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PHYSICAL REVIEW B 91, 155432 (2015)

Commensurism at electronically weakly interacting phthalocyanine/PTCDA heterointerfaces Marco Gruenewald,1 Christoph Sauer,2 Julia Peuker,1 Matthias Meissner,1 Falko Sojka,1 Achim Schöll,2 Friedrich Reinert,2 Roman Forker,1 and Torsten Fritz1,* 1

2

Friedrich-Schiller-Universität Jena, Institut für Festkörperphysik, Helmholtzweg 5, 07743 Jena, Germany Universität Würzburg, Experimentelle Physik VII & Röntgen Research Center for Complex Material Systems RCCM, 97074 Würzburg, Germany (Received 8 October 2014; revised manuscript received 6 March 2015; published 28 April 2015) Interfaces in multilayered electronic devices are of paramount importance, especially for layer thicknesses in the nanometer range. Among the interfacial processes are charge injection or extraction and excitonic dissociation, the latter being particularly relevant if molecular components are involved. Highly ordered superstructures are preferable to prevent undesired losses of charge carriers and/or excitons. Epitaxial organic-inorganic systems have already received eminent attention, but only few studies have dealt with organic-organic heterointerfaces so far. Here, we focus on the adsorption of metal-phthalocyanines (MePc, Me = Sn or Cu) on 3,4,9,10-perylenetetracarboxylic-dianhydride (PTCDA) in the form of stacked monolayers (ML) on Ag(111). Using scanning tunneling microscopy and low-energy electron diffraction we reveal an initial nonordered growth for dilute SnPc submonolayers and consecutively three condensed phases at coverages ranging up to 1 ML—each possessing a distinct commensurate registry with the underlying PTCDA. By applying in situ optical differential reflectance spectroscopy and photoelectron spectroscopy we find that neither the SnPc nor the CuPc phases exhibit significant electronic or optical coupling with the PTCDA interlayer. Therefore, our results demonstrate that commensurism does not necessarily imply chemisorption, as stated previously in the literature, but that physisorption may be accompanied by commensurate superstructures. DOI: 10.1103/PhysRevB.91.155432

PACS number(s): 68.43.−h, 68.37.−d, 78.20.−e, 79.60.−i

I. INTRODUCTION

Organic thin films have been already established in many applications, such as field-effect transistors (OFET), lightemitting diodes (OLED), and photovoltaic devices (OPVD) [1–3]. Recently, organic quantum well (OQW) structures have attracted much interest as they could play a central role in advanced organic-based applications, such as organic lasers and multilevel logical circuits [4–7]. In general, OQWs grant access to tailor-made properties by means of effects (e.g., quantum confinement), which are not accessible unless dealing with very thin layers with thicknesses ranging from one monolayer up to a few nanometers. However, the broad application of OQWs is still hindered by demanding preparation conditions, namely the need for both a high crystalline quality and well-defined interfaces [6,7]. It is obvious that controlling the molecular arrangement at such interfaces is of vital importance as it directly influences key physical properties such as electronic efficiency (via the anisotropic electron and hole mobilities), the energy alignment governing the interfacial charge transfer and interface dipoles, as well as optical properties, e.g., optical coupling between adjacent molecular transition dipole moments. In order to analyze the complex interaction mechanisms at these interfaces we focus on model systems, which provide access to an unbiased probing of effects at the molecular level named above. Ultrathin layers of metal-phthalocyanines (MePc) and perylene derivatives are used as they are known to form epitaxial layers on metals in a flat-lying adsorption geometry [8–10]. In the literature molecular adsorption is often distinguished as either physisorptive or chemisorptive, which can be re*

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1098-0121/2015/91(15)/155432(7)

garded as two extrema on the scale of the bonding strength. The main difference is that the latter case involves the formation of (partly) covalent bonds between substrate and adsorbate, which is manifested by a significant hybridization of electronic states and/or charge transfer (CT). There are numerous examples in the literature where the occurrence of commensurism is directly related to or even regarded as a univocal proof of a strong interaction between adsorbate and substrate [11–15]. From our point of view this is a prevalent misunderstanding, based on the disregard of (i) the (lateral) forces that adjust the adsorbate species in a certain registry with the substrate depend on the derivative of the interface potential, i.e., the corrugation, not its average value, and (ii) the overwhelming part of a strong adsorbate-substrate interaction can be non-site-specific [16]. Epitaxial registries, especially if organic layers are involved, are generally affected by a subtle balance between molecule-molecule (i.e., lateral) and molecule-substrate (i.e., vertical) interactions, corrugated on the atomic scale [16]. In case the latter are much stronger than the former, one might assume that commensurate epitaxial relations are preferred to point-on-line (pol), line-on-line (lol), or incommensurate superstructures. Such a scenario occurs, for example, upon adsorption of a 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) monolayer on Ag(111), where the molecules are affected by charge transfer from the metal and thus form partly covalent bonds with the substrate, giving rise to a site-specific adsorption energy gain [8,12,17–21]. This system is a prime example for a chemisorptive molecule-substrate interaction strong enough to bend the carboxylic C-O bonds and to pull the entire molecule toward the silver surface. It is noteworthy that on less reactive surfaces, such as Au(111), PTCDA also exhibits highly ordered monolayer structures while the electronic interaction at this interface is commonly classified

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MARCO GRUENEWALD et al.


as physisorptive with undeformed PTCDA molecules and no detectable charge transfer or covalent bond formation [20,21]. Remarkably, all known epitaxial relations of PTCDA monolayer domains on Au(111) are not commensurate, but pointon-line, although the lattice constants of Au and Ag are almost identical [9,22]. The arguments concerning the relation between the strength of the interaction and the type of epitaxy are sometimes also brought forward when organic-organic interfaces consisting of stacked flat-lying molecules are discussed: Since the electronic interaction is expected to be significantly weaker in those systems (as compared to molecule-metal heterointerfaces) owing to the closed-shell nature of their constituents, commensurate registries should rarely occur. Acknowledging the observation of less demanding line-on-line coincidences for a PTCDA monolayer on 1 ML hexa-peri-hexabenzocoronene (HBC) [23–25] and for other systems [26–29] seems to corroborate the above assumption that also in organic-organic heteroepitaxy the hierarchy of epitaxy types can be related directly to the adsorbate-substrate interaction strength. Nevertheless, two commensurate registries involving copper(II)-phthalocyanine (CuPc) have recently been found for the organic-organic heterointerfaces of CuPc/PTCDA/ Ag(111) and F16 CuPc/CuPc/Ag(111) [30,31]. This immediately raises the question whether the above plausibility argumentation can be really applied to such hetero-organic systems. Indeed, the electronic interaction between the stacked CuPc and PTCDA monolayers has been described previously as chemisorptive and brought in line with the observed commensurism [30]. It shall be elucidated here that commensurism at organicorganic heterointerfaces may occur for physisorbed (i.e., electronically weakly interacting) systems lacking appreciable signs for charge transfer or hybridization. To demonstrate this, we compare CuPc to tin(II)-phthalocyanine (SnPc) monolayers both deposited on a PTCDA monolayer on Ag(111) by characterizing the previously unknown commensurate structures of SnPc/PTCDA/Ag(111) in detail using low-energy electron diffraction (LEED) and low-temperature scanning tunneling microscopy (LT-STM). In order to analyze the bonding strength in situ differential reflectance spectroscopy (DRS) and photoelectron spectroscopy (PES) are applied. By the same methods we also reinvestigate the system CuPc/PTCDA/Ag(111). For SnPc as well as CuPc overlayers on 1 ML PTCDA on Ag(111) we find no indication for a chemisorptive bonding between the stacked organic layers because (i) the spectral character resembles that of nonhybridized molecules, and (ii) any features in DRS and PES, which would be indicative of a CT between the molecules are clearly absent in our data. Instead, the optical and electronic spectra of either MePc ML are fully consistent with those of monomers or molecules in weakly bonding environments, as known from molecules in solution or adsorbed on inert substrates. II. EXPERIMENTAL DETAILS

The Ag(111) surface was prepared by repeated Ar+ sputtering and annealing cycles in ultrahigh vacuum (UHV) at a base pressure in the low 10−10 mbar regime [32]. Molecular

films were deposited from effusion cells with growth rates of approximately 0.1 ML per minute while the substrates were kept at 300 K. The PTCDA monolayer was prepared by annealing of multilayers at approximately 600 K [30,33,34]. Subsequently, the MePc molecules were grown and monitored in real-time by means of DRS. The reciprocal space was analyzed by using a dual microchannel plate (MCP) LEED from OCI at sample temperatures between 300 K and approximately 20 K. The images have been corrected for the geometric distortion and for the primary electron energy error [35]. Axial distortions caused by tilting the sample off-normal were numerically corrected [36] prior to the analysis of the LEED patterns [37]. Within the experimental accuracy the PTCDA film structure remained unchanged upon deposition of the MePc overlayers. LT-STM images were acquired using a commercial apparatus (SPECS JT-LT-STM/AFM with KolibriSensorsTM ) operating at T = 1.1 K [38]. The optical setup is described in detail in Ref. [39]. Briefly, DRS measures in situ the film-thickness- and photonenergy-dependent reflectance change of a sample surface during deposition of an adsorbate film. For this, we use a 100 W halogen lamp with a stabilized power supply (Müller Elektronik-Optik). The reflected light is spectrally analyzed with a monochromator using blazed gratings and a liquid-nitrogen-cooled charge-coupled device (CCD) attached to it (Princeton Instruments Spec-10 100BR and Acton Research SpectraPro SP2356). The DRS signal is defined as: DRS(E,d) = [R(E,d) − R(E,d = 0)]/[R(E,d = 0)], where R(E,d) denotes the reflectance of the sample surface during film deposition and R(E,d = 0) represents the reflectance of the substrate. Subsequently, a numerical algorithm was used in order to extract simultaneously the real and imaginary parts of the complex dielectric function of the thin film from the DRS raw signal [39,40]. Multiple reflections and interference effects were taken into account. C1s core level PES measurements were performed at BESSY II at the undulator beam line UE52-PGM with ppolarized light, a beamline exit slit of 40 μm, and a cff value of 10, which results in an energy resolution better than 45 meV at 335 eV. Photoelectron intensities were measured in normal emission geometry with a VG-Scienta R4000 electron analyzer which was operated with an entrance slit of 300 μm and a pass energy of 20 eV leading to an energy resolution of 30 meV. hν was calibrated with the Fermi energy and an absolute accuracy of EB = 30 meV. The C1s core level PES intensities were normalized at the pre-peak plateau at EB ≈ 280 eV.

III. RESULTS AND DISCUSSION A. Structural characterization

We begin our structural analysis with the system 1 ML SnPc on 1 ML PTCDA/Ag(111), see Fig. 1(a). The unit cell parameters of 1 ML PTCDA on Ag(111) are well known [8]. The positions of the PTCDA spots remained constant upon SnPc growth. We fitted numerically a reciprocal lattice to the LEED data using all spots stemming from the SnPc overlayer simultaneously except for those very close or identical to PTCDA spots [37]. However, drawing the correct conclusions

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COMMENSURISM AT ELECTRONICALLY WEAKLY . . .


FIG. 1. (Color online) LEED and STM images of (a) 1.00 ML (phase P1), (b) 0.92 ML (phase P2), and (c) 0.62 ML (phase P3) of SnPc on 1.00 ML PTCDA/Ag(111). The LEED patterns are fully describable by single epitaxy matrices (Table I) and symmetry equivalents leading to simulated reflexes (open circles) superimposed on one half of the images. For comparison, LEED simulations of the underlying PTCDA (red dots) and the primitive directions of Ag(111) (yellow lines) are shown. The sample surfaces were slightly tilted to observe the (0 0) spot. The magnified insets in the upper right corners of the STM images depict the unit cell vectors and the bases consisting of Sn-up (bright center) and Sn-down (dark center) molecules in characteristic arrangements. Submolecular resolution is clearly achieved as shown in the lower left corner of the STM image in (a) with a Sn-down molecule in the center featuring pronounced nodal lines in the orbital structure.

for the epitaxy relation just from LEED needs some additional assumptions or complementary measurements in the case of organic-organic heteroepitaxy when various symmetry equivalent domains are observed: As the PTCDA unit cell itself does not exhibit mirror symmetry it is highly unlikely that the SnPc overlayer possesses more rotational symmetry equivalents than those of the PTCDA layer with respect to the Ag(111) substrate. In practice, although the SnPc LEED pattern can be simulated by different epitaxy matrices, it is most likely that only one of them is reasonable and therefore can be observed in real space. Taking this into account, we found commensurism (phase P1) in the temperature range from 300 K to ≈ 20 K between 1 ML SnPc and 1 ML PTCDA on Ag(111). To our knowledge, only two heterosystems of large aromatic molecules [both on Ag(111)] have been found to be commensurate so far: 1 ML CuPc/1 ML PTCDA [30] and 1 ML F16 CuPc/1 ML CuPc [31], where F16 CuPc is the perfluorinated derivative of CuPc. Accordingly, it is quite interesting whether the epitaxy relation depends on the choice of the MePc. Although CuPc and SnPc are rather similar, different commensurate epitaxy relations are preferred in 3 1 the heterostructures formed (CCuPc/PTCDA = (−2 1) [30], and 2 1 CSnPc/PTCDA = (−3 1) [phase P1]). Yet, the commensurate SnPc and CuPc unit cell areas are equal for the 1 ML phase in each case. The different structures must be induced by small geometric distortions (e.g., nonplanarity) of the phthalocyanine backbone caused by the incorporation of the relatively large tin atom in SnPc. Due to the nonplanarity two adsorption configurations of SnPc are feasible (Sn-up and

Sn-down) providing another degree of freedom for energy minimization [41–45]. In order to determine the adsorption geometry of individual molecules of the SnPc adlayer we acquired LT-STM images. Figure 1(a) depicts an overview scan showing unoccupied states predominantly located at the tin atoms. The Sn-up and Sn-down configurations are clearly discernible. We found six SnPc molecules in the unit cell (four Sn-up and two Sn-down), which renders the large superlattice found by LEED plausible. This motif is very different from any arrangement of SnPc in the known three-dimensional (3D) bulk structures [46,47]. Most likely, the regular arrangement of Sn-up and Sn-down molecules is caused by different adsorption sites. The lack of a moiré pattern in STM supports the finding of commensurism at the organic-organic interface. We further acquired highresolution LT-STM images that clearly feature submolecular resolution with distinct nodal lines in the orbital structure as shown in Fig. 1(a). A similar contrast has been reported previously for electronically decoupled phthalocyanines [48]. An identification of the individual adsorption sites of SnPc on 1 ML PTCDA/Ag(111) was not feasible. When trying to image simultaneously the underlying PTCDA layer for SnPc coverages below 1 ML the SnPc adlayer either forms two other distinct commensurate registries with respect to PTCDA or is disordered for too dilute submonolayers. Figure 1(b) depicts a typical LEED measurement with sharp spots ascribable to the PTCDA monolayer and to the 0.92 ML thick SnPc layer grown on top. Intriguingly, the LEED spots belonging to SnPc are now distinctively different from those of phase P1. However, they can also be explained by a commensurate

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TABLE I. Epitaxy relations of the three commensurate phases of SnPc on 1 ML PTCDA/Ag(111). c1 , c2 : SnPc lattice vectors; : unit cell angle between c1 and c2 ; θ : SnPc domain angle between c1 and the primitive substrate lattice vector a1 . The SnPc epitaxy matrices are given with respect to the PTCDA interlayer. For each matrix element an uncertainty of ±0.01 was determined, representing twice the standard deviation of the numerical lattice fit. The total scaling 3 5 error was found to be ±0.01. We used CPTCDA = (−6 1) to simulate the PTCDA/Ag(111) spots. Primitive silver lattice vectors enclose an angle of 120◦ . N is the number of SnPc molecules per unit cell. ˚ ||c2 ||/A ˚ phase ||c1 ||/A P1

31.8(5)

P2

31.9(4)

P3

25.2(4)

epitaxy matrix C N 2.00 1.00 42.1(6) 116.6(1) 36.6(1) 6 −3.01 1.00 2.01 1.00 53.6(6) 122.5(1) 36.7(1) 7 −4.01 1.01 2.00 0.00 18.9(3) 90.9(1) 73.3(1) 2 0.00 1.00 /deg

(a)

2 1 CSnPc/PTCDA = (−3 1) (phase P1). Consequently, the two MePc structures appear somehow mirrored to each other at the long vector of the PTCDA unit cell. However, both structures are not symmetry equivalent as the PTCDA unit cell is not perfectly rectangular. This is further illustrated in a real-space plot of the lattices in Fig. 2.

B. Optical spectroscopy

Referring to the structural similarities of SnPc and CuPc in the heterostructures we compare the optical spectra of both systems in order to characterize the electronic interactions at the organic-organic interfaces. The imaginary part of the dielectric function ε , extracted from the DRS [39,40], shall be discussed representing the light absorption behavior of the thin film. By

SnPc (phase P1) 2 1 -3 1 SnPc (phase P2) 2 1 -4 1

89°

a2 a1 1 nm PTCDA SnPc (phase P1) SnPc (phase P2) SnPc (phase P3) CuPc

θ/deg

2 1 phase P2 between both organic layers, given by CP2 = (−4 1). The corresponding unit cell now contains seven instead of six molecules, and the unit cell area of this SnPc overlayer is larger 2 1 by a factor of 6/5 as compared to CP1 = (−3 1). Hence the areal density of the P2 submonolayer structure is 3% lower than that of P1. LT-STM scans as shown in Fig. 1(b) reveal flat-lying molecules almost exclusively in Sn-up configuration, but with some defects. By further reducing the SnPc coverage another commensurate registry P3 with CP3 = (20 01) relative to the PTCDA interlayer occurs according to Fig. 1(c). While the phases P1 and P2 exist at 300 K, P3 is formed from the nonordered phase upon cooling to ≈ 100 K or less. The unit cell of P3 consists of one Sn-down and one Sn-up molecule only and is thus significantly smaller than those of the phases P1 and P2. The temperature- and coverage-dependent phase diagram shown in Fig. S1 [49] differs significantly from the respective CuPc heterostructures where commensurism is preferred also for lower coverages at T < 160 K and only one epitaxial relation was found [30]. The structural information extracted from LEED measurements is compiled in Table I. Despite the differences for the submonolayers both MePcs show peculiar similarities in their monolayer structures on 1 ML PTCDA/Ag(111). The CuPc adlayer is described by 3 1 CCuPc/PTCDA = (−2 1) [30], while the SnPc adlayer follows

(b)

SnPc (phase P3) 2 0 0 1 CuPc 3 1 -2 1

FIG. 2. (Color online) (a) Real space lattices and (b) LEED simulation of the three commensurate phases of SnPc (this work) as well as CuPc (Ref. [30]) for comparison, all of them grown on 1 ML PTCDA/Ag(111), respectively. The epitaxy matrices with respect to the PTCDA lattice (red dots) are given as well.

using a light beam at nearly normal incidence, the optical spectroscopy probes the in-plane components of optical transition dipole moments. Due to the relatively large separation between the centers of gravity of neighboring molecules within one molecular layer excitonic coupling between molecules of one kind is negligible [50]. However, if either a coupling between such dipoles belonging to molecules in different layers or a CT at the hetero-organic interface occurred, this should be clearly visible in the spectra [51,52]. Thus, even small charge rearrangements between the monolayers would have a noticeable impact on the optical properties, which allows us not only to distinguish the two extrema physisorption and chemisorption but also adsorption characteristics in between in terms of electronic interaction strength [53]. As a general trend of the influence on the optical spectra we observed previously for different systems either a pronounced broadening of the adsorbate spectrum (cf. Newns-Anderson model [54,55]) with increasing substrate-adsorbate interaction when dealing with metallic substrates [53] and/or new transitions of molecular ions on inert substrates due to integer CT [39,52]. In Fig. 3(a) the imaginary part ε of the dielectric function of the SnPc monolayer on top of 1 ML PTCDA/Ag(111) is depicted. The data were numerically extracted from the DRS shown in Fig. S2(a) [49] by applying a two-layer thin film model. The effective thicknesses of 1 ML PTCDA (dPTCDA = 0.32 nm [56,57]) and 1 ML SnPc (dSnPc = 0.34 nm [46,47]) were taken into account. For the SnPc ML we observe monomer behavior owing to a sharp peak at 1.71 eV followed by a small shoulder at 1.88 eV. We thus conclude that electronic interactions with the PTCDA interlayer are minor at most. Under closer scrutiny, the SnPc ε spectrum is much less broadened than for SnPc on Au(111) where a predominantly physisorptive behavior is expected [53]. In fact, the spectrum is even comparable to that obtained on an inert mica substrate, cf. Fig. 3(b). Consequently, we exclude a significant charge transfer between SnPc and PTCDA. It is worth noting that the interaction of optical transition dipole moments between the two different molecular layers is negligible as well. For comparison we repeated these measurements with CuPc instead of SnPc (Fig. S2(b) [49]). In Fig. 3(a) the ε spectrum of 1 ML CuPc on 1 ML PTCDA/Ag(111) is shown as well.

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(a)

Wavelength [nm] 700 600

500

1 ML SnPc on 1 ML PTCDA/Ag(111)

8

1 ML CuPc on 1 ML PTCDA/Ag(111)

ε''

12

Intensity [arb. units]

16

900 800


ε'' (normalized)

4 0 1

1 ML SnPc on mica

(b)

1.5

2.0 Energy [eV]

C1s PES hν = 335 eV φ = 0°

290

0.6 ML CuPc on mica

0

0.9 ML CuPc/1 ML PTCDA/Ag(111) 1 ML PTCDA/Ag(111) [α] CuPc multilayer [β] shifted by –0.65 eV [α] + [β]

2.5

FIG. 3. (Color online) (a) Imaginary part of the dielectric function ε of 1 ML SnPc and 1 ML CuPc, each on 1 ML PTCDA/Ag(111), numerically extracted from the DRS data. (b) Normalized ε spectra of SnPc [53] and CuPc on inert mica for comparison. Dashed lines indicate the shift of the ε peaks on 1 ML PTCDA/Ag(111) relative to those on mica caused by the different polarizabilities (dielectric backgrounds) of the substrates. The spectrum of 0.6 ML CuPc on mica is less affected by (weak) lateral intermolecular interactions and hence found at slightly higher energies than 1.0 ML CuPc (not shown). However, the latter already exhibits contributions from 3D clusters.

In accordance with SnPc we observe monomeric features indicated by a sharp peak at 1.81 eV and a shoulder at 2.00 eV. Consequently, we can also exclude a significant charge transfer at the organic-organic interface in these CuPc/PTCDA heterolayers. Moreover, the optical fingerprint is even less broadened than for a CuPc submonolayer on inert mica, cf. Fig. 3(b), presumably as a consequence of the high order in the CuPc film on 1 ML PTCDA/Ag(111), whereas the CuPc film on mica is disordered (cf. Fig. S3 [49]). Thus, apart from the obvious spectral shifts, the optical absorption spectra of both SnPc and CuPc on 1 ML PTCDA/Ag(111) are fully compatible with those of the two MePcs deposited on an inert substrate: the PTCDA interlayer serves as a decoupling monolayer [28], and signs for appreciable MePc-PTCDA hybridization and/or CT were not found. C. Photoelectron spectroscopy

Additional information on the nature of the interfacial bonding between the first MePc ML on 1 ML PTCDA/Ag(111) can be derived from photoelectron spectroscopy (PES). In PES a one-hole final state is created, which is sensitive to hybridization and charge transfer. At a molecule-metal interface where such electronic interactions occur, the line shape of the C1s core level PES spectra differs significantly from that of the same molecule in a weakly bonding environment such as bulk samples [18,58,59]. In the C1s core level PES data in Fig. 4 it can be seen that the line shape of the measured spectrum (open circles) is successfully reproduced by a mere linear combination of scaled reference spectra of a CuPc multilayer and 1 ML

288 286 Binding Energy [eV]

284

FIG. 4. (Color online) C1s PES of 0.9 ML CuPc/1 ML PTCDA/ Ag(111) (open circles) together with scaled reference spectra of 1 ML PTCDA/Ag(111) and a CuPc multilayer. The solid line is the sum of both reference spectra.

PTCDA/Ag(111). This demonstrates that the CuPc molecules behave as if they were in a purely homomolecular CuPc environment. The same finding has been reported for a 0.7 ML SnPc/1 ML PTCDA/Ag(111) sample [34]. Consequently, our PES data corroborate the results derived from DRS, namely that both systems, with either SnPc or CuPc on 1 ML PTCDA/Ag(111), do not exhibit significant hybridization or CT at their heteromolecular interface. Having discussed the rather weak electronic interaction at the SnPc/PTCDA and the CuPc/PTCDA interfaces, both on Ag(111), it seems contradictory that the former lowest unoccupied molecular orbital (F-LUMO) of the PTCDA monolayer shifts from −0.20 to −0.32 eV below the Fermi energy EF upon adsorption of 1 ML CuPc [30]. This shift means that the partial filling of the F-LUMO is significantly increased, which was previously attributed to an electron transfer from CuPc to PTCDA and hence interpreted as chemisorptive interaction [30]. However, in the view of our results outlined above this electron transfer is unlikely to occur between both organic layers. A remarkable effect of the adsorption of 1 ML CuPc is that the bonding distance d of the carbon atoms of PTCDA above the Ag(111) surface is reduced to 0.281 nm as compared to 0.286 nm for the pristine PTCDA ML on Ag(111) [30]. Therefore, we compare in Fig. 5 the literature values of the LUMO and F-LUMO position EL of PTCDA versus d on various (111)-oriented metals substrates. One has to bear in mind that it is generally difficult to simultaneously account for unoccupied (LUMO) and partially occupied (FLUMO) states and that a simple mathematical expression for EL (d) is not readily feasible. Nevertheless, the dashed line in Fig. 5 illustrates a clear tendency, namely that EL is reduced for decreasing d (note that on the less densely packed Ag(110) surface a similar trend was found for PTCDA by density functional theory (DFT) calculations [63] only with a quantitatively different slope). The system 1 ML CuPc/1 ML PTCDA/Ag(111), highlighted in red in Fig. 5, consistently follows this trend. Consequently, the shift of the F-LUMO position of the PTCDA monolayer upon CuPc adsorption can readily be linked to the reduced bonding

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LUMO w.r.t. Fermi-energy [eV]

1.5


Vads-sub that drives the molecules in the top layer (adsorbate) into an epitaxial registry with the underlying layer (substrate) via the lateral force

PTCDA on Au(111) measured with IPES

1.0

x,y Vads-sub . Fads-sub x,y = −∇

PTCDA

0.5 typical error bars: XSW PES 0.0 CuPc PTCDA on Ag(111) PTCDA on Ag(111) -0.5 with 1 ML CuPc on top PTCDA on Cu(111) 0.26 0.28 0.30 0.32 0.34 Bonding distance d [nm]

FIG. 5. (Color online) Compiled literature results for LUMO or former LUMO (F-LUMO) positions of PTCDA monolayers with respect to the Fermi energies EF on different metal substrates. F-LUMO and LUMO values were acquired with normal and inverse photoelectron spectroscopy (PES [20,30] and IPES [60]). The bonding distance d of the carbon atoms above the respective metal surface was determined using the x-ray standing-wave (XSW) technique [19,30,61,62]. The dashed line is a guide to the eye.

distance relative to the metal substrate, which indeed renders a charge transfer from Ag(111) to PTCDA plausible. In turn, a charge transfer at the MePc/PTCDA interface (Me = Sn, Cu) on Ag(111) is rather unlikely. In accordance with this conclusion only a minor charge density rearrangement across the CuPc/PTCDA interface, approximately 50 times smaller than that between PTCDA and Ag(111), was found by means of DFT calculations [64]. D. Summary

So far, we have characterized the hetero-organic interfaces between two different metal-phthalocyanines and 1 ML PTCDA on Ag(111) as electronically weakly interacting. A significant CT and the formation of covalent bonds at the CuPc/PTCDA and SnPc/PTCDA interfaces can be excluded from our optical and photoelectron spectra. This finding contravenes the aforementioned assumption, namely that the strength of the adsorbate-substrate interaction can be directly related to the type of epitaxy. Instead, it is the corrugation (i.e., the gradient in x and y direction) of the interaction potential

[1] S. Liu, W. M. Wang, A. L. Briseno, S. C. B. Mannsfeld, and Z. Bao, Adv. Mater. 21, 1217 (2009). [2] N. R. Armstrong, W. Wang, D. M. Alloway, D. Placencia, E. Ratcliff, and M. Brumbach, Macromol. Rapid Commun. 30, 717 (2009). [3] B. P. Rand, J. Xue, F. Yang, and S. R. Forrest, Appl. Phys. Lett. 87, 233508 (2005). [4] J. Faist, F. Capasso, D. L. Sivco, C. Sirtori, A. L. Hutchinson, and A. Y. Cho, Science 264, 553 (1994). [5] F. Lindner, K. Walzer, and K. Leo, Appl. Phys. Lett. 93, 233305 (2008).

(1)

While the lateral molecule-molecule interactions within the adsorbate layer naturally contribute to the superstructure formed [16], it follows from Eq. (1) that the absolute moleculesubstrate interaction strength (i.e., the spatial derivative of Vads-sub perpendicular to the plane of the interface, as well as the average value of Vads-sub itself) is insignificant for the driving forces determining the epitaxial relation. This situation can be rationalized mechanically by a glass slide glued to a smooth surface via a thin water film: Lateral motion is x,y Vads-sub ≈ 0), whereas picking almost freely feasible (−∇ up the glass slide vertically requires a considerable force z Vads-sub = 0). Having said so, it should be noted that (−∇ there is no a priori correlation between the average value of the molecule-substrate interaction Vads-sub and its lateral corrugation. IV. CONCLUSION

We have found three different commensurate phases in submonolayers and monolayers of SnPc on 1 ML PTCDA on Ag(111). Our optical and photoelectron spectra are consistent with a rather weak, physisorptive interaction between the phthalocyanine and the PTCDA layers for both SnPc and CuPc. Therefore, the occurrence of commensurism cannot be readily linked to the nature of the interfacial bonding and is in particular not a clear indication for a strong electronic interaction. Note added. Recently, it came to our attention that the issue of electronic interaction at the CuPc/PTCDA heterointerface is further addressed in Ref. [65]. ACKNOWLEDGMENTS

We thank the BESSY staff, E. Handick and M. Wiessner for experimental support. This work was financed by the Deutsche Forschungsgemeinschaft (DFG) through Grants No. FR875/9, No. FR875/11, and No. INST 275/256-1 FUGG (M.G., J.P., M.M., F.S., R.F., and T.F.) as well as No. GRK 1221, No. RE1469/9-1, and No. SCHO1260/4-1 (C.S., A.S., and F.R.).

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