ARTICLE pubs.acs.org/JPCC
Are Pentacene Monolayer and Thin-Film Polymorphs Really Substrate-Induced? A Molecular Dynamics Simulation Study Makoto Yoneya,*,† Masahiro Kawasaki,‡,§ and Masahiko Ando‡,§ † ‡
Nanosystem Research Institute of Advanced Industrial Science and Technology, 1-1-1 Umezono, Tsukuba 305-8568, Japan Hitachi Cambridge Laboratory, Hitachi Europe Ltd., J. J. Thomson Avenue, Cambridge CB3 0HE, United Kingdom ABSTRACT:
Molecular dynamics simulations were performed for mono- and multilayer pentacene films on a simple model surface to study polymorphs of the deposition-processed thin-films. The layer-by-layer deposition history was taken into account by successive layerstacking simulations of monolayer, bilayer and up to five-layered systems. We found that the tilt-free monolayer polymorph and the molecular tilting upon stacking layers on this monolayer were realized in the simulations even without any substrate interactions. These results imply that neither the monolayer nor the small angle tilted thin-film polymorph is substrate-induced as commonly explained. Our results also show a crucial dependence on the layer stacking history for the thin-film polymorph, which indicate that the thin-film polymorph would be induced with the layering history starting from the tilt-free monolayer.
’ INTRODUCTION Pentacene, the most intensely studied organic semiconductor, is known to have various crystallographic polymorphs, the occurrence of each depending sensitively on the temperature, thickness, processing histories, etc.1 In particular, the polymorphism in thin films, which is the most relevant for organic thin-film transistor applications, has attracted considerable interest for improvement in device performance. The thin-film polymorphs of pentacene are generally different from those of the single crystals. For example, the crystalline structure of the solution-processed pentacene thin films (thickness 100200 nm) was found2,3 to be similar to that reported by Campbell et al.4 [also known as the high-temperature (HT) polymorph],5 in contrast to the low-temperature (LT) polymorph structure observed for solution-processed single crystals.1 Contrary to the solution process, vapor deposition on silicon oxide substrate yields a “thin-film polymorph” that has a smaller molecular tilt angle than do the bulk polymorphs.6 This thin-film polymorph is often called substrate-induced polymorph, since it is found only in films thinner than 100 nm.6 An even thinner monolayer deposited on a silicon oxide substrate was found to be a completely molecular tilt-free (vertically standing) “monolayer polymorph” by grazing incidence X-ray diffraction (GIXD) measurement.7 Following the discussion for the thin-film polymorph above, this monolayer polymorph would r 2011 American Chemical Society
also be a substrate-induced polymorph, since the monolayer is the thin film at the thinnest limit. However, only limited work has been reported on the origin of these substrate-induced polymorphs.8 In this study, we performed molecular dynamics (MD) simulations to study these polymorphs of the deposition-processed films. Our purpose is to study the effects of the processing history aspects of the deposition process and not the deposition process itself, as was done in the work by Goose and Clancy.9 The layer-by-layer deposition history was simply taken into account by successive stacking simulations of monolayer, bilayer, and up to five-layered pentacene thin films on a simple model surface. We think even the simplest monolayer simulation would be worthwhile because the consistency of the results of the recent monolayer simulation10 with those of the experimentally reported tilt-free structure remains unclear.
’ MODEL AND METHOD The molecular model that we applied in this study is the same model used in the previous study.11 We applied the flexible (allatom) model of pentacene with the exception that all of the bond-stretching degrees of freedom were constrained to the Received: September 2, 2011 Revised: November 10, 2011 Published: December 02, 2011 791
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Figure 1. (a) Time evolution of the averaged stand-up angle of the pentacene molecules in monolayer simulations with various substrate potential strengths. (b) Initial and (c) final snapshots (viewed from the a-axis direction) of runs with silica substrate potential.
equilibrium bond lengths. For the inter- and intramolecular interaction, a general AMBER force field12 was applied. For the atomic charges of pentacene molecules, restrained electrostatic potential (RESP) charges,13 obtained by ab initio molecular orbital calculations at the B3LYP/6-31G(d) level in the Gaussian03 program,14 were used. By use of this pentacene model, crystal cell parameters were obtained within the maximum errors of 3% for the LT polymorphs, relative to the values obtained in an X-ray diffraction study.11 For the substrate surface potential, the integrated Lennard-Jones (93) potential,15 given by the following equation, was used, as in our previous study. ( 3 ) 2πFs εms σ ms 3 2 σ ms 9 σms Vs ðzÞ ¼ ð1Þ 15 z 3 z Combination rules, σms = (σm + σs)/2 and εms = (εmεs)1/2, are used between the interaction parameters for the atoms in the molecules (σm, εm) and surfaces (σs, εs). For the surface parameters, values of σs = 0.3 nm, εs = 0.8 kJ/mol, and Fs = 125/π nm3 (values taken from the literature16 for a silica surface) are used. Trajectories were produced by use of the MD simulation program package GROMACS (version 4.0)17 with the leapfrog time integration and LINCS bond constraint.18 The time integration step was set to 4 fs. Charge group-based twin-range 0.9 nm van der Waals and 1.9 nm electrostatic cutoff distances17 were applied to nonbonding interactions. We did not apply Ewald-like methods, which usually require three-dimensional periodic boundary, to the electrostatic interactions because we will apply two-dimensional periodic boundary conditions to the film simulations in the following.
Figure 2. Time variation of the cell parameters a, b, and γ in the runs shown in Figure 1 with silica substrate potential. Thin horizontal lines correspond to values obtained in a monolayer GIXD study.21
we have performed the simulation starting from this monolayer configuration at a constant temperature of 293 K and a pressure of 1 atm. In this simulation, the system was placed in a triclinic MD cell with the model substrate surfaces on the bottom and top of the MD cell under two-dimensional periodic boundary conditions in the ab directions. The simulation system was composed of the pentacene monolayer and the nitrogen gas phase over the monolayer, as in our previous study.11 For the temperature and pressure control, a NoseHoover thermostat19,20 and a ParrinelloRahman barostat21 were applied, allowing all six degrees of freedom in the MD cell. The nitrogen gas-phase volume ensured normal vapor pressure to keep the separation between the upper substrate and the pentacene thin film. Interaction between the gas and the pentacene molecules did not influence the thin-film structure because the interaction was much smaller than that between pentacene molecules. Time evolution of the averaged molecular stand-up angles θ from the substrate plane (i.e., θ = 90 “tilt angle”) is shown in Figure 1a. These angles were evaluated from the molecular vectors connecting two central positions of the pentacene’s outermost phenyl rings. The figure shows the molecularorientation transition from an initial stand-up angle value of
’ RESULTS AND DISCUSSION We first tried the pentacene monolayer simulation on the model substrate. The initial monolayer configuration was made by extracting the single (substrate-side) pentacene layer from the simulated 12-layer HT-polymorph pentacene film used in our previous study.11 The extracted monolayer was then copied in the ab-axis (substrate plane) directions to make the initial monolayer configuration (with 336 pentacene molecules) shown in Figure 1b. To obtain an equilibrium monolayer structure on the substrate under ambient (GIXD measurement)7 conditions, 792
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Figure 3. (a) Time evolution of averaged stand-up angle of pentacene molecules in bilayer simulations. (b) Initial and (c) final snapshots (viewed from the a-axis direction) of the runs with successive second layer stacking on the first monolayer polymorph. (d) Initial and (e) final snapshots of the runs with the extracted HT-polymorph bilayer.
approximately 69 to a final value of approximately 90 (tilt-free). The final monolayer configuration shown in Figure 1c could be assigned to the monolayer polymorph because the simulated monolayer also exhibits the in-plane compressed, more rectangular unit cell, as was experimentally reported.7 Figure 2 shows the compression and deformation (mostly occurring in the initial few tens of picoseconds) of the simulated unit cell parameters based on the MD cell shape. The averaged (over the last 100 ps) in-plane cell parameters were agreed within the maximum errors of 1% relative to the values obtained in a monolayer GIXD study.22 Figure 1a also shows the dependence of the reorientation behavior on the substrate surface potential strength. If we lower the surface potential by scaling Fs in eq 1 to be 0.1 or even 0 (i.e., without surface potential), the transition still occurred. In contrast, if we raise the surface potential 4-fold, the transition was suppressed, as seen in the figure. The results above indicate that the tilt-free monolayer polymorph is not a substrate-induced polymorph, as was previously explained. For example, Mannsfeld et al.7 denoted that the experimentally observed complete wetting of the pentacene monolayer on clean silicon oxide substrates is indicative of the moleculesubstrate interactions being stronger than the interlayer moleculemolecule interactions. They explained that the strong moleculesubstrate interactions render the tilt-free monolayer polymorph energetically beneficial. Burke et al.23 also stated that the monolayer polymorph of pentacene wets the surface due to the stronger moleculesubstrate interaction. However, these authors did not mention the intralayer molecule molecule interactions. We think the tilt-free structure of pentacene monolayer is strongly determined by the intralayer molecular interactions.24 The authors of the previous studies above would be confusing the origin of wetting with that of the tilt-free structure. Next, we studied how the film configuration will change as successive layers are stacked on the monolayer described above. First, the bilayer system was made by stacking a second layer on the monolayer in the c-axis direction with 2.3 nm gaps as shown in Figure 3b. We then re-equilibrated the system at 293 K and 1 atm of pressure. The upper plot of Figure 3a shows the corresponding time evolution of the averaged molecular stand-up
Figure 4. Time evolution of averaged stand-up angle of pentacene molecules in bilayer simulations with various substrate potential strengths.
angle. As seen in this plot, the molecular stand-up angle was spontaneously transformed from an initial value of approximately 90 to a final value of approximately 81 (9 tilting). The final angle is intermediate between those of the monolayer polymorph (ca. 90) and the bulk HT-polymorph (ca. 69). The snapshot after a 500 ps MD run shown in Figure 3c indicates a uniform stand-up angle (i.e., no detectable difference among the first and second layers) and “shifted”-type interlayer configurations (i.e., the molecules are not collinearly aligned between adjacent layers).11 All these properties are consistent with those of the thin-film polymorph.6,25,26 We also found that there was essentially no change in the tilting behavior described above with the lower values of 0.1 or even 0 substrate potential, as shown in Figure 4. Our results suggest that tilting with second layer stacking may not be substrate-induced but instead induced by interlayer molecular interactions. We then checked the dependence of tilting behaviors on the initial configurations. Another initial bilayer configuration was made by extracting the two (substrate-side) pentacene layers 793
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Figure 5. Time evolutions of averaged stand-up angle of pentacene molecules in the layer-by-layer stacking simulation containing up to five layers.
from the 12-layer HT-polymorph pentacene film, exactly as we did for the monolayer case, and then equilibrated the system in the same way. The lower line in Figure 3a corresponds to this case and shows that the stand-up angle basically remains at the initial HT-polymorph value. This result indicates that the initial configuration and history crucially affect the tilting behaviors with the layer stacking. We have performed successive layer-by-layer stacking to achieve up to five layers (a total of 1689 pentacene molecules) in a method similar to that described above. As shown in Figure 5, the average molecular stand-up angle was equilibrated to a value of approximately 76 (tilt ∼14), that is, still a smaller value than that of the bulk HT polymorph. To investigate the relationship between this film structure (with tilt ∼14) obtained above and the crystal structure of the thin-film polymorph obtained by the X-ray reciprocal space mapping (RSM) study ,27 we have done simulation starting from the latter as an initial film structure. The initial structures were made by stacking the unit cell 7, 6, and 12 times for the a-, b-, and c-axis directions, respectively. This 12layer film was put on the MD cell with substrates and gas phase as in the previous runs. Time variations of the averaged stand-up angle of the pentacene molecules and the in-plane cell parameters in this runs are shown in Figure 6. Initial pentacene tilt angle (ca. 4) changed to the angle ∼14 (Æθæ ∼76), that is, the equilibrated tilt angle in Figure 5. The averaged (over the last 100 ps) in-plane cell parameters in Figure 5 are also coincident with those of the run shown in Figure 5.28 Thus the final crystal structure of these runs shown in Figures 5 and 6 are the same even though they were obtained for largely different (but having similarity of finitely small-angle tilted) initial structures. This implies that the obtained structure would be one of the intrinsic metastable states of the system. Moreover, to induce this structure, starting from the finitely small-angle tilted initial structures, would be more crucial than the rest of the layer stacking history. We suppose the film structure with the molecular tilt angle ∼14 would correspond to the thin-film polymorph realized in our simulation model, although this tilt value larger than the reported tilt angle for the thin-film polymorph (46).25,26 We think the tilt values may vary depend on the surface potential and method of layer stacking (deposition), and our modeling in the current study was so simplified.
Figure 6. Time variations of averaged stand-up angle of pentacene molecules and cell parameters a, b, and γ in the runs starting from the crystal structure of the thin-film polymorph obtained by the X-ray RSM study.27
’ CONCLUSIONS We found that both the tilt-free monolayer on the model substrate and the molecular tilting upon stacking successive layers on this monolayer occurred in our simulations even in the absence of substrate interactions. Our simulation results indicate that the tilt-free monolayer and the tilting upon stacking layers are induced by the pentacene molecule’s intra- and interlayer interactions, respectively. These results imply that neither the “monolayer00 nor the “thin-film” polymorph is substrate-induced in the above meanings. Our results also show a crucial dependence of the thin-film polymorph on the layer stacking history starting from the tilt-free monolayer. In the deposition process, monolayer wetting enforces the layer stacking to start from the tilt-free monolayer polymorph, and this then leads to the thin-film polymorph in the later layer stacking. This would probably result in a polymorph difference between deposition- and solution-processed pentacene thin films. ’ AUTHOR INFORMATION Corresponding Author
*E-mail
[email protected]. Present Addresses §
Hitachi Central Research Laboratory, Hitachi Ltd., 1-280 Koigakubo, Kokubunji, Tokyo 1858601, Japan.
’ ACKNOWLEDGMENT This paper owes much to the thoughtful comments of Drs. Alberto Girlando and Massino Matteo of the University of Palma. We also thank Drs. Aldo Brillante, Raffaele Guido Della Valle, and Elisabetta Venuti of the University of Bologna for valuable discussions. ’ REFERENCES (1) Mattheus, C.; Dros, A.; Baas, J.; Oostergetel, G.; Meetsma, A.; de Boer, J.; Palstra, T. Synth. Met. 2003, 138, 475–481. 794
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(2) Duffy, C.; Andreasen, J.; Breiby, D.; Nielsen, M.; Ando, M.; Minakata, T.; Sirringhaus, H. Chem. Mater. 2008, 20, 7252–7259. (3) Natsume, Y.; Minakata, T.; Aoyagi, T. Synth. Met. 2009, 159, 338–342. (4) Campbell, R.; Robertson, J.; Trotter, J. Acta Crystallogr. 1962, 15, 289–290. (5) Siegrist, T.; Besnard, C.; Haas, S.; Schiltz, M.; Pattison, P.; Chernyshov, D.; Batlogg, B.; Kloc, C. Adv. Mater. 2007, 19, 2079–2082. (6) Bouchoms, I.; Schoonveld, W.; Vrijmoeth, J.; Klapwijk, T. Synth. Met. 1999, 104, 175–178. (7) Mannsfeld, S.; Virkar, A.; Reese, C.; Toney, M.; Bao, Z. Adv. Mater. 2009, 21, 2294–2298. (8) Drummy, L.; Martin, D. Adv. Mater. 2005, 17, 903–907. (9) Goose, J. E.; Clancy, P. J. Phys. Chem. C 2007, 111, 15653–15659. (10) Della Valle, R.; Venuti, E.; Brillante, A.; Girlando, A. ChemPhysChem 2009, 10, 1783–1788. (11) Yoneya, M.; Kawasaki, M.; Ando, M. J. Mater. Chem. 2010, 20, 10397–10402. (12) Wang, J.; Wolf, R. M.; Caldwell, J. W.; Kollman, P. A.; Case, D. A. J. Comput. Chem. 2004, 25, 1157. (13) Bayly, C. I.; Cieplak, P.; Cornell, W.; Kollman, P. A. J. Chem. Phys. 1993, 97, 10269. (14) Frisch, M. J. et al. Gaussian03, Revision C.02; Gaussian, Inc.: Wallingford CT, 2004. (15) Steele, W. Surf. Sci. 1973, 36, 317–352. (16) Pinilla, C.; Del Popolo, M.; Lynden-Bell, R.; Kohanoff, J. J. Phys. Chem. B 2005, 109, 17922–17927. (17) Lindahl, E.; Hess, B.; van der Spoel, D. J. Mol. Model. 2001, 7, 306. (18) Hess, B.; Bekker, H.; Berendsen, H. J. C.; Fraaije, J. G. E. M. J. Comput. Chem. 1997, 18, 1463. (19) Nose, S. Mol. Phys. 1984, 52, 255–268. (20) Hoover, W. Phys. Rev. A 1985, 31, 1695–1697. (21) Parrinello, M.; Rahman, A. J. Appl. Phys. 1981, 52, 7182. (22) Fritz, S.; Martin, S.; Frisbie, C.; Ward, M.; Toney, M. J. Am. Chem. Soc. 2004, 126, 4084–4085. (23) Burke, S.; Topple, J.; Gr€utter, P. J. Phys.: Condens. Matter 2009, 21, No. 423101. (24) The largest three interaction energies (average over the last 100 ps) in the bilayer system shown in Figure 3c were as follows: intralayer moleculemolecule interaction energy = 1.15 104 kJ/mol, interlayer moleculemolecule = 6.5 103 kJ/mol, and pentacene substrate = 1.49 103 kJ/mol (all values were per single pentacene layer with 336 molecules and negative sign means stabilizing interaction energy). (25) Yoshida, H.; Inaba, K.; Sato, N. Appl. Phys. Lett. 2007, 90, No. 181930. (26) Schiefer, S.; Huth, M.; Dobrinevski, A.; Nickel, B. J. Am. Chem. Soc. 2007, 129, 10316–10317. (27) Yoshida, H.; Sato, N. Phys. Rev. B 2008, 77, No. 235205. (28) The averaged in-plane cell parameter values of the five layers system (Figure 5) were a = 5.972 nm,b = 7.840 nm, and γ = 88.68. The values from the X-ray RSM study25 are a = 5.93 nm,b = 7.56 nm, and γ = 89.8. The run starting from this structure (Figure 6) resulted in the values a = 5.972 nm,b = 7.844 nm, and γ = 88.60. We checked that the last values in this system were kept in the extra 10 ns run (a = 5.972 nm, b = 7.845 nm, and γ = 88.61 in average over the last 1 ns).
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