Coordination Reactions and Layer Exchange Processes at a Buried

Article pubs.acs.org/JPCC

Coordination Reactions and Layer Exchange Processes at a Buried Metal−Organic Interface Min Chen,† Michael Röckert,‡ Jie Xiao,‡ Hans-Jörg Drescher,† Hans-Peter Steinrück,‡ Ole Lytken,‡ and J. Michael Gottfried*,† †

Fachbereich Chemie, Physikalische Chemie, Philipps-Universität Marburg, Hans-Meerwein-Str., 35032 Marburg, Germany Lehrstuhl für Physikalische Chemie II, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstr. 3, 91058 Erlangen, Germany

‡

S Supporting Information *

ABSTRACT: The reactive interface between phthalocyanine (2HPc) and a Cu(111) surface was investigated with X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption/reaction (TPD/TPR). 2HPc reacts with Cu(111) to form copper(II) phthalocyanine (CuPc). For 2HPc submonolayers, the reaction starts already below 240 K. Thin 2HPc multilayers (4 monolayers) are completely converted into CuPc at elevated temperatures (500 K). To understand how molecules can react even though they are initially not in contact with the Cu surface, TPD/TPR studies with a NiPc interlayer between Cu and 2HPc were performed. These studies reveal the operation of an exchange mechanism, by which CuPc or NiPc molecules from the first layer are replaced by 2HPc molecules from higher layers. Once a 2HPc molecule comes into contact with the Cu surface, the molecule can react with the surface, forming CuPc, which can then be replaced by another unreacted 2HPc molecule. These findings have important implications for metal−organic interfaces in organic electronic devices, in which the charge carrier injection at the electrodes depends on the electronic properties of the interface. metal surface.11−13,16−18 For the operational long-term stability of a device, the structural and chemical stability of the interface under the typical working conditions of the device (which can include elevated temperatures over extended periods of time) is of utmost importance. Redistribution of molecules between contact primer layer and organic bulk phase, mixing of metal and organic phase, or chemical reactions at the interface would drastically influence the work function, the overlap of wave functions between metal and organic phase, and thus the charge carrier transport properties. Fundamental studies of well-defined metal−organic interfaces have mostly used model systems consisting of monolayers of molecular organic semiconductors on noble metal surfaces at room temperature or below, i.e., under conditions where chemical reactions or interdiffusion of bulk materials do not occur. Organic electronic devices, however, often require the usage of reactive low-work-function metals such as calcium or aluminum, which readily react with the organic semiconductors’ functional groups or even their hydrocarbon core.19−32 Reactions are especially likely to occur when metals are vapor-deposited onto organic materials. However, large organic molecules can also react with surfaces of bulk metals. This has been shown for monolayers of phthalocyanines and porphyrins, which react with vapor-deposited metal atoms, but also with surfaces of bulk metals by coordination and oxidation of surface

1. INTRODUCTION Organic electronic devices such as light-emitting diodes (OLED) or field effect transistors (OFET) contain buried metal−organic interfaces in the form of contacts between metal electrodes and organic semiconductors.1,2 The efficiency of charge injection at these contacts depends crucially on the energy level alignment between occupied and unoccupied molecular levels relative to the Fermi level of the metal surface and on the wave function overlap between metal and organic phase.3 Active control of the interfacial energy level alignment is possible by influencing the interface dipole with a molecular monolayer placed directly at the boundary between metal and organic phase.4 Various studies of these contact primers on the basis of thiol-bonded self-assembled monolayers (SAMs) show that they are effective means for tuning the work function of the metal electrode and that they can significantly enhance charge injection in OFETs.3−7 Interfacial layers of planar aromatic molecules have also proven beneficial for the charge injection efficiency.8,9 For example, it has been shown that one monolayer of 6,13-pentacenequinone at the interface between pentacene and Ag influences the energy level alignment such that the hole injection barrier is lowered by 0.7 eV.9 Other interesting candidates for contact primers are large aromatic molecules and metal complexes such as metalloporphyrins and metallopthalocyanines because they combine a substantial orbital overlap with the states of the metal surface and a strong influence on the work function.10−15 The character of the surface chemical bond of these metal complexes varies substantially with the type of the coordinated metal ion and the © 2014 American Chemical Society

Received: February 24, 2014 Revised: March 29, 2014 Published: March 31, 2014 8501

dx.doi.org/10.1021/jp5019235 | J. Phys. Chem. C 2014, 118, 8501−8507

The Journal of Physical Chemistry C

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atoms, resulting in the formation of metal complexes.32−42 Complex formation changes the electronic structure of these organic semiconductors and thus is likely to affect the electronic properties of the interface.11,33,34 Furthermore, the presence of a bulk organic phase also allows for diffusion processes perpendicular to the interface. This can be the diffusion of (reacted) organic molecules from the interface into the bulk organic phase or even the diffusion of metal atoms into the organic phase, where they may get trapped by chemical reactions.20,32 A seminal study in this context was performed by Diller et al.,35 who showed that a multilayer of tetraphenylporphyrin on a Cu(111) surface reacts completely forming Cu(II) tetraphenylporphyrin. This finding raises the question how the molecules in the layers above the monolayer get into contact with Cu atoms: by diffusion of Cu atoms into the organic phase or by an exchange process, which brings unreacted molecules into contact with the surface and transports reacted molecules away from the surface? In this study, we use a combination of X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption/ reaction (TPD/TPR) to prove that an exchange of molecules provides indeed a possible pathway for multilayer metalation with atoms from a bulk metal. Driven by entropy, this may lead over time to the complete reaction of the organic semiconductor with the surface. In addition, we will also show that a contact primer monolayer, especially if it is not covalently bonded to the surface, may easily disappear by diffusion into the organic bulk material. These findings are potentially of crucial importance for the performance of real organic electronic devices.

heating in the range from 95 to 900 K. 2H-Phthalocyanine (2HPc), copper(II) phthalocyanine (CuPc), and nickel(II) phthalocyanine (NiPc) were thoroughly degassed in vacuo for 6 h at 450 K prior to deposition. In this report, “monolayer” denotes a complete, saturated adlayer of molecules in direct contact with the substrate surface at 300 K. The coverage of the adsorbed molecules was determined using a combination of XPS and low-energy ion scattering (LEIS) and was independently confirmed by STM, as explained previously.12 Convolutions of the N 1s spectra of 2HPc and its reaction product, CuPc, were performed by using the spectra of the pure, unreacted submonolayers of 2HPc and CuPc to model the spectra which contain contributions of both 2HPc and CuPc. The deconvolution of the 2HPc and CuPc N 1s signals neglects the small binding energy difference of 0.4 eV between the iminic nitrogen atoms in the peripheral mesopositions and in the center by combining them into one single peak.37,43

3. RESULTS AND DISCUSSION 3.1. X-ray Photoelectron Spectroscopy. TemperatureDependent Reactivity of 2HPc Submonolayers. Previous studies of adsorption and reaction of phthalocyanines and other tetrapyrroles with metal surfaces have mainly focused on the (sub)monolayer regime. To establish a link to this previous work, we first compare N 1s spectra of a phthalocyanine (2HPc) submonolayer and a 2HPc multilayer on a Cu(111) surface (Figure 1). The multilayer spectrum Figure 1a (bottom) contains two components: one for aminic nitrogen (−NH−, green line, 400.4 eV) at higher binding energy (BE)

2. EXPERIMENTAL DETAILS The XPS and TPD/TPR experiments were performed in an ultrahigh vacuum (UHV) system with a base pressure of 2 × 10−10 mbar. The XPS measurements were carried out with a Scienta ESCA-200 spectrometer equipped with an Al Kα X-ray source (1486.6 eV), an X-ray monochromator, and a hemispherical energy analyzer (SES-200). The overall energy resolution amounts to 0.3 eV. The reported XPS binding energies in this paper are referenced to the Fermi edge of the clean Cu surface (EB ≡ 0). For an improved signal-to-noise ratio for the topmost layers, photoelectrons were detected at an angle of 70° relative to the surface normal. For TPD/TPR, a Pfeiffer HiQuad QMA 400 quadrupole mass spectrometer was used. During TPD/TPR data acquisition, three channels for m/ z = 515, 571, and 576, corresponding to 2HPc, NiPc, and CuPc, respectively, were monitored quasi-simultaneously by computer-controlled multiplexing of the mass spectrometer. Reference TPD experiments with the pure substances and recording of all three masses showed the absence of significant crosstalk between different mass channels. The phthalocyanine molecules were evaporated from a home-built Knudsen cell evaporator at 640 K. The substrate was a Cu single crystal (purity >99.999%) with a thickness of 2 mm and a polished (111) surface, aligned to 8 monolayers) and of a 2HPc submonolayer (0.5 monolayers) on Cu(111) (b) after deposition at 160 K and (c−e) after heating to the indicated temperatures. For the CuPc reference spectrum ((f), 0.5 monolayers), CuPc was directly deposited onto the Cu(111) surface at 300 K. The spectra (b) and (c) were measured at the indicated temperatures and the others at 300 K. Line colors: green, aminic nitrogen (−NH−); red, iminic nitrogen (−N) in 2HPc; blue, iminic nitrogen in CuPc; gray, satellite. The satellites at 400.6 and 402.6 eV (values for the multilayer spectrum, bottom) are associated with shakeup processes (such as HOMO−LUMO excitation) and have previously been observed for related systems.48 8502



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and another for iminic nitrogen (−N, red line, 398.9 eV) at lower BE.32,44 The observed 3:1 ratio of the peak areas is in agreement with the molecular stoichiometry. To obtain information about the reactivity of a 2HPc submonolayer as a function of temperature, 0.5 monolayer of 2HPc was deposited at a sample temperature of 160 K. The corresponding N 1s spectrum (Figure 1b) was taken directly after deposition; its overall shape is practically identical to the 2HPc multilayer spectrum, except for slightly wider peaks in the submonolayer spectrum, which are attributed to a combination of inhomogeneity and lifetime broadening, as it is typical for adsorption on metal surfaces.45 The BE energy shift of 0.6 eV between monolayer and multilayer is attributed to the less efficient dielectric screening of the core hole in a matrix of other molecules, as compared to the screening by the metal surface in the monolayer. Such shifts have frequently been observed in photoelectron spectra of monolayers and multilayers of physisorbed atoms and molecules on metal surfaces.10,12,18,35,39,46,47 From the comparison of the 2HPc multilayer spectrum and the submonolayer spectrum taken at 160 K, we conclude that 2HPc does not undergo a chemical reaction with the Cu(111) surface at 160 K. Raising the temperature results in an intensity redistribution within the N 1s signal: The 2HPc related components lose intensity (green and red lines), while a new component (blue line), located between the two 2HPc related components, increases in intensity. This process starts between 160 and 240 K (Figure 1b,c), and it is complete by 450 K (Figure 1d,e), when the N 1s spectrum consists of a single peak at 398.7 eV. According to previous work, these changes are a clear sign that a fraction of the molecules has reacted with Cu atoms forming copper phthalocyanine (CuPc), according to the equation 2HPc + Cu → CuPc + H2(g).12 For comparison, a submonolayer spectrum of CuPc, which was directly deposited onto the Cu(111) surface, is shown in Figure 1f. As can be seen, position and width of the reference spectrum are almost identical to those of the 450 K spectrum, indicating that the changes in the spectra are related to the formation of CuPc. Since this metal complex contains only iminic N (−N), a single peak is observed. Note that the signal deconvolution of the spectra between 240 and 450 K in Figure 1 was performed by using tight restraints: The peak parameters were extracted from the spectra of the pure 2HPc and CuPc submonolayers (Figure 1a,f). Positions, widths, and stoichiometric ratio of these peaks were fixed, and only their intensities were allowed to vary. Even with these restraints, a good agreement between fit and experimental data is obtained, which confirms the interpretation given above. Temperature-Dependent Reactivity of 2HPc Multilayers. The results reported in the previous section raise the question whether layers beyond the monolayer can react also with the Cu surface. The latter would require transport of the molecules or metal atoms to make the reactants meet. This question was addressed by depositing 4 monolayers of 2HPc on Cu(111) and performing temperature-dependent XPS experiments, starting from 300 K. The N 1s spectrum in Figure 2a shows that, at room temperature, most of the 2HPc layer remains unreacted. Heating to 500 K (Figure 2b) for 2 min changes the situation: The shoulder related to aminic (−NH−) nitrogen around 400.4 eV disappears completely, and the whole signal evolves into a single peak at 398.9 eV, which is characteristic of CuPc; i.e., there is clear evidence for complete metalation. Simultaneously, the integral of the signal is reduced by 28%,

Figure 2. N 1s XP spectra taken after deposition of 2HPc onto Cu(111) at 300 K with an initial coverage of 4 monolayers and after heating to the indicated temperatures. All spectra were measured at room temperature after annealing the sample at the indicated temperatures for 2 min. Transition from multilayer to monolayer is accompanied by the typical peak shift to lower binding energy (marked by vertical lines), which is attributed to the more efficient core hole screening in the monolayer.10,12,18,35,39,46,47

which could be attributed to partial desorption. Taking attenuation effects into account, the residual intensity would correspond to a layer thickness of 2.5 monolayers. However, we will show in the following TPD/TPR section that the amount of desorption at 500 K is negligible: Extracting the activation parameters from the TPD data shown below and calculating the desorption rate at 500 K yields a total loss of 0.17 monolayers during the annealing time of 2 min. The observed reduction of the XPS intensity is therefore attributed to thermally activated dewetting, which has frequently been observed on organic multilayers.49−52 As has been shown previously,51 the transition from a smooth two-dimensional (2D) multilayer to the formation of three-dimensional (3D) crystallites leads to a reduction of the XPS intensity because the 3D crystallites cause a larger average attenuation of the photoelectrons, compared to a smooth multilayer.51 Therefore, we conclude that a thin multilayer (4 monolayers) of 2HPc on Cu(111) reacts completely with Cu atoms, forming CuPc. Desorption of the CuPc multilayer starts above 500 K, in agreement with the TPD/TPR data presented in section 3.2 and with the additional intensity loss in Figure 2c, and is complete by 650 K (Figure 2d). Heating to 700 K (Figure 2e) does not lead to a further reduction of the N 1s intensity; apparently, the CuPc monolayer does not desorb. 3.2. Temperature-Programmed Desorption and Reaction (TPD/TPR). TPD/TPR of a 2HPc Bilayer on Cu(111). TPD/TPR traces taken after deposition of 2.2 monolayers of 2HPc onto Cu(111) are shown in Figure 3. The masses of CuPc (m/z = 576 amu, Figure 3a) and 2HPc (515 amu, Figure 3b) were recorded quasi-simultaneously as explained in the Experimental Details section. As can be seen, the majority of the desorbing species is CuPc (69%), and the rest is 2HPc. Note that only layers above one monolayer desorb, according to XPS. Taking into account that the monolayer has reacted completely to CuPc by 450 K (see above), 83% of the bilayer is 8503



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Figure 4. TPD/TPR of 2 monolayers of 2HPc deposited onto 0.8 monolayer of NiPc on Cu(111) at 300 K. (a) CuPc (m/z = 576 amu), (b) NiPc (m/z = 571 amu), and (c) 2HPc (m/z = 515 amu).

Figure 3. TPD/TPR of 2HPc on Cu(111), taken with a heating rate of 5 K/s immediately after deposition of 2HPc. The initial coverage was 2.2 monolayers. (a) CuPc (m/z = 576 amu), (b) 2HPc (m/z = 515 amu).

converted into CuPc in this experiment. The fact that the reaction is not complete here (in contrast to the XPS result in section 3.1, Figure 2) is attributed to the longer reaction time in the XPS experiment. To clarify the question how molecules sitting above the monolayer can react with Cu atoms from the substrate, a submonolayer (0.8 monolayers) of nickel phthalocyanine (NiPc) was deposited onto the Cu(111) surface. This layer is stable on the Cu surface and does not desorb or react up to 700 K, as was verified by XPS and TPD (see the Supporting Information). Onto this NiPc submonolayer, two monolayers of 2HPc were deposited and a TPD/TPR experiment was performed. The resulting data in Figure 4 show desorption of CuPc as the majority species (Figure 4a, 76% of the desorbing species). Interestingly, there is also desorption of NiPc (Figure 4b, 13%) besides residual 2HPc (Figure 4c, 11%). Since NiPc (like the other two species) is unable to desorb from the monolayer (see also the Supporting Information), desorption of NiPc can only be explained by a temperature-driven exchange of molecules between the monolayer and the higher layers, as is schematically illustrated in the scheme on top of Figure 4. In a further experiment, 1.8 monolayers of NiPc were deposited onto the Cu(111) surface, followed by 1 monolayer of 2HPc. An excess of NiPc was used to ensure that the surface is initially completely covered by NiPc, which prevents any direct contact between 2HPc molecules and the Cu surface in the initial multilayer. The resulting TPD/TPR traces for this preparation are shown in Figure 5: Again, CuPc desorbs as the majority species (Figure 5a, 58%), accompanied by NiPc (Figure 5b, 38%) and 2HPc (Figure 5c, 4%). This result shows unambiguously that 2HPc reacts with the Cu(111) surface even if both reactants are in the beginning separated by a closed NiPc layer. Most likely, the exchange mechanism described above makes this reaction possible, even though it cannot be excluded that additionally Cu atoms diffuse into the organic layer and react there. However, diffusion of Cu atoms into the

Figure 5. TPD/TPR of one monolayer of 2HPc deposited onto 1.8 monolayers of NiPc on Cu(111) at 300 K: (a) CuPc (m/z = 576 amu), (b) NiPc (m/z = 571 amu), and (c) 2HPc (m/z = 515 amu).

organic multilayer would require a significant stabilization of the Cu atoms in the multilayer compared to an isolated gas phase Cu atoms, which has a heat of formation of 337 kJ/mol.53 These results raise the question why the exchange proceeds at much lower temperature than desorption, although both require that the bond of the first layer (monolayer) to the substrate is broken. The reason is that desorption and exchange differ fundamentally in their energetics and kinetics. Desorption requires a large amount of energy because it results in the complete loss of all bonds to the substrate. In contrast, the exchange of two layers does not change the energy of the system significantly because the initial state (in which one phthalocyanine is in contact with the Cu surface, another one is on top of the first molecule) is almost identical to the final state 8504



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reactants, 2HPc and Cu, get into contact to undergo this reaction. We have demonstrated that placing a layer of NiPc, which serves as an inert barrier between the Cu surface and the 2HPc multilayer, does not suppress the metalation reaction. In addition, partial desorption of NiPc is observed, although (sub)monolayers of NiPc do not desorb in the absence of the 2HPc overlayer. These findings clearly show that there is an exchange of molecules between the first layer, which is in direct contact to the metal surface, and the higher layers. In this way, 2HPc from the multilayers can get into contact with the Cu surface and react. The reaction product, CuPc, can migrate to the multilayer and desorb from there. The exchange of molecules between monolayer and multilayer has important implications for organic electronic, where organic monolayers have been proposed as contact primers for adjusting the energy level alignment at the interface between metal electrodes and organic semiconductors, with the aim to reduce barriers for charge carrier injection. If such a contact primer layer is replaced with molecules from the organic semiconductor by exchange processes, drastic changes of the charge carrier injection barriers may occur which affect the performance of the organic electronic device. The same holds true for chemical reactions at the interface, which tend to influence the electronic structure and thereby the charge carrier dynamics.

(except for small effects due to the different central ions). Besides this thermodynamic consideration, we also must compare the kinetics, i.e., the rates of desorption and exchange. Desorption has a very large activation barrier because the transition state is essentially the free molecule, and thus the desorption activation energy approximately equals the adsorption energy. Desorption is therefore very slow. The exchange, however, has a much lower barrier because the molecules remain all the time (van der Waals) bonded to the matrix of other molecules. In addition, while the bond of a firstlayer molecule to the metal surface breaks, another molecule from the second layer already forms a bond to the metal surface, thus reducing the energy barrier. Therefore, the exchange process can be much faster than desorption. It is perhaps helpful to consider the analogy to dissolution: Dissolution of many molecular or ionic solids proceeds at much lower temperatures than evaporation because the transition from the solid to the gas phase requires much more energy than the solid−solution transition. Some of our findings could potentially also be explained by an alternative mechanism, in which metal centers would be exchanged between the phthalocyanine ligands in different layers. The necessary temporary demetalation and remetalation makes this mechanism very unlikely considering the energetics and kinetics of the metalation reaction. We have previously reported extensive density functional theory (DFT) calculations of the metalation mechanism for the chemically very similar metalloporphyrins.34 The metalation reaction is strongly exothermic with 531 kJ/mol for Ni and 561 kJ/mol for Cu. These values also represent the activation barriers for the reverse reaction (demetalation) producing 2HPc and free metal atoms. With these barriers, temperatures above 2000 K would be necessary for sufficiently high exchange rates. (Note that barriers would be somewhat lower due to the fact the released metal atom is not free, but embedded in a matrix of molecules. Qualitatively, however, the argument will remain the same.) Moreover, the very low partial pressure of hydrogen (

Coordination Reactions and Layer Exchange Processes at a Buried

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