS spectra recorded on both a (5 × 5) and a (7 × 7) film of CoPc on Ag(100). For the sake of comparison, we have also plotted the spectrum of a multilayer

exptl

theo

175 248 321 382 430 461 727 761 940 1134

161 252 294 341 433 475 707 755 937 1128− 1153 1336

1340

assignment for an isolated molecule29

vibrational activity

iso NM OPB ring bre NM iso OPB ring NM OPB CCC OPB iso IPB CH Py OPB CH OPB NM CNmC IPB, iso def CH IPB

infrared raman infrared infrared infrared raman infrared infrared raman raman

iso NM str, CH CNmC IPB

raman

symmetry A2U A1G A2U A2U A2U B2G A2U A2U B2G B1GA1G A1G

a

Iso, isoindole; OPB, out-of-plane bending mode; bre, breathing mode; IPB, in-plane bending mode; Py, pyrrole; str, stretching mode.

From Table 1 we should first notice that the peaks observed on the (5 × 5) phase are all infrared active, whereas most of the additional peaks observed on the (7 × 7) phase present Raman activity. According to the HREELS dipole selection rules in the specular direction, which may be seen as those operating in infrared spectroscopy, observing an Raman-active mode suggests that they became infrared active because of a lowering of the molecular symmetry, because of O−M interaction.30 Such behavior has already been observed and discussed in the case of similar systems, such as ZnPc/Ag(110) and ZnPc/ Ag(100).31,32 Considering the correlation tables and according to the symmetry of the observed vibrational modes, the D4h molecular symmetry is most probably reduced to C2h or C2v for CoPc in the (7 × 7) phase. This supports the observations made on the STM image of the (7 × 7) phase, on which the two nonequivalent molecules appeared distorted with a 2-fold symmetry. Another striking issue to address is that at least two of the observed additional peaks, located at 1134 and 1340 cm−1, are strongly asymmetric. This specific profile can actually be interpreted as a Fano-like line shape, as evidenced by the good agreement between the experimental data and its fit (Figure 3b) using, for each of these features, the following analytic expression, corresponding to a generalization of Fanolike line-shape: L(ω) = L0 + cγωrω

Figure 3. (a) HREELS spectra recorded in specular condition (θi = θs = 62°) and at 3.0 eV primary energy, for the (5 × 5), (7 × 7), and a thick CoPc film. (b) Magnified view of the Fano-shaped peaks at around 0.14 and 0.17 eV together with their best fits (solid line).

[1 − (τ /γ )(ω 2 − ωr2)]2 [(γω)2 + (ω 2 − ωr2)2 ]

(1)

where L0 is an offset to take into account the experimental background, and c is an overall normalization constant; γ corresponds to the damping measured by the full-width at halfC


Article

The Journal of Physical Chemistry C maximum of the line shape; τ is the tunneling rate, and ωr is the fully renormalized Born−Oppenheimer vibrational frequency. The optimal fitting parameters used were ωr = 1153 cm−1 and γ = 90 cm−1, and the asymmetry parameter given by ωrτ is 0.74 for the first peak; ωr = 1371 cm−1 and γ = 97 cm−1 and the asymmetry parameter given by ωrτ is 0.99 for the second peak. These parameters, which are comparable to those determined in the case of ZnPc/Ag(110), reveal a strong nonadiabatic effect due to electron−phonon coupling.31,33−38 As demonstrated by Amsalem and co-workers, this type of coupling is concomitant to a partial filling of the lowest-unoccupied molecular orbital of the molecule due to a charge transfer from the substrate into the molecule.18,31 Interestingly, in the case of the (5 × 5) phase, for which no Fano-like line-shape is observed, it has been demonstrated that there is no partial filling of the LUMO under adsorption. In fact, when photoemission spectroscopy and theoretical calculations performed in the framework of the GW approximation are combined, it has been established that the charge transfer involves the unoccupied a1g molecular orbital, associated with the Co dz2 atomic orbital, rather than the eg molecular orbital that corresponds to the LUMO of the neutral molecule.4 This confirms that Fano-like line shapes are related to a partial filling of the LUMO of the molecule. Combining the STM and HREELS data, one can state that charge-transfer mechanisms are driven by the molecular arrangement and adsorption geometry. However, we do not know anything about the role played by the adsorption site. We know from the literature that on Ag(100) it is energetically more favorable for one single MPc (M = Fe, Co, Ni, Cu) molecule to adsorb on the hollow adsorption sites of the Ag surface. We also know that in the case of the (5 × 5) phase, the molecules present an adsorption geometry similar to that of an isolated one except from a slight azimuthal rotation of their molecular axis.22,27,28 Therefore, to further understand the differences observed in the STM, LEED, and HREELS between the (5 × 5) and the (7 × 7) molecular phases, we performed PED measurements on the (7 × 7) phase to thoroughly determine the local environment of a CoPc molecule in such a case. These measurements were carried out on the Co 2p3/2 core level of which a typical XPS spectrum taken with photon energy of 940 eV is shown in Figure 4a, together with its best fit

Figure 4b shows the anisotropic component χ of the experimental PED pattern, which bears the angular modulation of the diffraction pattern, and has been obtained as χ=

I(θ , ϕ) − I0(θ ) ⟨I(θ , ϕ) − I0(θ )⟩

(2)

In this equation, I(θ,ϕ) corresponds to the integral of the lowest binding energy Co 2p3/2 component used in the fitting procedure for given polar, θ, and azimuthal, ϕ, angles. I0(θ), which corresponds to the nondiffractive part, stands for the minimum value of I(θ,ϕ) for a given polar angle θ. Let us state here that this definition in not the usual one, but it has the advantage of enhancing the modulation of the anisotropy function.41,42 We believe our normalization to better show how different the pattern is from the mean value, which reflects directly the anisotropy. The overall anisotropy modulation of the PED signal exhibits a 2-fold symmetry with respect to the Ag⟨001⟩ direction, as indicated in the figure. The pattern presents several diffraction features stemming from the Co 2p photoelectrons backscattered by the Ag atoms underneath. To determine the relative position of the Co atoms with respect to the underlying Ag lattice, we compared the experimental PED patterns with the simulated ones. Photoelectron diffraction calculations were performed by the multiple scattering theory of photoelectrons using the Electron Diffraction in Atomic Clusters (EDAC) code.42 The atomic cluster employed consists of 349 Ag atoms for the Ag(100) substrate together with 1 atom of cobalt on top of it. Because of the low scattering cross section of C and N atoms with respect to Ag atoms, we first simulated the PED pattern by neglecting the molecular backbone to study solely the Co adsorption site. The simulations were carried out considering both the adsorption site of the Co atom and its distance from the last layer of the Ag(100) atomic cluster. From the STM images we know that there are two nonequivalent molecules within the (7 × 7) unit cell. From geometrical considerations, if one molecule is on an on-top site, the other one has to be on a hollow site and vice versa. On the other hand, if one molecule is on a bridge site, the other is expected to be on a bridge site as well. In this work we therefore privileged two different configurations; one with the Co atom evenly adsorbed on the on-top and hollow sites, and another one in which the Co atoms are adsorbed on the bridge sites. For each configuration, simulations were performed tuning the distance between the Co and the topmost Ag layer from 2.0 to 3.2 Å in steps of 0.1 Å. To determine the local adsorption geometry, and discriminate between possible arrangements of CoPc on Ag(100), we compared the experimental data χexp with the simulated anisotropy functions χsim calculated for different Co−surface distances and adsorption sites, including multiple sites. The similarity between the two χ functions was evaluated by computing the value of the reliability factor (Rf) described as follows:43,44

Figure 4. (a) Co 2p3/2 XPS spectrum together with its best fit, Ephoton = 940 eV. (b) PED pattern obtained of the (7 × 7) phase recorded on the Co 2p3/2 core level.

based on two different Gaussian peaks and a Shirley background. The lowest binding energy XPS feature at 778.6 ± 0.1 eV is attributed to the Co 2p3/2 core level line of the CoPc molecules adsorbed on Ag(100). The highest binding energy structure at 781.4 ± 0.1 eV arises from the multiplet structure observed in Co compounds.7,39,40

Rf =

∑ (χexp − χsim )2 ∑ [(χexp )2 + (χsim )2 ]

(3)

Figure 5 displays the reliability factor for each configuration as a function of the Co−surface distance. D


Article


each polar angle. We found that the best reliability factor is obtained again for a configuration in which the Co atoms are evenly distributed on the on-top and hollow sites and at a distance of 3.1 Å from the top Ag atomic layer. Given the broadness of the PED features, this result appears in good agreement with the configuration obtained using our anisotropy function. Therefore, for the refinement discussed below, we adopted the χ defined in eq 2. The fact that both the reliability factor and the visual resemblance are better in the case of the simulation using Co atom evenly adsorbed on the on-top and hollow sites suggests that this is the best configuration. Another important information to discuss is the Co−surface distance. According to the simulations, the Co atom stands at 3.0 ± 0.2 Å above the average distance of the topmost Ag atomic layer. This value is actually consistent with experimental and theoretical data from the literature demonstrating that for CoPc molecules adsorbed on Ag surfaces, the Co atom is at about 2.8−3.2 Å above the topmost Ag atomic layer.22,45,46 The second point to discuss concerns the adsorption sites. Interestingly, the configuration giving the best Rf includes two different adsorption sites, which is compatible with the fact that there are two nonequivalent molecules per unit cell, each presenting different orientations. To further refine the adsorption geometry, we intend to identify which specific azimuthal orientation of the molecules is associated with the on-top and hollow sites. With that purpose, we made some refinement in our cluster, introducing four nitrogen atoms surrounding the Co and corresponding to the pyrrole nitrogens of the macrocycle. According to the STM data, these N atoms, which define the molecular axis, are oriented either at ±5° or at ±30° from the Ag⟨011⟩ direction, as depicted in Figure 7.

Figure 5. Reliability factor as a function of the Co−surface distance.

When this procedure was followed, the lowest reliability factor, equal to 0.240, was obtained for a Co−surface distance of 3.0 Å and for a configuration in which the Co atoms are evenly adsorbed on the on-top and hollow sites. The relatively low value of the Rf factor provides a satisfactory agreement with the experimental PED pattern. Figure 6 exhibits the comparison

Figure 6. Top-left, experimental PED anisotropy; top-right, simulated PED anisotropy giving the best Rf; bottom-right, simulated PED anisotropy considering only bridge sites. Figure 7. Example of one atomic cluster used for the simulations. Blue, green, and gray balls correspond to nitrogen, cobalt, and silver atoms, respectively.

between the experimental PED pattern (top-left), the simulation giving the best Rf considering both hollow and top sites (top-right), and the simulation giving the best Rf and considering only bridge sites (bottom-right). Here again, the overall visual resemblance of the performed simulations with the experimental data seems to be better in the case of the simulation using two different adsorption sites. Given the relatively small difference between the different Rf values obtained for the different simulations and given the fact that we did not use the conventional anisotropy function, to double check our results and be more convincing we calculated also the anisotropy function for the data and the simulations commonly defined as the following:41,42 χ′ =

I(θ , ϕ) − I0′(θ ) I0′(θ )

Two adsorption configurations were tested. In configuration 1, the Co atoms are evenly distributed on the on-top and hollow site with their molecular axis at ±5° and ±30° from the Ag directions, respectively. In configuration 2, the Co atoms are evenly distributed on the on-top and hollow site with their molecular axis at ±30° and ±5° from the Ag directions, respectively. While there were no significant visual differences between the various simulations, as expected according to the relatively low scattering factor of the N atoms, the reliability factor did change after refinement as follow: Rf(conf 1) = 0.228 and Rf(conf 2) = 0.241. The lowering of the reliability factor using the first configuration indicates that we actually can discriminate the molecules adsorbed on the on-top sites from those on the hollow sites, which are oriented at ±5° and ±30° from the Ag directions, respectively. For the sake of discussion,

(4)

In this expression, I(θ,ϕ) is similar to the previous one and I0′ (θ) corresponds to the azimuthally averaged intensities at E


Article

The Journal of Physical Chemistry C let us highlight that in the adsorption configuration giving the best Rf, the molecules on the hollow site exhibit an adsorption geometry similar to that of a single molecule adsorbed on Ag(100), i.e., molecules on the hollow sites at 30° from the Ag’s directions.22,27,28 This difference observed in the local coordination of the Co atoms between these (5 × 5) and the (7 × 7) phases, i.e., only one adsorption site in the former case versus two different adsorption sites in the latter case, most probably is the origin of the difference observed in the loss spectra. As discussed before, this reveals different charge-transfer mechanisms that do not involve the same unoccupied molecular orbitals. The molecules display two orientations in the case of the (7 × 7) phase and only one for the (5 × 5) phase, which implies that the two molecules, or at least the two Co atoms, might display a different height because of different local reduction of the charge-density spillover of the metal.47−51 To assess this issue, we have calculated the reliability factor by changing separately the height of the Co atom in the two adsorption sites. The results, depicted in Figure 8, show that the configuration in

Figure 9. Ball and stick model of the local environment of the CoPc molecules adsorbed on Ag(100) and adopting a (7 × 7) reconstruction. Red, black, blue, green, and gray balls correspond to hydrogen, carbon, nitrogen, cobalt, and silver atoms, respectively.

adsorption sites, i.e., on-top and hollow. On the (5 × 5) phase, only one molecular orientation and one adsorption site, the hollow site, have been reported. HREELS data recorded on both phases suggest two different charge-transfer mechanisms: not involving the LUMO of the molecule in the case of the (5 × 5) phase, and involving the LUMO in the case of the (7 × 7) phase. The difference in the local coordination of the Co atoms between these two phases is most probably the origin of the difference observed in the loss spectra. One could further expect that there are locally two different charge-transfer mechanisms in the case of the (7 × 7) film. One involves the a1g molecular orbital associated with the Co dz2 atomic orbital and corresponding to the molecules adsorbed on the hollow site. The another one involves the eg molecular orbital, which corresponds to the LUMO of the neutral molecule and is associated with the CoPc adsorbed on the on-top sites. Finally, combining the STM, HREELS, and PED data, we can emphasize that charge-transfer mechanism depends on both the adsorption site as well as the adsorption geometry. More precisely, it is driven by the local coordination of the molecule and hence the spatial overlap between molecular orbitals and the charge density of the Ag substrate underneath.

Figure 8. Reliability factor as a function of the Co−Ag distance.

which both Co atoms are located at 3.0 ± 0.1 Å with respect to the underlying Ag substrate is indeed the most favorable one (circled portion in upper right part of Figure 8). Within our sensitivity, we cannot exclude that the backbone of the two types of molecules can be at different heights, but the fact that two Co atoms in different adsorption sites yield the same height above the surface can also be attributed to a cross-talk between adjacent molecules (likely mediated by the substrate), which levels the system to a common and intermediate height, as recently reported for 2D donor−acceptor mixed phases.42 From this work combining experimental and simulated data, it is finally possible to get a clear picture of the adsorption sites and geometry of CoPc molecules adsorbed on Ag(100), in the case of a (7 × 7) superstructure, as depicted in Figure 9.

■ ■

AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

■

REFERENCES

(1) Koch, N. Organic Electronic Devices and Their Functional Interfaces. ChemPhysChem 2007, 8, 1438−1455. (2) Ishii, H.; Sugiyama, K.; Ito, E.; Seki, K. Energy Level Alignment and Interfacial Electronic Structures at Organic/Metal and Organic/ Organic Interfaces. Adv. Mater. 1999, 11, 605−625. (3) Kronik, L.; Koch, N. Understanding Electronic Properties of Organic-Based Interfaces. MRS Bull. 2010, 35, 417−421. (4) Salomon, E.; Amsalem, P.; Marom, N.; Vondracek, M.; Kronik, L.; Koch, N.; Angot, T. Electronic Structure of CoPc Adsorbed on

CONCLUSION Combining STM and PED measurements carried out on the (7 × 7) phase of a CoPc film adsorbed on Ag(100), along with simulations performed in the framework of multiple scattering calculations, we demonstrate that the molecules are adsorbed at 3.0 ± 0.2 Å from the Ag surface, with two different azimuthal orientations and on two different, but well-identified, F


Article

The Journal of Physical Chemistry C Ag(100): Evidence for Molecule-Substrate Interaction Mediated by Co 3d Orbitals. Phys. Rev. B: Condens. Matter Mater. Phys. 2013, 87, 075407-1−075407-9. (5) Yamane, Y.; Gerlach, A.; Duhm, S.; Tanaka, Y.; Hosokai, T.; Mi, Y. Y.; Zegenhagen, J.; Koch, N.; Seki, K.; Schreiber, F. Site-Specific Geometric and Electronic Relaxations at Organic-Metal Interfaces. Phys. Rev. Lett. 2010, 105, 046103-1−046103-4. (6) Duhm, S.; Heimel, G.; Salzmann, I.; Glowatzki, H.; Johnson, R. L.; Vollmer, A.; Rabe, J. P.; Koch, N. Orientation-Dependent Ionization Energies and Interface Dipoles in Ordered Molecular Assemblies. Nat. Mater. 2008, 7, 326−332. (7) Auwärter, W.; Seufert, K.; Klappenberger, F.; Reichert, J.; WeberBargioni, A.; Verdini, A.; Cvetko, D.; Dell’Angela, M.; Floreano, L.; Cossaro, A.; et al. Site-Specific Electronic and Geometric Interface Structure of Co-Tetraphenyl-Porphyrin Layers on Ag(111). Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 81, 245403-1−245403-14. (8) Hosokai, T.; Machida, H.; Gerlach, A.; Kera, S.; Schreiber, F.; Ueno, N. Impact of Structural Imperfections on the Energy-Level Alignment in Organic Films. Phys. Rev. B: Condens. Matter Mater. Phys. 2011, 83, 195310-1−195310-7. (9) Yamane, H.; Yabuuchi, Y.; Fukagawa, H.; Kera, S.; Okudaira, K. K.; Ueno, N. Does The Molecular Orientation Induce an Electric Dipole in Cu-Phthalocyanine Thin Films? J. Appl. Phys. 2006, 99, 093705-1−093705-5. (10) Heimel, G.; Salzmann, I.; Duhm, S.; Koch, N. Design of Organic Semiconductors from Molecular Electrostatics. Chem. Mater. 2011, 23, 359−377. (11) Kera, S.; Yabuuchi, Y.; Yamane, H.; Setoyama, H.; Okudaira, K. K.; Kahn, A.; Ueno, N. Impact of an Interface Dipole Layer on Molecular Level Alignment at an Organic-Conductor Interface Studied by Ultraviolet Photoemission Spectroscopy. Phys. Rev. B: Condens. Matter Mater. Phys. 2004, 70, 085304-1−085304-6. (12) Functional Phthalocyanine Molecular Materials; Jiang, J., Ed.; Springer-Verlag: Berlin, Heidelberg, 2010. (13) Chaidogiannos, G.; Petraki, F.; Glezos, N.; Kennou, S.; Nespurek, S. Low Voltage Operating OFETs Based on SolutionProcessed Metal Phthalocyanines. Appl. Phys. A: Mater. Sci. Process. 2009, 96, 763−767. (14) Hohnholz, D.; Steinbrecher, S.; Hanack, M. Applications of Phthalocyanines in Organic Light Emitting Devices. J. Mol. Struct. 2000, 521, 231−237. (15) Kröger, I.; Stadtmuller, B.; Stadler, C.; Ziroff, J.; Kochler, M.; Stahl, A.; Pollinger, F.; Lee, T.-L.; Zegenhagen, J.; Reinert, F.; et al. Submonolayer Growth of Copper-Phthalocyanine on Ag(111). New J. Phys. 2010, 12, 083038-1−083038-23. (16) Kröger, I.; Bayersdorfer, P.; Stadtmüller, B.; Kleimann, C.; Mercurio, G.; Reinert, F.; Kumpf, C. Submonolayer Growth of H2Phthalocyanine on Ag(111). Phys. Rev. B: Condens. Matter Mater. Phys. 2012, 86, 195412-1−195412-8. (17) Stadtmüller, B.; Kroger, I.; Reinert, F.; Kumpf, C. Submonolayer Growth of CuPc on Noble Metal Surfaces. Phys. Rev. B: Condens. Matter Mater. Phys. 2011, 83, 085416-1−085416-10. (18) Giovanelli, L.; Amsalem, P.; Angot, T.; Petaccia, L.; Gorovikov, S.; Porte, L.; Goldoni, A.; Themlin, J.-M. Valence Band Photoemission from the Zn-Phthalocyanine/Ag(110) Interface: Charge Transfer and Scattering of Substrate Photoelectrons. Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 82, 125431-1−125431-8. (19) Gargiani, P.; Angelucci, M.; Mariani, C.; Grazia Betti, M. Metalphthalocyanine chains on the Au(110) surface: Interaction states versus d-metal states occupancy. Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 81, 085412-1−085412-7. (20) Peisert, H.; Biswas, I.; Aygül, U.; Vollmer, A.; Chassé, T. Electronic Structure of Cobalt Phthalocyanine Studied by Resonant Photoemission: Localization of Co-Related Valence Band States. Chem. Phys. Lett. 2010, 493, 126−129. (21) Petraki, F.; Peisert, H.; Latteyer, F.; Aygül, U.; Vollmer, A.; Chassé, T. Impact of the 3d Electronic States of Cobalt and Manganese Phthalocyanines on the Electronic Structure at the Interface to Ag(111). J. Phys. Chem. C 2011, 115, 21334−21340.

(22) Mugarza, A.; Robles, R.; Krull, C.; Korytar, R.; Lorente, N.; Gambardella, P. Electronic and Magnetic Properties of Molecule-Metal Interfaces: Transition-Metal Phthalocyanines Adsorbed on Ag(100). Phys. Rev. B: Condens. Matter Mater. Phys. 2012, 85, 155437-1− 155437-13. (23) Gao, L.; Ji, W.; Hu, Y. B.; Cheng, Z. H.; Deng, Z. T.; Liu, Q.; Jiang, N.; Lin, X.; Guo, W.; Du, S. X.; et al. Site-Specific Kondo Effect at Ambient Temperatures in Iron-Based Molecules. Phys. Rev. Lett. 2007, 99, 106402-1−106402-4. (24) Toader, M.; Hietschold, M. Tuning the Energy Level Alignment at the SnPc/Ag(111) Interface Using an STM Tip. J. Phys. Chem. C 2011, 115, 3099−3105. (25) Horcas, I.; Fernández, R.; Gómez-Rodríguez, J. M.; Colchero, J.; Gómez-Herrero, J.; Baro, A. M. WSXM: A Software for Scanning Probe Microscopy and a Tool for Nanotechnology. Rev. Sci. Instrum. 2007, 78, 013705-1−013705-8. (26) Floreano, L.; Cossaro, A.; Gotter, R.; Verdini, A.; Bavdek, G.; Evangelista, F.; Ruocco, A.; Morgante, A.; Cvetko, D. Periodic Arrays of Cu-Phthalocyanine Chains on Au(110). J. Phys. Chem. C 2008, 112, 10794−10802. (27) Mugarza, A.; Lorente, N.; Ordejon, P.; Krull, C.; Stepanow, S.; Bocquet, M.-L.; Fraxedas, J.; Ceballos, G.; Gambardella, P. Orbital Specific Chirality and Homochiral Self-Assembly of Achiral Molecules Induced by Charge Transfer and Spontaneous Symmetry Breaking. Phys. Rev. Lett. 2010, 105, 115702-1−115702-4. (28) Krull, C. Electronic Structure of Metal Phthalocyanines on Ag(100); Springer International Publishing: New York, 2014. (29) Liu, Z.; Zhang, X.; Zhang, Y.; Jiang, J. Theoretical Investigation of the Molecular, Electronic Structures and Vibrational Spectra of a Series of First Transition Metal Phthalocyanines. Spectrochim. Spectrochim. Acta, Part A 2007, 67, 1232−1246. (30) Ibach, H.; Mills, D. L. Electron Energy Loss Spectroscopy and Surface Vibrations; Academic Press: New York, 1982. (31) Amsalem, P.; Giovanelli, L.; Themlin, J.-M.; Angot, T. Electronic and Vibrational Properties at the ZnPc/Ag(110) Interface. Phys. Rev. B: Condens. Matter Mater. Phys. 2009, 79, 235426-1− 235426-10. (32) Dou, W.; Tang, Y.; Lee, C.; Bao, S.; Lee, S. Investigation on the Orderly Growth of Thick Zinc Phthalocyanine Films on Ag(100) Surface. J. Chem. Phys. 2010, 133, 144704-1−144704-7. (33) Langreth, D. C. Energy Transfer at Surfaces: Asymmetric Line Shapes and the Electron-Hole-Pair Mechanism. Phys. Rev. Lett. 1985, 54, 126−129. (34) Fano, U. Effects of Configuration Interaction on Intensities and Phase Shifts. Phys. Rev. 1961, 124, 1866−1878. (35) Kröger, J.; Lehwald, S.; Ibach, H. EELS Study of the Clean and Hydrogen-Covered Mo(110) Surface. Phys. Rev. B: Condens. Matter Mater. Phys. 1997, 55, 10895−10904. (36) Kröger, J. Electron−Phonon Coupling at Metal Surfaces. Rep. Prog. Phys. 2006, 69, 899−969. (37) Tautz, F. S.; Eremtchenko, M.; Schaefer, J. A.; Sokolowski, M.; Shklover, V.; Umbach, E. Strong Electron-Phonon Coupling at a Metal/Organic Interface: PTCDA/Ag(111). Phys. Rev. B: Condens. Matter Mater. Phys. 2002, 65, 125405-1−125405-10. (38) Temirov, R.; Soubatch, S.; Lassise, A.; Tautz, F. S. Bonding and Vibrational Dynamics of a Large π-Conjugated Molecule on a Metal Surface. J. Phys.: Condens. Matter 2008, 20, 224010-1−224010-10. (39) Scudiero, L.; Barlow, D. E.; Hipps, K. W. Physical Properties and Metal Ion Specific Scanning Tunneling Microscopy Images of Metal(II) Tetraphenylporphyrins Deposited from Vapor onto Gold (111). J. Phys. Chem. B 2000, 104, 11899−11905. (40) Lukasczyk, T.; Flechtner, K.; Merte, L. R.; Jux, N.; Maier, F.; Gottfried, J. M.; Steinrü c k, H.-P. Interaction of Cobalt(II) Tetraarylporphyrins with a Ag(111) Surface Studied with Photoelectron Spectroscopy. J. Phys. Chem. C 2007, 111, 3090−3098. (41) Kim, Y. J.; Westphal, C.; Ynzunza, R. X.; Wang, Z.; Galloway, H. C.; Salmeron, M.; Van Hove, M. A.; Fadley, C. S. The Growth of Iron Oxide Films on Pt(111): a Combined XPD, STM, and LEED Study. Surf. Sci. 1998, 416, 68−111. G


Article

The Journal of Physical Chemistry C (42) Garcia de Abajo, F. J.; Van Hove, M. A.; Fadley, C. S. Multiple Scattering of Electrons in Solids and Molecules: A Cluster-Model Approach. Phys. Rev. B: Condens. Matter Mater. Phys. 2001, 63, 0754041−075404-16. (43) Van Hove, M. A.; Tong, S. Y.; Elconin, M. H. Surface structure refinements of 2H-MoS2, 2H-NbSe2 and W(100)p(2 × 1)-O via new reliability factors for surface crystallography. Surf. Sci. 1977, 64, 85−95. (44) Chen, Y.; García de Abajo, F. J.; Chassé, A.; Ynzunza, R. X.; Kaduwela, A. P.; Van Hove, M. A.; Fadley, C. S. Convergence and Reliability of the Rehr-Albers Formalism in Multiple-Scattering Calculations of Photoelectron Diffraction. Phys. Rev. B: Condens. Matter Mater. Phys. 1998, 58, 13121−13131. (45) Baran, J. D.; Larsson, J. A.; Woolley, R. A. J.; Cong, Y.; Moriarty, P. J.; Cafolla, A. A.; Schulte, K.; Dhanak, V. R. Theoretical and Experimental Comparison of SnPc, PbPc, and CoPc Adsorption on Ag(111). Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 81, 0754131−075413-12. (46) Baran, J. D.; Larsson, J. A. Theoretical Insights into Adsorption of Cobalt Phthalocyanine on Ag(111): A Combination of Chemical and van der Waals Bonding. J. Phys. Chem. C 2013, 117, 23887− 23898. (47) Stadtmüller, B.; Schröder, S.; Bocquet, F. C.; Henneke, C.; Kleimann, C.; Soubatch, S.; Willenbockel, M.; Detlefs, B.; Zegenhagen, J.; Lee, T.-L.; et al. Adsorption Height Alignment at Heteromolecular Hybrid Interfaces. Phys. Rev. B: Condens. Matter Mater. Phys. 2014, 89, 161407-1−161407-6. (48) Hauschild, A.; Temirov, R.; Soubatch, S.; Bauer, O.; Schöll, A.; Cowie, B. C. C.; Lee, T.-L.; Tautz, F. S.; Sokolowski, M. NormalIncidence X-ray Standing-Wave Determination of the Adsorption Geometry of PTCDA on Ag(111): Comparison of the Ordered Room-Temperature and Disordered Low-Temperature Phases. Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 81, 125432-1−125432-12. (49) Ruiz, V. G.; Liu, W.; Zojer, E.; Scheffler, M.; Tkatchenko, A. Density-Functional Theory with Screened van der Waals Interactions for the Modeling of Hybrid Inorganic-Organic Systems. Phys. Rev. Lett. 2012, 108, 146103-1−146103-5. (50) Duhm, S.; Gerlach, A.; Salzmann, I.; Bröker, B.; Johnson, R. L.; Schreiber, F.; Koch, N. PTCDA on Au(111), Ag(111) and Cu(111): Correlation of Interface Charge Transfer to Bonding Distance. Org. Electron. 2008, 9, 111−118. (51) Henze, S. K. M.; Bauer, O.; Lee, T.-L.; Sokolowski, M.; Tautz, F. S. Vertical Bonding Distances of PTCDA on Au(111) and Ag(111): Relation to the Bonding Type. Surf. Sci. 2007, 601, 1566−1573.

H


Correlation between Charge Transfer and Adsorption Site in CoPc

Recommend Documents