Molecular Flexibility as a Factor Affecting the Surface Ordering of

organization. The addition of tetracene molecules as structural templates stabilizes certain ..... director of the nematic phase is close to a high sy...
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Molecular Flexibility as a Factor Affecting the Surface Ordering of Organic Adsorbates on Metal Substrates Serguei Soubatch, Ruslan Temirov, and F. Stefan Tautz* International UniVersity Bremen, School of Engineering and Science, P.O. Box 750561, 28759 Bremen, Germany ReceiVed May 22, 2006. In Final Form: August 3, 2006 The effect of molecular flexibility on the surface ordering of complex organic adsorbates is explored, using R,ωdihexylquaterthiophene (DH4T) and mixed DH4T|tetracene phases on Ag(111) as model systems. The structure of DH4T/Ag(111) interfaces is determined by the flexibility of the hexyl chains at either end of the quaterthiophene backbone: Above 273 K, DH4T forms a nematic liquid crystalline phase with a director close to the [112h] direction of the silver substrate. At 273 K, a reversible phase transition to a long-range ordered, point-on-line coincident phase is observed. However, this ordered state is still affected substantially by the flexible nature of DH4T, which materializes in a large number of local structural defects. If traces of DH4T are coevaporated with tetracene, inclusions of a 1:1 stoichiometric DH4T|tetracene phase are found in a tetracene/Ag(111) matrix (R-phase). In this mixed phase, the two surface enantiomers of pro-chiral DH4T on one hand and tetracene on the other form a complex stripe structure. The mixed phase shows a higher degree of order than present at the pure DH4T/Ag(111) interface, which also lacks chiral organization. The addition of tetracene molecules as structural templates stabilizes certain conformations of DH4T and thus, by balancing its structural flexibility, allows the surface-induced chirality of DH4T to become a decisive factor in determining the structure of the mixed phase.

1. Introduction Ordered structures of large organic adsorbates on inorganic surfaces are currently studied intensively, both because of their application potential (e.g., for charge carrier transport, if π-conjugated molecules are considered) and because of a fundamental interest in molecular self-assembly at surfaces.1 Generally speaking, interface structures between molecular and nonmolecular phases are determined both by intermolecular and molecule-substrate interactions. The challenge in the field of molecular self-assembly at surfaces is the design of molecular building blocks which optimize the resultant interface structure in a certain desired direction. Many possibilities to influence molecular architectures at surface exist, because molecular sizes, shapes and chemical functionalities can be varied in wide ranges. Molecules with a large degree of internal flexibility offer additional parameter space and therefore are of special interest in this context. In their bulk state, molecules with a substantial degree of flexibility often form liquid crystals; the potential for unconventional liquid crystalline behavior at surfaces adds additional interest to studying adsorption and ordering of flexible molecules on solid surfaces. Indeed, one may expect some unusual surface phases in the sway of three determining factors, namely the ordering influence of the substrate, intermolecular interactions, and (most interestingly in the present context) the mesogenic nature of the molecules which, due to the existence of a large number of soft internal degrees of freedom, implies the importance of thermal motion for the interface structure. Although the liquid crystalline (mesogenic) state of matter has been known since the end of 19th century, these materials have become of practical interest only in the 1960s.2 Since then liquid crystals have gained importance as materials for display * To whom correspondence should be addressed. Tel: +49 421 200 3223. Fax: +49 421 200 49 3223. E-mail: [email protected]. (1) De Feyter, S.; De Schryver, F. C. Chem. Soc. ReV. 2003, 32, 139-150. (2) Stephen M. J.; Straley, J. P. ReV. Mod. Phys. 1974, 46, 617-704.

applications. More recently, another intriguing application of liquid crystals has been found: Because many mesogenic organic molecules of anisometric shape (20-40 Å long) are also semiconducting, they can be utilized in organic electronic devices. Indeed, it has been shown that field effect mobilities in oligomers such as quaterthiophene (4T) and sexithiophene (6T) increase if two hexyl chains, inducing mesogenic behavior of the resulting molecules R,ω-dihexylquaterthiophene (DH4T) and R,ω-dihexylsexithiophene (DH6T), are grafted to either end of the semiconducting thiophene backbone.3-7 The promise of device applications further adds interest to the study of these flexible molecules at surfaces. R,ω-Dihexylquaterthiophene in particular is still one of the most prominent mesogenic materials for thin film transistor research,7-9 although a number of different oligothiophenes have been synthesized and studied.10-16 DH4T has also served as a model system for studying phase transitions to the liquid crystalline state.4,17 For all of these reasons, we have used DH4T (3) Ackermann, J.; Videlot, C.; Raynal, P.; El Kassmi, A.; Dumas, P. Appl. Surf. Sci. 2003, 212-213, 26-32. (4) Garnier, F.; Hajlaoui, R.; El Kassmi, A.; Horowitz, G.; Laigre, L.; Porzio, W.; Armanini, M.; Provasoli, F. Chem. Mater. 1998, 10, 3334-3339. (5) Garnier, F.; Yassar, A.; Hajlaoui, R.; Horowitz, G.; Deloffre, F.; Servet, B.; Ries, S.; Alnot, P. J. Am. Chem. Soc. 1993, 115, 8716-8721. (6) Dimitrakopoulos, C.; Furman, E.; Graham, F.; Hedge, S.; Purushothaman, S. Synth. Met. 1998, 92, 47-52. (7) Katz, H. E.; Lovinger, A. J.; Laquindanum, J. G. Chem. Mater. 1998, 10, 457-459. (8) Muck, T.; Wagner, V.; Bass, U.; Leufgen, M.; Geurts, J.; Molenkamp, L. W. Synth. Met. 2004, 146, 317-320. (9) Muck, T.; Fritz, J.; Wagner, V. Appl. Phys. Lett. 2005, 86, 232101232103. (10) Liu, P.; Nakano, H.; Shirota, Y. Liquid Cryst. 2001, 28, 581-589. (11) Azumi, R.; Go¨tz, G.; Debaerdemaeker, T.; Ba¨uerle, P. Chem. Eur. J. 2000, 6, 735-744. (12) Ponomarenko, S.; Kirchmeyer, S. J. Mater. Chem. 2003, 13, 197-202. (13) Barbarella, G.; Melucci, M.; Sotgiu, G. AdV. Mater. 2005, 17, 15811593 (14) Mena-Osteritz, E. AdV. Mater. 2002, 14, 609-616. (15) Mena-Osteritz, E.; Ba¨uerle, P. AdV. Mater. 2001, 13, 243-246. (16) Abdel-Mottaleb, M. M. S.; Go¨tz, G.; Kilickiran, P.; Ba¨uerle, P.; MenaOsteritz, E. Langmuir 2006, 22, 1443-1448.

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surface and a STM tip.21 Formation of vertical SAMs from oligothiophene molecules terminated with an active thiol anchoring group has been demonstrated recently.22 In contrast to this case, the most active molecular interaction potential of DH4T is located at the thiophene rings. Correspondingly, a planar configuration is expected on active metal surfaces. In the present study, we investigate the effect of the mesogenic nature of the flexible adsorbate DH4T on its interface structure with Ag(111). Nonmesogenic oligothiophenes have been wellstudied on Ag(111) and can serve as a reference.19,23-26 We have also investigated mixed phases of DH4T and tetracene on Ag(111). Tetracene molecules have very different mechanical properties from DH4T molecules: They are rigid, and we will see that the interplay between a rigid and a flexible molecule can lead to particularly interesting interface structures, if the former act as structural templates. 2. Experimental Methods

Figure 1. Molecular structure and electronic orbitals (highest occupied, HOMO and lowest unoccupied, LUMO) of R,ωdihexylquaterthiophene (a) and tetracene (b) as derived from Gaussian03 simulation (BPW91/cc-pVDZ calculation).

as a model to study the ordering behavior of flexible, mesogenic materials on single crystalline metal surfaces. The origin of the peculiar structural properties of DH4T and related materials is the simultaneous presence of a relatively rigid backbone (in the case of DH4T consisting of 4 thiophene rings) and two flexible capping units (here: hexyl-chains) on either side of the backbone18 (Figure 1a). The molecular structure is thus more adaptive than that of the backbone alone (4T, 6T, end-capped quaterthiophene EC4T,19 etc.) or even more rigid π-conjugated planar molecules (e.g., tetracene (Figure 1b) or pentacene). At approximately 353 K (80 °C) DH4T undergoes a phase transition from the crystalline to the liquid crystalline (LC) state, which in turn melts at approximately 453 K (180 °C).4,17 Between 353 and 453 K, DH4T exists in a tilted smectic phase.17 Thin films of DH4T on silica exhibit the same crystal structure as single crystals.18 On inert substrates (e.g., silica and glass), the molecules are oriented nearly upright. This orientation is preserved through the LC transition.17,18 On chemically more active substrates [e.g., molybdenum disulfide (MoS2)]20 DH4T adsorbs in a planar (i.e., parallel to the substrate) configuration. The same has been shown for other molecules of the same class on substrates of highly ordered pyrolytic graphite (HOPG).11 Of course, the adsorption orientation is not only determined by the chemical activity of the substrate, but also by molecular interaction potentials. If, for instance, active groups such as a thiol group (-S-H) are attached to the alkane chains, molecules tend to stand upright even on metal substrates such as Ag or Au, forming so-called self-assembled monolayers (SAMs). In this way, the number of S-Me bonds can be maximized. The chains themselves then interact with their neighbors and thus determine packing and tilt of the monolayer. If each molecule is terminated by two active groups, one on each side, it is even possible to form molecular bridges, either on the surface or between the (17) Amundson, K. R.; Katz, H. E.; Lovinger, A. J. Thin Solid Films 2003, 426, 140-149. (18) Moret, M.; Campione, M.; Borghesi, A.; Miozzo, L.; Sassella, A.; Trabattoni, S.; Lotz, B.; Thierry, A. J. Mater. Chem. 2005, 15, 2444-2449. (19) Seidel, C.; Soukopp, A.; Li, R.; Ba¨uerle, P.; Umbach, E. Surf. Sci. 1997, 374, 17-30. (20) Azumi, R.; Go¨tz, G.; Ba¨uerle, P. Synth. Met. 1999, 101, 569-572.

Molecular layers of DH4T and DH4T|tetracene were prepared by organic molecular beam deposition in an ultrahigh vacuum chamber with a base pressure of 5 × 10-11 mbar. The Ag(111) substrate was prepared by standard sputter-anneal cycles. Sputtering was performed with 0.8 keV Ar+ ions. Afterward the surface was annealed at 820 K. DH4T was deposited from a home-built effusion cell heated to 483 K onto the substrate held at 363 K (i.e., slightly above the mesogenic transition temperature of the bulk material). This substrate temperature is also used for the preparation of device structures based on DH4T.8 Mixed phases of DH4T and tetracene were prepared by co-deposition of the two species from the same crucible. In this case, the evaporation temperature and sample temperatures were set to 450 and 300 K, respectively. A low-temperature scanning tunneling microscope (LT STM) operating at 6-10 K and a low energy electron diffraction (LEED) optics were applied to study interface structures and ordering of the two types of molecular films discussed here.

3. Results and Discussion 3.1. DH4T on Ag(111). 3.1.1. Surface Phase Transition. As has been stressed earlier, the temperature of 353 K corresponds to the transition from the liquid crystalline state of bulk DH4T at high temperature to the crystalline state at low temperature. Remarkably, after the deposition of a monolayer of DH4T onto the Ag(111) substrate held at 363 K and subsequent cooling to room temperature (300 K), the monolayer does not exhibit any long range order. This is indicated by the absence of a LEED pattern derived from the molecular layer. Only upon further cooling to below 273 K does a sharp LEED pattern related to long range ordered molecular superstructure appear. Repeated annealing of the sample at 300 K and subsequent cooling reveals this transition to be a reversible phase transition. After several annealing and cooling cycles, the diffraction pattern becomes even sharper (Figure 2a). The observed LEED pattern of the long range ordered lowtemperature phase contains the contributions of 6 distinct patterns which are related to each other by 60° rotations. This 6-fold symmetry originates from the symmetry of the Ag(111) substrate. The oblique superstructure (see below) breaks the symmetry of (21) Li, Z.; Han, B.; Meszaros, G.; Pobelov, I.; Wandlowski, T.; Błaszczyk, A.; Mayor, M. Faraday Discuss. 2006, 131, 121-143. (22) Santos, E.; Schu¨hle, D. T.; Jones, H.; Schmickler, W. Langmuir 2005, 21, 6406-6421. (23) Soukopp, A.; Glo¨ckler, K.; Kraft, P.; Schmitt, S.; Sokolowski, M.; Umbach, E.; Mena-Osteritz, E.; Ba¨uerle, P.; Ha¨dicke, E. Phys. ReV. B 1998, 58, 1388213894. (24) Umbach, E.; Sokolowski, M.; Fink, R. Appl. Phys. A 1996, 63, 565-576. (25) Soukopp, A.; Glo¨ckler, K.; Ba¨uerle, P.; Sokolowski, M.; Umbach, E. AdV. Mater. 1996, 8, 902-906. (26) Meyerheim, H. L.; Gloege, Th.; Sokolowski, M.; Umbach, E.; Ba¨uerle, P. Europhys. Lett. 2000, 52, 144-150.

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Figure 2. DH4T phases on Ag(111): (a) LEED pattern observed at 100 K. (b) Overview of a disordered surface region (nematic phase) demonstrating preferential orientation of DH4T molecules along the [112h] direction of the Ag(111) substrate shown in the inset (15 Å × 15 Å). (c) Submolecular resolved real space STM image of DH4T molecules in the nematic phase. (d) Border region between a disordered and an ordered DH4T domain separated by a lacuna of uncovered Ag(111) surface. White arrows in panels b-d mark DH4T molecules which are bent in their quaterthiophene backbones.

the substrate surface. Rotations around the 6-fold axis and the mirror symmetry of the substrate create 12 symmetry-related domains with 6 distinct LEED patterns. The 6-fold symmetry of the molecular LEED pattern quite obviously indicates orientational registry of the molecular layer with the substrate and therefore a significant interaction between the two. Yet, the long range ordered phase of DH4T on Ag(111) forms at much lower temperature than the crystallization temperature in the bulk state. Taking into account the ordering influence of substrate, caused by the adsorbate/substrate interaction, one may have expected the opposite trend, namely a surface ordering temperature above the bulk ordering temperature. To clarify the nature of the phase transition, we have studied the local structure of DH4T films with low-temperature scanning tunneling microscopy (LT-STM). Figure 2d shows a 400 Å × 400 Å STM image in which both a long range ordered phase and a disordered phase are observed. Apparently, it is possible to freeze in the disordered state and observe it at temperatures as low as 10 K. Indeed, the fact that cycling the layer through the transition improves the LEED pattern implies that after a single passage through the transition some fraction of the layer remains disordered. The image in Figure 2d shows a frozen snapshot of the ordering front of a long range ordered domain. The frontier region of the ordered domain shows less order than the inner part. Moreover, the long range ordered domain is separated from the disordered one by a lacuna of uncovered Ag(111) substrate. The lacuna appears because the density of the ordered phase is substantially higher than that of the disordered one. If at the transition temperature an ordered island nucleates somewhere in a disordered domain, the corresponding part of the domain must shrink on ordering. At or close to equilibrium conditions, the disordered phase would then expand, unless kinetically hindered by a very strong intermolecular attraction (or strong bonding to the substrate) which would then essentially suppress the surface mobility of individual molecules in the disordered phase. If in the particular case of Figure 2d the time at temperatures with

sufficient molecular diffusion was too short for the disordered phase to advance toward the receding crystallization front, a lacuna must appear and survive as a metastable structural feature at low temperature. This mechanism supposes a certain “viscosity” of the disordered phase which prevents it from expanding instantaneously even at elevated temperatures. Such a viscosity could stem from attractive intermolecular interactions. Images such as the one in Figure 2d can in principle provide insight into the kinetics of the ordering process. In the very least, they can be used to study the structural properties of both the high and low-temperature phases. 3.1.2. High-Temperature Phase. Let us now consider the frozen snapshots of the disordered high-temperature phase in Figure 2b-d. If interpreted with care, these images can be used to deduce fundamental structural properties of the hightemperature phase, although in the stability region of this phase (i.e., above 273 K), one would expect a substantial degree of thermal motion. In many disordered regions of DH4T on Ag(111), one observes a preferential alignment of the molecules (Figure 2b,c); such domains of the high temperature phase may therefore be classified as a liquid crystalline phase of the nematic type. Although for many of the domains which we have observed in this work the director of the nematic phase is close to a high symmetry direction of the Ag(111) substrate, it is difficult to establish whether this agreement is systematic or coincidental. Nevertheless, a directing influence of the substrate on the molecules in the nematic phase seems likely, especially taking into account the strong orienting effect of the substrate in the long range ordered phase which is demonstrated by LEED (Figure 2a). Indeed, the average orientation of molecules in the nematic phase depicted in Figure 2c agrees well with the [112h] direction of the substrate, although we cannot affirm that this is true for all nematic domains. We note that there is no correlation between the director of the nematic phase and the scanning direction of the STM. Moreover, with very few exceptions the structure of the molecular layer is stable under scanning conditions. We can thus safely rule out any

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Figure 3. Structure analysis of the ordered low-temperature phase of DH4T on Ag(111). (a) Real space STM image. Lattice grid lines derived from panel (b) are shown in white. (b) Fourier-filtered STM image, obtained from the two-dimensional Fourier transformation of the image in (a) (shown in the upper inset of (b)). Unit cell vectors derived from the filtered image are shown in white. The lower inset gives the orientation of the Ag(111) substrate. (c and d) Statistical distribution of distances between the centers of DH4T molecules, measured along a and b directions.

influence of the scanning tip on the average direction of molecules in the nematic phase. LT STM images in the nematic phase resolve a clear submolecular contrast in DH4T molecules: Four bright protrusions correspond to the four pairs of lobes of the DH4T HOMO (highest occupied molecular orbital), located on the thiophene rings in the 4T backbone, whereas the hexyl chains appear as essentially structureless bars on either side of the backbone. Comparing the size of the observed protrusions (approximately 5 Å) with the dimensions of a DH4T molecule18 and the molecular geometry optimized by total energy calculations using the Gaussian package27 (Figure 1a), we may conclude that DH4T adsorbs with the molecular plane parallel to the substrate (i.e., not edge-on). The same adsorption configuration has been proposed for related molecules containing a thiophene backbone [e.g., 4T and EC4T] on Ag(111).19 In this flat orientation, one expects a strong adsorbate/substrate interaction leading to significant electronic coupling between molecule and substrate. For instance, the photoluminescence of quaterthiophene was recently found to be totally quenched on Ag(111).28 In the nematic phase, we observe many molecules in which the hexyl chains are bent with respect to the thiophene backbone. Because of the flexible nature of the alkyl chains, this is indeed not surprising, and this flexibility has been held responsible for the existence of the liquid crystalline phase. Remarkably, however, a number of molecules in the observed nematic phase are also (27) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision C.02; Gaussian, Inc.: Wallingford, CT, 2004. (28) Gebauer, W.; Langner, A.; Schneider, M.; Sokolowski, M.; Umbach, E. Phys. ReV. B 2004, 69, 155431-155438.

bent in their thiophene parts. Examples are indicated with arrows in Figure 2b-d. The observation of bent configurations of quaterthiophene backbones is interesting since it shows that 4T is not absolutely rigid and that molecular flexibility in DH4T is not restricted entirely to the alkyl chains. 3.1.3. Long Range Ordered Low-Temperature Phase. In addition to extensive regions of the nematic phase, long range ordered regions of DH4T are also observed in our LT STM topographs (Figure 2d). In these regions, the film is more compact, and it appears to largely consist of stretched out DH4T molecules (although mainly the thiophene backbones are visible in the image). STM images of the long range ordered phase also exhibit small bright spots, which will be discussed below. (A few of these bright spots are also visible in the nematic phase). As we will argue later in this section, these ordered domains correspond to the diffraction patterns observed in LEED below the phase transition at 273 K. A careful study of the local structure of the long range ordered DH4T domains reveals numerous local deviations from a perfectly periodic packing of molecules. Both a considerable spread of the molecular orientation angle (measured between the quaterthiophene backbone and some reference direction) and a distribution of lateral distances between neighboring molecules (measured as the center-to-center distance between the 4T backbones) are observed in the close-up image of a typical long range ordered domain in Figure 3a. Also, the bright spots form a pseudoperiodic array rather than a perfectly periodic one. Nevertheless, the tendency to form a highly ordered lattice is obvious if the phase is regarded at longer length scales. Indeed, a two-dimensional Fourier transformation of the real space STM image of Figure 3a, displayed in the inset to Figure 3b, results in a sharp reciprocal lattice pattern, confirming the long range order of the DH4T film. Filtering the as-measured image in Figure 3a by restricting the Fourier transform to the main spots including their immediate vicinity and back-transforming to real space reveals the underlying periodicity of the long range ordered DH4T phase (Figure 3b). From this image, unit cell vectors a and b of the ordered molecular superstructure can be derived: a ) 14.2 ( 0.2 Å, b ) 12.6 ( 0.2 Å, Θ ) 83°. This oblique unit cell contains one DH4T molecule. Remarkably, the

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orientation of one of the unit cell vectors, namely a, coincides perfectly (within experimental accuracy) with the diagonal of Ag(111) unit cell (i.e., [1h21h] the direction of the Ag single crystal). Measuring the distance between neighboring molecules along the a and b directions for more than 200 molecule pairs in 4 different long range ordered domains across the substrate surface, we gain a quantitative measure of the local disorder (Figure 3c). The fwhm of the nearest neighbor distance distribution amounts to approximately 20% (26 and 19% for a and b, respectively) of the most likely values (14.7 and 12.6 Å, respectively). This is well above the accuracy limit of our low drift LT STM. Locally, the unit cell vectors are thus a ) 14.7 ( 1.9 Å, b ) 12.6 ( 1.2 Å. With respect to the orientation, we find Θ variations of up to (10° around the average molecular direction as given by the Fourier filtered image (Figure 3b). We can thus conclude that long range ordered DH4T domains on Ag(111) at 10 K as observed here contain a large density of local defects, i.e., deviations from the average (equilibrium) molecular positions, conformations and orientations in the unit cells, but exhibit at the same time a well-defined lattice of unit cells. The layers therefore combine considerable disorder on the short range with good long range order. This situation differs from the usual situation in highly defected crystals or smectic liquid crystals in which short range order is preserved but long range order is poor. The best analogy to the observed state is a crystalline material close to its melting point, where all lattice objects make large (thermally excited) excursions from their equilibrium positions, while the long range order is still preserved. Note, however, that our DH4T layers were imaged far away from the LC transition. Of course, we cannot exclude that other areas of the sample are better ordered and that we are observing some metastable state here. However, we have never found in our experiments on DH4T/Ag(111) (near-)perfect periodic order of the type usually observed for stiff organic adsorbates. It thus seems as if soft degrees of freedom have been frozen in nonequilibrium positions. In this context, we note that the cooling rate between the disorder-to-order phase transition temperature and 10 K has been rather slow (about 10 h). This indicates that a perfectly ordered possible equilibrium state of DH4T on Ag(111) may be difficult to obtain in any real experiment. We now address the question whether the long range order observed in the STM images in Figure 3 is the same as the one responsible for the superstructure pattern observed in electron diffraction (Figure 2a). To this end, a simulated LEED image is generated from the structural parameters of the STM images. First, it has already been mentioned that the unit cell vector a from Figure 3b coincides with the [1h21h] direction of the substrate. Second, average lattice vectors of a ) 14.7 ( 1.9 Å and b ) 12.6 ( 1.2 Å have been determined, with an angle Θ ) 83°. From this information, the following structure matrix can be generated:29

[

4.00 ( 0.38 4.61 ( 0.44 -2.92 ( 0.37 2.92 ( 0.37

]

(1)

with the moduli of M21 and M22 being equal (by assumption). On the other hand, we can adjust the superstructure matrix (1) for optimum oVerall coincidence with the observed LEED image. This yields (29) To simulate the LEED pattern we used LEEDpat2 program by K. Hermann and M. A. Van Hove, see http://w3.rz-berlin.mpg.de/∼hermann/LEEDpat/.

[

4.0 4.4 -3.1 3.2

]

(2)

The simulated LEED pattern belonging to this second superstructure matrix is shown in Figure 4a, projected into the experimental image. One observes a good agreement although some deviations remain (e.g., in the inner ring of the double spots). Hence, we conclude that, within the experimental error, the long range ordered phases observed microscopically by STM and in diffraction are identical. The agreement between the observed and simulated LEED patterns for the central starlike group of spots (first-order spots) can be improved if the following matrix is used (cf. Figure 4b):

[

4.0 4.4 -3.0 3.0

]

(3)

With this matrix, the higher order spots are simulated slightly less accurately, but such deviations may result from distortions in the LEED experiment. Matrix (3) belongs to a superstructure having point-on-line coincidence of type I.30 Within experimental accuracy, we may therefore classify the observed superstructure as point-on-line coincident. The corresponding real space lattice is shown in Figure 4c. In this case the lattice parameters of the superstructure become: a*(Ag(111)) ) 2.9 Å, a ) 15.1 Å, b ) 12.2 Å, R(a* a) ) 150°, β(ba) ) 85.3°. To rationalize the remarkable type of order (point-on-line coincidence with large degree of short range disorder) found for DH4T on Ag(111) further, we compare our results to previous studies of similar molecules, namely end-capped derivatives EC3T, EC4T, EC5T, and EC6T.19,23-26 In these end-capped molecules, the thiophene rings at either end of the backbone are capped by one cyclohexane ring each. Various long range ordered phases have been observed for these molecules on Ag(111), but none is characterized by a significant short-range disorder as found here for DH4T. Even if complete disorder was observed in some limited surface region [e.g., for EC6T on Ag(111)],23 the adjoining ordered phase is perfectly periodic both at short and long length scales. Since the main difference between those molecules and DH4T is the flexibility associated with the hexyl side chains, we conclude that the specific features of DH4T ordering are driven by just this flexibility. Moreover, EC4T on Ag(111) is found to form ordered layers well above room temperature (nearly up to its dissociation temperature of 550 K),19 thus at much higher temperatures than DH4T/Ag(111). Since these two molecules differ only in the rigidity (flexibility) of the terminating groups, the “weaker” ordering of DH4T on Ag(111) is very likely caused by the presence of flexible alkyl chains in its molecular motif. We conclude the description of the low temperature phase by a short discussion of the bright spots which appear in STM images in Figures 2 and 3. These spots decorate the short edges of DH4T in the low temperature phase. However, most often only one spot is observed between two adjacent DH4T molecules. Two possible reasons may explain this feature. First, the gap between two molecules may be decorated (passivated) by a residual gas molecule. These adsorbates might then be imaged as bright protrusions by the STM. However, taking into account the high areal density of these dots, which is often close to the density of DH4T molecules, the low pressure of residual gases, and the fact that the areal density of these protrusions does not depend on time, this explanation can safely be ruled out. Rather, we explain these features as arising from the terminal CH3 groups of the hexyl chains. The bright appearance of these end groups (30) Hooks, D. E.; Fritz, T.; Ward, M. D. AdV. Mater. 2001, 13, 227-241.

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Figure 4. Superposition of the LEED diffraction pattern of an ordered DH4T monolayer at 100 K (Figure 2a) with two simulated reciprocal lattices: (a) superstructure matrix (2) and (b) superstructure matrix (3) (point-on-line coincidence). A real space model of the point-on-line structure is displayed in panel (c) (open circles: Ag atoms, gray circles: superstructure lattice points.

may indicate that they are pointing out of the surface plane into the vacuum. If this is the case, it is noteworthy that only one of the two facing CH3 groups of adjacent molecules is actually raised above the surface plane. The other may be pushed underneath the hexyl chain of the neighboring molecule. As mentioned already, we also observe bright spots in the frozen nematic phase, albeit with much lower density than in the ordered phase. It is indeed conceivable that also in this phase a CH3 group sticks out of the surface occasionally. 3.2. DH4T|tetracene on Ag(111). We have already mentioned that the lack of flexibility of pristine or endcapped oligothiophenes leads to much more robust interface order. We now ask the question whether the addition of a rigid molecule into the DH4T layer can increase the degree of local order in DH4T on Ag(111). One may envisage the rigid species to act as an “internal” structural template which forces DH4T into well-defined structural configurations. For a trial experiment to test this concept, we have chosen tetracene as the rigid species. Tetracene is a molecule consisting of 4 benzene rings and lacking flexible substituents (cf. Figure 1b). Recently, it has been shown that tetracene forms commensurate ordered layers on Ag(111).31 Two structural modifications have been reported, namely the R-phase and the β-phase. If tetracene is deposited on Ag(111) at room temperature and then cooled to 100 K, the R-phase is obtained. In the present work, we prepared a mixed DH4T|tetracene phase by codeposition of DH4T and tetracene from the same glass crucible. For this purpose, small amounts of the DH4T were added to the tetracene source material. Accordingly, the dominant phase formed after deposition from this source and subsequent cooling is the R-phase of tetracene. However, besides extensive regions of the tetracene/Ag(111) R-phase, we also found small areas covered by a complex phase consisting of alternating stripes of DH4T and tetracene molecules (Figure 5). Our first finding thus is that tetracene and DH4T indeed assemble on Ag(111) into a structurally well-defined, well-ordered mixed phase. We note that the mixed phase turns out to be better ordered than the low temperature phase of pure DH4T. The mixed phase has a 1:1 stoichiometry. Apparently the mobility of DH4T molecules on Ag(111) is large enough for this phase to be able to form in only a few extended domains on the substrate surface which are spaced far apart (microns), otherwise we would expect a more homogeneous (random) distribution of DH4T molecules as sparse dopants or contaminants in the matrix of the ordered tetracene R-phase. Remarkably, we also neither (31) Langner, A.; Hauschild, A.; Fahrenholz, S.; Sokolowski, M. Surf. Sci. 2005, 574, 153-165.

Figure 5. STM images of the DH4T|tetracene mixed phase. (a) Overview image. The DH4T and tetracene stripes run almost perpendicularly between two Ag(111) substrate steps which are visible in the upper right and lower left corners of the image. In the upper left corner, the tetracene R-phase comes into view. (b) Closeup image of the DH4T|tetracene mixed phase. The unit cell is indicated in black. The arrows mark packing faults of DH4T enantiomers, further discussion in main text.

observe the complete segregation of DH4T from tetracene into pure DH4T islands nor trapping of DH4T at substrate steps or surface defects. Apparently, the mixed phase is stabilized by a sizable attractive interaction between the two molecular species.

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Upon close examination of STM images, further interesting structural details of the mixed phase (Figure 5) become evident. The tetracene molecules can be separated in two groups having slightly different orientations. At the same time, the DH4T molecules also can be divided into two groups, differing in their handedness. Along the tetracene and DH4T rows in Figure 5b, molecules of two different groups alternate, with the exception of a few stacking faults. To rationalize the observed structure of the mixed DH4T|tetracene phase, let us first consider the molecular structure of a single DH4T molecule. Although a free DH4T molecule is not chiral, it becomes chiral as soon as it adsorbs on a surface in planar configuration. This effect of surface-induced chirality is well-known from other molecules.32-38 It originates in the impossibility to flip a given molecule on its other side once it has adsorbed on the surface with one of its two faces down. Such a flip would require a 180° rotation around the long axis of the molecule, hence the surface bond would have to be broken temporarily, and this requires a large activation energy which usually is not available by thermal excitation. For this reason, two distinct surface species of DH4T on Ag(111) exist, which differ in the handedness of the “propeller” formed by the two hexyl chains.39 These two species cannot be transformed into each other by any symmetry operation in which the ring system stays flat on the surface at all times. Since the probability for a gas-phase molecule to adsorb on the surface with one or the other face down is the same, the relative abundance of the two surface species (enantiomers) must be 1:1. The mixed phase of DH4T and tetracene constitutes a clear case of chiral recognition: Two distinct types of -DH4T-Tczigzag chains are observed; they are marked in red and blue in Figure 6, and each type of chain is formed with one of the two possible DH4T enantiomers only. Although the tetracene molecule itself is not chiral, it apparently plays an important role in the chiral selection of DH4T molecules for the various sites within the mixed phase. This can already be concluded from the fact that pure DH4T does not seem to show this chiral selection at all (at least we have not observed such a behavior in our study of DH4T/Ag(111)). The role of tetracene can be described as follows: On its short side, each tetracene molecule is coordinated with hexyl chains from two DH4T molecules belonging to the same chirality; its long sides are again coordinated by hexyl chains with identical chirality (these chains run nearly parallel to the tetracene backbone), but this time belonging to opposite DH4T enantiomer. The four hexyl chains around a tetracene molecule therefore form a chiral hollow into which the tetracene molecule fits well. The tetracene orientation is hence determined by intermolecular interactions with its neighbors. In this way, two distinct tetracene orientations exist in the mixed phase. They alternate along the tetracene chains. It is worth noting in this context that in the pure R-phase of tetracene all molecules are oriented in exactly the same direction, driven by the silver substrate.31 In the present case of the mixed phase, the (32) Hel-Or, Y.; Peleg, S.; Avnir, D. Langmuir 1990, 6, 1691-1695. (33) Tao, F.; Bernasek, S. L. J. Phys. Chem. B 2005, 109, 6233-6238. (34) Weigelt, S.; Busse, C.; Petersen, L.; Rauls, E.; Hammer, B.; Gothelf, K. V.; Besenbacher, F.; Linderoth, T. R. Nat. Mater. 2006, 5, 112-117. (35) Weckesser, J.; De Vita, A.; Barth, J. V.; Cai, C.; Kern, K. Phys. ReV. Lett. 2001, 87, 096101-096104. (36) Bo¨hringer, M.; Schneider, W.-D.; Berndt, R. Angew. Chem., Int. Ed. 2000, 39, 792-795. (37) France, C. B.; Parkinson, B. A. J. Am. Chem. Soc. 2003, 125, 1271212713. (38) Chen, Q.; Richardson, N. V. Nat. Mater. 2003, 2, 324-328. (39) This propeller structure derives from the tilt of the hexyl chains relative to the long axis of the quaterthiophene backbone. This tilt is present in the relaxed structure of the molecule, as calculated by Gaussian (Figure 1). It is also observed in the STM, cf. Figures 5b and 6.

Soubatch et al.

Figure 6. In this close-up image of the DH4T|tetracene mixed phase, the two different surface enantiomers of DH4T are marked in blue and red (panels a and b). The two different surface orientations of tetracene are also marked in corresponding colors. Arrows in (a) mark two packing faults in DH4T stripes: Two neigbouring red rows and two neigbouring blue rows instead of the normal bluered-blue-red alternation.

intermolecular interaction between DH4T and tetracene molecules must thus be strong enough to successfully compete with the adsorbate/substrate interaction of tetracene on Ag(111). In fact, the intermolecular interaction between the chiral DH4T and achiral tetracene, enforcing a parallel orientation of the formers’ hexyl chains and the tetracene backbone, may play an important role in the enantioselectivity of the various sites to be occupied by DH4T molecules within the structure of the mixed phase. After the above qualitative discussion of the interface structure, we now turn to its quantitative aspects, namely the exact dimensions of the unit cell. This will allow us to determine the relationship between the substrate and superstructure lattices. The unit cell of the mixed phase is displayed in Figure 5b. After what has been said above it is obvious that each primitive unit cell must contain two DH4T and two tetracene molecules. From the STM, the unit cell vectors can be determined as a ) 25.2 ( 0.2 Å and b ) 24.7 ( 0.2 Å; the angle between these two basis vectors amounts to approximately 101°. Comparing the orientation of DH4T|tetracene unit cell vectors and the known orientation of the tetracene R-phase unit cell with respect to the Ag lattice,31 we can determine the angles between the Ag(111) [1h10] direction and vectors a and b as 22° and 123°, respectively. The corresponding superstructure matrix is

[

9.94 ( 0.08 3.76 ( 0.03 -0.51 ( 0.01 8.25 ( 0.07

]

(4)

This matrix reveals that the mixed phase is incommensurate with the Ag(111) surface. A more precise structural analysis

Ordering of Organic Adsorbates

demands for integral diffraction methods, which are not available in our case due to the low surface fraction occupied by the DH4T|tetracene mixed phase. Apparently, the improved order in the mixed layer, as compared to the low-temperature phase of pure DH4T, comes at the cost of loosing even the low degree of substrate registry observed for DH4T alone. Nevertheless, some orienting influence of the substrate is still noticeable, as must be concluded from the fact that the mean direction of tetracene molecules, although being affected by the intermolecular interaction as pointed out above, are still close to the [1h10] direction of Ag(111). Although it is indisputable that the degree of order in the mixed phase is superior to the low-temperature phase of pure DH4T, there are still numerous packing faults (as mentioned above) in the rather complex structure of the mixed phase. Examples have been marked with arrows in Figures 5b and 6. From these examples, it is clear that packing faults are induced by misplaced DH4T enantiomers. Finally, we return to the LT phase of pure DH4T. For reasons described above, a 1:1 stoichiometry of the two DH4T surface enantiomers should be present also in this phase. However, molecular chirality is not a decisive factor in determining its structure (although occasionally it is possible to classify a molecule as belonging to one or the other enantiomer). Rather, the flexibility of the hexyl chains is the dominating property of DH4T in the pure phase. It therefore appears that tetracene, by its own rigidity, stabilizes DH4T structurally and (thus) allows chiral recognition to become a decisive factor in determining the structure of the mixed phase.

4. Conclusion We have presented here the results of our study concerning the structure of molecular films made from the mesogenic DH4T molecule. Using two different films on Ag(111), namely a pure

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DH4T phase and mixed DH4T|tetracene phase, we have demonstrated the complementary effects of two specific features of the structural motif of DH4T (i.e., its flexibility and its surfaceinduced chirality). We have found that in pure phases of DH4T the influence of structural flexibility dominates. At high temperature, this influence expresses itself through the existence of a nematic liquid crystalline phase. At about 273 K, this phase experiences a reversible phase transition to an ordered, most probably pointon-line coincident state. However, this ordered state is still influenced by the flexible nature of the molecules, which materializes in a large number of local structural defects. These defects effectively hinder chiral selection between DH4T molecules to become a determining factor of the evolving structure. In this situation, we have tried out the concept of a molecular template which, if mixed with the parent phase, can balance the structural flexibility of the latter. Indeed, adding tetracene, a rigid molecule of well-adjusted size and shape, to the DH4T matrix, the structural flexibility of DH4T is checked, with the result that the potential of local chiral recognition in DH4T can unfold. Remarkably, we find that the presence of a nonchiral molecule enhances and bears out chiral recognition of a chiral molecule, by stabilizing its structure. In conclusion, our results show a wide scope for structural engineering of molecular interfaces on the basis of flexible building blocks. This potential may be further explored in future work. Acknowledgment. This work was supported by the Deutsche Forschungsgemeinschaft (German Research Foundation). We thank Professor V. Wagner (International University Bremen) for providing the purified DH4T. LA061440S