Copper Phthalocyanine as Contact Layers for Pentacene Films Grown

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Copper Phthalocyanine as Contact Layers for Pentacene Films Grown on Coinage Metals Alexander Mänz, Alrun Aline Hauke, and Gregor Witte J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b10324 • Publication Date (Web): 02 Jan 2018 Downloaded from http://pubs.acs.org on January 3, 2018

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The Journal of Physical Chemistry

Copper Phthalocyanine as Contact Layers for Pentacene Films Grown on Coinage Metals Alexander Mänz, Alrun Aline Hauke and Gregor Witte* Molekulare Festkörperphysik, Philipps-Universität Marburg, 35032 Marburg, Germany ABSTRACT The metal-semiconductor interface determines the efficiency of charge carrier injection into any organic electronics device. Control of this interface, its structure and morphology is therefore essential for device improvement. In this study, we analyze the approach of controlling semiconductor morphology at this interface by insertion of a copper phthalocyanine (CuPc) monolayer as a primer between Ag(111), Au(111) and Cu(100) surfaces and the organic semiconductor pentacene (PEN). Controlled monolayer formation is facilitated by thermal desorption of excess multilayers, monitored via thermal desorption spectroscopy (TDS), X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM), and the growth of PEN on the resultant monolayer primers is investigated by near-edge X-ray absorption spectroscopy (NEXAFS), atomic force microscopy (AFM) and STM. While well-ordered CuPc monolayers with flat-lying molecules are formed on Au(111) and Ag(111), no long-range order is observed on Cu(100). Subsequently deposited PEN molecules initially adopt a recumbent orientation with their long axis oriented parallel to the surface, while upon further deposition this structure is metastable as molecules adopt an upright orientation beyond the bilayer and form (001) oriented films. Although the recumbent orientation of the CuPc primer layer is not transferred to thicker PEN films, which is attributed to the geometrical inequality of the two molecules, a distinct dewetting, as found for PEN films grown on bare metal surfaces, is efficiently suppressed. This effect is reproducible even for polycrystalline Au surfaces, which resemble the situation of metal contacts in devices. 1. INTRODUCTION The metal-semiconductor interface is perhaps the most important interface in any organic electronics device with metal electrodes: its structure, morphology, chemical composition and energy barrier will ultimately determine whether charge carriers can be injected into / extracted out of the organic semiconductor (OSC). 1,2 The control of these parameters by insertion of a functional molecular interlayer at the interface is therefore a promising approach to enhance device performance. In their pioneering work, Alloway et al. used partly fluorinated alkanethiol self-assembling monolayers (SAMs) to tune the work function of Au surfaces. 3,4 Further work involving the tuning of metal work functions by SAMs, including extensive theoretical studies 5,6, however, has demonstrated certain drawbacks to this approach: aliphatic SAMs, due to their large HOMO-LUMO gaps, provide only poor charge carrier transport 7, while aromatic SAMs with smaller gaps and thus better transport properties are generally less soluble and accordingly difficult to process. 8 While oligo-phenylenethiol SAMs have been demonstrated to provide better results in both respects 9-12, there is still the remaining issue of the molecular length of SAMs in general. As the charge transport from the metal into the OSC still appears to be a tunneling process through the molecular interlayer 13-16, SAM thicknesses of approximately 1nm lead to a significant reduction in tunneling probability. An interesting alternative may be the use of vacuum-processed small molecules as functional interlayers. 1719 The expected advantages of this approach are the reduced thickness of such contact primer monolayers compared to SAMs and the potentially larger contact area provided by flatlying molecules in comparison to uprightly oriented, covalently bound organothiol SAMs, which have only a point contact to the metal. This concept has recently also been used for contacting single molecules in break junction studies. 20 Even more interestingly, several studies have reported an orienta-

tion inheritance for organic heterostructures 21,27, which in this context may be used to control molecular orientation and morphology. Since the charge carrier transport is highly anisotropic in crystalline OSCs, this orientation control must be utilized e.g. for vertical transistor architectures. 28 From this it follows that using orientational template (contact) layers might indeed become an important tool for organic thin film functionalization, provided such a templating effect can also be induced in hetero-structures of structurally less similar molecules. Another issue related to this approach is the fabrication of well-defined, ordered monolayers similar to those obtained for SAMs: While one may of course easily deposit a nominal layer thickness equivalent to one monolayer, this often results in the growth of multilayer islands coexisting with an incomplete monolayer. These islands are difficult to see e.g. in scanning tunneling microscopy (STM), as one may accidentally drag the weakly bound multilayer molecules away from the measurement area when approaching the sample with the STM tip. A more controllable method of monolayer formation is thus required, as demonstrated already for the case of pentacene (PEN) on coinage metal surfaces. 29-31 The structural and morphological properties of such a monolayer, as well as those of subsequently deposited OSC layers, then need to be understood before one can begin to study small molecule monolayers as functional interlayers for metal-OSC interfaces. In the present study, the influence of the heteromolecular interface structure on the morphology and crystallinity of an organic thin film is analyzed. Chemisorbed monolayers of copper phthalocyanine (CuPc), a common representative of the large group of phthalocyanines, on the coinage metal substrates Au, Ag and Cu are used as a contact layer model system for the subsequent growth of PEN multilayers. In order to ensure reproducible preparation conditions, a robust monolayer preparation protocol by multilayer desorption is intro1

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duced, followed by the characterization of on-top adsorbed PEN mono- to multilayers. Finally, the structure of PEN films grown on highly ordered CuPc monolayers on single crystal metal surfaces are compared to films grown on defective CuPc monolayers formed on polycrystalline Au substrates.

2. EXPERIMENTAL For the present study, various metal substrates have been examined. A Cu(100) single crystal was used as well as Au(111) and Ag(111) samples consisting of 100 – 200 nm metal films that were epitaxially grown under high vacuum conditions onto freshly cleaved and carefully degassed mica substrates, as described elsewhere. 8 Prior to organic film deposition all substrates were pre-treated in situ by repeated cycles of Ar-ion sputtering (800 eV) and annealing (Ag, Au: 750 K, Cu: 860 K) until sharp (1x1) low energy electron diffraction (LEED) patterns with a low background signal and no traces of contamination were found by STM or X-ray photoelectron spectroscopy (XPS). In addition, polycrystalline gold substrates were also used, which were prepared by sputter deposition of 200 nm Au onto polished Si(100) wafers covered with a native oxide layer. Copper(II)-phthalocyanine (CuPc, SIGMA-ALDRICH, purity: 99%) and pentacene (PEN, SIGMA-ALDRICH 99.9%) thin films were prepared via organic molecular beam deposition (OMBD) from an aluminum crucible of a resistively heated Knudsen cell. Typical deposition rates were 10 Å/min, monitored by a quartz crystal microbalance (QCM), and the substrates were kept at room temperature during deposition. The microstructural order of the molecular thin films was characterized in situ by scanning tunneling microscopy (STM, OMICRON VT STM XA), operated in constant current mode and using etched tungsten tips. As complementary measurements, the film morphology was characterized by atomic force microscopy (AFM, Agilent SPM 5500), performed in tapping mode (fres = 325 kHz) under ambient conditions. X-ray diffraction (XRD) was employed to analyze the crystalline structure of (multilayer) films, using a Bruker D8 Discovery diffractometer with monochromatized Cu Kα radiation (λ = 1.540 Å) and a LynxEye silicon strip detector. To study the thermal stability of the organic films, thermal desorption spectra (TDS) were recorded using a quadrupole mass spectrometer (Balzer QMG 220) equipped with a Feulner cup positioned close to the sample surface. All spectra were acquired during a computercontrolled linear heating rate of β = 0.5 K/s, while the temperature was measured by a thermocouple attached directly to the sample surface. C1s near-edge x-ray absorption spectroscopy (NEXAFS) and XPS measurements were performed at the HE-SGM dipole beamline of the synchrotron storage ring BESSY II of the Helmholtz Center Berlin (Germany), which provides linearly polarized light (polarization factor: 0.91) and an energy resolution at the carbon K-edge of about 300 meV. All NEXAFS spectra were recorded in partial electron-yield (PEY) mode. To determine the average molecular orientation relative to the sample surface, NEXAFS spectra were recorded at different angles of incidence (30°, 55°, and 90°). Details on the experimental setup and data evaluation are provided in the literature. 32

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3.1 CuPc Monolayer Preparation. First, the thermal stability of CuPc (Fig. 1a) multilayers on Ag(111) was studied to determine the usable temperature range for post-deposition thermal treatments. This aims to explore the feasibility of CuPc monolayer preparation by multilayer desorption. By comparing the intensities of the C1s signal and Ag Auger electrons in temperature-dependent XPS measurements (Fig. 1c, red), the desorption process of CuPc multilayers can be monitored. For this purpose, samples are heated at the desired temperature for one minute and then cooled down prior to the XPS measurement. By selecting an incident photon energy of 650 eV, the C1s photoelectron signal (binding energy EB = 285.3 eV) is collected simultaneously with the Ag MNN Auger electron signal (Ekin = 351 eV), as shown in Fig. 1b). For temperatures above 525 K, a strong decrease in the C- to Ag-signal intensity ratio can be observed. This corresponds to the onset of multilayer desorption. Interestingly, this transition can also be observed by the naked eye as the disappearance of the blue film. This process stops at 570 K, where a C1s signal is still observed without any further decrease in intensity for increasing temperature (at least up to 600 K). This remaining intensity is therefore attributed to the chemisorbed CuPc monolayer, which is confirmed by the NEXAFS and STM data presented below. The multilayer desorption process is also monitored directly by TDS measurements (Fig. 1c, blue). Due to the limited mass range of the spectrometer (≤ 300 amu) the molecular ion signal could not be measured directly. Instead, various CuPc fragments (masses 63 amu (Cu+), 128 amu C8N2H4+, 192 amu (C8N2H4Cu+) and 288 amu (CuPc2+) were analyzed.

Figure 1: a) CuPc and its structural formula. b) Temperaturedependent XP spectra of a CuPc multilayer film (10 nm). c) Red: Quotient of the C- and Ag-intensities over temperature. Blue: Thermal desorption spectra of CuPc multilayers on Ag(111)/mica, β = 0.5 K/s, m/z = 63 amu (Cu).

3. RESULTS AND DISCUSSION


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Figure 2: a) Full C1s NEXAFS spectrum of CuPc multilayers (30nm) on Ag(111) recorded at θ = 55°. b) Schematic representation of the angles θ and α. c) Corresponding dichroism analysis for the π*-region of the data shown in a). d, e) Dichroism analysis for the CuPc monolayer on Ag(111) and Au(111) in the same spectral region. All mass signals show the same desorption behavior with the desorption onset at 520 K. Leaving aside dehydrogenation, one can therefore exclude fragmentation of the CuPc carbon backbone or separation of the central Cu atom when heating the sample up to 600 K. This is in line with previous reports 33 and also corroborated by our STM data. Resulting CuPc monolayers have been investigated by carbon edge-NEXAFS measurements (Fig. 2) in order to get comprehensive information on the molecular orientation. Fig. 2a) shows a typical normalized C1s NEXAFS spectrum of a 30 nm CuPc film on Ag(111) that was recorded at an incident angle of θ = 55° of the field vector relative to the surface normal. In previous analyses of the NEXAFS signature of CuPc 34,35, the dominating peaks at excitation energies below 290 eV were identified as excitations into unoccupied π*-orbitals (so-called π*-resonances), while resonances appearing above the continuum edge (dashed line) were attributed mainly to σ*-resonances. As the transition dipole moment, T, of the π*-resonances is perpendicular to the aromatic ring plane of the phthalocyanines, this allows a determination of the average molecular tilt angle α from the quantitative analysis of the dichroism of the leading resonances, as described in detail elsewhere. 32 Fig. 2c) shows the region of π*-resonances for 30 nm CuPc on Ag(111). The sample exhibits an almost planar, only slightly tilted orientation, which is indicated by a non-vanishing intensity for θ = 90° (i.e. normal incidence). The quantitative analysis of the dichroism yields a tilt angle of α = 23°, showing that CuPc adopts a recumbent orientation in multilayers, as found previously for CuPc on Au(100). 36 In Fig. 2d,e), spectra of CuPc monolayers prepared by multilayer desorption (by heating at 550 K for 5 minutes) on Ag(111) and Au(111) are shown. Comparing the NEXAFS signature of the CuPc monolayers on both noble metal surfaces with that of the multilayer shows a notable broadening (especially of the most intense π*-resonance at 285.2 eV) for the Ag(111) due to interaction with the metal substrate 37 and only a slight broadening on Au(111), reflecting the weak binding on Au. Using x-ray standing

Figure 3: CuPc monolayers on a) Au(111), b) Ag(111) and c) Cu(100) surfaces prepared by multilayer desorption at 530 K. Tunneling parameters: a) I = 1nA, Ubias = 0.3V; b) I = 250 pA, Ubias = 0.6 V; c) I = 30 pA, Ubias = 33 V. Au(111) and Ag(111) samples were measured at room temperature, while the Cu(100) sample was measured at a substrate temperature of 110 K. Insets in a) and b) show correlated averaged images, the inset in c) is a magnified view. waves (XSW) it was found that the vertical adsorption height of CuPc/Au(111) (3.3 Å) is larger than for CuPc/Ag(111) (3.0 Å), which reflects the stronger adsorption on silver. 38 Both monolayers reveal a distinct dichroism with a nearly vanishing π* intensity for normal beam incidence (red curve), which indicates a planar adsorption geometry. The quantitative analysis yields molecular tilt angles below 5°. Deviations from a perfectly planar orientation are attributed to different adsorption geometries occurring at surface defects, as they also contribute to the average orientation. For a more sophisticated view on the microscopic molecular ordering, STM micrographs were obtained for CuPc monolayers prepared on Ag, Au and Cu substrates by desorption of excess multilayers. As shown in Fig. 3, STM micrographs of such prepared CuPc monolayers on Ag(111) and Au(111) reveal homogeneous


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and densely packed CuPc molecules, spreading over terraces with more than 100 nm diameter, with an almost quadratic molecular area of (190 ± 5) Å2 on Au(111) and (194 ± 5) Å2 on Ag(111). These compare nicely to previously reported data for directly adsorbed CuPc monolayers (without multilayer desorption), where the molecules adopt an surface unit cell size of 194 Å2 on Au(111) and 192 Å2 on Ag(111). 39-41 Interestingly, the simultaneous imaging of densly packed substrate steps and the molecular rows of CuPc allows to identify their relative orientations. The CuPc monolayer's unit cell vector deviates from the Ag 〈110〉 high-symmetry direction by 3°, as found previously by SPA-LEED measurements reported by Kröger et al. 40 For CuPc on Au(111), the surface reconstruction is visible and molecules align in rows along the Au 〈110〉 azimuth. It is interesting to note that after air exposure and subsequent annealing, CuPc monolayers on Au(111) still show the same characteristic molecular rows in STM, with a lying molecular configuration also still visible in NEXAFS. As a third coinage metal substrate, copper (Cu) was examined. Since the growth of CuPc on Cu(111) has been found to show a significant degree of disorder, 42 we have chosen a Cu(100) surface to provide a better symmetry matching behavior. For Cu(100), STM measurements show no long-range periodic arrangement of CuPc molecules on the surface, merely a certain degree of local order. This result could be anticipated already from the fact that measurements had to be performed at substrate temperatures of 110 K in order to obtain molecular resolution, a fact which by itself already suggests a disordered surface. The disorder is attributed to the much larger adsorption energy on the Cu(100) surface compared to the nobel metal surfaces. We therefore assume strongly diminished molecular diffusion lenghts, resulting in hit-and-sticklike growth. This is in line with previous reports of directly adsorbed monolayers (without multilayer desorption) of copper and iron phthalocyanines on Cu(100) surfaces.39,43,44 In conclusion, multilayer desorption provides well-defined and long-range ordered CuPc monolayers on Ag(111) and Au(111) substrates. In the case of Cu(100) surfaces, the CuPc monolayer's ordering is strongly disturbed. Such samples will thus be used as an example of a defective and ill-defined CuPc primer layer. 3.2 Molecular orientation of PEN on CuPc monolayers. Next, we have studied the subsequent growth of PEN films on CuPc primer monolayers supported by noble metals using C1s NEXAFS spectroscopy. This enables a non-invasive in-situ characterization of the PEN adlayers without any risk of tip-assisted distortion. Although this technique provides no direct information on the crystalline assembly, the NEXAFS dichroism yields information on the average orientational order for a large sample area (≥ 1 mm2), including also non-crystalline regions. In contrast, only highly ordered or periodic structures are captured by XRD and STM (which might benon-representative for the entire film). Fig. 4 summarizes the C1s NEXAFS spectra of such PEN films grown at room temperature with nominal coverages ranging from the submonolayer regime (0.2 nm)to multilayers (30 nm). The NEXAFS PEN signature is identified on the basis of previous studies, which show the presence of π*-resonances at energies below 287 eV. 30,45 While flat-lying CuPc molecules reveal vanishing π*-resonance intensities when measuring at normal incidence (cf. red curves in Fig. 2d,e), distinct NEXAFS signals are seen in this spectral region for the 0.2 nm PEN films under normal incidence. As depicted in Fig. 4

Figure 4: a), c) and d) C1s X-ray absorption spectra for PEN thin films of different thicknesses adsorbed on a CuPc monolayer on Ag(111). e), g) and h) PEN thin film growth on CuPc monolayer on Au(111) substrates with qualitatively equal characteristics. The color coding is the same as in Fig. 2. Panels b) and f) show respective 55° spectra of CuPc monolayers and PEN multilayers.

a) and e) (red curves), their signature concurs with that of undisturbed thick PEN films (cf. Fig. 4 d, h). Since the C1s NEXFAS signatures of CuPc and PEN are largely overlapping (as shown for the allows drawing the following conclusions: (i) On average, PEN molecules are not flat-lying on the CuPc contact layer, but instead are 55° spectra in Fig. 4 b, f), a quantitative analysis of the PEN-related dichroism is generally hampered. However, the appearance of distinct NEXAFS signals with clear PEN signature at normal incidence tilted. (ii) The PEN molecules are not in direct contact to the metal surfaces, as this would result in a distinct broadening of π*resonances and disappearance of their fine-structure, as demonstrated before for PEN monolayer films on Au(111) and Ag(111). 29,30 The latter observation allows in particular to exclude a replacement reaction, as it was found e.g. for CuPc and perfluoropentacene (PFP) or perfluorinated CuPc (F16CuPc) and PEN. 46 With further increasing film thickness, the NEXAFS signature of PEN becomes more pronounced also at grazing incidence (especially the first π*-resonance of PEN below 284 eV is visible for the 4 nm films, cf. Fig. 4 c,g), which indicates a slightly tilted standing molecular orientation. For coverages of 30 nm PEN on top of the CuPc primer layer, the NEXAFS signals reveal a pronounced dichroism with the largest π*-resonance intensities for normal incidence. The quantitative analysis of the dichroism yields an average molecular tilt angle α of about 83°, which indicates an upright molecular orientation, as found for PEN multilayer films grown on SiO2 substrates.47 By contrast on bare noble metal surfaces (Au(111) and Ag(111)) PEN adopts a recumbent molecular orientation. 29,30 This demonstrates an effective screening of the metal surface by the ordered CuPc monolayer.


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Figure 6: X-ray diffraction in Bragg-Brentano geometry of 30 nm PEN films on CuPc monolayers on Ag(111), Au(111) and Cu(100).

Figure 5: Morphological characterization of PEN/CuPc heterolayers. a) STM micrograph of nominally 0.2 nm PEN on CuPc/Ag(111). The substrate temperature for this scan was 110 K and the tunnel parameters were Ubias = -2 V, I = 10 pA; b) schematic representation of PEN orientation on the substrate; c) to f) AFM topography micrographs of PEN on CuPc monolayer on different substrates: c) AFM micrograph of 4 nm PEN on CuPc/Ag(111); d)f) 30 nm PEN coverage on Ag(111), Au(111) and Cu(100). All scale bars for c)-f) are 300 nm.

3.3 Morphology of PEN adlayers on CuPc monolayers. The NEXAFS data for nominal 0.2 nm PEN deposited on the CuPc monolayer on Ag(111) and Au(111) suggests the formation of a meta-stable bilayer of PEN with the long axis of the molecules parallel to the substrate surface, but not exactly flat-lying. This, indeed, can be confirmed by examining the respective layers by STM. Fig. 5a) illustrates this for the case of PEN on CuPc/Ag(111): The STM data shows a molecular orientation with the long axis of PEN parallel to the substrate. A closer inspection of the width-tolength ratio suggests a slight tilt of the molecules, as shown in Fig. 5b). Further PEN deposition to a nominal thickness of 4 nm, however, takes place with a change in molecular orientation for these subsequently deposited layers, as again suggested by the NEXAFS data. AFM measurements on such films on Ag(111) clearly indicate the formation of the pyramidal islands with characteristic step heights of 1.5 nm typically associated with standing PEN molecules (see Fig. 5c). This growth

mode is also found for samples with a nominal thickness of 30 nm PEN on CuPc monolayers grown on Ag(111) and Au(111). Corresponding micrographs and height profiles are shown in Fig. 5 d) and e), which also exhibit the characteristic steps of 1.5 nm, reflecting the upright orientation of PEN molecules. This type of growth is known for PEN on weakly interacting substrates such as SiO2. 47 In the case of a Cu(100) surface as substrate (Fig. 5 f) AFM micrographs of 30 nm PEN on a CuPc monolayer also show pyramidshaped dendritic structures with step heights of 1.5 nm, which indicates uprightly-oriented molecular configuration. In contrast to Ag(111) and Au(111), however, on the Cu(100) PEN multilayers reveal notably shorter terraces as well as an increased roughness. In our interpretation, this is directly caused by the strong disorder of the underlying CuPc monolayer shown previously (cf. Fig. 1c). This case resembles the situation found for PEN on microscopically rough substrates such as C60. 48 In summary, we find a rather disordered growth of PEN in a metastable bilayer directly at the PEN/CuPc interface. For all metal substrates and PEN film thicknesses of several tens of nanometers, we find that subsequent PEN layers adopt a standing configuration known from much less interacting substrates such as SiO2 or SAMtreated gold. Thus, the CuPc primer layer prevents a substrateadsorbate interaction, enabling an upright molecular orientation of PEN, which is in stark contrast to the recumbent orientation of PEN on bare noble metals. 29,30 Furthermore, this standing orientation points towards a strong degree of electronic decoupling of the PEN adlayers from the metal substrates, a factor which has been suggested as important for efficient charge carrier injection across this interface in a recent theoretical model by Oehzelt et al. 17 3.4 Crystallographic aspects. In order to identify the crystallographic phase adopted in PEN multilayers, X-ray diffraction in Bragg-Brentano geometry has been applied to the above-shown multilayer films with PEN film thicknesses of 30 nm. Fig. 6 shows survey diffractograms for such films on a CuPc monolayer on Ag(111)/mica, Au(111)/mica and Cu(100) substrates. Beside the substrate reflexes, reflexes corresponding to the thin film phase of


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PEN can be identified, as shown magnified for the (001) reflex region. For films grown on a CuPc monolayer on Cu(100), the (001) reflex of the PEN thin film polymorph is dominant but comparably weak in intensity. XRD further allows to quantify the mosaicity of the PEN multilayers through analysis of their rocking curves. By deriving the quotient Q of the measured rocking width (i.e. out-of-plane mosaicity) of the PEN(001)TF-reflex and the rocking width of the corresponding underlying metal substrate, we thus gain a measure of how well the PEN molecules align with the crystalline order given by the substrate: Q equals 1 for perfect molecular arrangement and increases for increasing out-of-plane mosaicity of the adlayer. We obtain a quotient of Q = 1.1 for Ag(111)/mica substrates, Q = 1.3 for Au(111)/mica substrates and Q = 1.9 for a Cu(100) single crystal substrate. The calculation underlines the difficulties of preparing long-range ordered PEN multilayer films on CuPc monolayers on Cu(100) surfaces. By contrast, the value of Q =1.1 for PEN multilayer films on CuPc monolayers on Ag(111) surfaces reveals a considerably smaller out-of-plane mosaicity. 3.5 Polycrystalline substrates. To study the influence of the crystalline order of the substrate and to investigate a metal surface that more closely resembles the metal contacts in actual organic electronic devices, we also prepared the CuPc monolayer (by multilayer desorption) with subsequently adsorbed 30 nm thick PEN films on polycrystalline Au surfaces. AFM micrographs and XRD specular scans are shown in Fig. 7. In the former, we observe PEN islands with significantly smaller diameter than on previously discussed samples. The lateral extent of the characteristic PEN terraces is also much smaller, making the islands appear somewhat blurry and hampering the analysis of individual step heights. As shown in the inset, fiber-like structures are found in addition to the blurry islands. Corresponding XRD measurements show weak (001)reflexes as well as second order reflexes of PEN in the thin film phase. In the (001)TF-plane, PEN molecules are known to adopt a standing configuration. Besides the thin film reflexes, minor amounts of PEN in the Siegrist phase and (022) orientation are found. In previous studies, fiber-like structures of PEN growth have been observed for untreated polycrystalline Au surfaces.30 This might be an indicator for the onset of a 3D crystallization process which is free of any influence of the substrate. Meanwhile, a comparison of the data obtained by AFM for 30 nm PEN on CuPc monolayers deposited on Au(111) (Fig. 5e, RMS roughness 3.1 nm) and polycrystalline Au (Fig. 7a, RMS roughness 8.7 nm) shows that for both cases the CuPc monolayer, just like a SAM, efficiently suppresses the dewetting typically observed for PEN on bare metal surfaces50, in particular bare gold30 (where films exhibit roughnesses of more than 40 nm), and thus enables smoother PEN films. This decrease in surface roughness is particularly relevant for organic electronic devices such as PEN-based Zener diodes 51 or bottom-contact organic field-effect transistors52, where the quality of subsequently deposited thin layers as well as device characteristics such as contact resistance or breakthrough stability depend critically on semiconductor morphology and roughness. 4. SUMMARY In this study, we investigated the influence of a CuPc monolayer, prepared on coinage metal surfaces, on the subsequent growth of PEN films with thicknesses from the submonolayer to the multilayer regime. For this purpose, a multilayer desorption approach was used in order to form controlled, well-defined CuPc monolayers on the

Figure 6: a) AFM micrograph (topography) of 30 nm PEN on CuPc monolayer on polycrystalline gold on SiO2. Inset shows large area scan. b) XRD measurement of the same sample shows a weak (001) reflex of PEN in thin film phase (TF) and a weak (022) reflex of the Siegrist (S) phase.

respective metal substrates. This multilayer desorption process of CuPc films was characterized via XPS and TDS. Remaining CuPc monolayers have been found to be close-packed and long-range ordered in the case of Ag(111) and Au(111) substrates, while on.Cu(100) the layer shows significantly higher degrees of disorder. NEXAFS measurements reveal a disordered growth of a metastable PEN bilayer on CuPc monolayers on Ag(111) and Au(111), which remains intact upon further PEN deposition. This disordered bilayer has to be considered also for charge transport considerations in organic electronic devices. Upon further PEN deposition, no clear inheriting effect of molecular orientation from CuPc monolayers into PEN multilayers is observed, evidently due to the very dissimilar shapes of the two molecules (four-leaf clover versus elongated platelet). Instead, AFM and XRD measurements show PEN growing in thin film polymorph and upright orientation for films of 30 nm thickness on all substrates, even on Cu(100) and polycrystalline Au. The CuPc monolayer further suppresses the dewetting of PEN multilayers normally found on preparation by multilayer desorption thus obviously provides a reliable method to induce OSC growth in polymorphs known for low-interaction substrates even on metal surfaces. The result are smoother layers that are more easily compatible with vacuum-based organic electronics device fabrication, in addition to providing electronic decoupling of the OSC from the metal electrodes. Regarding the broad variety of phthalocyanine derivatives, this might be a new route to tailor the metal-organic interface, modify charge injection barriers and establish an immersion-free alternative surface coating technique to SAMs. These expectations will have to be validated by complementary electronic characterization of the phthalocyanine/metal interface, which is planned as a future work for the sys-


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tems presented here. In this respect, the above data must act as a reliable basis of structural information to which the electronic data can be correlated in order to yield a full understanding of the interface properties of phthalocyanine monolayers on metals and their applicability as contact primers for organic electronic devices.

Author Information Corresponding Author *(G.W.) E-Mail: [email protected] Telephone: +49 6421 28-21384 ORCID: Gregor Witte: 0000-0003-2237-0953 Notes The authors declare no competing financial interest.

Acknowledgements We acknowledge financial support provided by the German Science Foundation (DFG) through the collaborative research center "Structure and Dynamics of Internal Interfaces'' (SFB 1083, TP A2) and thank the Helmholtz-Zentrum Berlin (electron storage ring BESSY II) for provision of synchrotron radiation at the beamline HE-SGM.

References (1) Ishii, H.; Sugiyama, K.; Ito, E.; Seki, K.; Energy level alignment and interfacial electronic structures at organic/metal and organic/organic interfaces. Adv. Mater. 1999, 11, 605-625. (2) Braun, S.; Salaneck, W. R.; Fahlman, M.; Energy-level alignment at organic/metal and organic/organic interfaces. Adv. Mater. 2009, 21, 1450-1472. (3) Alloway, D. M.; Hofmann, M.; Smith, D.L.; Gruhn, N. E.; Graham, A. L.; Colorado, R.; Wysocki, V. H.; Lee, T. R.; Lee, P. A.; Armstrong, N. R.; Interface dipoles arising from self-assembled monolayers on gold: UV-photoemission studies of alkanethiols and partially fluorinated alkanethiols. J. Phys. Chem. B 2003, 107, 11690-11699. (4) Alloway, D. M.; Graham, A. L.; Yang, X.; Mudalige, A.; Colorado, R.; Wysocki, V. H.; Pemberton, J. E.; Lee, T. R.; Wysocki, R. J.; Armstrong, N. R.; Tuning the effective work function of gold and silver using ω-functionalized alkanethiols: varying surface composition through dilution and choice of terminal groups. J. Phys. Chem. C 2009, 113, 20328-20334. (5) Heimel, G.; Romaner, L.; Zojer, E.; Bredas, J. L.; Toward control of the metal-organic interfacial electronic structure in molecular electronics: A first-principles study on self-assembled monolayers of π-conjugated molecules on noble metals. Nano Lett. 2007, 7, 932-940. (6) Heimel, G.; Romaner, L.; Zojer, E.; Bredas, J. L.; The interface energetics of self-assembled monolayers on metals. Accounts Chem. Res. 2008, 41, 721-729. (7) Bock, C.; Pham, D. V.; Kunze, U.; Käfer, D.; Witte, G.; Wöll, C.; Improved morphology and charge carrier injection in pentacene field-effect transistors with thiol-treated electrodes. J. Appl. Phys. 2006, 100, 114517. (8) Käfer, D.; Witte, G.; Cyganik, P.; Terfort, A.; Wöll, C.; A comprehensive study of self-assembled monolayers of anthracenethiol on gold: solvent effects, structure, and stability. J. Am. Chem. Soc. 2006, 128, 1723-1732.

(9) Schmidt, C.; Witt, A.; Witte, G.; Tailoring the Cu(100) work function by substituted benzenethiolate self-assembled monolayers. J. Phys. Chem. A 2011, 115, 7234-7241. (10) Bowers, C. M.; Liao, K. C.; Zaba, T.; Rappoport, D.; Baghbanzadeh, M.; Breiten, B.; Krzykawska, A.; Cyganik, P.; Whitesides, G. M.; Characterizing the metal-SAM interface in tunneling junctions. ACS Nano 2015, 9, 1471-1477. (11) Masillamani, A. M.; Crivillers, N.; Orgiu, E.; Rotzler, J.; Bossert, D.; Thippeswamy, R.; Zharnikov, M.; Mayor, M.; Samori, P.; Multiscale charge injection and transport properties in selfassembled monolayers of biphenyl thiols with varying torsion angles. Chem. Eur. J. 2012, 18, 10335-10347. (12) Holmlin, R. E.; Haag, R.; Chabinyc, M. L.; Ismagilov, R. F.; Cohen, A. E.; Terfort, A.; Rampi, M. A.; Whitesides, G. M.; Electron transport through thin organic films in metal-insulator-metal junctions based on self-assembled monolayers. J. Am. Chem. Soc. 2001, 123, 5075-5085. (13) DiBenedetto, S. A.; Facchetti, A.; Ratner, M. A.; Marks, T. J.; Molecular self-assembled monolayers and multilayers for organic and unconventional inorganic thin-film transistor applications. Adv. Mater. 2009, 21, 1407-1433. (14) Fan, F. R. F.; Yang, J. P.; Cai, L. T.; Price, D. W.; Dirk, S. M.; Kosynkin, D. V.; Yao, Y. X.; Rawlett, A. M.; Tour, J. M.; Bard, A. J.; Charge transport through self-assembled monolayers of compounds of interest in molecular electronics. J. Am. Chem. Soc. 2002, 124, 5550-5560. (15) Karthäuser, S.; Control of molecule-based transport for future molecular devices. J. Phys. Condens. Matter 2011, 23, 013001. (16) Engelkes, V. B.; Beebe, J. M.; Frisbie, C. D.; Lengthdependent transport in molecular junctions based on SAMs of alkanethiols and alkanedithiols: effect of metal work function and applied bias on tunneling efficiency and contact resistance. J. Am. Chem. Soc. 2004, 126, 14287-14296. (17) Oehzelt, M.; Koch, N., Heimel, G.; Organic semiconductor density of states controls the energy level alignment at electrode interfaces. Nat. Commun. 2014, 5, 4174. (18) Oehzelt, M.; Akaike, K.; Koch, N.; Heimel, G.; Energy-level alignment at organic heterointerfaces. Sci. Adv. 2015, 1, e1501127. (19) Otero, R.; de Parga, A. L. V.; Gallego, J. M.; Electronic, structural and chemical effects of charge-transfer at organic/inorganic interfaces. Surf. Sci. Rep. 2017, 72, 105-145. (20) Mol, J. A.; Lau, C. S.; Lewis, W. J. M.; Sadeghi, H.; Roche, C.; Cnossen, A.; Warner, J. H.; Lambert, C. J.; Anderson, H. L.; Briggs, G. A. D.; Graphene-porphyrin single-molecule transistors. Nanoscale 2015, 7, 13181. (21) Hinderhofer, A.; Schreiber, F.; Organic-organic heterostructures: concepts and applications. ChemPhysChem 2012, 13, 628643. (22) Breuer, T.; Witte, G.; Controlling nanostructures by templated templates: inheriting molecular orientation in binary heterostructures. ACS Appl. Mater. Interfaces 2015, 7, 20485-20492. (23) Koller, G.; Berkebile, S.; Krenn, J.; Netzer, F.; Oehzelt, M.; Haber, T.; Resel, R.; Ramsey, M.; Heteroepitaxy of organicorganic nanostructures Nano Lett. 2006, 6, 1207-1212. (24) Oehzelt, M.; Koller, G.; Ivanco, J.; Berkebile, S.; Haber, T.; Resel, R.; Netzer, F. P.; Ramsey, M. G.; Organic heteroepitaxy: psexiphenyl on uniaxially oriented α-sexithiophene. Adv. Mater. 2006, 18, 2466-2470. (25) Chen, W.; Huang, H.; Chen, S.; Chen, L.; Zhang, H. L.; Gao, X. Y.; Wee, A. T. S.; Molecular orientation of 3,4,9,10-perylenetetracarboxylic-dianhydride thin films at organic heterojunction interfaces. Appl. Phys. Lett. 2007, 91, 114102. (26) Evans, D. A.; Steiner, H. J.; Vearey-Roberts, A. R.; Dhanak, V.; Cabailh, G.; O'Brien, S.; McGovern, I. T.; Braun, W.; Kampen,


7


T. U.; Park, S.; Zahn, D.R.T.; Perylenes and phthalocyanines on GaAs(001) surfaces. Appl. Surf. Sci. 2003, 212, 417-422. (27) Gordan, O. D.; Hermann, S.; Friedrich, M.; Zahn, D. R. T.; Optical properties of 3,4,9,10-perylenetetracarboxylic dianhydride/copper phthalocyanine superlattices. J. Appl. Phys. 2005, 97, 063518. (28) Lüssem, B.; Günther, A.; Fischer, A.; Kasemann, D.; Leo, K.; Vertical organic transistors. J. Phys. Condens. Matter 2015, 27, 443003. (29) Käfer D., Witte, G.; Evolution of pentacene films on Ag(111): growth beyond the first monolayer. Chem. Phys. Lett. 2007, 442, 376-383. (30) Käfer, D.; Ruppel, L.; Witte, G.; Growth of pentacene on clean and modified gold surfaces. Phys. Rev. B 2007, 75, 085309. (31) Söhnchen, S.; Lukas, S.; Witte, G.; Epitaxial growth of pentacene films on Cu(110). J. Chem. Phys. 2004, 121, 525-534. (32) Breuer, T.; Klues, M.; Witte, G.; Characterization of orientational order in π-conjugated molecular thin films by NEXAFS. J. Electron Spectrosc. Relat. Phenom. 2015, 204, 102-115. (33) Thussing, S; Jakob, P.; Structural and vibrational properties of CuPc/Ag(111) ultrathin films. J. Phys. Chem. C 2016, 120, 9904-9913. (34) De Francesco, R.; Stener, M.; Fronzoni, G.; Theoretical study of near-edge x-ray absorption fine structure spectra of metal phthalocyanines at C and N K-edges. J. Phys. Chem. A 2012, 116, 2885-2894. (35) Nardi, M. V.; Detto, F.; Aversa, L.;Verucchi, R.; Salviati, G.; Iannotta, S.; Casarin, M.; Electronic properties of CuPc and H2Pc; an experimental and theoretical study. Phys. Chem. Chem. Phys. 2013, 15, 12864-12881. (36) Peisert, H.; Knupfer, M.; Schwieger, T.; Auerhammer, J. M.; Golden, M. S.; Fink, J.; Full characterization of the interface between the organic semiconductor copper phthalocyanine and gold. J. Appl. Phys. 2002, 91, 4872. (37) Grand, J.-Y.; Kunstmann, T.; Hoffmann, D.; Haas, A.; Dietsche, M.; Seifritz, J.; Möller, R.; Epitaxial growth of copper phthalocyanine monolayers on Ag (111). Surf. Sci. 1996, 366, 403414. (38) Kröger, I.; Stadtmüller, B.; Kleimann, C.; Rajput, P.; Kumpf, C.; Normal-incidence x-ray standing-wave study of copper phthalocyanine submonolayers on Cu(111) and Au(111). Phys. Rev. B 2011, 83, 195414. (39) Lippel, P. H.; Wilson, R. J.; Miller, M. D.; Wöll, C.; Chiang, S.; High-resolution imaging of copper-phthalocyanine by scanningtunneling microscopy. Phys. Rev. Lett. 1989, 62, 171-174. (40) Kröger, I.; Stadtmüller, B.; Stadler, C.; Ziroff, J.; Kochler, M.; Stahl, A.; Pollinger, F.; Lee, T.-L.; Zegenhagen, J.; Reinert, F.;

Page 8 of 9

Kumpf, C.; Submonolayer growth of copper-phthalocyanine on Ag(111). New J. Phys. 2010, 12, 83038. (41) Stadtmüller, B.; Kröger, I.; Reinert, F.; Kumpf, C.; Submonolayer growth of CuPc on noble metal surfaces. Phys. Rev. B 2011, 83, 085416. (42) Karacuban, H.; Lange, M.; Schaffert, J.; Weingart, O.; Wagner, T.; Möller, R.; Substrate-induced symmetry reduction of CuPc on Cu(111): an LT-STM study. Surf. Sci. 2009}, 603, L39-L43. (43) de Oteyza, D. G.; El-Sayed, A.; Garcia-Lastra, J. M.; Goiri, E.; Krauss, T. N.; Turak, A.; Barrena, E.; Dosch, H.; Zegenhagen, J.; Rubio, A.; Wakayama, Y.; Ortega, J. E.; Copper-phthalocyanine based metal-organic interfaces: the effect of fluorination, the substrate, and its symmetry. J. Chem. Phys. 2010, 133, 214703. (44) Rehman, R.; Dou, W.; Qian, H.; Mao, H.; Floether, F.; Zhang, H.; Li, H.; He, P.; Bao, S.; Adsorption behavior of iron phthalocyanine at the initial stage on Cu(100) surface. Surf. Sci. 2012, 606, 1749-1754. (45) Fratesi, G.; Lanzilotto, V.; Floreano, L.; Brivio, G. P.; Azimuthal dichroism in near-edge x-ray absorption fine structure spectra of planar molecules. J. Phys. Chem. C 2013, 117, 6632-6638. (46) Wakayama, Y.; de Oteyza, D. G.; Garcia-Lastra, J. M.; Mowbray, D. J.; Solid-state reactions in binary molecular assemblies of F16CuPc and pentacene. ACS Nano 2011, 5, 581-589. (47) Bouchoms, I. P. M.; Schoonveld, W. A.; Vrijmoeth, J.; Klapwijk, T. M.; Morphology identification of the thin film phases of vacuum evaporated pentacene on SiO2 substrates. Synth. Met. 1999, 104, 175-178. (48) Breuer, T.; Witte, G.; Diffusion-controlled growth of molecular heterostructures: fabrication of two-, one-, and zero-dimensional C60 nanostructures on pentacene substrates. ACS Appl. Mater. Interfaces 2013, 5, 9740-9745. (49) Peisert, H.; Biswas, I.; Knupfer, M.; Chasse, T.; Orientation and electronic properties of phthalocyanines on polycrystalline substrates. Phys. Status Solidi B 2009, 246, 1529-1545. (50) Beernink, G.; Strunskus, T.; Witte, G.; Importance of dewetting in organic molecular-beam deposition: pentacene on gold. Appl. Phys. Lett. 2004, 85, 398-400. (51) Kleemann, H.; Gutierrez, R.; Lindner, F.; Avdoshenko, S.; Manrique, P. D.; Lüssem, B.; Cuniberti, G.; Leo, K.; Organic Zener diodes: tunneling across the gap in organic semiconductor materials. Nano Lett. 2010, 10, 4929-4934. (52) Pereira, A.; Bonhommeau, S.; Sirotkin, S.; Desplanche, S.; Kaba, M.; Constantinescu, C.; Diallo, A. K.; Talaga, D.; Penuelas, J.; Videlot-Ackermann, C.; Alloncle, A. P.; Delaporte, P.; Rodriguez, V.; Morphological and crystalline characterization of pulsed laser deposited pentacene thin films for organic transistor applications. Appl. Surf. Sci. 2017, 418, 446-451.


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Copper Phthalocyanine as Contact Layers for Pentacene Films Grown

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