Thermal Stability and Interlayer Exchange Processes in Heterolayers

Article pubs.acs.org/JPCC

Thermal Stability and Interlayer Exchange Processes in Heterolayers of CuPc and PTCDA on Ag(111) Sebastian Thussing and Peter Jakob* Fachbereich Physik und Wissenschaftliches Zentrum für Materialwissenschaften, Philipps-Universität Marburg, Renthof 5, Marburg 35032, Germany S Supporting Information *

ABSTRACT: The structural properties and thermal evolution of heterolayers comprising copper-phthalocyanine (CuPc) and 3,4,9,10-perylene-tetracarboxylicdianhydride (PTCDA) on Ag(111) were investigated using FT-IRAS, SPALEED, and TDS. The bilayer systems have been prepared at low temperatures (T ≃ 80 K) and annealed to successively higher T. The layered arrangement of CuPc deposited onto PTCDA/Ag(111) displays unusual thermal stability up to 450−500 K when intermixing sets in. For layers with a reversed stacking sequence, exchange processes of first- and second-layer molecules take place already at 200−300 K. PTCDA shows a stronger tendency to occupy first-layer sites, displacing CuPc from the direct contact with the metal substrate; continued annealing, however, leads to a depletion of PTCDA within the deposited organic film. This apparent paradoxon is resolved by comparing two related quantities, the adsorption energy per area (for parallel adsorbed molecular species) and the adsorption energy per molecule. As a consequence of the dissimilar “footprint” areas of CuPc and PTCDA, and the larger overall size of CuPc, the former quantity is larger for PTCDA, while the latter quantity is larger for CuPc. The experimental observations regarding layer exchange, intermixing, and modifications in thin film composition and structure are then readily explained.

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INTRODUCTION In recent years, the investigation of organic thin films has become an attractive field of research as physical properties of this material class may be tuned selectively to improve the performance of electronic and optoelectronic devices. The focus gradually shifted from the study of monomolecular films to systems comprising different types of molecules favoring either a release or an accumulation of charge. Adequate combinations of such donor−acceptor type of molecules then may yield dramatic enhancements in the performance of OLEDs and increasingly higher efficiencies of photovoltaic cells.1,2 Thereby it is the interface between the first layer of molecules and the metal surface that influences the growth behavior of deposited molecules and determines the quality of grown films as well as the electronic properties of these organic layers.3 Moreover, the combination of unlike molecular species opens additional degrees of freedom regarding the internal structure of grown films and orientation of involved species. This is particularly important at the interface where the two compunds meet and where the efficiency, for example, of charge transfer depends crucially on the local arrangement of the molecular species in this region. It is thus of foremost importance to acquire an in-depth understanding of elemental processes, to identify relevant parameters and develop strategies as to control these. In organic heterolayer systems, donor and acceptor types of molecules are combined so that their physical properties, for © 2017 American Chemical Society

example, electronic structure and structural stability of the interface, can be analyzed. In this respect, the orientation of the molecular species may play a crucial role whereby upright standing molecules enhance and flat lying molecules block charge transfer across the interface.4 For ultrathin layers, especially for bilayer systems, the sequence of donor and acceptor type molecules has a strong influence on the electronic levels of the constituents within the hetero organic interface.5 It is therefore essential to understand the physical properties in such systems, which after all opens a way to modify and optimize future applications. The most basic approach to realize an organic−organic heterointerface is to deposit a monolayer of a particular organic molecule on a metal surface and add an additional monolayer of a different species on top of it. Note that for such “few-layer” interfaces (and at variance to organic heterointerfaces comprising thick films), the stacking sequence plays a crucial role as chemical interactions with the metal substrate are restricted to first-layer molecules; this is because of the weak coupling between first- and second-layer molecules in bilayer systems, which is dominated by van der Waals interactions.6−12 Electronic levels of molecular species are therefore distinctly different depending on whether they are occupying first- or second-layer sites. Evidently, these kinds of heterolayers Received: March 13, 2017 Revised: May 31, 2017 Published: June 1, 2017 13680

DOI: 10.1021/acs.jpcc.7b02377 J. Phys. Chem. C 2017, 121, 13680−13691

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The Journal of Physical Chemistry C represent systems with two different interfaces: first, between the metal substrate and the chemically interacting first layer (contact layer), and second, between first-layer and the merely van der Waals bonded second-layer species whose electronic properties are close to isolated molecules or bulk material thereof. In this respect, we note that molecular thin films comprising a single species only likewise constitute a system with two interfaces, as electronic properties of first- and secondlayer species are distinctly different. Regarding lateral ordering, the site preference (corrugation of the molecules−substrate interaction) represents an essential factor, which is enhanced by the chemical interaction of firstlayer species with the metal substrate. For second-layer species, the tendency for lateral ordering is substantially lower, and, even though occasionally enhanced electrostatic interactions exist between those layers, it is much more difficult to establish a correlation between first- and second-layer periodicities.7,11−13 For film thicknesses beyond the first layer, the arrangement of the molecules is primarily influenced by intermolecular interactions. In addition to the frequently observed parallel orientation of second-layer species,5,6,8,9,12−14 and depending on the respective bulk crystal structure, inclined geometries of the individual molecules or upright standing molecules may be encountered. An ideal combination to act as a model system to study elemental processes at organic heterointerfaces is CuPc (copper-phthalocyanine) and PTCDA (3,4,9,10-perylene-tetracarboxylic-dianhydride). For both molecular species, the knowledge base regarding their structural and electronic properties of the respective homogeneous layers is substantial.15−20 Moreover, their electronic characteristics qualify them to act as a donor (CuPc) and acceptor (PTCDA) pair. Finally, the planar and highly symmetric molecular structure places them in first row for fundamental research-based projects such as ours; in real devices, alternative compounds seem a better choice, though. According to the literature, and on the basis of scanning tunneling microscopy (STM), low energy electron diffraction (LEED), and normal incidence X-ray standing wave spectroscopy (NIXSW) data, the PTCDA and CuPc molecules of the respective contact layers are adsorbed parallel to the Ag(111) surface;15,21 for PTCDA/Ag(111), some minor warping of the molecular plane has been reported and attributed to a chemical interaction between the carboxyl group at the corners of PTCDA and the substrate Ag atoms.19,21,22 For PTCDA and CuPc bilayers, the parallel orientation is retained.17,18 Thereby, the presence of an additional layer has an only minor effect on the long-range order of the respective monolayer systems. For PTCDA bilayers, an identical herringbone pattern with two inequivalent molecules is observed,18 while for CuPc a slight lateral compression of the point-online (POL) phase of CuPc/ Ag(111) is found.17 In both cases, a fixed registry between firstand second-layer molecules exists. Things become more complex for heterolayers comprising CuPc and PTCDA. For the stacked CuPc/PTCDA/Ag(111) bilayer, a structure model has been suggested on the basis of NIXSW and STM data, which likewise comprise a parallel orientation of both species;9,13 this model is fully corroborated by the IR data presented in this study. Interestingly, STM and SPA-LEED investigations showed that flat lying CuPc molecules on top of the PTCDA/Ag(111) monolayer display a commensurate long-range order, with respect both to the PTCDA layer and also to the Ag(111) substrate surface.13

Moreover, the deposition of CuPc on top of the PTCDA monolayer reduces the distance between PTCDA and the Ag(111) surface by about 0.1 Å, as verified by NIXSW;9 in parallel, the binding energy of the first-layer PTCDA F-LUMO is slightly increased (∼120 meV);13 that is, the presence of second-layer CuPc induces an enhanced charge transfer into the stacked heterolayer. This is a surprising observation as only weak van der Waals forces should prevail between CuPc and PTCDA, according to DFT calculations8 and DRS measurements.7 Specifically, Gruenewald et al.7 concluded, on the basis of DRS measurements, that CuPc and PTCDA electronic levels of the CuPc/PTCDA/Ag(111) bilayer stay largely decoupled; that is, coupling between first- and second-layer species is negligible. Efforts to produce a well-defined and stable bilayer with inverse stacking sequence, that is, PTCDA/CuPc/Ag(111), were unsuccessful so far, as both molecules intermix upon preparation at room temperature.23 Specifically, for 0.6 ML PTCDA deposited onto 0.9 ML CuPc/Ag(111) at room temperature, STM images yield intermixed areas for the most part; only occasionally ordered domains, most likely due to second-layer PTCDA molecules adopting their preferred herringbone type of arrangement, that is, similar to the wellknown PTCDA/Ag(111) monolayer phase,18 are found. It is well-known that structural arrangements of molecular thin films may change as different kinetic constraints are imposed during the growth process. To our knowledge, all heterolayers comprising CuPc and PTCDA have been prepared at room temperature. This is why we decided to use our advanced and versatile preparation capabilities and investigate this material combination in a much more extended temperature range. In the present study, two different stacking sequences of the CuPc and PTCDA bilayer system have been prepared at low temperature (T ≃ 80 K). Fourier-transform infrared absorption spectroscopy (FT-IRAS), spot-profile-analysis low-energy electron diffraction (SPA-LEED), and thermal desorption spectroscopy (TDS) have been used to explore the thermal evolution of the grown layers. In particular, specific information on the location (first or second layer) and the nature of the molecular species (CuPc or PTCDA) within the heterolayers could be derived. By examining these characteristic vibrational bands of PTCDA and CuPc molecules, the structural integrity of the stacked layers has been monitored, thermally induced interlayer exchange processes identified, and variations in the overall composition of the heterolayer detected.

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EXPERIMENTAL SECTION The experiments were performed in an ultrahigh vacuum chamber (base pressure p = 5 × 10−11 mbar), which contained facilities for Fourier-transform infrared absorption spectroscopy (Bruker IFS 66v/s), spot-profile analysis low energy electron diffraction, and thermal desorption spectroscopy. All infrared spectra (spectral range of 600−4000 cm−1) were taken at a resolution of 2 cm−1 and using a LN2 cooled MCT (HgCdTe) detector at a surface temperature of 80 K. Electron diffraction measurements have been performed with an Omicron SPALEED operated at typical electron energies of 30 eV. For desorption experiments, several masses of our quadrupole mass spectrometer (Pfeiffer, QMG 700, mass range 0−1024 u) were collected “simultaneously” (with a time delay of 200 ms between data points associated with the individual masses), while heating the sample linearly at a rate of 1 K/s. 13681


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The Journal of Physical Chemistry C Temperatures have been measured using a K-type thermocouple (laser-)welded to the edge of the Ag(111) single crystal. The sample was mounted to a liquid He or N2 cooled cryostat and could be heated resistively. CuPc (PTCDA) was deposited from a homemade evaporator held at 500 K (530 K) leading to typical deposition rates of 0.2 monolayers per minute in both cases. During deposition, the background pressure remains at ≤1 × 10−10 mbar. CuPc (PTCDA) contact layers were grown at a sample temperature Tsample of 300 K (400 K), if not specified otherwise. Before the experiments were performed, the sample was generally cleaned by Ar+ sputtering (700 eV, 1 μA, Tsample = 380 K, Δt = 30 min) and subsequently annealed to 780 K (Δt = 5 min). A quantitative evaluation of desorbing CuPc and PTCDA has been achieved by comparison to the thermal desorption traces of the respective pure layers. For both molecules, welldefined features associated with the desorption of second-layer species (bilayer) are found. Because the parallel molecular orientation is retained for CuPc and PTCDA mono- and bilayers, the integrated signals of desorbing second-layer species have been set to 1 monolayer (ML).

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RESULTS AND DISCUSSION CuPc and PTCDA Bilayer Systems. In this section, various bilayer systems comprising PTCDA and CuPc on Ag(111) will be introduced and characterized regarding their structural and vibrational properties. In Figure 1a−c, the SPALEED images of these layers are depicted; the corresponding IR spectra are shown in Figure 1e, along with spectra of the PTCDA/Ag(111) and CuPc/Ag(111) monolayers. To ease identification, we have added layouts of the individual layers with red (blue) bars denoting PTCDA (CuPc) molecules (molecular structures are delineated in Figure 1d). The grown bilayers are thermally stable at room temperature, and, according to our SPA-LEED data, they all display a distinct long-range order. Figure 1a shows the LEED pattern of the PTCDA bilayer on Ag(111). In accordance with the literature,18 the observed longrange order is found to be identical to the PTCDA/Ag(111) monolayer. In Figure 1b, 1 ML CuPc has been deposited onto a well-ordered PTCDA/Ag(111) monolayer. In addition to the generic LEED pattern associated with the PTCDA/Ag(111) monolayer, additional reflexes are detected for the stacked CuPc/PTCDA/Ag(111) heterolayer. In accordance with recent work by Stadtmüller et al.,13 this is attributed to an overlayer structure of CuPc on top of PTCDA/Ag(111), which is commensurate with respect to each other and to Ag(111). This finding, along with our IR spectra in Figure 1e, suggests that the CuPc and the PTCDA layers form a stacked array with a welldefined organic−organic interface. The observation of two overlapping LEED patterns in Figure 1b furthermore indicates that the scattered electrons transmit information related to both the uppermost layer as well as the underlying contact layer. The LEED pattern in Figure 1c refers to a CuPc bilayer on Ag(111), for comparison. The matrix describing the pattern differs slightly from the CuPc/Ag(111) monolayer15,17 and can be assigned to a POL-phase.17 The observation of just a single set of reflections (as opposed to a combined LEED pattern containing reflexes associated with first and second-layer CuPc) is interpreted as a clear indication for a long-range order, which comprises both CuPc layers. It is apparent that the addition of a second CuPc layer leads to a modification of the CuPc/ Ag(111) monolayer unit cell size.17

Figure 1. SPA-LEED images (Ekin = 30 eV) of various highly ordered bilayer systems: (a) PTCDA/PTCDA/Ag(111); (b) CuPc/PTCDA/ Ag(111); and (c) CuPc/CuPc/Ag(111). PTCDA and CuPc (molecular structures depicted in (d)) have been deposited at sample temperatures of 400 and 300 K, respectively. In panel (e) and starting from top to the bottom, infrared absorption spectra of 1 ML PTCDA, 2 ML PTCDA, 1 ML CuPc on top of 1 ML PTCDA, 2 ML CuPc, and 1.05 ML CuPc, all of them grown on Ag(111), are displayed (spectral range 640−910 cm−1); to ease identification of the various molecular layers, schematic layouts thereof have been included. For better clarity of presentation, the spectra are shifted vertically. All data were collected at 80 K.

To characterize the grown layers and identify individual species, we take advantage of the high sensitivity and selectivity of FT-IRAS; the corresponding IR spectra are summarized in Figure 1e. The displayed spectral range 640−910 cm−1 covers predominantly out-of-plane vibrational modes of the deposited PTCDA and CuPc molecules.24−26 It is evident that, from their specific vibrational signature as well as the correlated 13682


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The Journal of Physical Chemistry C appearance of distinct bands, a mode assignment regarding the identity of the molecular species, as well as their location (first or second layer), can be attained. Despite some incidental overlapping of modes, the associated complex line shapes and contributing modes are, in general, readily disentangled. Comparison of the spectra in Figure 1e shows that, upon variation of the local environment of the respective molecular species, the individual modes are subject to slight frequency shifts; in these cases, the corresponding bands are connected by vertical lines. As reference layers, we have also included the spectra of the PTCDA/Ag(111) and CuPc/Ag(111) monolayers (top and bottom curves, respectively). Characteristic bands for PTCDA/Ag(111) are located at 712.4, 785.4, 791.4, 818.8, 831.5, and 841.6 cm−1; vibrational modes associated with second-layer PTCDA are found at 742.0, 815.8, and 878.6 cm−1. These values compare favorably with the less wellresolved bands reported in the literature, using high-resolution electron energy loss spectroscopy (HREELS).27,28 For the CuPc/Ag(111) contact layer, modes are located at 719.0 and 765.4 cm−1. Equivalent new bands related to second-layer CuPc are located at about 735 and 778−779 cm−1. As evidenced by the three top curves, deposition of a second layer (PTCDA or CuPc) causes only slight frequency shifts of the PTCDA monolayer modes and, of course, the emergence of extra modes specific to second-layer PTCDA or CuPc. Apparently, these shifts are not very specific regarding the nature of the (purely physisorbed) second-layer species. Actually, similar shifts can be observed if an extra xenon layer is deposited onto PTCDA/Ag(111) (not shown). Equivalently, for second-layer CuPc (the respective vibrational modes are labeled by blue vertical lines), comparison of the three bottom spectra reveals that the nature of the layer underneath affects second-layer vibrational bands only marginally as well (Δν ≃ 1 cm−1). Figure 2 shows infrared absorption spectra of 1 ML PTCDA (top curve), 2 ML PTCDA (center curve), and a CuPc/ PTCDA stacked bilayer (bottom curve), all of them adsorbed on Ag(111). The layers have been annealed to T ≃ 400 K, and they display a distinct long-range order characterized by the LEED patterns in Figure 1. Within the displayed spectral range of 1150−1810 cm−1, exclusively in-plane vibrational modes of PTCDA and CuPc are expected.24−26 According to the surface selection rule for metal substrates, parallel oriented dipoles are screened as a result of image charges induced in the surfacenear region of the substrate. This effect renders prominent inplane vibrational modes of both molecules virtually invisible when adsorbed with their π-conjugated backbones oriented parallel to the metal surface. Our observation of quite strong absorption bands in this spectral region is ascribed to interfacial dynamical charge transfer (IDCT).17,20,29−33 Interestingly, the vibrational bands associated with the PTCDA/Ag(111) monolayer are subject to only very minor changes if an extra layer of either CuPc or PTCDA is deposited. In particular, the IDCT characteristics of the PTCDA/Ag(111) monolayer is only marginally affected; the only noteworthy modification concerns the 1240 cm−1 band, which becomes slightly narrower in the case of CuPc/PTCDA as compared to PTCDA/PTCDA. The similarity of spectral features in Figure 2 thus demonstrates that the PTCDA/Ag(111) contact layer is only very weakly influenced by second-layer species. The total lack of additional modes associated with secondlayer molecules can be taken as clear evidence for a parallel orientation of these and also for the absence of a distortion of

Figure 2. Infrared absorption spectra of various molecular layers comprising a PTCDA/Ag(111) contact layer. Specifically, the PTCDA monolayer has been covered either by (a) vacuum, (b) 1 ML PTCDA, or (c) 1 ML CuPc. PTCDA/Ag(111) has been deposited at 400 K so that a well-ordered template for the second layers is formed; similarly, proper ordering of second-layer species is ensured by annealing to 400 K after deposition. The displayed spectral range of 1150−1810 cm−1 is representative of CuPc and PTCDA in-plane modes. All spectra were collected at 80 K.

their molecular planes (which would be quite common for firstlayer molecules). Moreover, our findings confirm that IDCT is restricted to the layer in direct contact with the metal substrate. This is in accordance with the idea that the interaction of second-layer molecules with the contact layer solely comprises van der Waals and electrostatic interactions. The virtually identical curves in Figure 2 further suggest that second-layer species alter the density of states of the PTCDA/Ag(111) contact layer at εF only marginally. CuPc/PTCDA/Ag(111) Heterolayer Interfaces. Thermal Stability. Figure 3 shows thermal desorption spectra of a stacked bilayer system comprising one monolayer (ML) of CuPc postdeposited onto a complete PTCDA/Ag(111) contact layer, with the QMS signals of masses 575 u (CuPc) and 392 u (PTCDA) being collected in parallel while heating the sample. To specify the initial stacking sequence of such heterolayer systems, we will use the nomenclature CuPc/ PTCDA/Ag(111). As a reference, the desorption trace of 3.3 ML PTCDA/ Ag(111), which yields desorption of the PTCDA bulk/ multilayer at 484 K, the bilayer at 505 K, and of the monolayer at 646 K, has been included in Figure 3 (gray curve). In the latter case, molecular desorption is accompanied by dissociation, which causes an abrupt drop of the signal at T ≃ 650 K. In this respect, we note that our observation of PTCDA/Ag(111) monolayer desorption is at variance to reports in the literature,18,21 stating that PTCDA on Ag(111) does not desorb but dissociates completely. According to our quantitative analysis, 1/3 ML of the PTCDA monolayer does in fact desorb. Thereby, the absence of dissociation and thermal stability of PTCDA/Ag(111) up to annealing temperatures of Tann = 600 K is deduced from IRAS results, which show virtually identical spectra (not shown). The onset of PTCDA bilayer desorption can be estimated to T ≃ 450 K (cf., the gray curve in Figure 3 and in Figure S1); 13683


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The Journal of Physical Chemistry C

Vibrational Analysis and Interlayer Exchange. As mentioned above, our observation of distinct bands associated with the various layers facilitates their assignment (first- and secondlayer CuPc or PTCDA). In this way, heteromolecular bilayer systems can be analyzed in terms of their composition and thermal stability, for example, whether thermally induced interlayer exchange processes or phase transformations occur. In Figure 4 is summarized the thermal evolution of the stacked CuPc/PTCDA/Ag(111) heterolayer system. The data

Figure 3. Thermal desorption spectra (heating rate dT/dt = 1 K/s) of a stacked CuPc/PTCDA/Ag(111) heterolayer. The solid and dashed lines refer to desorption of PTCDA and CuPc, respectively. The gray curve has been included for comparison and represents desorption of a pure PTCDA/Ag(111) thin film (3.3 monolayer thickness); this curve has been scaled down by a factor of 0.2 to fit the frame.

this value is notably lower than has been found for CuPc/ Ag(111) (T ≃ 500 K).17 Desorption of CuPc molecules in direct contact to Ag(111) only develops at T ≈ 650 K and, similar to PTCDA, is likewise accompanied by thermal decomposition, which limits the amount of intact desorbing CuPc to about 0.1 ML.17 For CuPc/PTCDA/Ag(111), the feebleness of PTCDA desorption at T < 500 K indicates that only very few secondlayer PTCDA molecules are present under these conditions, that is, the initial stacking sequence is retained. The observation of PTCDA desorption at T = 556 K, however, provides unequivocal evidence that interlayer exchange processes do occur at elevated T. This observation has two major implications: (i) CuPc forces first-layer PTCDA to switch to the second layer; because the temperatures required for this process to occur are higher than those of second-layer PTCDA desorption, that is, PTCDA molecules will desorb instantaneously after exchange is completed, the interlayer exchange rate can be directly deduced from the PTCDA desorption signal. The somewhat irregular (i.e., nonexponential) shape of the PTCDA desorption trace at 450−520 K may well correspond to early encounters of such layer exchange processes, possibly at step edges or domain boundaries. Note that at step edges exchange between first- and second-layer sites can be achieved without the need of an actual switching between layers, but simply by means of lateral motion. (ii) Second-layer CuPc (dashed line in Figure 1) becomes depleted, which reduces the CuPc desorption signals at T > 500 K accordingly. The desorption temperature of residual second-layer CuPc thereby is identical to desorption of the CuPc/Ag(111) bilayer.17 Note, however, that CuPc for the most part undergoes interlayer exchange so that very little CuPc desorbs and the majority of CuPc molecules remains in direct contact to Ag(111) up to dissociation. This is why the total amount of desorbing PTCDA exceeds the corresponding CuPc yield by far.

Figure 4. Thermal evolution of the stacked CuPc/PTCDA/Ag(111) heterolayer system. The displayed signals indicated in the figure refer to the integrated intensities of characteristic vibrational modes of PTCDA and CuPc (P1, 710 cm−1; C1, 765 cm−1; C2, 735 cm−1). All signals have been normalized to the respective integrated intensities of pure PTCDA and CuPc layers on Ag(111). CuPc was deposited at 80 K on top of a fully saturated and highly ordered PTCDA/Ag(111) monolayer and subsequently annealed to successively higher temperatures. The corresponding IR spectra were recorded after recooling to 80 K.

points have been extracted from individual IR spectra, and they denote the abundance of various species (CuPc/PTCDA first or second layer) after the sample has been annealed to the indicated temperatures (for details, e.g., selected IR spectra thereof, see Figure S2). It is evident that interlayer exchange is negligible for T < 500 K as intensity variations associated with first-layer PTCDA and second-layer CuPc are very minor. The only exception concerns the range (i) below 200 K; specifically, cold deposited CuPc initially forms a disordered second layer, which gradually adopts a parallel orientation with clearly discernible long-range order upon annealing (see Figure 1b). The intensities of PTCDA modes thereby remain constant, most likely because the PTCDA/Ag(111) monolayer has already seen a temperature of 400 K before CuPc is admitted. We note that thermal desorption of either PTCDA or CuPc can be categorically excluded at such low T < 200 K (see Figure 3). According to our LEED observations, the pattern in Figure 1b, representative for a well-ordered CuPc on PTCDA/ Ag(111) heterolayer, is retained in the temperature range (ii), that is, Tann = 200−450 K. The absence of vibrational modes associated with CuPc in direct contact with the Ag substrate, as well as the constant intensity of CuPc second-layer bands, are clear indications for a negligible intermixing at Tann < 500 K. Annealing to T > 500 K, range (iii), causes dramatic spectral changes. Specifically, vibrational modes associated with firstlayer PTCDA, as well as second-layer CuPc, abruptly lose 13684


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by far, the latter corresponding to 0.2 ML at most. Moreover, the general appearance of PTCDA and CuPc desorption signals at T > 500 K is similar; the only difference relates to the PTCDA peak at 542 K, which is now notably smaller than that in Figure 3 at 556 K. Interestingly, the 542 K peak in Figure 5 and the dominant peak at 556 K for CuPc/PTCDA/Ag(111) in Figure 3 display identical slopes, which suggests a common origin. Detailed views and a comparison of the various rising slopes of the PTCDA desorption traces are shown in Figure S1. This is unexpected because, for CuPc/PTCDA/Ag(111) layers, desorption of PTCDA at T > 500 K has been associated with first-layer PTCDA displaced to the second layer and desorbing instantaneously. In principle, the reverse deposition sequence employed in this section should yield PTCDA exclusively in second-layer sites (in the absence of interlayer exchange). As will be demonstrated using IRAS data (see below), such an interlayer exchange does in fact occur, and a notable fraction of the initial second-layer PTCDA on top of CuPc/Ag(111) switches to the first layer at T ≈ 200−400 K. The extent of this exchange is, however, limited, which is why first-layer PTCDA is less abundant in the layer of Figure 5 as compared to Figure 3. Related to that, substantial desorption of second-layer PTCDA occurs at T < 500 K for the PTCDA/CuPc/Ag(111) initial stacking. Specifically, the strong PTCDA peak at 492 K can be ascribed to desorption of PTCDA molecules, which did not undergo layer exchange and kept their second-layer sites. This conclusion is based on the rising slope of the PTCDA desorption signal, which agrees perfectly with the corresponding trace of the pure PTCDA/Ag(111) bilayer. Note that direct comparison of the two PTCDA desorption traces in Figure 5 is misleading due to the ×0.2 scaling factor applied to the gray curve. This correlation, however, becomes evident when comparing the two traces at identical amplification settings and/or on a log-scale (see Figure S1a). The quantities of PTCDA desorbing in the temperature ranges 430−515 and 515−700 K and attributed to the desorption of second- and first-layer PTCDA, respectively, are found to be almost identical. Hence, the amount of PTCDA subject to layer exchange can be estimated to about 0.5 ML. The different desorption processes, that is, (i) PTCDA desorbing directly from the second layer and (ii) PTCDA displaced from the first to the second layer and desorbing instantaneously, can thus be identified from their different appearance and T-ranges within the TD spectra. Vibrational Analysis and Interlayer Exchange. In Figure 6, 0.5 ML PTCDA has been deposited onto a fully covered CuPc/ Ag(111) monolayer. Individual IR spectra have been arranged in a false-color 2D-plot to describe the thermal evolution of spectral features associated with the various molecular species encountered in the course of the annealing series. We have chosen a PTCDA coverage of 0.5 ML instead of a saturated second layer because interlayer exchange proceeds up to completion for that layer. For 1 ML of extra PTCDA deposited onto the CuPc/Ag(111) monolayer, only a fraction of secondlayer PTCDA undergoes interlayer exchange with first-layer CuPc, while the remaining part continues to stay in the second layer and, according to Figure 5, desorbs at 450−500 K. The great benefit of the 2D-plot in Figure 6 is that the temperature ranges, which are critical in terms of transformations within the layer, are readily identified from the correlated increase and decay of vibrational mode intensities.34 Establishing such correlations also helps to reveal the identity of the respective molecular species. Because we are not facing any

intensity. In parallel, vibrational features associated with the CuPc/Ag(111) monolayer emerge, which we attribute to interlayer exchange between CuPc and PTCDA. In fact, such exchange processes should additionally produce second-layer PTCDA; associated signals thereof are, however, missing completely. This absence of second-layer PTCDA in IR spectra despite vivid layer exchange is ascribed to desorption of PTCDA molecules after they were displaced from first-layer sites. Such a scenario implies that second-layer PTCDA desorption proceeds at lower T than interlayer exchange, which is in accordance with our observation of PTCDA bilayer desorption at T ≃ 500 K (Figure 3, gray curve). Our model therefore presumes a dynamic equilibrium of PTCDA and CuPc occupying first- or second-layer sites at T ≃ 500 K, in conjunction with a distinctly lower adsorption energy (and desorption temperature) for second-layer species in the case of PTCDA as compared to CuPc. This leads to a gradual depletion of the overall PTCDA content within the grown film. The conjectured layer exchange CuPc ↔ PTCDA in particular explains the much more prominent desorption yield of PTCDA as compared to CuPc in Figure 3. The driving force leading to interlayer exchange of CuPc and PTCDA in the stacked CuPc/ PTCDA/Ag(111) bilayer will be discussed below. PTCDA/CuPc/Ag(111) Heterolayer Interfaces. Thermal Stability. In this section, the reversed stacking sequence, that is, PTCDA deposited on top of a CuPc/Ag(111) monolayer, has been prepared and analyzed in a similar way as the CuPc/ PTCDA/Ag(111) heterolayer system. Figure 5 shows thermal

Figure 5. Thermal desorption spectra (heating rate dT/dt = 1 K/s) of an initially stacked PTCDA/CuPc/Ag(111) heterolayer. The solid and dashed lines represent PTCDA and CuPc desorption signals, respectively. The gray curve corresponds to the desorption of 3.3 ML PTCDA/Ag(111), and it has been scaled down by a factor 0.2 to fit into the frame.

desorption spectra of this system; similar to Figure 3, the QMS signals of masses 575 u (dashed curve) and 392 u (solid curve) associated with desorbing CuPc and PTCDA have been collected while linearly heating the sample. Despite a significantly different overall appearance of the desorption traces when compared to those of the CuPc/ PTCDA/Ag(111) system in Figure 3, we noticed a number of parallels. Most importantly, we find that the amount of associatively desorbing PTCDA again exceeds the CuPc signal 13685


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comprising PTCDA and CuPc represents a thermodynamically stable arrangement. According to the literature, for example, reports by Stadtmüller et al.,35−37 the two molecular species represent a donor−acceptor pair, which is stabilized by electron transfer (via the metal substrate) from CuPc to PTCDA. Our findings regarding interlayer exchange upon moderate annealing are also in agreement with reports in the literature, suggesting intermixing of the two species upon PTCDA postdeposition onto a CuPc/Ag(111) monolayer performed at room temperature (300 K).23 (iii) 300−500 K: The layer produced after annealing to about room temperature consists of a mixed PTCDA + CuPc monolayer and with CuPc representing the only second-layer species. According to the 2D color plot in Figure 6, the initial second-layer PTCDA molecules have completely disappeared and migrated to the first layer. (iv) 500−600 K: The thermodynamically stable layering of PTCDA + CuPc (intermixed first layer) and CuPc (second layer) is subject to a gradual transformation at T > 500 K. Even though the seemingly obvious process should be desorption of second-layer CuPc, this conclusion is deceptive as, according to Figure 5, it is PTCDA that represents the dominant desorbing species. Moreover, the vibrational spectrum obtained after annealing to 600−650 K does not exhibit any sign for adsorbed PTCDA. Rather the vibrational features observed at the end of our annealing series can unequivocally be ascribed to CuPc/ Ag(111). Another piece of unambiguous evidence for complete PTCDA desorption comes from LEED data, which yields the sharp reflexes of the CuPc/Ag(111) c-phase (see Figure 8). In Figure 7a, the thermal evolution of the layer in Figure 6, specifically, the abundances of the various species, is depicted. In this schematic plot, the various temperature ranges discussed above are indicated by dashed vertical lines; cartoons illustrate the structural characteristics of the produced layers, as well as relevant processes. It is apparent that the initial stacking is stable only up to temperatures of 200 K, when interlayer exchange processes set in. The abundances of first-layer CuPc and second-layer PTCDA gradually decrease until, at 300 K, they are replaced by a layer characterized by a mixed (CuPc + PTCDA) first layer, partially covered by second-layer CuPc. These conclusions have been drawn from IRAS data, which are summarized in Figure 6 as well as in Figure S3. At T ≥ 500 K, thermal desorption of first-layer PTCDA sets in, which proceeds via a two-step process; first, CuPc ↔ PTCDA interlayer exchange leads to a temporary energetically unfavorable configuration with PTCDA occupying second-layer sites, which is followed by an additional process, the actual desorption of second-layer PTCDA. Not unexpectedly, the vacated first-layer sites of PTCDA get replenished by secondlayer CuPc. This is in accordance with our finding that, after completion of the thermal annealing series, the initial PTCDA/ CuPc/Ag(111) layer has converted to a CuPc/Ag(111) commensurate phase (see series of SPA-LEED images in Figure 8), with no PTCDA left. The thermal evolution of a full PTCDA monolayer deposited onto 1 ML CuPc/Ag(111) at 80 K is displayed in Figure 7b, with identical labeling of first- and second-layer species as in Figure 7a. In this series, the vibrational mode intensities corresponding to first-layer CuPc and PTCDA are strongly attenuated (or they overlap with strong modes of second-layer species), and a proper quantitative evaluation is difficult; the corresponding data have therefore been omitted in Figure 7b.

Figure 6. 2D-plot with color coded infrared absorption intensities representing the thermal evolution of an initially stacked PTCDA/ CuPc/Ag(111) bilayer. 0.5 ML PTCDA was deposited at 80 K on top of a fully saturated highly ordered CuPc monolayer (POL-phase). The stacked layer was then annealed to successively higher T (and recooled to 80 K for data taking).

chemical reactions or configurational modifications (e.g., reorientation of adsorbed species), except for interlayer exchange processes (and thermal desorption) to occur, that is, the parallel orientation of all participating species is largely retained, the interpretation of the annealing sequence in Figure 6 is straightforward. Thereby, it is convenient that the spectral features of all conjectured species (first/second-layer PTCDA and CuPc) are known from independent measurements. Characteristic mode frequencies for PTCDA and CuPc,17 both with their π-conjugated backbones oriented parallel to the surface, are as follows: PTCDA (first layer), 710 cm−1; PTCDA (second layer), 740, 815, 870−880 cm−1; CuPc (first layer), 720, 765 cm−1; and CuPc (second layer), 735, 780 cm−1. These line positions of vibrational modes may be subject to slight frequency shifts induced by differences in the local environment and usually accompanied by intensity variations. In Figure 6, the thermal evolution can be briefly subdivided into four temperature ranges, which are characterized by either transformations within the heterolayer or by stable conditions of the thus obtained layers. (i) 100−200 K: The initial layering, PTCDA on top of the CuPc/Ag(111) monolayer, is largely retained except for some slight rearrangements leading to improvements in the parallel orientation of cold deposited PTCDA upon mild annealing (this is deduced from a weakening of PTCDA in-plane vibrational modes). (ii) 200−300 K: This temperature range is governed by substantial rearrangements between CuPc and PTCDA molecular species; apparently, the grown heterolayer represents a nonequilibrium situation, and by provision of sufficient thermal energy to overcome kinetic barriers, the molecular film proceeds toward thermal equilibrium, which is represented by PTCDA occupying first-layer sites. As mentioned above, this process is active only until about 0.5 ML of PTCDA has switched to the first layer; that is, the layer exchange PTCDA ↔ CuPc does not proceed to completion even if a full second layer of PTCDA is available. Rather it seems that a mixed layer 13686


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Figure 7. Thermal evolution of the initially stacked heterolayers of (a) [0.5 ML PTCDA]/[1 ML CuPc]/Ag(111) and (b) [1 ML PTCDA]/ [1 ML CuPc]/Ag(111). The displayed signals (see legend in panel (b)) reflect the abundances of CuPc and PTCDA after annealing to increasingly higher temperatures. Specifically, the chemical identity (CuPc or PTCDA) and the location (first or second layer) are extracted from the integrated intensities of characteristic vibrational modes (P1, 710 cm−1; P2, 810 cm−1; C1, 765 cm−1; C2, 735 and/or 778 cm−1). All signals have been normalized to the respective integrated intensities of pure PTCDA and CuPc layers on Ag(111). Deviation from ideal values is due to screening effects, etc., of the dynamical dipole moments. PTCDA was deposited at 80 K on top of a fully saturated and highly ordered CuPc/Ag(111) monolayer (POLphase) and subsequently annealed to successively higher temperatures. The corresponding IR spectra were recorded after recooling to 80 K.

Figure 8. Series of SPA-LEED patterns, describing the thermal evolution of the stacked PTCDA/CuPc/Ag(111) heterolayer; 1 ML PTCDA was postdeposited (Tsample = 80 K) on top of a fully saturated CuPc/Ag(111) contact layer. (a) Initial, highly ordered CuPc/ Ag(111) monolayer (POL-phase), prepared at 300 K; (b) layer in (a) with 1 ML PTCDA deposited on top and annealed to 200 K thereafter; (c) layer in (b) after annealing to 420 K for 60 min; (d) layer in (b) after annealing to 600 K; (e) layer in (d) after annealing to 650 K; and (f) SPA-LEED pattern of the commensurate CuPc/ Ag(111) phase, for comparison. All images have been obtained at a sample temperature of Tsample = 80 K and at Ekin = 30 eV.

between neighboring PTCDA (the molecule’s quadrupole moment may additionally contribute to that). This will result in island formation, and layer exchange processes should occur primarily at the rim of these islands; alternatively, individual PTCDA may detach from islands and undergo exchange at a higher rate due to fewer neighboring species and accordingly less configurational constraints. (ii) Sterical hindrance introduced by the capping PTCDA molecules may block first-layer CuPc from migrating toward the second layer. Vice versa, the associated lack of available first-layer sites should hinder second-layer PTCDA from accessing the more favorable sites in direct contact with the Ag(111) surface. At Tann = 450−500 K, the total amount of PTCDA is gradually lowered due to desorption of second-layer PTCDA (see Figure 5). After reaching temperatures above 500 K, those PTCDA molecules that have captured first-layer sites during previous annealing steps begin to desorb as well (according to Figure 5, PTCDA desorption is completed only after reaching temperatures of 650 K). As (pure) PTCDA in direct contact to

It is apparent that the initial layering sequence, that is, PTCDA occupying second-layer sites, is significantly more stable for the layer in Figure 7b as compared to that in Figure 7a. For 0.5 ML PTCDA postdeposited onto a complete CuPc/ Ag(111) monolayer, all PTCDA molecules switch to the first layer, and this process starts already at T > 200 K. For 1 ML PTCDA on CuPc/Ag(111), annealing temperatures of Tann > 300 K are needed to induce any exchange as deduced from the declining abundance of second-layer PTCDA as well as the emergence of vibrational bands associated with second-layer CuPc; thereby, a notable fraction of PTCDA remains in the second layer and forms PTCDA bilayer islands. Two effects may be held responsible for the enhanced stability regarding PTCDA ↔ CuPc layer exchange for a saturated PTCDA second layer: (i) PTCDA molecules embedded within a 2D-network of second-layer PTCDA experience a stabilization by means of hydrogen bonding 13687


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The Journal of Physical Chemistry C Ag(111) does not desorb at Tann < 600 K, we conclude that the PTCDA desorption at 500−580 K must be catalyzed by the presence of second-layer CuPc. We suggest that desorption of first-layer PTCDA is initiated by layer exchange in the course of which PTCDA is displaced to the second layer and CuPc molecules regain first-layer sites. This is actually the main cause for the low overall desorption yield of CuPc. In accordance with the diminishing PTCDA content, a long-range order characteristic of pure CuPc on Ag(111), the c-phase, is formed after completion of the annealing series, that is, after the sample has been heated to 650 K (see Figure 8). Structural Analysis. A series of SPA-LEED patterns observed in the course of annealing the PTCDA/CuPc/ Ag(111) heterolayer is displayed in Figure 8. In this series, one extra monolayer of PTCDA has been deposited at 80 K onto the CuPc/Ag(111) contact layer (see Figure 8a), and subsequently annealed to increasingly higher temperatures (Figure 8b−e). At Tann = 200 K (Figure 8b), the reflexes associated with the initial CuPc POL-phase are still discernible, but they are strongly attenuated due to the PTCDA second layer. Interestingly, this PTCDA layer seems to develop a lateral ordering, which corresponds to the well-known herringbone structure of PTCDA/Ag(111).18 Specifically, the observed ringlike features in Figure 8b can be interpreted as the LEED pattern of PTCDA/Ag(111), however, lacking any azimuthal order or preference (for details, see Kröger et al.15 and Figure S5). We stress that, according to our IR data, no layer exchange between CuPc and PTCDA has yet occurred at this stage of annealing; the ring-like pattern thus stems from second-layer PTCDA. Upon more extensive thermal annealing to about 400 K, this ring-like pattern transforms into the well-known discrete spots of PTCDA/Ag(111) (Figure 8c). According to our IRAS data (see Figure 7), PTCDA ↔ CuPc layer exchange is fully operative at this temperature, and we conclude that it is not the “floating” second-layer PTCDA that has improved its azimuthal order; rather, the discrete spots most likely are induced by PTCDA in direct contact to Ag(111) (after replacing first-layer CuPc by means of interlayer exchange) and second-layer PTCDA on top of these ordered domains of PTCDA/Ag(111). In this respect, we note that identical LEED patterns exist for mono- and bilayer PTCDA/Ag(111) (see Figure S5). This conclusion is corroborated by our IR spectra, which show spectral signatures resembling those encountered for wellordered PTCDA double layers. Further support comes from observations of Stadtmüller et al. using STM23 who observed a herringbone pattern of PTCDA/Ag(111) for second-layer PTCDA after 0.6 ML PTCDA has been postdeposited onto 0.9 ML CuPc at room temperature. According to Henneke et al.,38 domains of pure PTCDA and the (2PTCDA + CuPc) mixed brickwall structure (MBW) may form for PTCDA-rich mixtures on Ag(111). We therefore suggest that, in the course of our annealing series, the generic herringbone structure is realized by first-layer PTCDA, and that PTCDA molecules left in the second layer accumulate on top of first-layer PTCDA molecules, thereby adopting an identical long-range order. This model presumes that the formation of a congruent PTCDA double layer is energetically favored as compared to PTCDA being positioned randomly, or on top of a CuPc/ Ag(111) contact layer. We note that Tann = 420 K (Figure 8c) is still well below the desorption temperatures of CuPc and PTCDA so that a total coverage of 2 ML is maintained. Our model comprising the formation of bilayer PTCDA islands is

further corroborated by the identical slopes of the TD-peak at 492 K and of the corresponding peak of pure PTCDA layers in Figure 5 (see also Figure S1). Within our annealing series, none of the mixed CuPc− PTCDA monolayer phases on Ag(111)37−39 have been observed; however, the absence of the respective LEED reflexes might simply mean that mixed domains of CuPc and PTCDA are disordered (and in this way do not add extra spots to the observed SPA-LEED patterns). We do, however, observe a distinct novel overlayer structure after annealing to 600 K (Figure 8d). Such high temperatures largely take care of second-layer CuPc and PTCDA species so that the ordered arrangement comprises first-layer CuPc and PTCDA molecules only. The unit cell size (see Figure S6 for details) amounts to 4.9 −0.3⎤, as 560 Å2, and a matrix description thereof is ⎡⎣11.9 15.2 ⎦ verified by the LEEDpat simulation package.40 A description of phenomena related to the formation of long-range ordered overlayers and the analysis of LEED data can be found here.41−43 We note that continued annealing of the layer in Figure 8d leads to the formation of the commensurate CuPc/Ag(111) “cphase”; this new mixed phase should thus be CuPc-rich. According to the unit cell size and the “footprint” areas of CuPc (≃200 Å2) and of PTCDA (≃120 Å2) for parallel oriented molecular species, a composition of (2 CuPc + 1 PTCDA) per unit cell is deduced. Interestingly, the first- and second-order spots along the b⃗2* direction (defined in Figure S6) are virtually missing, and prominent scattering intensity is found for thirdand fifth-order spots only. We suspect that the irregular intensities of the individual LEED beams are due to the particular arrangement of the three molecular species within the unit cell; a thorough analysis of the individual reflexes’ intensities would require intricate theoretical methods to be applied, which is beyond the scope of this experimental study. As mentioned above, by annealing to Tsample = 650 K (Figure 8e), that is, to temperatures when desorption has reduced the amount of adsorbed PTCDA to negligible values, the wellknown LEED pattern of the CuPc/Ag(111) c-phase15,17 is found. An according SPA-LEED pattern of pure CuPc/ Ag(111) has been included in Figure 8f for comparison. This finding confirms that (i) the majority of the desorbing species is PTCDA and (ii) in the course of the entire processing sequence, CuPc desorption is weak but nonzero, thereby reducing the CuPc coverage to about 0.8−0.9 ML (starting at 1.0 ML, i.e., with a CuPc/Ag(111) POL-phase in Figure 8a). Thermodynamic Aspects. From a general perspective, both types of interlayer exchange processes of the PTCDA/CuPc/ Ag(111) heterolayer observed in the temperature range 200− 500 K can be associated with the trend of the bimolecular system to reach thermal equilibrium. At T ≃ 300 K, thermal equilibrium merely concerns the organic heterolayer, that is, the stacking sequence of molecular species, which includes interlayer exchange processes but neglects thermal desorption. At T > 500 K, thermal equilibrium also encompasses the surrounding gas phase, which additionally brings desorption processes into play. As will be explained in the following, the relevance of different thermodynamic entities associated with the molecule− metal interaction does change in the course of the performed annealing series. Specifically, at T < 450 K, that is, when molecular desorption of either PTCDA or CuPc is negligible, it is the adsorption energy per area that decides on which of the 13688


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The Journal of Physical Chemistry C two molecular species will occupy first-layer sites; this will lead to a preference of PTCDA in the contact layer. At T > 500 K, molecular desorption becomes relevant, and the lower adsorption energy per molecule (not per unit area) for PTCDA causes a gradual vanishing of PTCDA from the deposited organic film. As interlayer exchange has already started at these temperatures, the high rate of second-layer PTCDA desorption will inevitably lead to a depletion of firstlayer PTCDA by means of the correlated transfer of secondlayer CuPc to the first layer. In this way and independent of the initial stacking sequence, the thin film composition progresses toward a dominance of CuPc on the Ag(111) surface at elevated sample temperatures >500 K; eventually, this leads to the formation of the characteristic, nominally “PTCDA-free” CuPc/Ag(111) commensurate phase. The adsorption energies of PTCDA/Ag(111) and CuPc/ Ag(111) listed in Table 1 have been taken from DFT

acceptor (PTCDA) types of species, the two molecular entities may experience an energy gain by means of charge transfer from CuPc to PTCDA via the Ag-surface,35 which stabilizes the mixed arrangement.

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CONCLUSIONS In this study, the thermal evolution and structural properties of stacked heterolayers comprising bilayers of CuPc and PTCDA on Ag(111) have been investigated using FT-IRAS, SPA-LEED, and TDS. Particular emphasis was laid on the quality and thermal stability of the organic−organic interface, which allowed a trustworthy identification of intermixing and molecular exchange processes. Specifically, we find that both types of layered arrangements follow distinctly different pathways in their thermal evolution, depending on adsorption energies and kinetic barriers associated with thermally activated processes. In a quantitative analysis of IR absorption spectra, specifically our observation of strong out-of-plane modes and negligible inplane modes, a parallel orientation of first-and second-layer PTCDA as well as of CuPc is deduced, which is in accordance with structure models of monolayer and bilayer films suggested in the literature.9,13,17,46 Thereby, the effect of an additional layer on the vibrational spectra of the contact layer primarily concerns out-of-plane modes. In particular, the interfacial dynamical charge transfer associated with the excitation of totally symmetric in-plane modes is only marginally altered. For CuPc on PTCDA/Ag(111), we find an exceptional high thermal stability of this particular stacking sequence, which is subject to interlayer exchange only upon annealing to T > 450 K. Such processes cause most first-layer PTCDA to be displaced to the second layer and to desorb intact, whereas less volatile second-layer CuPc molecules remain adsorbed, capture first-layer sites in direct contact to the Ag substrate, and eventually dissociate at T > 650 K. The well-ordered and thermally stable CuPc/PTCDA/Ag(111) heterolayer represents a model system comprising donor and acceptor types of molecules, which is an ideal choice for further investigations, for example, regarding charge transfer processes and elemental excitations at the interface. For the reversed stacking sequence, that is, PTCDA on CuPc/Ag(111), we find that the stacking sequence and associated interlayer exchange displays some unusual behavior, which is related to the fact that PTCDA replaces CuPc from first-layer sites, even though the molecular adsorption energy is considerably higher for CuPc. Specifically, two sequential interlayer exchange processes have been identified at 200−300 K and at Tann > 450 K, respectively. Our observations clearly demonstrate that a kinetic barrier exists regarding interlayer exchange, not only for the CuPc/ PTCDA/Ag(111) heterolayer, but also for the reversed stacking. In particular, PTCDA molecules embedded within extended long-range ordered islands or domains display low yields regarding interlayer exchange, even at 300 K. Lower coordinated PTCDA (isolated molecules or when located at the rim of islands), on the other hand, is more susceptible to such exchange, which starts already at T > 200 K. In the course of thermal annealing to 200−300 K, PTCDA migrates to the first layer and lifts out CuPc from the first toward the second layer; this process is driven by the adsorption energy per surface area that is higher for PTCDA as compared to CuPc. This preference of PTCDA to occupy

Table 1. Adsorption Energy per Molecule Eads (in eV) and Adsorption Energy per Surface Area Eads/A (in eV/nm2), as well as Desorption Temperatures of Bilayer and Monolayer Species for PTCDA and CuPc on Ag(111)a PTCDA CuPc

Eads

Eads/A

Tbilayer (K) des

Tmonolayer (K) des

2.86 4

2.4 2.1

505 560

∼650 >650

a

The adsorption energies have been calculated by Ruiz et al.44 for PTCDA and by Huang et al.45 for CuPc. The area per PTCDA or CuPc molecule that enters the quantity Eads/A has been derived from the long-range ordered phases of the respective saturated molecular monolayer phases on Ag(111).15,18 Characteristic desorption temperatures of PTCDA and CuPc mono- and bilayers are taken from Figures 3 and 5 and from our previous work on CuPc/Ag(111).17

calculation using the PBE + vdWsurf approach.44,45 To derive the adsorption energies per area, Eads/A, we evaluated the 2D unit cell sizes of the respective saturated monolayer phases of PTCDA and CuPc on Ag(111). For CuPc on Ag(111), a pointon-line phase is found, which yields a primitive unit cell size of 192 Å2;15,17 for PTCDA/Ag(111), a herringbone phase comprising two PTCDA per unit cell (area A = 238.7 Å2) has been reported,18 which yields 120 Å2 per PTCDA molecule. It is evident that the adsorption energy per molecule is notably lower for PTCDA as compared to CuPc. When taking the respective molecular sizes into account, however, a reversed situation is encountered; that is, Eads/A is now lower for CuPc as compared to PTCDA. Moreover, repulsive intermolecular interactions within dense CuPc layers15 could additionally reduce the CuPc adsorption energy; for PTCDA on Ag(111), island formation prevails, which indicates a strengthening of the adsorption energy for dense layers.19 The respective energy difference of Eads/A should therefore further increase in favor of PTCDA. One may ask why, given the fact that the adsorption energy per unit area is larger for PTCDA as compared to CuPc, the interlayer exchange of both species remains incomplete for the PTCDA/CuPc/Ag(111) stacked double layer system. Specifically, the process ceases after about 0.5 ML PTCDA have captured the preferred first-layer sites. We suspect that the situation encountered after PTCDA molecules have transferred to the first layer is more complex, because a mixed PTCDA− CuPc heterolayer may form.35−39 For donor (CuPc) and 13689


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The Journal of Physical Chemistry C first-layer sites is in perfect agreement with the high thermal stability of the stacked CuPc/PTCDA/Ag(111) bilayer. In the temperature range of Tann > 450 K, that is, when thermal desorption processes come into play, the adsorption energy per molecule is the relevant quantity that determines the evolution of the deposited heterolayer. Its value is notably higher for CuPc as compared to PTCDA, in accordance with the lower desorption temperature of PTCDA bilayer species as compared to CuPc. The more volatile nature of PTCDA then leads to a loss of PTCDA within the contact layer and a recapturing of these sites by CuPc molecules (which had been edged out from their initial first-layer sites to the second layer in the earlier stages of the annealing series). The unusual back and forth kind of exchange processes of PTCDA and CuPc, occupying either first- or second-layer sites, provides unequivocal evidence that the thermal evolution of heterolayer systems comprising different types of molecules is controlled by two quantities. Primarily, it is the adsorption energy per area of the individual molecules that decides on the occupancy of preferred sites. As soon as thermal desorption sets in, the adsorption energy per molecule dictates which of the molecular species gets depleted; in this way, the overall composition of the heterolayer is altered, which naturally will affect the occupancy of the preferred sites. Depending on the processing conditions, either one of the two thermodynamic quantities may take the leading role and define the outcome thereof, for example, what kind of structural arrangement prevails.

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(2) Blochwitz, J.; Pfeiffer, M.; Fritz, T.; Leo, K. Low Voltage Organic Light Emitting Diodes Featuring Doped Phthalocyanine as Hole Transport Material. Appl. Phys. Lett. 1998, 73, 729−731. (3) Barth, J. V.; Costantini, G.; Kern, K. Engineering Atomic and Molecular Nanostructures at Surfaces. Nature 2005, 437, 671−679. (4) Chen, W.; Chen, S.; Chen, S.; Huang, Y. L.; Huang, H.; Qi, D. C.; Gao, X. Y.; Ma, J.; Wee, A. T. S. Orientation-Controlled Charge Transfer at CuPc/F16CuPc Interfaces. J. Appl. Phys. 2009, 106, 064910. (5) Borghetti, P.; de Oteyza, D. G.; Rogero, C.; Goiri, E.; Verdini, A.; Cossaro, A.; Floreano, L.; Ortega, J. E. Molecular-Level Realignment in Donor-Acceptor Bilayer Blends on Metals. J. Phys. Chem. C 2016, 120, 5997−6005. (6) Chen, W.; Huang, H.; Chen, S.; Gao, X. Y.; Wee, A. T. S. LowTemperature Scanning Tunneling Microscopy and Near-Edge X-ray Absorption Fine Structure Investigations of Molecular Orientation of Copper(II) Phthalocyanine Thin Films at Organic Heterojunction Interfaces. J. Phys. Chem. C 2008, 112, 5036−5042. (7) Gruenewald, M.; Sauer, C.; Peuker, J.; Meissner, M.; Sojka, F.; Schöll, A.; Reinert, F.; Forker, R.; Fritz, T. Commensurism at Electronically Weakly Interacting Phthalocyanine/PTCDA Heterointerfaces. Phys. Rev. B: Condens. Matter Mater. Phys. 2015, 91, 155432. (8) Egger, D. A.; Ruiz, V. G.; Saidi, W. A.; Bucko, T.; Tkatchenko, A.; Zojer, E. Understanding Structure and Bonding of Multilayered MetalOrganic Nanostructures. J. Phys. Chem. C 2013, 117, 3055−3061. (9) Stadtmüller, B.; Willenbockel, M.; Schröder, S.; Kleimann, C.; Reinisch, E. M.; Ules, T.; Soubatch, S.; Ramsey, M. G.; Tautz, F. S.; Kumpf, C. Modification of the PTCDA-Ag Bond by Forming a Heteromolecular Bilayer Film. Phys. Rev. B: Condens. Matter Mater. Phys. 2015, 91, 155433. (10) Häming, M.; Greif, M.; Sauer, C.; Schöll, A.; Reinert, F. Electronic Structure of Ultrathin Heteromolecular Organic-Metal Interfaces: SnPc/PTCDA/Ag(111) and SnPc/Ag(111). Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 82, 235432. (11) Huang, H.; Chen, W.; Chen, S.; Qi, D. C.; Gao, X. Y.; Wee, A. T. S. Molecular Orientation of CuPc Thin Films on C60/Ag(111). Appl. Phys. Lett. 2009, 94, 163304. (12) Kleimann, C.; Stadtmüller, B.; Schröder, S.; Kumpf, C. Electrostatic Interaction and Commensurate Registry at the Heteromolecular F16CuPc-CuPc Interface. J. Phys. Chem. C 2014, 118, 1652−1660. (13) Stadtmüller, B.; Sueyoshi, T.; Kichin, G.; Kröger, I.; Soubatch, S.; Temirov, R.; Tautz, F. S.; Kumpf, C. Commensurate Registry and Chemisorption at a Hetero-Organic Interface. Phys. Rev. Lett. 2012, 108, 106103. (14) Häming, M.; Greif, M.; Wiener, M.; Schöll, A.; Reinert, F. Characterization of Ultra-Thin Organic Hetero-Interfaces - SnPc/ PTCDA/Ag(111). Surf. Sci. 2010, 604, 1619−1622. (15) Kröger, I.; Stadtmüller, 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. (16) Gottfried, J. M. Surface Chemistry of Porphyrins and Phthalocyanines. Surf. Sci. Rep. 2015, 70, 259−379. (17) Thussing, S.; Jakob, P. Structural and Vibrational Properties of CuPc/Ag(111) Ultrathin Films. J. Phys. Chem. C 2016, 120, 9904− 9913. (18) Kilian, L.; Umbach, E.; Sokolowski, M. Molecular Beam Epitaxy of Organic Films Investigated by High Resolution Low Energy Electron Diffraction (SPA-LEED): 3,4,9,10-Perylenetetracarboxylicacid-Dianhydride (PTCDA) on Ag(111). Surf. Sci. 2004, 573, 359−378. (19) Kilian, L.; Hauschild, A.; Temirov, R.; Soubatch, S.; Schöll, A.; Bendounan, A.; Reinert, F.; Lee, T.-L.; Tautz, F. S.; Sokolowski, M.; et al. Role of Intermolecular Interactions on the Electronic and Geometric Structure of a Large π-Conjugated Molecule Adsorbed on a Metal Surface. Phys. Rev. Lett. 2008, 100, 136103. (20) Tautz, F. Structure and Bonding of Large Aromatic Molecules on Noble Metal Surfaces: The Example of PTCDA. Prog. Surf. Sci. 2007, 82, 479−520.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b02377. Comparison of thermal desorption spectra plotted in a linear and logarithmic scale; additional IR spectra obtained after annealing CuPc/PTCDA and PTCDA/ CuPc stacked layers to successively higher T; and SPALEED images of the mixed (2CuPc + PTCDA) phase on Ag(111): experimental and simulated LEED patterns (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +49-6421-2824328. E-mail: [email protected]. ORCID

Peter Jakob: 0000-0001-6478-309X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge support from the Deutsche Forschungsgemeinschaft DFG (Germany) through the collaborative research center “Structure and Dynamics of Internal Interfaces” (SFB 1083).

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REFERENCES

(1) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Polymer Photovoltaic Cells: Enhanced Efficiencies via a Network of Internal Donor-Acceptor Heterojunctions. Science 1995, 270, 1789−1791. 13690


Article

The Journal of Physical Chemistry C (21) Glöckler, K.; Seidel, C.; Soukopp, A.; Sokolowski, M.; Umbach, E.; Böhringer, M.; Berndt, R.; Schneider, W.-D. Highly Ordered Structures and Submolecular Scanning Tunnelling Microscopy Contrast of PTCDA and DM-PBDCI Monolayers on Ag(111) and Ag(110). Surf. Sci. 1998, 405, 1−20. (22) Hauschild, A.; Karki, K.; Cowie, B. C. C.; Rohlfing, M.; Tautz, F. S.; Sokolowski, M. Molecular Distortions and Chemical Bonding of a Large π-Conjugated Molecule on a Metal Surface. Phys. Rev. Lett. 2005, 94, 036106. (23) Stadtmüller, B.; Gruenewald, M.; Peuker, J.; Forker, R.; Fritz, T.; Kumpf, C. Molecular Exchange in a Heteromolecular PTCDA/CuPc Bilayer Film on Ag(111). J. Phys. Chem. C 2014, 118, 28592−28602. (24) Kobitski, A.; Scholz, R.; Zahn, D. Theoretical Studies of the Vibrational Properties of the 3,4,9,10,-Perylene Tetracarboxylic Dianhydride (PTCDA) Molecule. J. Mol. Struct.: THEOCHEM 2003, 625, 39−46. (25) Tautz, F. S.; Sloboshanin, S.; Schaefer, J. A.; Scholz, R.; Shklover, V.; Sokolowski, M.; Umbach, E. Vibrational Properties of Ultrathin PTCDA Films on Ag(110). Phys. Rev. B: Condens. Matter Mater. Phys. 2000, 61, 16933−16947. (26) 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. Acta, Part A 2007, 67, 1232−1246. (27) Tautz, F. S.; Sloboshanin, S.; Shklover, V.; Scholz, R.; Sokolowski, M.; Schaefer, J. A.; Umbach, E. Substrate Influence on the Ordering of Organic Submonolayers: a Comparative Study of PTCDA on Ag(110) and Ag(111) Using HREELS. Appl. Surf. Sci. 2000, 166, 363−369. (28) Eremtchenko, M.; Bauer, D.; Schaefer, J. A.; Tautz, F. S. Polycyclic Aromates on Close-Packed Metal Surfaces: Functionalization, Molecular Chemisorption and Organic Epitaxy. New J. Phys. 2004, 6, 4. (29) Persson, B. N. J. Vibrational Energy Relaxation at Surfaces: O2 Chemisorbed on Pt(111). Chem. Phys. Lett. 1987, 139, 457−462. (30) 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. (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. (32) Braatz, C. R.; Ö hl, G.; Jakob, P. Vibrational Properties of the Compressed and the Relaxed 1,4,5,8-Naphthalene-Tetracarboxylic Dianhydride Monolayer on Ag(111). J. Chem. Phys. 2012, 136, 134706. (33) Rosenow, P.; Jakob, P.; Tonner, R. Electron-Vibron Coupling at Metal-Organic Interfaces from Theory and Experiment. J. Phys. Chem. Lett. 2016, 7, 1422−1427. (34) Braatz, C. R.; Esat, T.; Wagner, C.; Temirov, R.; Tautz, F. S.; Jakob, P. Switching Orientation of Adsorbed Molecules: Reverse Domino on a Metal Surface. Surf. Sci. 2016, 643, 98−107. (35) Stadtmüller, B.; Lüftner, D.; Willenbockel, M.; Reinisch, E. M.; Sueyoshi, T.; Koller, G.; Soubatch, S.; Ramsey, M. G.; Puschnig, P.; Tautz, F. S.; et al. Unexpected Interplay of Bonding Height and Energy Level Alignment at Heteromolecular Hybrid Interfaces. Nat. Commun. 2014, 5, 3685. (36) 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. (37) Stadtmüller, B.; Schröder, S.; Kumpf, C. Heteromolecular Metal-Organic Interfaces: Electronic and Structural Fingerprints of Chemical Bonding. J. Electron Spectrosc. Relat. Phenom. 2015, 204 (Part A), 80−91. (38) Henneke, C.; Felter, J.; Schwarz, D.; Tautz, F. S.; Kumpf, C. Controlling the Growth of Multiple Ordered Heteromolecular Phases by Utilizing Intermolecular Repulsion. Nat. Mater. 2017, 16, 628−633.

(39) Stadtmüller, B.; Henneke, C.; Soubatch, S.; Tautz, F. S.; Kumpf, C. Tailoring Metal-Organic Hybrid Interfaces: Heteromolecular Structures with Varying Stoichiometry on Ag(111). New J. Phys. 2015, 17, 023046. (40) Hermann, K. E.; Van Hove, M. A. LEEDpat utility, Version 4.2; FHI and HKBU: Berlin/Hong Kong, 2014. (41) Woodruff, D. A.; Delchar, T. A. Modern Techniques of Surface Science; Cambridge University Press: Cambridge, UK, 1994. (42) Hooks, D. E.; Fritz, T.; Ward, M. D. Epitaxy and Molecular Organization on Solid Substrates. Adv. Mater. 2001, 13, 227−241. (43) Hillier, A. C.; Ward, M. D. Epitaxial Interactions Between Molecular Overlayers and Ordered Substrates. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54, 14037−14051. (44) Ruiz, V. G.; Liu, W.; Tkatchenko, A. Density-Functional Theory with Screened van der Waals Interactions Applied to Atomic and Molecular Adsorbates on Close-Packed and Non-Close-Packed Surfaces. Phys. Rev. B: Condens. Matter Mater. Phys. 2016, 93, 035118. (45) Huang, Y. L.; Wruss, E.; Egger, D. A.; Kera, S.; Ueno, N.; Saidi, W. A.; Bucko, T.; Wee, A. T. S.; Zojer, E. Understanding the Adsorption of CuPc and ZnPc on Noble Metal Surfaces by Combining Quantum-Mechanical Modelling and Photoelectron Spectroscopy. Molecules 2014, 19, 2969−2992. (46) Umbach, E.; Sokolowski, M.; Fink, R. Substrate-Interaction, Long-Range Order, and Epitaxy of Large Organic Adsorbates. Appl. Phys. A: Mater. Sci. Process. 1996, 63, 565−576.

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Thermal Stability and Interlayer Exchange Processes in Heterolayers

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