Incommensurate Epitaxy of Tetrathiophene on Potassium Hydrogen

Crystal Growth & Design , 2006, 6 (8), pp 1826–1832. DOI: 10.1021/cg0601453. Publication Date (Web): July 7, 2006 ... dynamics simulations of the de...
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Incommensurate Epitaxy of Tetrathiophene on Potassium Hydrogen Phthalate: Insights from Molecular Simulation Valentina

Marcon,†

Guido

Raos,*,†

Marcello

Campione,‡

and Adele

Sassella‡

Dipartimento di Chimica, Materiali e Ingegneria Chimica “Giulio Natta”, Politecnico di Milano, Via L. Mancinelli 7, I-20131 Milano, Italy, and Dipartimento di Scienza dei Materiali and CNISM, UniVersita` degli Studi di Milano Bicocca, Via R. Cozzi 53, I-20125 Milano, Italy

CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 8 1826-1832

ReceiVed March 15, 2006; ReVised Manuscript ReceiVed June 1, 2006

ABSTRACT: The (010) surface of potassium hydrogen phthalate (KAP) has a strong orientational effect on the growth of crystalline tetrathiophene (4T) films from the vapor phase, even if there is complete incommensurism between their crystal lattices. Here we report on molecular dynamics simulations of the deposition of 4T on KAP, with several degrees of coverage and different initial conditions. We analyze both the behavior of preassembled crystalline monolayers of one tetrathiophene polymorph and the dynamics of molecules randomly distributed on the organic substrate. The results clearly show the presence of two preferred orientations of the long axes of the 4T molecules, in agreement with experiment. The aggregation of molecules into clusters is also observed at higher degrees of coverage. Again, the molecules are parallel to each other along the preferred orientations, and even a second layer adopts the same alignment. I. Introduction Conjugated polymers and oligomers are being widely investigated for their promising electronic and optoelectronic applications.1 Devices such as organic thin film transistors (OTFT), light emitting diodes (OLED), and photovoltaic cells typically consist of one or more active layers which can be obtained by different techniques, such as spin coating, casting, physical vapor deposition, etc. These methods often produce amorphous or polycrystalline materials, with little control over the conformational and positional order. This disorder can affect the performance of the devices, since the mobility of charge carrierssa typical parameter of importance for OFTF applicationssis very sensitive to molecular order and the extent of intermolecular π-π orbital overlap. The development of sophisticated growth techniques such as organic molecular beam epitaxy (OMBE) has allowed the growth of thin films with a close control over their morphological and structural properties.2,3 Most of the experimental studies conducted so far have concentrated on film growth on relatively simple inorganic substrates, such as Si/SiO2. The technological drive toward “all-organic” electronics provides a strong motivation for exploring the possibility of growing ordered organic thin films on polymeric or molecular substrates. There are two important factors which may differentiate the general behavior of organic and inorganic substrates. The first and most obvious one is the difference in surface chemistry (e.g., polarity and polarizability, acid-base interactions, etc.). The second one is the frequent occurrence in organic crystals of large lowsymmetry unit cells, as opposed to small high-symmetry cells for simple inorganic solids. Therefore, the existence of an epitaxial relationship (either full commensurism or different sorts of partial coincidence4) between the crystal lattices of the substrate and overlayer is expected to be relatively rare when both materials are molecular solids. Naively, it might be expected that the lack of any epitaxial registry would frustrate all attempts to growth highly ordered organic thin films over large areas. * To whom correspondence should be addressed. Tel: +39-02-23993051. Fax: +30-02-2399-3080. E-mail: [email protected]. † Politecnico di Milano. ‡ Universita ` degli Studi di Milano Bicocca.

The present work focuses on tetrathiophene (4T) deposited on potassium hydrogen phthalate (also known as potassium acid phthalate, KAP). This is an organic lamellar crystal exposing molecularly flat (010) surfaces5 with a high affinity toward 4T. Optical studies, X-ray diffraction, and atomic force microscopy (AFM) measurements on 4T thin films grown on KAP show the great influence of the substrate surface on the order and the orientation of the 4T crystallites, which can take the form of either “needles” or “islands” (see Figure 1).6-8 This occurs despite the absence of simple epitaxial relationships between the unit cells of these materials: they are incommensurate, even allowing for minor adjustments in their lattice vectors.8 Therefore, the origin of the orientation of 4T growth by KAP cannot be explained by general geometric arguments but requires a more detailed understanding of the energetics and dynamics of the molecule-substrate interaction. In this paper we use molecular dynamics (MD) simulation to gain some insight into several aspects of the deposition of 4T molecules on the (010) surface of KAP. The static behavior of 4T crystalline monolayers on KAP has already been investigated by energy minimization in a preliminary study.8 In section II we describe the force field (FF) and the simulation methods. We then go on to discuss our simulation results. Part 1 of section III is concerned with the behavior of a single molecule on the surface. The following parts of section III deal with the dynamics of a certain number of 4T molecules interacting with the surface: in part 2 we investigate the behavior of preformed crystalline monolayers, while in part 3 we simulate the deposition of several molecules with random initial configurations. Conclusions follow. II. Simulation Methodology All simulations were performed with the TINKER molecular modeling package.9 We have used a previously developed FF to describe 4T.10 The most critical elements in our oligothiophene FF are the inter-ring torsion potential, which was derived from high-level ab initio calculations (MP2/aug-cc-pVTZ),11 and the nonbonded interaction parameters, which consist of point charges on all atoms from gas-phase ab initio calculations12 and Lennard-Jones parameters borrowed from OPLS-AA.13 The present version of the FF is both simpler and more satisfactory than a previous one,12 which adopted (with suitable modifications) the MM3 parameters for the nonbonded energy.14

10.1021/cg0601453 CCC: $33.50 © 2006 American Chemical Society Published on Web 07/07/2006

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Figure 2. Motion of a molecule of tetrathiophene on the KAP surface during the MD simulations at 300 and 350 K. In both cases, the initial coordinate of the molecular center of mass is (0.0,0.0). Figure 1. AFM image (10 × 10 µm2) of a 15 nm thick film of 4T deposited at room temperature by OMBD on KAP(010). The orientation of the substrate unit cell is given within the image. The 4T(001) islands and 4T(010) needles are clearly visible as light gray shapes and white segments, respectively. A structural model explaining the orientation of twinned needles indicated by white arrows is also reported. The needles are elongated along the short axis (a axis) of the 4T/LT cell. The FF for KAP fully relies on the OPLS-AA parametrization implemented within TINKER, for both bonded and nonbonded parameters. This was tested by comparison with its crystal structure. KAP has an orthorhombic unit cell (space group Pca21) with lattice parameters: a ) 9.614 Å, b ) 13.330 Å, c ) 6.479 Å.15 Since these data were obtained by X-ray diffraction at room temperature, where thermal expansion and molecular motions may be important, simulation data were generated by MD rather than energy minimization. We performed NPT simulations (i.e., with a constant number of particles, pressure, and temperature) at T ) 300 K and P ) 1 atm for 2 ns on a 2 × 2 × 2 supercell of the crystal. We calculated the electrostatic interactions using both a cutoff method, with a cutoff value of 12 Å, and the Ewald sum. Even though the latter is in principle more accurate, the best results for the average lattice parameters and density were obtained with the simpler and computationally cheaper cutoff method: a ) 9.116 Å (-5.2%), b ) 13.097 Å (-1.8%), c ) 6.699 (+3.4%), F ) 1.696 g/cm3(+3.8%). Thus, the present FF and the cutoff method reproduce quite satisfactorily the behavior of both KAP and 4T,10 and they were adopted for all subsequent simulations. From the unit cell, we obtained a periodic slab of crystalline KAP with an area of 53.5 × 54.7 Å2 and a thickness of about 27 Å. The surface exposed to deposition is the (010) surface of KAP. Its outermost layer consists of aromatic rings, while the first layer of potassium cations is found about 8 Å below. Although the complete simulation box has full three-dimensional periodic boundary conditions, each slab is separated from its periodic image by about 40 Å of void space along the z axis. Since this distance is much larger than the cutoff, we have an effective two-dimensional system with zero slab-slab interactions. The behavior of crystalline 4T on KAP, already studied with static minimizations,8 was further investigated by MD simulations on systems with preordered monolayers of 4T deposited on the surface. The monolayers were obtained by cutting a thin slab along different planes of the “low temperature” (LT) crystal polymorph:16,17 in one case the surface of the 4T/LT crystal in contact with KAP is the (010) surface, in another case it is the (001) surface. We also investigated the possibility of self-organization of the 4T molecules on the surface, without any predefined arrangement. To this purpose we deposited between 1 and 12 4T molecules on the surface, in random initial configurations. In some simulations, tetrathiophene molecules were deposited on both sides of the KAP slab, with two different initial configurations. Molecules on opposite sides of the KAP slab do not interact because of our choice of the nonbonded interaction cutoff. Thus, a single MD run provided us with two effectively independent simulations, allowing us to check the consistency of their results.18 Note that the KAP substrate was allowed to move in all the MD simulations, but the surface shows only small thermal fluctuations and its crystalline order is preserved. Except for the simulations of the KAP crystal described above, all MD simulations were performed in the NVT ensemble (i.e., with a

constant number of particles, volume, and temperature). The temperature was controlled by the Berendsen method19 with a coupling time of 2.0 ps (and, in the NPT simulation of the KAP crystal, of 0.2 ps for the pressure). The MD equations were integrated with the velocity-Verlet scheme and a 2.0 fs time step, with fixed C-H bond lengths. The time length of each simulation is between 4 and 8 ns.

III. Results and Discussion We first discuss in some detail the behavior of a single 4T molecule on the KAP surface. This system provides information about translational and rotational diffusion on the substrate and also about the conformation and orientation of the molecules with respect to the c axis of KAP (cKAP, corresponding to the x axis of our simulation box). We then go on to present the simulations with ordered and disordered initial configurations of several 4T molecules. We analyze many-molecule ordering effects by calculating quantities such as the 〈P2〉 nematic order parameter,18 which quantifies the degree of parallelism of the molecules, as well as selected molecule-molecule and moleculesubstrate distances. III.1. Behavior of a Single Molecule. To characterize the mobility and preferred orientation of 4T on the KAP (010) surface, we performed two independent simulations (8 ns each) of a single 4T molecule, respectively, at T ) 300 and T ) 350 K. Figure 2 shows that the projection of the molecular center of mass on the plane of the substrate roughly undergoes a twodimensional random walk. Even though the MD runs are too short to extract reliable diffusion coefficients,20 it is clear that (a) even at room temperature, the molecule is sufficiently mobile to translate over a distance several times larger than its length, and (b) its mobility is fairly isotropic, since the overall displacements along the x axis ()cKAP) and y axis are comparable. It is remarkable that the molecule-substrate interaction energy is quite anisotropic, unlike the diffusion coefficient. Figure 3 shows the analysis of the average potential energy as a function of the θ angle between the long molecular axis and cKAP, obtained by dividing the range between 0 and 90° in steps of 10° (because of the symmetry of a 4T molecule and of the underlying KAP substrate, the angles θ, -θ, 180° - θ, and 180° + θ are equivalent). The potential energy profile shows a minimum located between 50 and 60°, with a depth of about 2 kcal/mol. This is a significant fraction of the total 4T/KAP interaction energy (=19 kcal/mol) and is about 3 times larger than the thermal energy (RT ) 0.6 kcal/mol at 300 K). A similar profile was also obtained in the simulation at 350 K. In agreement with Boltzmann’s principle, this low-energy orientation with respect to the substrate is preferentially populated by the 4T molecule, as can be seen in Figure 4 (upper part), where

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crystal,16 our MD simulations include intramolecular flexibility through the torsion potential between the thiophene moieties. The upper part of Figure 6 shows that there is a significant

Figure 3. Average potential energy of tetrathiophene on the KAP surface, from the MD run at 300 K. The zero of the potential energy corresponds to the KAP slab and the 4T molecule at infinite distance.

Figure 6. Histogram of the populations of the dihedral angles (upper panel) and analysis of the energy with respect to the number of cis dihedrals of the 4T molecule (lower panel). Both plots were obtained from the MD simulation of a single molecule at 300 K.

Figure 4. Distribution (upper panel) and time evolution (lower panel) of the orientational angle of a 4T molecule, during the MD run at 300 K.

there is a sharp peak around 55°. Figure 5 shows that when the 4T molecule adopts this particular orientation, it lies comfortably within a groove formed by the outermost H atoms of the phenyl rings of KAP. The lower part of Figure 4 shows, however, that the molecule is not at all “locked” at this specific angle. Instead, it is highly mobile and makes frequent orientational transitions and large-amplitude fluctuations about this direction. Besides exploring the KAP surface by translational and orientational diffusion, the 4T molecule also enjoys significant conformational freedom. Indeed, while this molecule is known to adopt an exactly or nearly trans-planar conformation in the

portion of cis angles (0° e |φ| e 90°, where φ is the S-CC-S torsion), in addition to the more populated trans angles (90° e |φ| e 180°). The histogram also shows that adjacent thiophenes are almost never coplanar (i.e., the populations of φ = 0° and φ = 180° are negligible). This reflects the shape of the gas-phase torsion potential, whose minima fall at φ = 147° (trans disorted) and φ = 37° (cis disorted, 0.36 kcal/mol higher in energy).11 The effect of the conformational disorder can also be seen in the lower part of Figure 6, where the average energy values are plotted versus the number of cis dihedral angles in the molecule. The average energy increases almost linearly from 0% to 100% of cis conformation, where all three torsion angles are distorted. Interestingly, the total energy increment on going from an all-trans to an all-cis conformation is about 2 kcal/ mol, which is roughly double the value predicted on the basis of the gas-phase torsion potential (3 × 0.36 kcal/mol). This indicates that the interaction with the KAP substrate becomes less favorable when the molecular conformation switches from trans to cis. III.2. Results of MD on Crystalline Monolayers. We investigated the dynamic behavior of two crystalline monolayers

Figure 5. Snapshots of a single 4T molecule in a typical low-energy configuration. The outermost hydrogens forming the channels along the 〈101〉 directions of KAP have been highlighted.

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Figure 7. Initial (left) and final (right) snapshots for the simulation of the 4T crystalline monolayer, with the (010) surface of the LT polymorph in contact with the KAP substrate. The snapshot in the middle is an intermediate configuration at 1.17 ns, where light blue molecules temporarily form a second layer.

of the LT polymorph, obtained by cutting thin slabs along different planes of the LT crystal and depositing them on the (010) surface of KAP. In one case, KAP is in contact with the (010) surface of 4T/LT (“lying molecules”), and in the other case it is in contact with the (001) surface (“standing molecules”). Previous X-ray and electron diffraction work6,8 has demonstrated that these two arrangements are associated respectively with the 4T “needles” and “islands”, which are clearly visible in Figure 1. We first describe the MD simulation of the (010)-like monolayer, which lasted 4 ns at 300 K. In the initial configuration the 4T molecules form an average angle of 58° with cKAP (see Figure 7). This is one of the two minima found in our previous work (the other is at 131°, about 1 kcal/mol higher in energy), where we scanned by energy minimizations all possible relative orientations of the substrate and monolayer.8,21 The initial and final snapshots of the simulation, shown in Figure 7, might suggest that the system did not undergo major changes during the simulation. Only a small shift of half a monolayer can be observed. However, inspection of the whole MD trajectory shows that the system has a rather dynamic behavior. In fact, during this first 2.5 ns of simulation, the crystal-like order of the monolayer is largely destroyed. Some molecules form an angle with cKAP of about 120°, instead of the 58° angle of the starting configuration. Some others form a second layer, on top of the first one. Eventually, however, the system reaches the configuration shown on the right-hand side of Figure 7 and retains it for the following 1.5 ns. It is important to stress, however, that although this configuration appears to be quite stable, we cannot expect such a small cluster to retain it forever. Overall, the system is rather mobile and longer simulations may well show transitions to other states, such as those seen in the first half of this MD run. The loss and recovery of orientational order is also demonstrated by the time dependence of the 〈P2〉 parameter, calculated over each frame in the trajectory (Figure 8). The final average orientation of the 4T molecules with cKAP is 57°, with a 〈P2〉 parameter of 0.98 (averages calculated over the last 1.5 ns). The average distance between the centers of mass of two neighboring 4T molecules is 6.1 Å, and the average distance of the center of mass of the 4T molecules from the outermost hydrogens of the KAP surface is 2.3 Å. We now describe the results on the (001)-like monolayer. In the previous static studies,8 we found two minima at 0 and 180° with similar energies (see the left-hand side of Figure 9). We performed NVT MD simulations at 300 K for 4 ns, with both initial configurations. In both cases the final configuration is more disordered than in the (010) monolayer. Three different

Figure 8. Time evolution of the 〈P2〉 orientational order parameter, during the simulations of the (010)-like monolayer and the two (001)like monolayers.

arrangements of molecules can be observed in Figure 9. Some molecules (shown in light blue) are still roughly vertical to the KAP surface, as in the starting crystalline monolayer. A second set of molecules (red) have adopted a flat configuration with respect to the surface and have an orientation similar to those already found for the single 4T molecule and the (010) monolayer. Finally, there is also an intermediate layer of more disordered molecules (blue). The overall orientational order parameter fluctuates about an average value of 〈P2〉 = 0.8 (Figure 8). The graph in Figure 10 shows the correlation between the average distance of the center of mass of 4T molecules and their average orientation with cKAP. For simplicity, the angles with cKAP were all brought in the range between 0 and 90° (for angles greater than 90°, the value 180° - θ is reported). We can observe that the molecules which are standing on the surface, having the center of mass more than 5 Å away from it, form with cKAP an angle below 30°. The lying molecules, with their center of mass between 2 and 3 Å from the KAP surface, form with cKAP an angle between 40 and 60°. The molecules in the intermediate layer do not seem to adopt any special orientation. We conclude this section by observing that the different structural stabilities of the (010) and (001) monolayers seem to point to different requirements for their initial formation. As a rough estimate, it appears that the critical crystalline nucleus may contain, at room temperature, less than 10 molecules in the first case but requires more than 20 in the second (we have disregarded the possible dependence of the critical size of the nucleus on the degree of coverage). The first estimate follows

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Figure 9. Initial (left) and final (center and right) snapshots for the simulation of the 4T crystalline monolayer, with the (001) surface of the LT polymorph in contact with the KAP substrate. The molecules in the starting configuration have a 0° orientation with respect to cKAP. The 4T molecules in the final snapshots have been colored according to the inclination with respect to the surface.

which is expected on the basis of our single-molecule simulations, the molecules also aggregate within a relatively short time. In the simulations with 2, 3, and 4 molecules we observe formation of a single cluster, in which the molecules share the same orientation. Figures 11 and 12 show representative

Figure 10. Relationship between the distance of the center of mass of 4T molecules from the KAP surface and the orientation with respect to cKAP. The points correspond to the last configuration achieved in two independent MD simulations, starting from 0 and 180° (see text).

from the visual observation that a (010)-like monolayer of only 10 molecules appears to be quite “stable” over time, even if this stability does not correspond to a single well-defined configuration, but there are instead considerable dynamic fluctuations. The second estimate follows from the observation that even a preformed (001)-like cluster containing 23 molecules (see Figure 9) is unable to retain a configuration (or an esemble of configurations) with a comparable degree of order. III.3. Results of MD on Several Molecules with Random Initial Configurations. Having established that small preordered and preoriented crystalline monolayers are sufficiently stable over the time scale of an MD run, we decided to consider the possibility of spontaneous self-organization by simulating several 4T molecules with random initial configurations. To this purpose we deposited between 2 and 12 molecules on the KAP surface, and we ran NVT simulations at 300 K for 7 ns. The analysis of the last 400 frames collected during the simulation time shows that, in all cases, the molecules lie flat on the surface and they tend to adopt two orientations around 55 and 125° with respect to cKAP. Besides achieving this orientational order,

Figure 12. Final configuration (at 5.92 ns) from an MD simulation with eight molecules at 300 K. The light blue molecules form a second layer, on top of the dark blue layer. This simulation is distinct from that shown in Figure 11.

snapshots from two independent simulations with 8 4T molecules (they actually correpond to opposite sides of the same KAP slab). Figure 11 shows that the molecules are initially well separated (first frame); they then aggregate into a cluster with a common orientation with respect to cKAP (second frame, taken within the first 2 ns), and finally they remain aggregated but split into two different subclusters, with six of them at 54° and the other two at 128° (third frame, taken at 7 ns). This behavior is observed also in the other simulation (Figure 12). We observe again the coexistence of the two main orientations, in which however the cluster at 54° contains two molecules forming a second layer. The two layers are quite distinct, since their average distances from the KAP surface are equal to 2.4 and 6.1 Å, respectively. Analogous results were found also in the two simulations with 12 molecules. Figure 13, showing the final configuration

Figure 11. Snapshots from an MD simulation with eight molecules at 300 K. The picture on the left is the initial configuration, the one in the middle refers to an early stage of the simulation (0.87 ns), and the one on the right refers to the final part of it (5.92 ns).

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Figure 13. Final snapshot from an MD simulation with 12 molecules at 300 K. The molecules are shown in different colors, depending on their orientation.

from one of them, demonstrates the formation of one large cluster divided into two subclusters, one at 54° and the other at 126° with respect to cKAP. In all these simulations, the calculated 〈P2〉 parameter within a cluster or subcluster is very high, being equal to 0.95 or higher. Despite this large orientational order, the 4T molecules retain a certain degree of conformational disorder (i.e., an appreciable population of cis angles). By an analysis analogous to that of Figure 6 for the single-molecule simulations, we observe again an almost linear increase of the average potential energy with the number of cis torsion angles. Other things being equal, the most stable configurations are the ones with the smallest numbers of cis conformations. IV. Conclusions Our simulations of 4T on the (010) surface of KAP were motivated by the desire to understand the origin of the strong orientational effect of this substrate on the growth of crystalline 4T films by gas-phase deposition.6-8 This effect, which is clearly demonstrated by Figure 1, is somewhat puzzling, since there is complete incommensurism (lattice mismatch) between the crystals of KAP and of 4T (in particular, of its LT polymorph). Therefore, this “incommensurate epitaxy” cannot be explained by general geometric arguments (e.g., a match between simple rational combinations of the lattice vectors4) but requires a detailed description of the molecule-substrate interaction. The MD simulations correctly reproduce the strong orientational effect of the KAP surface on 4T. The effect is already present at the single-molecule level, before their aggregation to form precrystalline nuclei or thin films. The molecular alignment is along the 〈101〉 crystallographic directions of KAP, which correspond to the directions formed by H atoms protruding from the substrate surface. This confirms that the driving force for the orientation of the 4T domains is embodied by the crystallochemical properties of the substrate surface, such as its atomic scale corrugation. Our single-molecule simulations also show, however, that the substrate-molecule interaction is not strong enough to prevent translational, orientational, or conformational diffusion of the latter. This special combination of strong orientation and good molecular mobility is certainly conducive to the formation of well-developed crystalline thin films of 4T. In analyzing the behavior of preformed 4T monolayers on KAP, we considered both a (010)-type monolayer (observed in the needles of Figure 1 and characterized by “lying” 4T molecules) and two (001)-type monolayers (observed in the islands of Figure 1 and characterized by “standing” 4T molecules). The former appears to be more stable, since it retains an almost crystalline arrangement during the simulation (see Figure 7). Instead, the (001) monolayers are more unstable, with

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some molecules switching from a vertical to a planar arrangement, with the typical distances and orientation of a (010) monolayer (see Figure 9). This observation allows us to hypothesize that the critical nucleus for the crystallization of the (001) islands is larger than for the (010) needles and suggests interesting possibilities for the control of their structure and morphology by playing with variables such as the substrate temperature or deposition rate. Our simulations of several 4T molecules (from 2 to 12, over an area of 290 Å2) with a random initial distribution show the spontaneous tendency of the molecules to form well-ordered clusters. The molecular arrangement of these clusters resembles that of the (010) thin films, even though their dynamic nature prevents us from classifying them as truly crystalline. Incidentally, this “fluctional” character is believed to be a general feature of crystallization nuclei of both small-molecule and polymeric substances,22 and indeed we believe that they may rightly be identified as such. Visual inspection of the final snapshots from some simulations (Figures 11-13) also provides evidence of an incipient twinning of these aggregates and suggests a detailed molecular model for the interface between the needlelike crystals growing from them. Figure 1, where several twinned needles have been detected by AFM, demonstrates the qualitative agreement between the simulation and experimental results. Acknowledgment. This research was supported by the MIUR through the PRIN2004 and PRIN2005 programs and the NEMAS Center of Excellence at Politecnico di Milano. Part of the computer time was made avalilable by CILEA (Milano). References (1) (a) Garnier, F. Acc. Chem. Res. 1999, 32, 209. (b) Katz, H. A.; Bao, Z. J. Phys. Chem. B 2000, 104, 671. (c) Dimitrakopoulos, C. D.; Malenfant, P. R. L. AdV. Mater. 2002, 14, 99. (d) Ko¨hler, A.; Wilson, J. S.; Friend, R. H. AdV. Mater. 2002, 14, 701. (2) Forrest, S. R. Chem. ReV. 1997, 97, 1793-1896. (3) Sassella, A.; Campione, M.; Papagni, A.; Goletti, C.; Bussetti, G.; Chiaradia, P.; Marcon, V.; Raos, G. Strategies for two-dimensional growth of organic molecular films. Chem. Phys. 2006, 325, 193206. (4) (a) Hooks, D. E.; Fritz, T.; Ward, M. D. AdV. Mater. 2001, 13, 227241. (b) Mannsfeld, S. C. B.; Leo, K.; Fritz, T. Phys. ReV. Lett. 2005, 94, 056104. (5) Borc, J.; Sangwal, K. Surf. Sci. 2004, 555, 1-10. (6) Sassella, A.; Besana, D.; Borghesi, A.; Campione, M.; Tavazzi, S.; Lotz, B.; Thierry, A. Synth. Met. 2003, 138, 125-130. (7) Sassella, A.; Campione, M.; Moret, M.; Borghesi, A.; Goletti, C.; Busetti, G.; Chiaradia, P. Phys. ReV. B 2005, 71, 201311(R). (8) Campione, M.; Sassella, A.; Moret, M.; Papagni, A.; Trabattoni, S.; Resel, R.; Lengyel, O.; Marcon, V.; Raos, G. Organic-Organic Epitaxy of Incommensurate Systems: Quaterthiophene on Potassium Hydrogen Phthalate Single Crystals. Submitted for publication. (9) Ponder, J. W. TINKER: Software Tools for Molecular Design, version 4.1; Washington University School of Medicine, Saint Louis, MO, 2003. (10) Marcon, V.; Raos, G. J. Am. Chem. Soc. 2006, 128, 1408-1409. (11) Raos, G.; Famulari, A.; Marcon, V. Chem. Phys. Lett. 2003, 379, 364-372. (12) Marcon, V.; Raos, G. J. Phys. Chem. B 2004, 108, 18053-18064. (13) Jorgensen, W. L.; Maxwell, D. S.; Tirado-Rives, J. J. Am. Chem. Soc. 1996, 118, 11225-11236. (14) Li, J.-H.; Allinger, N. L. J. Am. Chem. Soc. 1989, 111, 8576-8582. (15) Eremina, T. A.; Furmanova, N. G.; Malakhova, L. F.; Okhrimenko, T. M.; Kuznetsov, V. A. Crystallogr. Rep. 1993, 38, 554. (16) Siegrist, T.; Kloc, C.; Laudise, R. A.; Katz, H. E.; Haddon, R. C. AdV. Mater. 1998, 10, 379-382. (17) The LT polymorph of 4T is characterized by a monoclinic unit cell (P21/c space group, Z ) 4) with a ) 6.085 Å, b ) 7.858 Å, c ) 30.483 Å, β ) 91.810°.

1832 Crystal Growth & Design, Vol. 6, No. 8, 2006 (18) Marcon, V.; Raos, G.; Allegra, G. Macromol. Theory Simul. 2004, 13, 497-505. (19) Berendsen, H. J. C.; Postma, J. P. M.; van Gunsteren, W.; Di Nola, A.; Haak, J. R. J. Chem. Phys. 1984, 81, 3684-3690. (20) The mean-square displacement of the molecular center of mass roughly follows the Einstein relation 〈∆x2(t) + ∆y2(t)〉 ) 4Dt (note that 4 ) 2 × d is the appropriate prefactor for diffusion in d ) 2 dimensions). The difficulty in providing a quantitative estimate of the diffusion coefficient D is mainly due to the fact that, in the present simulations, we do not benefit from the advantage of averaging over the mean-square displacements of several molecules.

Marcon et al. (21) We point out that the symmetry of a 4T/LT monolayer is lower than that of a single 4T molecule. Therefore, while the orientation of the latter with respect to cKAP can be specified by giving an angle in the [0°, 90°] range (see Figures 3 and 4), the orientation of the former requires an angle in the [0°, 180°] range (i.e., the θ and 180° - θ angles are no longer equivalent). (22) (a) Oxtoby, D. W. Acc. Chem. Res. 1998, 31, 91-97. (b) Leyssale, J.-M.; Delhommelle, J.; Millot, C. J. Am. Chem. Soc. 2004, 126, 12286-12287. (c) Allegra, G.; Meille, S. V. AdV. Polym. Sci. 2005, 191, 87-135.

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