pubs.acs.org/Langmuir © 2009 American Chemical Society
Line-on-Line Organic-Organic Heteroepitaxy of Quaterrylene on Hexa-peri-hexabenzocoronene on Au(111) Daniel Kasemann,† Christian Wagner,† Roman Forker,† Thomas Dienel,† Klaus M€ullen,‡ and Torsten Fritz*,† †
Institut f€ ur Angewandte Photophysik, Technische Universit€ at Dresden, 01062 Dresden, Germany, and ‡ Max-Planck-Institut f€ ur Polymerforschung, Ackermannweg 10, 55128 Mainz, Germany Received May 18, 2009
In a recent paper, we discussed the optical properties of a heterostructure consisting of a highly ordered monolayer of quaterrylene (QT), electronically decoupled from the gold substrate by a predeposited epitaxial monolayer of hexa-perihexabenzocoronene (HBC). Here we now present the detailed structural investigation of this organic double-layer system. We show that the structure of the heterosystem can be identified as line-on-line coincidence (lol), a new type of epitaxy discovered by us previously for the system 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) on HBC on highly oriented pyrolytic graphite (HOPG). Additionally, we provide evidence on the basis of advanced potential energy calculations that indeed energetic gain drives this lol growth mode.
Introduction The past several years have witnessed a huge interest in organic thin films, for both single-molecule electronics and, more generally, organic-based devices consisting of several layers.1-3 The investigation of highly ordered layers grown by organic molecular beam epitaxy (OMBE) represents a rewarding approach to improving our understanding of the physics at organic-organic and organic-inorganic interfaces. OMBE not only provides welldefined and reproducible layer systems but also allows the preparation of atomically thin organic films so that the processes occurring directly at the interface(s) can be studied in detail.4-7 Herein miniaturization refers to not only smaller lateral structures but also the third dimension. In other words, future devices will employ thinner and thinner layers, therefore being more and more governed by interface effects. Moreover, despite the fact that all state-of-the-art (opto-) electronic devices contain inorganic quantum wells (or at least epitaxial heterostructures), only little is known about true organic-organic heteroepitaxy, comprising subsequently grown epitaxial layers of two different organic species on a single crystalline substrate. Here we use the term “true” in the sense that there is a mathematically well-defined relation between the lattices of the two organic materials deposited on top of each other, as opposed to a mere subsequent deposition of two organic species without such a relationship. Indeed, the overwhelming part of the literature on highly ordered organic films is exclusively concerned with the growth of a single molecular species on an inorganic substrate; the structural aspects of organic-organic interfaces on the molecular level have only *To whom correspondence should be addressed. E-mail: torsten.fritz@iapp. de. (1) Forrest, S. R. Nature 2004, 428, 911–918. (2) Walzer, K.; Maennig, B.; Pfeiffer, M.; Leo, K. Chem. Rev. 2007, 107, 1233– 1271. (3) Koch, N. ChemPhysChem 2007, 8, 1438–1455. (4) Forrest, S. R. Chem. Rev. 1997, 97, 1793–1896. (5) Karl, N.; G€unther, C. Cryst. Res. Technol. 1999, 34, 243–254. (6) Hooks, D. E.; Fritz, T.; Ward, M. D. Adv. Mater. 2001, 13, 227–241. (7) Schreiber, F. Phys. Status Solidi A 2004, 201, 1037–1054. (8) Mannsfeld, S. C. B.; Leo, K.; Fritz, T. Phys. Rev. Lett. 2005, 94, 056104. (9) Mannsfeld, S. C. B.; Fritz, T. Mod. Phys. Lett. B 2006, 20, 585–605.
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recently attracted substantial interest.8-19 Despite the still increasing number of papers, reports on true epitaxy are rather scarce.8,9 In the majority of publications, a possible epitaxial relation is either not discussed at all or denoted as “incommensurate”, as no common type of epitaxy (i.e., commensurism or point-on-line coincidence6) could be identified,10-17 yet other authors exclusively identify kinetic growth as the driving force behind the observed mutual alignment.18,19 Only recently, a new type of epitaxy [“line-on-line” epitaxy (lol)] has been discovered which explains the molecular arrangement in organic heteroepitaxial systems encompassing flat aromatic hydrocarbons.8,9 Here we present the structural investigation of a new organic doublelayer system consisting of a highly ordered monolayer of quaterrylene (QT) on an epitaxially grown monolayer of hexa-perihexabenzocoronene (HBC) on a Au(111) surface. In contrast to the studies mentioned above, we discuss an example of true epitaxial alignment between two different organic monolayers grown subsequently on Au(111). We will also provide evidence that the growth is governed by energetic gain. In a recent paper, we discussed the optical properties of such a heterostructure.20 While the respective structures of single HBC (10) Chen, W.; Huang, H.; Chen, S.; Chen, L.; Zhang, H. L.; Gao, X. Y.; Wee, A. T. S. Appl. Phys. Lett. 2007, 91, 114102. (11) Chen, W.; Huang, H.; Chen, S.; Gao, X. Y.; Wee, A. T. S. J. Phys. Chem. C 2008, 112, 5036–5042. (12) Barrena, E.; De Oteyza, D. G.; Sellner, S.; Dosch, H.; Osso, J. O.; Struth, B. Phys. Rev. Lett. 2006, 97, 076102. (13) De Oteyza, D. G.; Barrena, E.; Osso, J. O.; Sellner, S.; Dosch, H. Chem. Mater. 2006, 18, 4212–4214. (14) De Oteyza, D. G.; Barrena, E.; Sellner, S.; Osso, J. O.; Dosch, H. Surf. Sci. 2007, 601, 4117–4121. (15) Oehzelt, M.; Koller, G.; Ivanco, J.; Berkebile, S.; Haber, T.; Resel, R.; Netzer, F. P.; Ramsey, M. G. Adv. Mater. 2006, 18, 2466–2470. (16) Koller, G.; Berkebile, S.; Krenn, J. R.; Netzer, F. P.; Oehzelt, M.; Haber, T.; Resel, R.; Ramsey, M. G. Nano Lett. 2006, 6, 1207–1212. (17) Piot, L.; Marchenko, A.; Wu, J.; M€ullen, K.; Fichou, D. J. Am. Chem. Soc. 2005, 127, 16245–16250. (18) Campione, M.; Raimondo, L.; Sassella, A. J. Phys. Chem. C 2007, 111, 19009–19014. (19) Yang, J.; Wang, T.; Wang, H.; Zhu, F.; Li, G.; Yan, D. J. Phys. Chem. B 2008, 112, 3132–3137. (20) Forker, R.; Kasemann, D.; Dienel, T.; Wagner, C.; Franke, R.; M€ullen, K.; Fritz, T. Adv. Mater. 2008, 20, 4450–4454.
Published on Web 09/25/2009
DOI: 10.1021/la901760j
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and QT monolayers on Au(111) have already been reported,21,22 we use here the results of a comprehensive refined analysis of the system HBC on Au(111). The structure of the organic-organic heterosystem [QT on HBC on Au(111)] was imaged with molecular resolution by room-temperature scanning tunneling microscopy (STM), and the lattice constants were further refined by means of low-energy electron diffraction (LEED) analysis. We find that the QT molecules grow indeed lol on the ordered HBC layer on Au(111). This is, to the best of our knowledge, only the second documented occurrence of lol epitaxy. In the case of organic-inorganic epitaxy, the arrangement of the molecular layer with respect to the inorganic substrate can be generally classified according to different categories.6,8,9 These epitaxy modes can be described and identified solely by the epitaxial matrix, which provides the transformation between the substrate and the adsorbate lattice vectors. For commensurate growth [“point-on-point” (pop)], the epitaxial matrix has to consist exclusively of integer values. For “point-on-line” (pol) epitaxy, at least one column of integer values is required.23 Consequently, every adsorbate lattice point lies on primitive substrate lattice lines, i.e., Æ10æ lines. For the case of organic-organic epitaxy, however, we recently reported a new epitaxial class, the lol coincidence, discovered for the system PTCDA on HBC on highly oriented pyrolytic graphite (HOPG).8,9 It is important to note that this type of epitaxy cannot be readily identified by the epitaxial matrix as in this case typically no integer numbers are contained at all. To determine whether the overlayer is arranged according to lol epitaxy, an analysis of the reciprocal lattice vectors is required instead. For lol, one arbitrary reciprocal substrate lattice vector has to end on a reciprocal adsorbate lattice point. In real-space, lol epitaxy can be visualized by the coincidence of two higher indexed lattice lines (hence the name line-on-line).
Experimental Section Both sample preparation and characterization were completely conducted under ultra-high-vacuum (UHV) conditions. The gold single crystal was cleaned by several cycles of Ar+ sputtering (600 eV) and annealing (600 C). After verifying its surface quality by STM and LEED, we thermally deposited the organic molecules from Knudsen cells. At a base pressure of 3 10-10 mbar, the sublimation temperatures are 350 C for QT and 430 C for HBC. The deposition rate for the HBC layer was ∼0.3 monolayer/min. One HBC monolayer corresponds to a nominal thickness of ∼0.34 nm. The QT deposition rate had to be ∼0.1 monolayer/ min to achieve ordered monolayers. During evaporation, the samples were held at room temperature. HBC was purified by two cycles of gradient sublimation. QT was purchased from the Institut f€ ur PAH Forschung (Greifenberg, Germany) and used as provided. Both species were thoroughly degassed in situ for several hours prior to deposition to remove the remaining contaminants. By carefully adjusting the evaporation parameters and verifying the layer structure by STM, we determined the evaporation time needed for one closed monolayer. The molecular layers shown in the STM images have not been annealed. The HBC single layer was slightly heated prior to LEED analysis and QT deposition (to 80 C). (21) Sellam, F.; Schmitz-H€ubsch, T.; Toerker, M.; Mannsfeld, S.; Proehl, H.; Fritz, T.; Leo, K.; Simpson, C.; M€ullen, K. Surf. Sci. 2001, 478, 113–121. (22) Franke, R.; Franke, S.; Wagner, C.; Dienel, T.; Fritz, T.; Mannsfeld, S. C. B. Appl. Phys. Lett. 2006, 88, 161907. (23) Alternatively, for a hexagonal substrate, integer values for both sums over the rows of the epitaxial matrix also fulfill the pol condition, provided that the hexagonal substrate is described by a lattice angle of 60. If a 120 angle is chosen between the primary substrate vectors, both differences of the rows of the epitaxial matrix will be integers.
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Figure 1. STM image of an ordered monolayer of HBC on Au(111) (50 nm 50 nm, 1 V, 50 pA). This image shows a domain with a domain angle δ of 1.7. The skeletal formula of HBC is shown in the top left corner. The inset shows the LEED pattern of a monolayer of HBC on Au(111) (Ekin = 12.0 eV). The unit cells of the two domains are sketched (solid blue, 0 structure; dashed-dotted green, 30 structure). The red circles highlight the (1, 1)0 and (2, 0)30 spots and vice versa. These pairs of LEED spots clearly show different spacings, thus indicating indeed different HBC structures. The STM investigations were conducted using a commercial Omicron UHV STM-1 instrument operating at room temperature. The STM images have been drift corrected, but no additional filtering was applied. The LEED experiments were performed using a four-grid Omicron UHV device (SPECTA-LEED). The patterns were taken at low emission current to prevent damage to the organic layer. The images were electronically recorded with a CCD camera connected to a frame grabber card. To improve image quality, a series of 25 images was averaged. To eliminate optical distortion, the images were corrected by a reference function derived from the well-known Si(77) surface reconstruction. The brightness of the LEED patterns presented here is inverted. The electron beam voltage was corrected by the method proposed by G€ unther.24 In addition to the LEED images presented here, series of patterns in the range of 7-60 eV have been recorded to achieve more precise lattice vectors by including higher-order LEED spots. The LEED analysis was conducted by geometrical and kinematic LEED theory with the tool LEEDsim (sim4tec GmbH).25
Results and Discussion HBC molecules grow flat-lying and in an almost hexagonal arrangement on the Au(111) substrate.21 They form large highly ordered domains at room temperature, even without annealing. Figure 1 shows an STM image with molecular resolution of one monolayer of HBC on the reconstructed Au(111) surface. The nearly hexagonal arrangement of the molecules can nicely be inferred. These layers have been explored by LEED to derive the lattice vectors. The inset in Figure 1 shows a typical LEED image (24) G€unther, C. Organische Molekularstrahlepitaxie: Ordnungsprinzipien grosser Aromaten auf Schichthalbleitern; Logos Verlag: Berlin, 1998. (25) LEEDsim, version 1.38. www.sim4tec.com.
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Article Table 1. Structural Data for the Heterostructure QT on HBC on Au(111) Derived from LEED
domain
domain angle δa (deg)
lattice parameters a = 14.2 ( 0.1 A˚ b = 14.0 ( 0.1 A˚ γ = 61.8 ( 0.5 a = b = 14.3 ( 0.1 A˚ γ = 58.0 ( 0.5
0 HBC
1.7 ( 0.5
30 HBC
29 ( 0.5
QT on 0 HBC
-47 ( 1
a = 9.4 ( 0.2 A˚, b = 20.9 ( 0.4 A˚ γ = 77 ( 1
QT on Au(111)c
26 ( 1
a = 8.9 ( 0.3 A˚, b = 19.6 ( 0.6 A˚ γ = 78 ( 1
34 ( 1 86 ( 1
epitaxy matrixb ! 3 2:7 -3 5:8 ! 5 0 0 5 ! -0:71 0:56 0:19 1:39 ! 1:92 3 -7:79 6 ! -0:33 3:33 -8:11 5:11 ! -3 3:37 -6 -1:44
a
The domain angles of 0 HBC, 30 HBC, and QT on Au(111) are given vs the Au[112] direction; that of QT on HBC on Au(111) is given vs HBC[10]. The epitaxial matrices are given with respect to the Au(111) substrate for the HBC layers and in reference to the 0 HBC structure for the QT layer. c From ref 22 for the sake of comparison. b
of one ordered monolayer of HBC on Au(111). The LEED pattern can be fully explained by two very similar yet unequal HBC structures and their rotational equivalents. As the reconstructed gold surface does not exhibit a perfect hexagonal arrangement, the different rotational domains lead to a splitting of the LEED spots clearly observable in the LEED patterns. The two HBC structures will be labeled by their domain angle δ (angle of [10]HBC with respect to [112]Au): the one already published in ref 21 as “0 structure” (blue solid lines in Figure 1) and a second one, rotated by ca. 30 (green dashed-dotted lines in Figure 1), henceforward dubbed the “30 structure”. While the measured domain angle for the first structure is not precisely 0 but 1.7 ( 0.5, this structure will still be denoted 0 structure for the sake of simplicity. The obtained lattice parameters are summarized in Table 1, together with the corresponding epitaxial matrices. Even though the given absolute uncertainties for both HBC structures overlap, we are dealing with two different sets of lattice constants clearly shown by the simultaneously imaged higher-order LEED spots in Figure 1. The red circles highlight the (1, 1)0 and (2, 0)30 spots and vice versa. These pairs of LEED spots show different spacings, thus indicating different HBC structures. Moreover, as so-called on-axis growth occurs (domain angle ≈ 0), the 6-fold symmetry of the surface can lead to only six rotationally equivalent domains, not 12. Since the epitaxial matrix of the 30 structure consists of integer values only, it can be identified as commensurate. For the 0 structure, the matrix exhibits only one column of integer values; thus, this structure grows in a pol mode on the gold surface. Next, this densely packed HBC monolayer on Au(111) was utilized as a substrate, and one monolayer of QT molecules was deposited on top. Figure 2a shows STM images of the doublelayer system QT on HBC on Au(111). One can immediately identify the QT molecules grown densely packed in molecular rows similar to the structure known from the first QT monolayer on Au(111).22 The underlying HBC molecules are not evident at first glance. To identify the periodic structures contributing to the image, one can employ fast Fourier transformation (FFT) (Figure 2b). FFT reveals contributions of the ordered QT molecules (red dashed), and an additional hexagonal structure (blue solid). As the position of our gold single crystal in the STM is the same for every measurement, we know the orientation of the underlying gold substrate, even if it is not visible in this STM image. With the [112]Au direction (sketched in the FFT in Langmuir 2009, 25(21), 12569–12573
Figure 2b), we can identify the hexagonal pattern as underlying HBC molecules arranged in the 0 structure. This substantiates the fact that the STM image indeed shows an epitaxial doublelayer of highly ordered QT molecules on HBC on Au(111). One may wonder why FFT is needed to identify the two different lattices in Figure 2a, expecting to see a contrast modulation in the real-space image as well. Indeed, the contrast modulation exists, but it is too weak to be seen by the naked eye, especially due to the noise present in the picture. As a matter of fact, STM images of organic-organic heterosystems at room temperature often appear more blurred than those of a single molecular layer on, e.g., Au, which is mainly for two reasons. First, the modulation of the electronic landscape of the underlying substrate (a dissimilar molecular layer vs a metal surface) has the same length scale as the layer to be imaged. This leads to a variation in the tunneling current from molecule to molecule, as the heterosystems are not commensurate, and the two different molecules are not perfectly on top of each other. Second, the binding forces between the two molecular layers in the heterosystem are much weaker than to a metal substrate, giving rise to a non-negligible mobility of the molecules in the uppermost layer. To accurately determine the lattice vectors of the organic-organic heterosystem, a LEED analysis was performed as well. Figure 2c gives a typical LEED pattern of this doublelayer system. It exhibits contributions of the two hexagonal HBC structures already seen in Figure 1. The unit cell of the 0 HBC structure is sketched in solid blue; that for the 30 structure has been omitted for the sake of clarity. The remaining LEED spots can be fully explained by only one QT structure (red dashed) and its equivalent rotational domains. We would like to stress the point that the presence of the two different structures, i.e., of the HBC and the QT lattice, exclusively in LEED patterns would not undoubtedly allow to identify the structure grown as heterostructure, as the two lattices could be present in adjacent sample areas. However, as the FFT analysis discussed above is performed on STM images of such double-layer systems, the heteroepitaxial growth is unambiguously evidenced. The lattice parameters derived for the heterostructure are summarized in Table 1. The HBC domains turn out to have exactly the same structure as the bare HBC layers on gold. The 30 domain contributes only with a low intensity. This leads to the conclusion that only a small part of the investigated region is covered by the 30 structure. DOI: 10.1021/la901760j
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Figure 3. (a) Reciprocal lattice vectors for the QT unit cell (red dashed) and the 0 HBC lattice (blue solid). The [12]* HBC vector ends on the [21]* QT lattice point. (b) In real-space, this transfers to coincidence of the [12] HBC lattice line with the [21] QT lattice line (indicated by the green dashed-dotted line). (c) Model of the mutual alignment in the QT-HBC heterolayer. The coincident lattice lines are parallel to the x-axis. (d) Inverse FFTs of the two separate lattices depicted in Figure 2b rotated clockwise by an angle of 14, evidencing that there is indeed one set of equally spaced lines on which all molecular centers (QT and HBC) are situated.
Figure 2. (a) Two different STM images of an ordered monolayer of QT (the skeletal formula of QT is sketched in the top left corner) on a monolayer of HBC on Au(111) (66 nm 33 nm, 1.0 V, 75 pA; 48 nm 24 nm, 1.0 V, 55 pA). It clearly shows the molecular rows of QT similar to those in ref 22. (b) FFT of the STM images (depicted here for the lower STM image in panel a) shows contributions of both the QT molecular lattice (red dashed) and the hexagonal HBC lattice (blue solid) to the STM image. This proves that the STM image indeed shows a double-layer structure. (c) LEED pattern of such a double-layer at an electron energy Ekin of 8.0 eV with the unit cells of QT and HBC (blue solid, 0 HBC phase; red dashed, QT). The spots between the blue HBC spots stem from the 30 HBC structure (compare to Figure 1).
As already stated above, the QT layer grows on the HBC layer in a motif similar to the one directly on Au(111).22 In other words, the packing of the molecules within the layer is similar on Au(111) and on HBC. However, the lattice vectors are slightly enlarged (by ∼6%), and the domain angles differ as 12572 DOI: 10.1021/la901760j
well. The angle between the [10]QT vector and the [112]Au direction is 49.0 ( 0.5 which is far from the domain angles for QT molecules directly on gold as given in ref 22. Thus, we conclude that the ordering of the QT layer is not merely driven by the gold substrate. Since we have learned from the FFT analysis that the QT structure grows on a 0 HBC domain, the epitaxial matrix given for the QT layer in Table 1 is given with respect to the 0 HBC structure. This matrix clearly shows that we have neither commensurate growth nor point-on-line epitaxy as defined by Hooks et al.6 To check for line-on-line epitaxy, we have to examine the reciprocal and real-space lattice vectors for the system “QT on 0 HBC” given in panels a and b of Figure 3. As we can see, in reciprocal space the [21]* QT vector ends on the [12]* 0 HBC lattice point. In real-space, this transfers to a coincidence of the [21] QT lattice line with the [12] HBC lattice line. In Figure 3b, these lines are sketched in green (dashed-dotted). The green circles represent the axis intercepts given by the Miller indices. Thus, we conclude that the QT layer is indeed arranged in a line-on-line manner on the 0 HBC layer. Having considered the structural information derived from the LEED image until now, we can further prove that the STM image shows exactly the same structure. As FFT displays the reciprocal space just like LEED does, one can analyze Figure 2b by means of a LEED simulation software.25 If one takes the HBC structure as built in scale to enhance the relative accuracy, it is obvious that the QT structure obtained from the LEED image nicely fits the FFT spots (Figure 2b shows the FFT with an overlaid LEED simulation). Langmuir 2009, 25(21), 12569–12573
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Figure 4. Potential energy calculation with a QT domain of 9000 undistorted QT molecules scanned over a free-standing HBC film. The x-axis is parallel to the coincident [21] QT and [12] HBC lattice lines (black dashed-dotted lines in Figure 3c). The energetic landscape shows a clearly stripy behavior, indicating that the resulting energetic gain of 0.14 kcal/mol is independent of the x-position of the QT domain. This nicely illustrates the nature of the observed lol epitaxy.
To further illustrate the nature of lol epitaxy, a real-space model of the growth of QT on HBC is given in Figure 3c. The lattices in panels c and d of Figure 3 are arranged such that the coincident [21] QT and [12] HBC lattice lines (black dashed-dotted lines) are parallel to the x-axis. The precise orientation of the molecules in the model was first roughly derived from STM images (Figure 2a) and then refined by force field calculations further described below. Hence, the observed lol structure is characterized by an almost identical azimuthal orientation of the carbon rings in QT and HBC. The relative position of the QT molecules with respect to the HBC lattice is, however, mostly arbitrary, and Figure 3c shows only one possible cutout (note that there is no such thing as a supercell or simple periodicity in lol epitaxy): the position of the QT domain on the x-axis (i.e., on the coincident lattice lines) is irrelevant as in this direction the QT and HBC lattices are incommensurate. This is confirmed in a potential energy (PE) calculation at the force field level (Figure 4), where the position of a QT domain of 9000 undistorted QT molecules was scanned over a free-standing HBC film.8,9,26 The obtained result shows an energetic gain of 0.14 kcal/mol per QT molecule, independent of the x-position of the QT domain.27 While the PE is independent of the x-position, a distinct dependency on the y-position of the QT domain is found. The energetic minimum is obtained for a relative molecular orienta(26) PowerGrid, version 1.22. www.sim4tec.com. (27) The energy is given with respect to the incommensurate energy that has been calculated according to the procedure described in refs 8 and 9.
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tion between the QT overlayer on the HBC layer as given in Figure 3c. These results of the PE calculations are completely confirmed by a detailed analysis of a STM image of the organic-organic heterolayer. An inverse FFT of the respective spots in Figure 2b leads to two real-space images containing solely either the QT or the HBC molecules. Via a comparison of cutouts of each image taken at identical positions (Figure 3d), it is evident that there is indeed one set of equally spaced lines on which all molecular centers (QT and HBC) are situated. The lol epitaxy is thus characterized by a one-dimensional pattern in the potential energy of the QT-HBC interface. It is worth noting that both HBC structures on Au(111) as well as the QT structure on HBC reported here differ noticeably from their respective known bulk phases. Both HBC and QT bulk structures are characterized by three-dimensional herringbone patterns; i.e., there are no crystal planes containing flat-lying HBC or QT molecules.28,29
Conclusions In conclusion, we demonstrated the feasibility of growing wellordered, truly heteroepitaxial layers with molecularly flat interfaces composed of structurally dissimilar organic molecules, namely, QT and HBC. By means of LEED analysis as well as FFT of STM images, the structure of the heterosystem was identified as line-on-line epitaxy, thus proving that the growth of QT is mainly governed by the interaction with the HBC layer rather than the Au(111) substrate. It is shown by advanced potential energy calculations of realistically large QT domains on HBC that the lol structure found indeed represents an energetic minimum in the molecule substrate potential. A distinct position of the QT layer with respect to the HBC lattice was identified in the direction perpendicular to the coincident lattice lines, while no such distinct position exists along these lines. The coincidence between the respective lattice lines could be directly confirmed by an inverse FFT analysis of a STM image. The demonstrated heteroepitaxial system of two organic molecular species is a first yet crucial step toward organic quantum wells, and we anticipate that highly ordered organic heterostructures will soon gain similar importance as their inorganic counterparts, therefore potentially leading to entirely new classes of organic-based applications. Acknowledgment. We acknowledge financial support from the Deutsche Forschungsgemeinschaft (Grants FR 875/6, FR 875/9, and FR 875/10). R.F. was supported by the Studienstiftung des deutschen Volkes. (28) Goddard, R.; Haenel, M. W.; Herndon, W. C.; Kr€uger, C.; Zander, M. J. Am. Chem. Soc. 1995, 117, 30–41. (29) Kerr, K. A.; Ashmore, J. P.; Speakman, J. C. Proc. R. Soc. London, Ser. A 1975, 344, 199–215.
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