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2 Surface Science 603 (2009) Contents lists available at ScienceDirect Surface Science journal homepage: a-sexithiophene on Cu(110) and Cu(110) (2 1)O: An STM and NEXAFS study M. Oehzelt a,b, *, S. Berkebile b, G. Koller b, J. Ivanco b,1, S. Surnev b, M.G. Ramsey b a Institute of Experimental Physics, Johannes-Kepler-University Linz, Altenbergerstraße 69, A-4040 Linz, Austria b Institute of Physics, Karl-Franzens-University Graz, Universitätsplatz 5, A-8010 Graz, Austria article info abstract Article history: Received 26 September 2008 Accepted for publication 4 December 2008 Available online 13 December 2008 Keywords: a-sexithiophene STM NEXAFS XPS The adsorption of a-sexithiophene (6T) on Cu(110), Cu(110) (2 1)O and the mesoscopically patterned Cu O striped surface have been studied by STM (scanning tunnelling microscopy), XPS (X-ray photoelectron spectroscopy) and NEXAFS (near edge X-ray absorption fine structure). The molecular resolution of the STM allowed to determine the orientation and local order of the molecules in the submonolayer and monolayer regime. It is shown that the 6T molecules align with their long molecular axis along the densely packed copper rows on Cu(110) and along the Cu O rows on the Cu(110) (2 1)O surface. On the striped phase with alternating copper and Cu O regions the molecules adsorb first on the Cu regions and after complete filling of these regions, on the Cu O. The orientation is the same on both areas as on the respective pristine surfaces with the only exception that the molecules reorient by 90 if the width of the copper regions is smaller than the molecular length. The NEXAFS measurements allowed for a determination of the adsorption geometry of the molecules: while 6T lies flat on the surface on clean copper, the molecular planes are inclined with an angle as high as 39 with respect to the substrate on (2 1)O. For the latter, this inclination angle is 4 higher than in the bulk crystal structure of 6T observed for thicker films to release stress and allow commensurability with the substrate lattice, while for the former it is a result of the aromatic system bonding to the Cu(110) surface, as confirmed by XPS. Ó 2008 Elsevier B.V. All rights reserved. 1. Introduction Organic semiconductors have attracted considerable interest in the last decades and as a result remarkable progress in the field of organic electronics has been made [1 5]. Especially their potential use in low-cost devices such as organic solar cells, light emitting diodes or thin film transistors have encouraged many investigations including basic research studies [6 10]. The majority of device-relevant properties depend crucially on the crystal quality of the organic films [11]. Moreover, the optoelectronic properties are highly anisotropic and depend on the orientation of the molecules within these films [12 14]. Therefore, it is highly desirable to obtain organic layers with high order and a well-defined orientation. The early stages of growth are particularly important for the control of the thin film properties which are influenced by the delicate interplay of molecule molecule interactions and molecule substrate interactions. Although, the interface of organic material to the metal contact is important, fundamental investigations of adsorption and growth are few [2,15]. * Corresponding author. Address: Institute of Experimental Physics, Johannes- Kepler-University Linz, Altenbergerstraße 69, A-4040 Linz, Austria. address: martin.oehzelt@jku.at (M. Oehzelt). 1 Present address: Institute of Physics, Technical University Chemnitz, Reichenhainer Straße 70, D Chemnitz, Germany. One prominent class of organic semiconductors is the family of oligothiophenes with a-sexithiophene (6T) being one of the most prominent transistor materials [16]. Recently surface-science studies focused on 6T deposited on Au(110) [17], Au(111) [18,19], Ag(100) [20], Ag(110) [21,22] and Cu(110) [23,24] have appeared along with a STM study focused on the initial growth of 5T on Cu(110) and Cu(110) (2 1)O [25]. While on the three- or fourfold symmetric surfaces a variety of different orientations and packings within the monolayer are observed, the two-fold symmetric (110) metal surfaces tend to align rod-like molecules uniaxially. This tendency seems to be quite general, but there is no general rule in which direction the long molecular axis is oriented. In many cases the rod-like molecules align along the densely packed atomic rows at the surface, which is the [1 10] direction. Typical examples are: 6T on Au(110) [17], pentacene [26,27], 5T on Cu(110) [25] and 6P on Cu(110) [28,29]. In the perpendicular direction [001] pentacene on Au(110) [30], 6T on Ag(110) [21,22] and on Cu(110) [23,24] have been reported. Oddly it has been reported that, the 6T molecules which consist of six thiophene rings apparently orients across the copper rows [23,24], while the 5T molecules (built of five thiophene rings) aligns along it [25]. These very unusual results drew our attention to this system. Another motivation to study this system are the previous results on 6P (para-hexaphenyl), a molecule consisting of six phenyl rings. On the pristine copper surface the molecules are aligned along the /$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi: /j.susc

3 M. Oehzelt et al. / Surface Science 603 (2009) copper rows in [1 10] direction, but already small traces of oxygen which form even in reasonable UHV conditions ( mbar) giving rise to an emerging striped phase (less than a few% of the surface) can reorient the molecules over large areas of the clean copper in [001] direction [28,31]. This reorientation upon small traces of oxygen can maybe explain the different results reported. Additionally, 6P is able when deposited on CuO to release stress in the first monolayer by tilting [28]. The question remains if a similar rod-like molecule like 6T has the same ability to release stress. Recently we have reported an X-ray diffraction (XRD) study on the bulk structure of 6T films grown on Cu(110), Cu(110) (2 1)O [32] and the striped Cu CuO surfaces [33]. In all these cases the bulk film consists mainly of crystallites with the 6T(010) net planes parallel to the substrate. For 6T on CuO this 6T(010) orientation is exclusively observed. Azimuthally these crystallites are oriented in such a way that all long molecular axes are oriented along the CuO rows which is the [001] copper crystal direction [32]. On clean copper the in-plane orientation of the molecules is found to be ±5 away from the Cu [1 10] direction, the direction of the close packed copper rows, perpendicular to the orientation observed in the CuO case, and in contrast to a previous study [23,24]. One of the motivations for this study is to answer the question of what happens if two perpendicular orientations are present on the substrate at the initial growth stage (in the monolayer regime). The investigation of the structure of thick films showed that for an around 1:1 Cu:CuO covered striped phase the CuO orientation wins. The dominating crystal orientation in this film has exactly the same orientation as is observed for films on a full reconstructed CuO surface. This result is somehow unexpected as the first molecules deposited on such a surface nucleate on the clean copper regions. Of course due to the inhomogeneity of the substrate template impurities of other orientations are also observed. In this paper, we want to present STM, XPS and NEXAFS data on the initial nucleation and subsequent growth of 6T on clean and oxygen reconstructed Cu(110) surfaces. 2. Experimental All experiments were performed in ultra high vacuum (UHV) with base pressures in the region of low mbar or better. The near edge X-ray absorption fine structure (NEXAFS) and X- ray photoelectron spectroscopy (XPS) measurements were performed using the Mustang end-station at the RG-BL (linear polarization of 0.95) of BESSY II. XPS spectra were collected using a SPECS 150 spherical electron analyzer with an overall resolution of around 200 mev, as measured from the copper crystal Fermi edge at RT. The NEXAFS measurements were performed with photon angles of incidence from normal (h = 0 ) to grazing (h = 80 ) with the polarization plane containing both the [001] and [1 10] substrate azimuths. All chambers have basic surface cleaning facilities which are used to clean the substrates with repeated cycles of argon ion sputtering and annealing at 800 K. Along the Cu(110) surface two other surfaces were employed for the growth studies: (a) dosing of 10 L [1 Langmuir (L) = Torr s] of oxygen on the clean Cu(110) crystal forms a p(2 1) added-row reconstruction which chemically polishes the surface [34]. The resulting terraces are up to hundreds of nanometers wide and separated by monoatomic steps. In addition, oxygen exposure not only flattens out the substrate but also passivates it. (b) The striped Cu CuO phase was prepared by exposing the clean copper surface to 1.5 L of oxygen which generates a periodic structure of alternating regions of clean Cu and Cu O regions [34]. 6T molecules (Syncom, B.V.) were evaporated from a molecule evaporator, which, after thorough outgassing, allowed film growth at residual pressures below mbar. The substrate was held at room temperature during deposition and nominal growth rates of 1 Å/min were used, as monitored with a quartz microbalance. All STM measurements were done at room temperature in a home-build UHV chamber equipped with an Omicron micro-stm internally damped by a viton stack and externally damped by air-legs. 3. Results and discussion The surfaces used in this study were chosen because they are anisotropic with a surface corrugation to orient the molecules. The Cu(110) single crystal has a surface unit cell with dimensions of 2.55 Å 3.6 Å and close packed copper rows in [1 10] direction (Fig. 1a left part). The second surface is generated by adsorption of oxygen on a previously cleaned Cu(110) surface. The adsorbed oxygen reconstructs the surface to the so-called (2 1)O addedrow structure with a surface unit cell of 3.6 Å 5.1 Å and copper oxygen rows in [001] direction (Fig. 1a right part). The third surface is created by adsorbing less oxygen than is necessary to fully reconstruct the Cu(110) surface which generates a regular array of clean copper regions and (2 1)O regions with a periodicity of about 90 Å [34]. In Fig. 2a submonolayer coverage and in Fig. 2b the monolayer coverage of 6T deposited on Cu(110) are displayed. At room temperature imaging the clean copper substrate and the surface with low 6T coverage is difficult. Clearly in both cases 6T molecules adsorbed on the Cu(110) surface are aligned in the [1 10] direction. Despite the uniaxial orientation there is no ordered overlayer structure. The next-neighbor distance in [1 10] direction is around 26 ± 1 Å, the spacing in [001] direction is around 8 9 Å. Even in the densest monolayer the lateral spacing of the molecules (8 9 Å) is considerably greater than the molecules van der Waals width of 6.8 Å. The molecules are dispersed on the surface and seem to repel rather than attract each other. A similar behavior has also been observed for para-sexiphenyl (6P) where the molecular electronic structure was changed by the substrate giving rise to a substrate mediated repulsion [28,31]. Interestingly, unlike 6P, the adsorption of 6T is strong enough to limit diffusion already at room temperature, allowing individual molecules to be observed at submonolayer coverages (6P molecules, in contrast, appear as streaks much longer than a molecule when not densely packed). On the Cu(110) (2 1)O surface submonolayer coverages could not be imaged presumably due to the high mobility of Fig. 1. The Cu(110) surface is depicted by the white balls in (a). In the right part of (a) the (2 1)O added-row structure is shown by the grey balls (Cu in the next higher layer) and oxygen shown as the smaller black balls. Note that the close packed rows on clean copper are in [1 10] direction, while the copper oxygen rows are oriented along [001], as indicated by the double arrows. The molecular structure and the van der Waals dimensions of the a-sexithiophene molecule are given in (b).

4 414 M. Oehzelt et al. / Surface Science 603 (2009) Fig. 2. STM images of 6T adsorbed on Cu(110) with submonolayer coverage (a) and monolayer coverage (b) (azimuthal directions of the substrate are indicated by the arrows). Tunnelling parameters: bias voltage V t = +1.0 V, tunnelling current I t = 1nA in (a) and V t = +1.5V, I t = 1nA in (b). the molecules on this substrate. Higher 6T exposure on the (2 1)O reveals a molecular layer with a higher order than on the clean copper. Again the molecules are uniaxially aligned, but this time in [001] direction, which is along the CuO rows (see also Fig. 1a). The completed monolayer is shown in Fig. 3. The presence of the molecules is seen as stripes parallel to [1 10] with a spacing of approximately the molecular length. Only high magnification with somewhat counter intuitive tunnelling conditions that retract the STM tip further away from the substrate allows to resolve the individual molecules as seen in Fig. 3. Fourier transforming this image reveals the characteristic intermolecular distances and the high degree of order. In [1 10] direction, the molecules are commensurate with the substrate corrugation and have a next-neighbor distance of 5.1 ± 0.2 Å spacing. In the direction of the long molecular axis, that is the [001] direction, the distance is 27.0 ± 0.5 Å. The molecules are organized in stacks along their short axis and in this direction their distance is well-defined and given by the substrate corrugation. In contrast, the order between these stacks is less defined. This effect is also reproduced by the Fourier transformed image: the stacking of the molecules produces a sharp point, while the intensity of the perpendicular direction is smeared out and seen as streaks. This lack of order can be associated with Fig. 3. STM image of the monolayer coverage of 6T on Cu(110) (2 1)O (substrate directions are indicated by the arrows). Tunnelling parameter: bias voltage V t = +2.0 V, tunnelling current I t = 1nA. The insert is the Fourier transformation of the STM image and reveals the characteristic distances between the molecules. the mobility of the molecules perpendicular to the primary surface corrugation. On the striped Cu CuO surface the 6T molecules adsorb initially on the clean copper regions as can be seen in the submonolayer coverage image of Fig. 4a. Again the long molecular axis is aligned along the [1 10] direction whenever the width of the copper regions is wide enough to allow it, i.e. they are oriented in the same direction as on the pristine copper surface. However, when the copper stripe width is smaller than the molecular length, the orientation is changed to [001]. This behavior can be clearly seen in the upper right corner of Fig. 4a. In general, the molecules are dispersed all over the copper regions, but mostly in clusters consisting of at least two molecules with the molecules side-by-side. Interestingly, the molecules are not strictly aligned, but there is a spread of a couple of degrees around the principal surface direction [1 10]. This small deviation in the azimuthal alignment has also been seen in the XRD results of thick films grown on Cu(110) [33]. Additional molecules deposited on this striped surface first fill up the copper regions after which they start to adsorb on the CuO regions. This behavior is strongly indicative of a higher adsorption energy on copper than on CuO. The completed monolayer is shown in Fig. 4b, where the presence of molecules on the CuO regions are indicated by the contrast stripes with a periodicity of the approximate molecular length similar to that in Fig. 3. For submonolayer coverages on the Cu CuO striped surface, the movement of 6T molecules or dimers can be followed in subsequent STM images at room temperature. In Fig. 5, five consecutive images of a strip of copper with eight 6T molecules on it is shown (the region is indicated by the dashed box in Fig. 4a. As indicated by the arrow the dimer pair is seen to move on the copper between the (2 1)O regions. The movement on the copper regions is exactly along the [1 10] surface direction, which is along the direction of close packed copper rows (see Fig. 5). Whether this displacement is thermal diffusion or caused by the interaction with the STM tip can not be determined, although it should be noted that the scan direction is approximately 45 to the direction of movement. The molecules that are moving are indicated by circles in Fig. 4a. Over the large area of Fig. 4a only a small fraction of approximately 5% of the molecules are changing their positions. Nevertheless it is clear that the diffusion barrier along the close packed copper rows is smaller than across them, as all displacements of the molecules were seen strictly along the [1 10] direction. It is interesting that it is mainly dimers that are seen to move, which suggests that the dimer units may be important in film growth and may also be related to the various 6T monolayer structures on Ag(001) that seem to be made of dimer pairs [20].

5 M. Oehzelt et al. / Surface Science 603 (2009) Fig. 4. STM images of 6T adsorbed on a Cu CuO striped surface. A submonolayer coverage is shown in (a). The completed monolayer is shown in (b). The ellipses in (a) indicate molecules that are seen to move and the dotted rectangle marks the spot of the images detailed in Fig. 5. Tunnelling parameters: bias voltage V t = +1.0 V, tunnelling current I t = 1nA in (a) and (b). Fig. 5. Displacement of molecules on the copper regions of the Cu CuO striped surface. The surface region where these images are taken are indicated by the dotted rectangle in Fig. 4a. The 6T-dimer is displaced along the [1 10] surface direction, the direction of the close packed copper rows. The STM tip scanning direction is diagonal and indicated by the double arrow. The differences in the bonding in the monolayers on Cu and Cu O are clearly expressed in both the C1s XPS and spectral details of the C1s absorption edge, as seen in Fig. 6 and 7. For the thick 6T films, the C1s appears as two emissions separated by 0.6eV (Fig. 6a and 7a). The absolute energies of these emissions are different on the two substrates due to the different interfacial dipoles for each case. The higher binding energy peak can be associated with the linking (a) carbons, while the lower binding energy feature arises from the b and end carbons in analogy to bithiophene [35,36]. The monolayer C1s on Cu O has the same spectral appearance as the multilayer (Fig. 7a), and no significant effects of bonding to the substrate can be discerned. Of note is that there is no evidence for changes in the b carbon emissions that might have been expected for this tilted trans molecule with half of the b carbons in close proximity to the substrate and half away from it. The XPS results on clean Cu are in stark contrast, as seen in Fig. 6a. Here the double peak of the multilayer collapses to a single peak in the monolayer with a distinct asymmetric tail to higher binding energy associated with electron-hole pair creation in the presence of the metal substrate [35]. Clearly the carbon atoms of the monolayer molecule moiety all have a similar environment consistent with the molecules being bonded flat on the Cu substrate. This collapse is also seen in the NEXAFS spectra of the monolayer (Fig. 6c) with the multiplicity of features in the C1s? p * transition observed in the multilayer (Fig. 6b, an assignment of this structure can be found in [37]) reducing to a single broad peak. Clearly, on Cu, the molecule substrate interactions are dominant while on Cu O it is molecule molecule interactions. However, even for the latter, commensurability with the substrate is a factor, as will be discussed in the following. Additionally angular dependent NEXAFS was used to determine the area averaged orientation of the molecules, complementary to the STM measurements that are inherently local. Fig. 6c shows the spectra for a monolayer thick 6T film grown on the clean Cu(110) surface. The strongest p * intensity for grazing incidence (h =80 ) for both azimuths ([001] and [1 10]) and a vanishing p * intensity for normal incidence (h =0 ) again for both perpendicular azimuths clearly indicates that the monolayer molecules have their aromatic planes parallel to the surface. A closer analysis following the methods of Ref. [38] of the p * transition intensity angular behavior reveals an aromatic plane tilt angle a of 0 ± 10, as shown in the inset of Fig. 6c. In such a case, the orientation of the long molecular axis can not be determined from the NEXAFS measurements alone, but the STM measurements have shown with no doubt that the molecules are oriented along the [1 10] azimuth. The vanishing p * intensity clearly rules out that domains of molecules which have an significant tilt angle with respect to the substrate are present on the surface. These results are in agreement with the STM images and confirm that the areas shown are representative for the whole surface. For comparison, the NEXAFS of a multilayer film (14 nm) have been included in Fig. 6b. Here, the average tilt angle is determined to be a =42±2 (see inset). The near absence of p * intensity only for polarization parallel to [1 10] show this to be the orientation of the long molecular axis. In summary, the 6T molecules are flat lying in the monolayer and oriented in [1 10] direction on the clean Cu(110) surface in both the monolayer and the multilayer. The measurements on the Cu(110) (2 1)O surface reveal a different picture (Fig. 7b and c). Here, a difference in the angular behavior of the NEXAFS between the two perpendicular azimuths is clearly present in the monolayer (Fig. 7c). While the angular

6 416 M. Oehzelt et al. / Surface Science 603 (2009) Fig. 6. (a) Shows the C1s XPS spectra for a monolayer and multilayer (14 nm) of 6T on Cu(110). They have been normalized to the peak height and the energy is referenced to E F. NEXAFS spectra for the multilayer of 6T on Cu(110) and the monolayer of 6T on Cu(110) are shown below in (b) and (c), respectively. The NEXAFS spectra were measured for angles from normal incidence (h =0 ) to glancing incidence (h =80 ) in the two substrate azimuthal directions [1 10] and [001]. Circles in the insets of (b) and (c) show plots of the p * -orbital intensity (area beneath the thick line labeled p * ) as a function of the photon incidence angle (h). Lines corresponding to different aromatic plane tilt angles (a) are given with the data points. The inset of (b) shows lines corresponding to a =42 (black solid), a =40, and a =44 (both dotted). The inset of (c) shows lines corresponding to a =0 (black solid), a =10 and a =20 (both dotted). Fig. 7. (a) Shows the C1s XPS spectra for a monolayer and multilayer (30 nm) of 6T on Cu(110) (2 1)O. They have been normalized to the peak height and the energy is referenced to E F. NEXAFS spectra for the multilayer of 6T on Cu(110) (2 1)O and the monolayer of 6T on Cu(110) (2 1)O are shown below in (b) and (c), respectively. The NEXAFS spectra were measured for angles from normal incidence (h =0 ) to glancing incidence (h =70 or 80 ) in the two substrate azimuthal directions [1 10] and [001]. The insets of (b) and (c) show plots of the p * -orbital intensity (area beneath the thick line labeled p * ) as a function of the photon incidence angle (h). Lines corresponding to different aromatic plane tilt angles (a) are given with the data points. The inset of (b) shows lines corresponding to a =41 (black solid), a =38, and a =44 (both dotted). The inset of (c) shows lines corresponding to a =40 (black solid), a =35 and a =45 (both dotted).

7 M. Oehzelt et al. / Surface Science 603 (2009) effects are strong in [001] direction, they are more or less negligible in the [1 10] direction. The absence of p * intensity for polarization parallel to [001] shows this to be the orientation of the long molecular axis. The small variation in the p * intensity along the [1 10] azimuth determines the molecular inclination angle to be around the so-called magic angle of a =45. The experimental values of the C1s? p * transition intensities (see inset) confirm the already qualitatively obtained result that the molecular plane is tilted by about 45 ± 3 with respect to the substrate surface. NEXA- FS measurements have also been done for an approximately 30 nm thick 6T film and are displayed in Fig. 7b. The orientation of the long molecular axis remains unchanged compared to the monolayer, whereas the gradual p * intensity increase of subsequent NEXAFS spectra with increasing incidence angle along the [1 10] azimuth shows that the average molecular tilt angle is no longer at the magic angle. The transition intensity plot (see inset) reveals that the molecular tilt angle has significantly decreased from the monolayer to a =41±3. To summarize the situation on CuO: in the monolayer the sideby-side distance of the molecules is commensurate with the substrate (=5.1 Å) and the tilt angle of the molecular planes is increased by 4 compared to the multilayer case. Thick films of 6T on CuO grow in the single crystal structure [39] with the 6T(020) planes parallel to the substrate [32]. Within this bulk plane the side-by-side distance of the molecules is 5.58 Å and the tilt angle of the molecular planes is 35. Although there is a systematic shift in the absolute value of the molecular tilt angle determined by the NEXAFS measurements, the decrease in the tilt angle from the monolayer to the thick film is significant and reproducible. This structural change can be understood in terms of a simple geometric model. While the 6T multilayer exists in the single crystal structure, the molecules at the surface are tilted by 4 about their long molecular axis compared to the multilayer phase to decrease the side-by-side distance on the surface from 5.58 Å to 5.1 Å. The only restriction for this rotation is that the distance of neighboring molecular planes remains constant (see Fig. 8) and therefore the van der Waals distance is not compromised. Interestingly, already this slight modification of the molecular packing can release a lattice mismatch as high as 8.6%. For a simple compression of the lattice, while keeping the molecular orientation constant, pressures in the GPa region would be required [40,41]. With the results presented here some previous results known from the literature can be understood. While the orientation of 6T presented here and 5T [25] agrees on the three observed surfaces, the interpretation of the adsorption geometry is different. On clean copper both materials are lying flat on the surface, but on (2 1)O we have clearly shown with NEXAFS that the molecules are inclined with respect to the surface (edge-on). Qualitatively the contrast of the STM images for both studies are similar and, therefore, we think that the previous interpretation that the molecular planes lie flat, based on STM measurements alone, is incorrect and was prematurely made. In addition, the inclination of the 5T molecules on CuO would solve the discrepancy in coverages without the need to assume a different sticking coefficient for the two surfaces, as the authors suggested. More material is clearly needed to complete a monolayer of tilted molecules than for flat lying molecules. Further the commensurability of the molecules in the 2D islands on CuO (5.1 Å) would compromise the van der Waals distances of neighboring molecules (6.8 Å) considerably if a flat lying orientation is assumed (see also Fig. 1b). All these indications lead to the conclusion that the molecules lie tilted on the copper oxygen surface. In previous NEXAFS studies of 6T deposited on Cu(110) the molecules were determined to be oriented in [001] direction [23,24]. In contrast, to these results our studies have determined that on clean copper the 6T molecules are oriented in [1 10] direction. In this study, the STM images on clean copper and even on the copper regions of the striped phase have shown this orientation. The molecules will align in [001] direction only if the majority of the copper surface is reconstructed with oxygen. It can be deduced from this study that in case of oxygen contamination an almost fully reconstructed surface is needed to change the orientation of the molecules from [1 10] to [001]. Possible explanations for the opposite orientation obtained in the previous studies are either a highly oxygen contaminated substrate surface or a highly stepped clean Cu surface. 4. Conclusions In summary we have shown in this study that uniaxially aligned 6T films were observed on Cu(110) and Cu(110) (2 1)O. The molecules orient in such a way that their long axis follows the densely packed rows at the surface. On the striped Cu CuO surface the molecules maintain the orientation of the respective pristine surfaces. Only if the width of the copper regions is smaller than the molecular length, the long axis changes its orientation to the one of CuO. In addition, to the STM measurements which allowed the determination of the molecular orientations, NEXAFS measurements determined the adsorption geometry of the molecules. Here we found that the molecules are flat lying on the copper surface, while they have a significant tilt on the (2 1)O surface. Comparing the tilt angle of the monolayer to that of the multilayer showed that the tilt in the monolayer is 4 higher than in the multilayer case. This effect could be explained by adopting a simple geometric model illustrating a molecular stress release mechanism. Acknowledgements: Fig. 8. Models of the different 6T structures grown on Cu(110) (2 1)O in the monolayer and the multilayer. The dotted line in the multilayer model shows the orientation of the monolayer molecules. Compared to the monolayer, the molecules are rotated by 4 while preserving the distance d of their molecular planes. For the monolayer model also the substrate is shown. Open circles show copper atoms while the grey circles represent the CuO rows of the surface reconstruction. This work has been supported by the Austrian Science Fund (FWF). We like to acknowledge the assistance of G.-N. Gavrila with the MUSTANG end-station at RG-BL of BESSY II. References [1] J. Roncali, P. Leriche, A. Cravino, Adv. Mater. 19 (2007) [2] N. Koch, Chem. Phys. Chem. 8 (2007) [3] J.E. Anthony, Chem. Rev. 106 (2006) 5028.

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