Electrostatic Interaction and Commensurate Registry at the

Article pubs.acs.org/JPCC

Electrostatic Interaction and Commensurate Registry at the Heteromolecular F16CuPc−CuPc Interface Christoph Kleimann,†,‡ Benjamin Stadtmüller,†,‡ Sonja Schröder,†,‡ and Christian Kumpf*,†,‡ †

Peter Grünberg Institut (PGI-3), Forschungszentrum Jülich, 52425 Jülich, Germany Jülich-Aachen Research Alliance (JARA) − Fundamentals of Future Information Technology, 52425 Jülich, Germany

‡

ABSTRACT: Tailoring the properties of molecular thin films and interfaces will have decisive influence on the success of future organic electronic devices. This is equally true for metal−organic and hetero−organic contacts as they occur, for example, in donor− acceptor systems. Here, we report on the structure formation and interaction across such a heteromolecular interface. It is formed by monolayers of F16CuPc and CuPc stacked on a Ag(111) surface. We investigated the lateral and vertical structure using spot-profile analysis low energy electron diffraction and normal incidence X-ray standing waves, and performed pair potential calculations to understand the driving forces for the structure formation. Most surprisingly, for one phase we found a commensurate registry between the two organic layers, usually a sign for a strong (chemisorptive) interaction often involving metallic states of the surface. However, because the organic bilayer is not commensurate with the underlying Ag substrate in our case, the dominating factor must be the intermolecular interaction. Pair potential calculations suggest a site-specific adsorption that leads to a commensurate registry at the heteromolecular interface. The adsorbate system was further characterized by measuring adsorption heights, indicating flat-lying molecules and a CuPc−F16CuPc layer spacing of 3.06 Å.

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INTRODUCTION Because of their flexibility and the possibility for low cost mass fabrication, organic materials are of increasing importance for electronic devices. Although some successful thin film applications like organic light emitting diodes (OLEDs),1 organic field effect transistors (OFETs),2 and organic photovoltaic cells (OPVCs)3−5 exist already, the development and optimization of new devices still calls for a better understanding of fundamental processes at the interfaces occurring in these systems. Consequently, the study of thin organic films on metal surfaces has attracted increasing attention within the last decades.6−19 The general goal is to control the self-organized growth of the molecular films by selecting the substrate and adsorbate materials and by varying growth parameters such as the sample temperature. Simple metal−organic interfaces have clearly been in the central focus of these investigations so far. However, recently the attention turned toward heteromolecular adsorbate systems20,21 and organic−organic interfaces.22−24 In all cases, the structural ordering of the first molecular adsorbate layer is of particular interest, because it dominates the electronic interaction with the substrate and influences the growth of consequent layers. The main subject of this work is phthalocyanine (Pc) molecules, which exist in many variations, mainly differing by the central metal atom or functional group. A second approach for changing the properties of these molecules is fluorination, that is, replacing the terminating hydrogen atoms by fluorine, which changes the electronic properties due to the large © XXXX American Chemical Society

electron affinity of this atomic species. In this work, we concentrate on the formation of heteromolecular interfaces in mixed films consisting of CuPc and its perfluorinated counterpart F16CuPc. In particular, we investigated the bilayer system F16CuPc/CuPc/Ag(111) using spot-profile analysis low energy electron diffraction (SPA-LEED) and normal incidence X-ray standing waves (NIXSW). The results regarding both lateral and vertical structure were supported by pair potential calculations and allow conclusions on the interaction strength between the organic layers and to the surface. The commensurate registry of both molecular layers with each other as well as a vertical layer distance smaller than typical values for organic van der Waals crystals indicate an unexpectedly strong interaction.

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EXPERIMENTAL DETAILS All measurements were performed in UHV systems with a base pressure below ∼5 × 10−10 mbar. Clean Ag(111) surfaces were prepared by multiple cycles of Ar+ ion sputtering and annealing at T ≥ 720 K. This standard treatment ensures large terraces on the Ag(111) surface, which were checked by (SPA-)LEED. For the NIXSW experiments, we also checked the cleanliness of the surface by X-ray photoelectron spectroscopy (XPS). Afterward, organic molecules were deposited by organic molecular beam epitaxy from dedicated Knudsen cells. This was monitored by Received: November 17, 2013 Revised: December 27, 2013

A

dx.doi.org/10.1021/jp411289j | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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recording the ion current signal of a typical fragment of the molecule by a quadrupole mass spectrometer (QMS). After calibration, the integrated signal was used as a measure for the deposited amount of molecules and hence allows one to determine coverages. During some depositions, we also recorded short in situ SPA-LEED scans to observe the formation and transition of structural phases. In all experiments, we at first deposited a multilayer film of CuPc directly on the Ag(111) surface and annealed the sample at T ≥ 553 K. This desorbs all but the first layer and leaves a densely packed CuPc monolayer film behind, the properties of which are very wellknown.18,25 On this monolayer film, we deposited the desired amount of F16CuPc, which was then investigated further by SPA-LEED or NIXSW. While SPA-LEED reveals the unit cell size and superstructure matrixes with very high precision (the error in the entries of a superstructure matrix is smaller than 0.04), with NIXSW, vertical distances between the surface and the adsorbed molecules can be determined. The method is chemically sensitive; that is, the vertical positions of atomic species can be measured individually (as long as their core level photoelectron signals can be distinguished in XPS). In brief, in a NIXSW experiment the photon energy of an incident synchrotron (X-ray) beam is scanned through the Bragg condition of a bulk reflection (the Ag(111) in our case). The Bragg-reflected wave interferes with the incident wave and forms a standing wave field, which is shifting through the crystal by one-half the Bragg plane spacing (corresponding to one-half of the wavelength of the standing wave) while scanning through the Bragg condition. This causes a characteristic change of the X-ray intensity at the positions of individual atoms during the scan, which is reflected in the X-ray absorption yield of the atomic species. The shape of the absorption yield curves versus energy is therefore characteristic for the vertical position of the corresponding atoms relative to the bulk Bragg planes. Experimentally, absorption yield curves are obtained by measuring the (partial) XPS yield of the atomic species of interest, which allows an element specific analysis. The yield curves are fitted by the fundamental equation: Y = 1 + R + 2 R ·F H cos(Φ − 2πP H)

Our NIXSW measurements were performed at beamline ID32 of the European Synchrotron Radiation Facility (ESRF) in Grenoble, which was equipped with a conventional LEED system and a hemispherical electron analyzer (SPECS PHOIBOS 225), mounted at an angle of ∼90° relative to the incident X-ray beam. Absorption yield curves were measured on the basis of C1s, N1s, and F1s partial yield core level spectra. To prevent beam damage in the organic film, the measurements on the bilayer system were performed at T = 50 K, and the acquisition time on each spot of the sample was limited to ∼20 min. The scans on different spots of the sample were summed in the subsequent analysis for each element to achieve better statistics. The spectra were analyzed using the CasaXPS software31 with fitting models based on detailed off-Bragg XPS measurements; see below. The resulting absorption yield curves were fitted with the Torricelli software.32

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SPA-LEED RESULTS F16CuPc layers on a compact CuPc monolayer film on Ag(111) have been investigated with SPA-LEED at room and low temperature (RT and LT) for different F16CuPc coverages Θ2. (The coverage for CuPc was Θ1 = 1 ML in all cases.) We observed the formation of three different phases, two at RT (with low and high coverage) and one at LT. Corresponding SPA-LEED images are shown in Figure 1. Note that the

(1)

where R is the reflected X-ray intensity, Φ is the phase of the standing wave field (changing from π to 0 during the scan), and FH and PH are the fitting parameters coherent fraction and coherent position, respectively. PH represents the average vertical position of the atomic species relative to the nearest Bragg plane in units of the lattice spacing. It is related to the corresponding average adsorption height DH by PH = (DH mod dhkl)/dhkl. Even though the modulo operation induces a mathematical uncertainty for DH, usually only one value for DH corresponds to physically reasonable bonding distances. The second parameter FH is an ordering parameter, which describes the distribution of adsorption heights for the atoms of the species in question. A value of 1 indicates that all atoms are lying perfectly aligned on the same adsorption height; a value of 0 corresponds to complete (vertical) disorder in the sense of a homogeneous distribution of atoms between two Bragg planes. Often the parameters FH and PH are plotted as polar vectors whereby FH corresponds to the length and PH to the polar angle of the vector. This so-called Argand representation enables, for example, the separation of different components by simple vector operations like vector subtraction; see below. For further experimental details, please refer to the literature.26−30

Figure 1. SPA-LEED pattern for F16CuPc/CuPc/Ag(111) observed at different F16CuPc coverages and temperatures. The CuPc coverage was Θ1 = 1 ML in all cases. At RT, the F16CuPc layer only orders at a coverage close to Θ2 = 1 ML. Phase (c) was observed at 100 K for F16CuPc coverages between 0.5 and 1 ML. Part (d) shows the molecular structure of the F16CuPc molecule with its three chemically different carbon species, which are numbered accordingly.

characteristic diffraction spots of the CuPc monolayer structure are visible in all LEED patterns. This is most clearly to be seen in the LEED image of the RT low coverage phase (Figure 1a), because all other features are diffuse in this pattern; that is, all sharp spots in this pattern stem from the CuPc/Ag(111) structure. In the other images, they are superimposed by other B



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This unit cell is oblique with |A⃗ | = 18.52 Å, |B⃗ | = 32.60 Å, and α = 72.34°. Its area is 575.41 Å2, 3 times as large as the unit cell of the underlying CuPc layer. It is striking that also all diffraction spots of the densely packed monolayer-phase of CuPc/Ag(111) (i.e., of the first layer of this system) can be indexed using this unit cell. This can be seen in Figure 2, or by comparing the superstructure matrix for the bilayer system Mbilayer with that of the CuPc/Ag(111) structure MCuPc.18 The transition matrix T, which reflects the registry between the two molecular layers in this system, is defined by the relation:

sharp diffraction spots. This demonstrates that the F16CuPc molecules do not destroy the lateral order of the CuPc layer underneath. At RT and low coverage (Θ2 < 1 ML), the only signal in the SPA-LEED image stemming from the F16CuPc molecules in the second layer is a diffuse ring of scattered intensity around the (0,0)-spot. This indicates that the F16CuPc layer is disordered as long as it is dilute and not densely packed. Such a behavior, that is, the formation of gas-like phases at low coverages, was also found for CuPc, SnPc, and H2Pc on Ag(111).17,18,22,33−35 A closer inspection of the ring reveals that it is not uniform but shows an intensity modulation. Upon increasing the F16CuPc coverage to 1 ML, new fundamental spots appear while the diffuse intensity disappears. This ordered phase is rather instable; annealing at T > 390 K transforms the layer into the disordered phase. Most likely this is due to intermixing of the molecules from both layers. Note that the order of the first layer is preserved during this process, indicated by the LEED pattern that becomes very similar to that shown in Figure 1a. In the second layer, the lateral order is lost, although desorption can be excluded as a reason for this transition due to the moderate temperature. The order of the bilayer structure is also instable in the electron beam; it becomes gradually disordered during longer SPA-LEED scans. The third phase occurs when cooling the disordered gas phase to temperatures below 110 K. A star-like pattern becomes visible, formed by short streaks that actually consist of LEED spots broadened in certain directions. Higher orders of these superstructure spots are faint and sometimes hardly detectable, as it is often seen for incommensurate superstructures. We now turn to a deeper investigation of the ordered RT phase at high coverages (1 ML), that is, to the stacked bilayer system of F16CuPc/CuPc/Ag(111) consisting of closed, densely packed layers (corresponding to the SPA-LEED image in Figure 1b). As illustrated in Figure 2 by calculated

⎛ 0.96 0.98 ⎞ ⎛ 4.75 − 0.22 ⎞ ⎟. ⎜ ⎟ Mbilayer = T ·MCuPc = ⎜ ⎝−1.01 2.02 ⎠ ⎝ 2.61 5.60 ⎠

Most interestingly, within the error bars, all elements of the transition matrix T are integer. This means that the two organic layers in this system are commensurate with each other, and indicates a strong interaction between the two layers and the existence of prominent adsorption sites. Please note that the situation is different from the case of CuPc/PTCDA/ Ag(111).22 In that system, both CuPc and PTCDA layer are commensurate with the Ag(111) substrate, and consequently also with each other. In the system studied here, although the organic layers are commensurate with each other, they only show a point-on-line registry with the substrate. Therefore, the reason for commensurability must lie in the interaction between the two different organic molecules, and a significant influence of the substrate is unlikely.

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NIXSW RESULTS The SPA-LEED results discussed above raise the question of the strength of the interaction between the two organic layers, CuPc and F16CuPc. The most interesting geometric parameter in this context is their vertical distance, which can be measured by NIXSW with a very high precision, typically better than ΔDH ≈ 0.05 Å. However, regarding data interpretation our system is challenging, because both types of molecules are chemically very similar, and it will be difficult to distinguish the different atomic species of both molecules (except for fluorine, of course, which is only present in the upper organic layer). We therefore analyze the data mainly by comparing it with the simpler and well-known reference system CuPc/Ag(111). To verify the results for the reference system, we at first have repeated the NIXSW experiment on a close-packed monolayer of CuPc on Ag(111). The results are very close to our previous ones,18 as can be seen in Table 1. It should also be mentioned that, due to limited beamtime, the reference measurement on CuPc/Ag(111) was performed at RT, and the one on the stacked bilayer system at LT. However, as demonstrated earlier,18 neither adsorption heights nor coherent fractions of the individual species differ significantly for RT and LT. After the measurement of CuPc on Ag(111) was completed, F16CuPc was deposited directly (without preparing a new CuPc film), and a second XSW data set was collected. We have recorded XSW data based on F1s, N1s, and C1s core levels. Some typical XPS spectra are shown in Figure 3a−c. In Figure 4, the corresponding XSW data are shown, based on an integrative analysis of the core level region of each species, that is, reflecting the averaged results for all carbon, nitrogen, or fluorine species, respectively. In panel (a), the absorption yield data are shown, with corresponding fitting curves. The fit parameters are given as numbers, and are shown in their Argand representation in panel (b). Each symbol represents the

Figure 2. SPA-LEED pattern of the ordered RT phase of F16CuPc/ CuPc/Ag(111). In the upper-right half of the image, calculated positions of LEED spots are indicated by red circles for all six rotational and mirror domains. Reciprocal unit cell vectors are drawn as white arrows for one domain.

LEED spots (red circles), this superstructure has a point-online registry36 with the Ag surface. The corresponding superstructure unit cell is ⎛ 7.13 5.29 ⎞ ⎟ Mbilayer = ⎜ ⎝ 0.48 11.51⎠

(3)

(2) C



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Table 1. NIXSW Resultsa CuPc/Ag(111) core level

FH

PH

Cu2p3/2 N1s C1s

0.77(2)/0.53(8)18 0.64(3)/0.57(8)18 0.63(2)/0.48(6)18

DH [Å]

0.28(1)/0.258(3)18 0.27(1)/0.297(3)18 0.31(1)/0.310(2)18 F16CuPc/CuPc/Ag(111)

3.02(5)/2.97(4)18 3.00(5)/3.04(4)18 3.09(5)/3.08(3)18

core level

FH

PH

DH [Å]

F1s N1s C1s

0.52(5) 0.97(20) 0.91(20)

0.62(1) 0.62(2) 0.60(2)

6.16(2) 6.16(5) 6.11(5)

a

Fit parameters FH and PH as well as the resulting adsorption heights DH are listed for the 1 ML structure of CuPc/Ag(111) and the bilayer structure F16CuPc/CuPc/Ag(111). Parameters for the latter are based on the Argand vector analysis, which explains the rather large error in the coherent fractions of the C and N species for this structure, for details, see text. The results for CuPc/Ag(111) are compared to those from ref 18.

satellites) lie very close to the C−F component (in case of S3 the positions are almost identical), which probably is the reason for the fact that our XPS-component analysis did not give reasonable results. It was not possible to unambiguously separate all CuPc components (in particular these satellites) from the C−F component in our PES spectra. The second possible way to interpret our data will be applicable, if the adsorption heights for one of the two layers are known from another experiment. This is the case for our system and was the reason for performing the reference measurement on the CuPc/Ag(111) system mentioned above. Hence, we subtract the first layer signal from that measured for the bilayer to obtain the second layer result. Changes within the CuPc layer, which are induced by the additional adsorption of F16CuPc, cannot be detected in this way, but they are expected to be small because no changes in the SPA-LEED pattern were found. In similar systems, changes of the adsorption height in the range of 0.05 Å have been found.23 For this type of analysis, we make use of the Argand diagram, which is a polar diagram representing the coherent position and fraction as polar angle and vector length, respectively, corresponding to the formula Y = FH exp(2πPH). In this representation, the superposition of different contributions to the absorption yield can simply be calculated as the normalized vector sum of the Argand vectors of the individual components. In our case, the resulting Argand vector Y corresponding to the measured yield consists of two components, one from each organic layer (YF16CuPc and YCuPc, respectively):

end of an Argand vector, the length and polar angle of which correspond to the coherent fraction and position, respectively. We have performed several measurements on different spots on our sample. These individual results can be seen as open symbols, and its average as a full symbol. The statistics of these measurements gives an idea of the precision of the measurement. We used the standard deviations of the results as error bars for the fit parameters FH and PH. For our system, only the interpretation of the F1s result is straightforward, as fluorine is the only species that is present in the second layer only. The coherent position PH = 0.62 corresponds to an average adsorption height of DH = 6.16 Å above the Ag surface for this species. Because of the modulo operation (see Experimental Details), also values of 3.81 or 8.51 Å would be possible (DH ± 2.35 Å), however, not realistic for molecules in the second layer. The coherent fraction of FH = 0.52 indicates flat-lying molecules in the second layer, probably with a certain degree of disorder. Because the atoms are located at the outer edge of the molecules, even a relatively small tilt of the molecules produces quite different adsorption heights, and therefore a considerably lower coherent fraction would be expected in that case. Because the C1s and N1s regions contain contributions from both the CuPc and the F16CuPc layers, the according XSW results are an average of the two layers in the sense of the vector sum of both components in the Argand diagram (for details, see below). The values for the coherent positions of C and N lie relatively close to those found for the CuPc/Ag(111) structure, and the coherent fractions are relatively large. Both indicate a rather small contribution from F16CuPc, that is, a rather small coverage for this layer. For separating the signals from both layers, there are two possible ways of analyzing the data. The first (and, in principle, the preferable one) is to separate the XPS signals from species of both layers by their core level shifts. However, for Cu and N this is not possible, because no significant chemical shifts can be expected for these species in CuPc and F16CuPc. For C we have tried to separate the signals, as can be seen in Figure 3c and d. The fluorine-bonded carbon atoms in F16CuPc are expected to show a rather clear core level shift as compared to their counterparts in CuPc being bonded to H. Also, in fact, in the difference spectrum shown in Figure 3d, a rather large component is visible, which we adapted to our XPS fitting model in Figure 3c (dark green curve). However, unfortunately, some satellites of other carbon species (S1, 2, and 3, orange, gray, and cyan curves, most likely shake-up and charge transfer

Y = a ·YF16CuPc + (1 − a) ·YCuPc

(4)

where a is the fraction of the total yield stemming from the upper layer. This fraction of course depends on the secondlayer coverage Θ2, but also on the damping of the signal from the first layer by the second due to absorption. It can be calculated by: a = Θ2 /[(1 − Θ2) + Θ2 ·exp( −d /λ) + Θ2]

(5)

The first term in the denominator corresponds to the uncovered part of the first layer, the second to the covered part, for which damping has to be considered (exponential factor), and the third part stands for the second layer. Apparently, the precise F16CuPc coverage is important for calculating the parameter a. We carefully estimated the coverage from the difference in the normalized photoelectron yield of the C1s core level before and after the deposition of the F16CuPc layer, whereby inelastic scattering of the photoD



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Figure 4. Results of the NIXSW analysis based on XPS peak integration: (a) Electron yield curves for the F1s, N1s, and C1s integrated regions are shown together with the best fits. The resulting fit parameters are given as numbers, and (in b) plotted in the Argand diagram for every individual XSW measurement (open symbols) as well as for their average (full symbols). The experimental error can be estimated from the statistical spread of the data points.

empirical formula reported by Seah and Dench37 for organic compounds: λ [nm] =

⎞ ⎛ 1000 49 ·⎜ + 0.11· E kin [eV] ⎟ 3 2 ρ [kg/m ] ⎝ (E kin [eV]) ⎠ (6)

Using a bulk density of ρ = 2118 kg/m , taken from literature,38 and a kinetic electron energy of Ekin = 2350 eV (obtained from the photon energy and the C1s binding energy), we obtained λ = 2.52 nm. For a 1 ML (d = 3 Å) thick F16CuPc layer, we therefore expect an attenuation of the XPS signal by 11%. The observed increase in the C1s electron yield of 15% due to the deposition of F16CuPc therefore corresponds to a second layer coverage of Θ2 = 0.15/0.89 = 0.17 ML. From this value and eq 5, we obtain a = 0.147. Finally, knowing Y and YCuPc from the respective experiments, we achieve the result by substituting eq 4 for YF16CuPc: 3

Figure 3. XPS data and fitting models for the components of the (a) F1s, (b) N1s, and (c) C1s regions, taken at an off-Bragg photon energy of 2640 eV. (d) Comparison of C1s spectra for CuPc/Ag(111) and F16CuPc/CuPc/Ag(111). The difference spectrum identifies the C−F component.

electrons was considered. The inelastic mean free path λ for electrons in F16CuPc films was calculated from the semiE


The Journal of Physical Chemistry C YF16CuPc =

1 ·(Y − (1 − a) ·YCuPc) a

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respectively. The height of the Cu atoms in F16CuPc is not shown because no NIXSW data for Cu2p were measured due to limited beamtime. Noteworthy, the distance between the organic layers is smaller than the layer distance in the system CuPc/PTCDA/Ag(111), which was found to be 3.22 Å.22 This is probably the result of the stronger electrostatic interaction between CuPc and F16CuPc as compared to CuPc and PTCDA, caused by the electronegativity of the F atoms. It should also be mentioned that the results of the vector construction depend on the attenuation of the first layer photoelectrons by the second layer, which was only roughly estimated. However, by trying different values for the attenuation, the error could be estimated. We found that only the coherent fraction is significantly influenced by changes of this value. FH seems to be generally overestimated in the vector constructions. The more important fit parameter PH is only slightly affected as long as the true attenuation does not differ very strongly from the estimation.

(7)

This is also illustrated in the Argand diagram in Figure 5 and yields the coherent fraction FH = 0.97 and the coherent position

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PAIR POTENTIAL CALCULATIONS The experimental findings reported so far revealed information on the vertical adsorption distances of both adsorbate layers as well as on unit cell sizes and lateral structural correlations between the layers. In particular, we found that for the highcoverage RT structure both adsorbate layers are commensurate to each other, which indicates a strong correlation between the molecules of both layers. We have investigated this aspect further by pair potential calculations based on van der Waals and electrostatic interactions. It has been shown that physisorptive interaction can be sufficiently described by such calculations, even in the case of a small amount of charge transfer between surface and adsorbate.40,41 In our case, we are interested in the interaction between two molecular layers CuPc and F16CuPc. Charge transfer effects are not expected for this system, so that the Ag substrate can promptly be neglected. The code and its application on molecular adsorbate layers are described elsewhere.40 In Figure 7, the results of the pair potential calculations are shown. In panel (a), we calculated the interaction energy between one single F16CuPc molecule and a compact layer of CuPc molecules. It is plotted as a color-coded contour plot versus two translational parameters Δx and Δy indicating the lateral position of the F16CuPc molecule relative to the CuPc molecule below. The molecules in the CuPc layer are fixed at positions according to the monolayer structure of CuPc/ Ag(111), the unit cell and the molecular orientation of which are known from SPA-LEED,18 STM,42 and ARPES/orbital tomography.43 For the map shown in Figure 7a, we assumed an identical azimuthal orientation of both types of molecules, and a vertical distance of Δz = 3.1 Å, corresponding to the result of the NIXSW measurement reported above. Note that this map is one out of a series of many calculations performed for different vertical distances and azimuthal rotations. Altogether we have performed calculations on a grid given by Δx,Δy = −14 to 27 Å, Δz = 2.5 to 4.0 Å, and θ = 0 to 90° with a grid spacing of 0.2 Å for Δx and Δy, 0.1 Å for Δz, and 2.5° for θ. θ is the relative azimuthal orientation of the F16CuPc and CuPc molecules. For Figure 7a, we have selected the Δx−Δy map for Δz = 3.1 Å and θ = 0 because it contains the absolute potential minimum at (Δx,Δy) = (0,0), that is, for on-top adsorption of F16CuPc on CuPc molecules. Note that the entire potential surface in this plane contains only negative values spanning from −1.5 eV (red) to −3.4 eV (blue color code); that is, the interaction

Figure 5. Argand analysis for (a) the N1s and (b) the C1s NIXSW results. The components from the CuPc (red symbols) and F16CuPc layers (green symbols) can be separated as described in the text.

PH = 0.62 for N1s and FH = 0.91 and PH = 0.60 for C1s. The corresponding adsorption heights of 6.16 and 6.11 Å for N1s and C1s, respectively, are in good agreement with the value for the fluorine atoms of 6.16 Å; see also Table 1. This indicates that the F16CuPc molecules are indeed flat-lying when adsorbed on CuPc/Ag(111). The vertical adsorption geometry, as measured by NIXSW, is sketched in Figure 6. The adsorption heights DH of the atomic species in both molecular layers are drawn on scale with the atomic radii. Both the covalent and the van der Waals radii39 are shown by the solid and dashed circles,

Figure 6. On-scale illustration of the adsorption heights of all investigated atomic species in F16CuPc on CuPc on Ag(111). Solid and dashed circles represent covalent and van der Waals bonding radii.39 F



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strongly smeared out around a bridge-site above two CuPc molecules of the first layer. In particular, the on-top adsorption sites above molecule “B” and “C” are not favorable, so that we can conclude that these second F16CuPc molecules do not adsorb on-top of molecules of the first layer. This finding is in agreement with the fact that no simple (1 × 1) structure is formed by the F16CuPc layer, as one might have expected by merely considering the pair potential map for the first F16CuPc molecules (Figure 7a). These pair potential calculations, together with the SPALEED results, suggest the following scenario for the adsorption of F16CuPc on the CuPc monolayer structure: At small coverages and RT, the adsorbed F16CuPc molecules find a potential surface with a relatively large number of possible adsorption sites, and have the space and thermal energy to move and rotate on the surface almost freely. This causes the SPA-LEED pattern shown in Figure 1a with a diffuse ring of intensity stemming from F16CuPc (beside the pronounced LEED pattern of the well-ordered CuPc/Ag(111) monolayer structure). Upon increasing the coverage, the molecules are more strongly sterically hindered in their rotation and translation, and finally the great majority finds the most favorable adsorption site corresponding to on-top adsorption on CuPc, that is, the deepest minimum in the potential map Figure 7a. In this coverage regime, the potential landscape changes and favors the adsorption of further F16CuPc molecules at other adsorption sites, presumably close to bridge-sites. The interplay of the interaction across the two organic layers (i.e., the F16CuPc−CuPc interaction) and the intermolecular interaction within the F16CuPc layer obviously force the bilayer system in a very well-ordered, commensurate registry forming a (1 1|−1 2) superstructure unit cell (with respect to the CuPc layer) and gives rise to the LEED image shown in Figure 1b. The Ag(111) surface has no significant influence on the structure formation in the bilayer, because the two molecular layers are commensurate with each other, but not with the underlying substrate. Upon cooling the sample at a coverage smaller than 1 ML, the molecules, which are still almost freely moving and rotating, become trapped at different minima in the potential surface given by the CuPc/Ag(111) structure, apparently also in the more shallow minima. This leads to a rather poorly ordered superstructure causing the streaky LEED pattern (Figure 1c). Most likely the streaks originate from row-like structures or domains occurring due to the fact that the minima in the potential surface form a distorted rectangular grid. This causes a preferred alignment of the molecules in rows, whereby the starlike shape of the streaks stems from different rotational and mirror domains due to the 3-fold symmetry of the Ag(111) surface.

Figure 7. Pair potential maps for the interaction of one F16CuPc molecule with (a) a closed layer of CuPc (corresponding to the “first F16CuPc molecules” adsorbing on the CuPc/Ag(111) system) and (b) with the closed CuPc layer and some previously adsorbed F16CuPc molecules, which are located at on-top sites above the CuPc molecule labeled “A”. In all cases, the vertical distance (height difference) between the F16CuPc and the CuPc layer is Δz = 3.1 Å.

between the molecules is always attractive. For other values of Δz and θ, other more shallow minima are found, but most of them are also visible in this map, for example, at (Δx,Δy) = (−0.2,7) and (Δx,Δy) = (7,−0.2). It should also be mentioned that for smaller vertical distances Δz, the potential turns rather quickly to positive values, indicating repulsive (nonbonding) interaction between the molecules. Figure 7a shows the potential for F16CuPc molecules adsorbing on the CuPc layer without interaction with other F16CuPc molecules. This assumption is reasonable as long as the density of F16CuPc molecules is small. When this precondition is no longer fulfilled, that is, when these “first F16CuPc molecules” have adsorbed and found their ideal (ontop) adsorption site, the potential landscape changes for the adsorption of further F16CuPc molecules. We have simulated this situation under the assumption of the formation of a (1 1|−1 2) superstructure unit cell as it was found experimentally. Figure 7b shows the corresponding potential map for one F16CuPc molecule interacting with the underlying CuPc layer, and with F16CuPc molecules located at on-top adsorption sites (above the CuPc molecule labeled “A”). It can be seen that the potential minima for these “second F16CuPc molecules” are

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SUMMARY AND CONCLUSIONS We have investigated the heteromolecular adsorbate system F16CuPc/CuPc/Ag(111) by means of SPA-LEED, NIXSW, and pair potential calculations. F16CuPc was deposited with different coverages up to 1 ML on a compact monolayer structure of CuPc/Ag(111). The F16CuPc molecules adsorb flat-lying and initially (at low coverages) form a disordered overlayer, which transforms into a long-range ordered structure when the coverage approaches 1 ML. Most remarkably this overlayer is commensurate with the underlying CuPc layer, but shows only point-on-line registry with the Ag substrate, as found in SPA-LEED. In this aspect, the system under study G



Article

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differs from the mixed bilayer structure of CuPc/PTCDA/ Ag(111) for which two molecular layers were found, which are commensurate with each other and with the substrate.22 Pair potential calculations revealed the origin of the commensurate structure: A relatively strong electrostatic attraction (beside van der Waals forces) dominates the structure formation and forces the first F16CuPc molecules onto on-top sites right above the CuPc molecules. Therefore, the F16CuPc on CuPc adsorption is dominated by physisorptive forces, in particular by the electrostatic interaction between the terminating atoms F and H of the respective molecules and their different partial charges. On the other hand, the strong (repulsive) intermolecular interaction within the F16CuPc layer caused by the electronegative F atoms inhibits an identical positioning and orientation of the further F16CuPc and the CuPc molecules, which would result in a (1 × 1) superstructure. At low temperature, the order of the second organic layer is much worse. A streaky SPA-LEED pattern was found, which indicates a significant anisotropic disorder in the structure. Also here the pair potential calculations contributed significantly to the understanding of the structure formation. They revealed a rectangular network of possible different adsorption sites that are possibly obtained by the F16CuPc molecules, which (1) is the reason for the rather poor order in the film and (2) favors a row-like alignment of the molecules giving rise to the streaks in the LEED pattern. By NIXSW, we measured the adsorption heights of both molecular layers. These experiments confirmed the flat-lying geometry of the molecules and revealed an intermolecular distance between the F16CuPc and CuPc molecules of ΔDHav = 3.06 Å (averaged from the results for the individual species). This is close to the values expected from calculated van der Waals distances for the atomic species involved44 (only for carbon an overlap of van der Waals radii would be possible if the C atoms of both molecules were located directly on-top of each other), but also smaller than typical layer distances in crystals of similar organic molecules ranging from 3.2 to 3.4 Å.38,45,46

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AUTHOR INFORMATION

Corresponding Author

*Tel.:+49(0)2461-61-1452. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank M. Willenbockel, B. Detlefs, and J. Zegenhagen for their support during the NIXSW beam time. We also acknowledge financial support from the Deutsche Forschungsgemeinschaft (KU 1531/2-1) and the European Synchrotron Radiation Facility (ESRF).

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Electrostatic Interaction and Commensurate Registry at the

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