Evidence for π-interactions in Stacked Polymers by STM Simulations

Transcription

1 pubs.acs.org/jpcc Evidence for π-interactions in Stacked Polymers by STM Simulations Alain Rochefort,*, Stephane Bedwani,*, and Alejandro Lopez-Bezanilla*, Departement de genie physique and Regroupement quebecois sur les materiaux de pointe (RQMP), Ecole Polytechnique de Montreal, Montreal, Quebec H3C 3A7, Canada Oak Ridge National Laboratory, One Bethel Valley Road, Oak Ridge, Tennessee , United States ABSTRACT: The influence of π-interactions in thin layers of stacked polymers has been studied with first-principles DFT calculations and STM simulations. Improving π π coupling between polymeric layers in the stack contributes to enlarging the size of STM contrasts and to attenuating the fine atomic structure features in the topmost superficial polymer layer. In addition, the tunneling transport within the stack is strongly influenced by the distance separating the polymer layers within the stack. Consequently, it appears very difficult to determine the structural properties (stack height, interlayer distance) of thin-layer stacks only on the basis of STM contrast measurements. For multiple-layer stacks, we have clearly identified two different tunneling regimes where one is assisted by π-coupling that enhances the tunneling current, whereas the other is a pure tunneling transport mechanism. Our results suggest that STM could constitute a powerful technique to probe the existence of π-electron interactions in π-conjugated stacks. INTRODUCTION The molecular organization of π-conjugated materials has a dramatic influence on the electrical performance of organic semiconductor thin film devices, such as transistors, 1 3 photovoltaic cells, 4 6 and other applications. 7,8 Furthermore, the presence of π-electron coupling within assembled molecules and polymers has recently received a significant amount of attention since this creates a charge carrier channel with a potentially high mobility The pioneering work of Sirringhaus et al. 13 on regioregular poly(3-hexylthiophene-2,5-diyl) (rrp3ht) has demonstrated the existence of two-dimensional charge transport and, more specifically, the clear improved charge mobility in the π-stacking direction of well-organized P3HT domains where significant π-electron coupling is expected. 14 Although the magnitude of the π-electron coupling could be controlled through a judicious choice of the chemical composition of the materials to facilitate cofacial π π stacking, 15,16 the most effective approach usually remains to proceed in a careful annealing of the active phase. 17,18 On the other hand, the effects of π-stacking on the electronic structure of the materials have received more direct spectroscopic evidence of their existence during the past decade The use of scanning tunneling microscopy (STM) and spectroscopy (STS) techniques leads to common features of π-stacked systems in which the band gap is narrower than for isolated species, 20,23 and where the charge transport appears more efficient. 19,24 In parallel, the photophysical properties of ordered P3HT have been also extensively studied at both theoretical 25 and experimental 26 levels. The existence of structural ordering in rrp3ht solution is mainly associated with the formation of H-aggregates that leads to the emergence of new absorption bands that are significantly red shifted with respect to the main absorption peak observed in diluted and disordered solution. 26 From these observations, it seems reasonable to anticipate that well-ordered rrp3ht polymers have a higher band dispersion and a smaller optical band gap than the disordered phase. The stacking of π-conjugated polymers may result into a lamella structure that can be orientated parallel or normal to the substrate. 12,13,17 Although the normal orientation can be more straightforwardly analyzed by STM and STS because the π π overlap regions are directly accessible by the probe, the π π regions can only be indirectly probed in the parallel orientation. For this last orientation, there is an additional inherent experimental limitation, such as only a few layers of stacked polymer can be effectively probed. Hence, the few STM/STS experiments on organized multilayers recently performed were only considering a stack of two layers in which the second layer is randomly oriented over the first layer. 19,24 On the basis of the variation of the tunnel current (or the relative height) for the second layer, the studies have observed significant electronic interactions between the two polymer layers of the π-stack. 19,24 Furthermore, as given the growing availability of accurate X-ray diffraction data for several stacked polymers, 27,28 those results suggest that STM could be used to perform a more quantitative analysis of the π π coupling and transport properties of π-stacked polymers. Received: May 24, 2011 Revised: July 12, 2011 Published: August 15, 2011 r 2011 American Chemical Society dx.doi.org/ /jp204832q J. Phys. Chem. C 2011, 115,

2 Figure 3. Variation of the PDOS of carbon atoms from layer 2 of a three-layer stack as a function of the interlayer distance (d π ). Figure 1. Molecular model of the unit cell considered for a polythiophene multilayer stack on graphite. Figure 4. Influence of the stack thickness on the PDOS of carbon atoms from the first adsorbed PTP layer. Figure 2. Variation of the projected density of states (PDOS) of carbon atoms from polythiophene with the polymer graphite distance. In summary, the electronic structure and, more importantly, the charge transport properties of both molecular and polymer stacks showing substantial π π interactions are drastically different to those of isolated species. Although several techniques can indirectly detect their existence, a more direct approach is still needed. By means of firstprinciples calculations and STM simulations, we have evaluated the strength of STM to characterize the presence and the magnitude of π-electron interactions in highly organized π- stacked polymer. The results we obtained for a simple polythiophene polymer, and our subsequent interpretation of the π-coupling, can be extended to any stack of conjugated polymer or oligomer. COMPUTATIONAL DETAILS We have performed first-principles density functional theory (DFT) computations with the SIESTA software package 29 to determine the electronic properties of polythiophene (PTP) stacks adsorbed on a graphite substrate. The DFT calculations were carried out within the local density approximation (LDA). Although LDA is well known for underestimating the band-gap (E g ) value for organic semiconductors, 30 such deviations have a negligible effect on our final conclusions, which are based on the relative variation of E g. All computations were performed using periodic boundary conditions in conjunction with norm-conserving dx.doi.org/ /jp204832q J. Phys. Chem. C 2011, 115,

3 Figure 5. Comparison of the shape of the LUMO of the isolated polymer (a) to the STM contrast for a single PTP adsorbed layer (b). The variation of electronic properties of both graphite and PTP before and after adsorption is shown in (c) through the PDOS of carbon atoms. Troullier Martins pseudopotentials and a double-ζ polarized linear combination of numerical atomic orbital basis sets. The pseudopotentials generated with the ATOM software 29 for the different atomic species reproduce the all-electron eigenvalues and excitation energies of multiple atomic configurations within a 1 mry error. STM simulations were performed with our SPAGS-STM (Strongly Parallel Adaptive Grid Solvers STM) software to evaluate topographic mode images and scanning tunneling spectra (STS). The software includes several algorithmic strategies, such as parallel computation of the tunnel currents 31 and adaptive grids that minimize the probing sites needed to obtain a highresolution image. 32 In STM simulations, the tunnel current was computed within a scattering approach based on the Landauer B uttiker formalism 33 along with an extended H uckel theory (EHT) Hamiltonian. 34 EHT parameters used for STM simulations were evaluated from DFT calculations following the procedure of Cerda and Soria. 35 The polymer chains within the stack are rotated by 180 to each other in an ABA pattern. The distance (d ss ) between the first PTP chain (the closest to graphite) and the graphite surface was fixed at 3.5 Å while the interlayer distance (d π ) within the stack was varied (3.5 e d π e 4.2 Å) to determine the influence of the PTP packing on the π-electron coupling. Such an ABA configuration was found to be the most stable structure in several cases of polythiophene-based stacks. 14 Because the orientation of the polymer chain along the surface has a weak influence on electronic properties of PTP, 36 we have only considered the caseofwell-alignedptp chains along the armchair direction of the graphite surface. The unit cell used to perform DFT and STM simulations is shown in Figure 1 for the case of three PTP layers. In the DFT calculations, because the interaction of the physisorbed polymer stack with the substrate is expected to be relatively weak, and that we are mostly interested in the electronic properties of the stack, we have modeled the graphite surface by a single graphene layer. 37 For STM simulations, the unit cell (Figure 1) corresponds to the bottom part of a tunnel junction that sits on a semi-infinite electrode graphite, and the upper part of the junction contains the STM tip and is attached to a semi-infinite metal (Pt) electrode. Except for the case of single PTP layer STM simulations, the first polymer layer over graphite was fixed at d ss =3.5Åwhiletheinterlayerdistance (d π )betweenlayers1 2and2 3wassimultaneouslyvaried. Figure 6. Influence of the interlayer distance d π on the resulting STM contrasts of different stacks of polythiophene (I = 0.1 na, V = 1.5 V). RESULTS AND DISCUSSION Electronic Properties of π-stacked Polythiophene. To distinguish the π π interactions originating from the polymer chains within the stack from the interaction between the stack and the graphite substrate, Figure 2 shows the variation of the projected density of states (PDOS) in the vicinity of the Fermi energy for carbon atoms at several polymer graphite distances (d ss ). First, the PDOS of isolated PTP gives sharp π (E < E F ) and π* (E > E F ) bands, where the DFT-LDA calculated band gap (E g = 1.45 ev) is, as expected, underestimated with respect to the experimental value (2.00 ev). Nevertheless, because we are mainly interested in the variation of the π-electron coupling, that is, in the relative changes in the band-gap value, this discrepancy of LDA does not have an impact on our general conclusions. In agreement with the previous work of Dubois et al., 36 the presence of graphite has a relatively weak influence on the electronic properties of PTP, even when the polymer becomes significantly close to the substrate. From a height of Å, the band gap decreases only from 1.32 to 1.20 ev due to the weak π-electron overlap between frontier electrons located on the polymer backbone and the delocalized π-electrons on graphite. This weak π-electron coupling is also revealed through the small increase in band dispersion for both π and π* bands as the PTP graphite distance d ss decreases. Hence, we may conclude that graphite interacts weakly with PTP, introducing only a weak dispersion of its π and π* states dx.doi.org/ /jp204832q J. Phys. Chem. C 2011, 115,

4 Figure 7. Influence of the interlayer distance d π on the resulting STM contrasts for a three-layer stack (I = 0.1 na, V = 1.5 V). Figures 3 and 4 clearly demonstrate the importance of interlayer interaction on the magnitude of the π-coupling within the polymer stack. Figure 3 shows the projected DOS of carbon atoms from the second layer of a three PTP layers stack for PTP interlayer distances (d π ) of 3.5, 3.7, and 4.2 Å. The height of the PTP stack (between layer 1 and graphite) over graphite was always fixed at 3.5 Å. The decrease in the band-gap value when d ss varies from 4.2 down to 3.5 Å observed in Figure 3 ( 0.40 ev) reveals an improved π-electron coupling as the interlayer distance decreases. At the larger interlayer considered (d π = 4.2 Å), the π-coupling is more important within the stack (E g = 1.04 ev) than between PTP and graphite for shorter d ss distances; E g is 1.20 ev at d ss = 3.5 Å. A similar behavior is observed in Figure 4, where we show the effect of stacked PTP on the electronic properties of the first layer. The single adsorbed layer gives rise to abandgapofe g =1.20eV,whereasthepresenceoftwoadditional stacked PTP layers contributes to decrease the band gap down to 0.73 ev. The variation of electronic structure properties observed here are in excellent agreement with previous works in which a variation of the interlayer distance has been considered in molecular and polymer stacks. 14,38,39 In addition, the presence of the multiplet features in the PDOS (see Figures 3 and 4) originates from two sources. First, an increasing overlap between the frontier orbital of the different PTP layers destabilizes the HOMO-band states while it stabilizes the LUMO-band states. 14 These variations in orbital overlap create the additional peaks observed in the vicinity of the Fermi energy. Second, although the distance between the PTP layers is reduced, the one-dimensional character along the polymer backbone direction still remains quite significant. Hence, we observe a dx.doi.org/ /jp204832q J. Phys. Chem. C 2011, 115,

5 Figure 8. Analysis of the calculated STM profiles along (middle panel) the backbone direction for a three-layer polythiophene film for d π = 3.3 Å, and across (bottom panel) the polymer chain for a three-layer stack separated by different interlayer distances (d π ). Arrows in the bottom panel indicate a decreasing interlayer distance, 3.3 Å e d π e 4.2 Å. STM images were computed at I = 0.1 na and V = 1.5 V. reduction of the band gap, but the nature of these weakly bound states near E F still shows the well-known van Hove singularities associated with 1D systems. Finally, the π-coupling between PTP and graphite remains relatively weak even at short distances between the adsorbed PTP layer and the surface. In contrast, the π-electron coupling is much more significant within stacked polymer layers where we have calculated a sharp decrease in the band gap of PTP from the isolated case (1.46 ev) to three PTP layers stacked by d π = 3.5 Å (0.66 ev). Analysis of STM Contrast. The comparison between the LUMO wave function obtained from a DFT calculation on isolated polythiophene and an STM simulation for a single adsorbed layer model is given in Figure 5. The simulated STM contrasts are consistent with the nature of the LUMO of an isolated PTP. Nevertheless, the contribution from sulfur to the LUMO wave function is much less apparent in the resulting STM image. This behavior indicates that the sulfur atom is not an efficient medium for electron transport. This trend is in agreement with a previous observation on π-stacks of 4,4 0 -biphenyldithiol 38 and with a more recent work on polythiophene crystal 40 that revealed a very low conductance for the bands containing a significant contribution from the sulfur states. In addition, the zigzag structure revealed in our STM simulations is also perceptible in the high-resolution STM images obtained by Grevin et al. on rrp3ht films. 41 Figure 6 shows the influence of the interlayer distance d π on the STM contrasts for stacks containing up to three polythiophene layers. The STM images for a single layer are nearly independent of the height of PTP over the graphite surface; the STM tip follows a similar corrugation pattern and remains at virtually the same tip sample distance in the three PTP graphite distances considered. More drastic variations in STM contrasts are observed along the formation of the polythiophene stacks represented in Figure 6. Because the height of the first PTP layer has a weak influence on the resulting STM contrasts, the first layer was fixed at 3.5 Å for the stacks containing two and three layers. As the interlayer distance d π decreases, the STM contrasts appear broader in the direction perpendicular to the PTP backbone and the atomic details also become much broaden. These trends can be visualized along the formation of the multilayer stack for a given d π, and for a given stack, but at different d π values. Figure 7 gives a different perspective of the influence of the packing distance d π on the resulting STM contrasts for the specific case of a three layers PTP stack. As mentioned above, the broadening of the contrast across the polymer increases with a decreasing d π to a point where we distinguish STM contrasts originating from neighboring unit cells. The three-layer models clearly show that the atomic details of the STM images become sharper and more intense for larger d π values. The increasing broadening with a decreasing d π can be easily explained by an increasing amount of π π coupling between PTP layers that contributes to disperse the π-band states and to repel the π-electrons from the interplanar region. 14 This variation in STM contrasts suggests that the STM technique can be used to probe the presence of π-electron interactions in molecular stacks made of π-conjugated systems. A more extended analysis of the STM contrast along the polymer backbone direction is performed in Figure 8 for a stack with d π = 3.3 Å.The periodicity between maxima along line A or C is 8.55 Å, which is in very good agreement with the molecular periodicity between equivalent thiophene units of 8.52 Å. A similar agreement is found between the calculated STM image (4.28 Å) and the theoretical (4.27 Å) periodicity of the C C bonds separating every thiophene unit. In contrast to the representation of the LUMO of isolated polythiophene, the corrugation of the C C bonds along the A and C lines is more important than that for the C C bonds along line B. A similar analysis was performed for the STM profiles along the perpendicular direction of the backbone but where we dx.doi.org/ /jp204832q J. Phys. Chem. C 2011, 115,

6 Figure 9. Variation of transport properties in a stacked polymer containing (a) one, (b) two, and (c) three layers with various interlayer distances d π. The distance between the graphite and the PTP layers (d ss ) is fixed at 3.5 Å except for the case of the single-layer model (a) where d ss was varied from 3.3 to 4.2 Å. considered the influence of d π on the broadening of the STM contrasts. The lower panel of Figure 8 shows the different profiles calculated along the red line identified by the large arrow in the upper panel for 3.3 Å e d π e 4.2 Å. The zone near the backbone edges is associated with the spreading of the π-states and reflects the magnitude of the π π coupling that increases as d π decreases. From d π = Å, the lateral size of STM contrasts for the polymer stack increases by nearly 4.0 Å. The zone of maximal tunnel current (or height) near 15 Å gives additional information on the transport properties through the stack. The calculated height from the STM contrast shows a decrease of 0.7 Å, although the real difference in the stack height when d π varies from 4.2 to 3.3 Å is about 1.8 Å. This discrepancy will be discussed in the next section. The changes observed in the spreading of π-states across the PTP as well as the loss of atomic details in the tightly packed polythiophene stack can be explained through a quite simple mechanism. The polythiophene polymer is characterized by the presence of delocalized π-conjugated electrons of a given thickness on both sides of the PTP backbone. When the π-electron clouds between two parallel layers overlap, the electron electron repulsion increases and the π-electron cloud that is sandwiched within the interlayer zone is spread through the edges. An increasing amount of π-electron states across the backbone direction enhances the tunneling of electrons through the stack and also contributes to screen the atomic details of the polythiophene layers underneath. Decomposition of the Tunneling Current. In the present section, we provide a simple model of the calculated tunneling current through the different polythiophene stacks considered. Figure 9 establishes the relation between the calculated stack height determined from the maximal STM corrugation and the theoretical (real) stack thickness that is equivalent to a distance given by d ss +(n 1)d π, where n is the number of layers. The dotted line represents an ideal linear relation between calculated and theoretical height values in which the height value calculated at d π = 3.7 Å is taken as a reference. The PTP stack containing a single layer is quite interesting; the STM tip follows a similar corrugation pattern independently of the height of PTP over the graphite surface. In the absence of chemical bonds between PTP and the graphite surface, the system can be described by a simple junction made of two tunneling barriers, as shown on the left panel of Figure 9a. As pointed out above, the molecular levels (or states) of the polymer through which electrons can tunnel are not affected by a variation of the height of the layer over graphite in the range of Å. The resulting tunnel current depends on the total width of the tunnel barrier formed from the upper (L 1 ) and the lower (L 2 ) gap between the polymer layer and the respective electrode. Hence, dx.doi.org/ /jp204832q J. Phys. Chem. C 2011, 115,

7 Figure 10. Influence of the fixed tunnel current used in the topological imaging mode on the determination of stacked polymer height for highly packed polymer thin film (current in na, 10 9 A). for a given tip graphite distance, the summation of L 1 and L 2 remains constant, such as the total probability of tunneling is proportional to T µ e γ 1 L 1 e γ 1 L 2 ¼ e γ 1 ðl 1 þ L 2 Þ ð1þ where γ 1 is the decay factor associated with the electronic structure of PTP. This result clearly indicates that the STM technique cannot allow a precise measurement of the height of a physisorbed layer. The difference between the measured heights essentially depends on the balance between the so-called through molecule (TM) contribution that is changing from a molecule or polymer to another, while the through space (TS) contribution is remaining nearly constant. 42,43 More surprising results occur for multilayered systems containing two or three PTP layers, as shown in Figure 9b,c. In these cases, we may distinguish two types of regimes associated with different potential barriers: one is similar to the one described for the single-layer system (γ 1 ) and a second one (γ 2 ), in which the tunneling is assisted by the presence of π-electron coupling between PTP layers that strongly depends on the magnitude of d π. Equation 1 can be modified to include this new contribution, such as T µ e γ 1 ðl 1 þ L 2 Þ e γ 2 d π ð2þ where γ 2 is the decay factor associated with the π-states of PTP that overlap within the stack and d π is the interlayer distance. The right panels of systems containing two and three layers show essentially the same trend: a change of the STM regime in the calculated height of PTP stacks when the interlayer distance d π becomes larger than 3.7 Å. Beyond that d π value, the height of the STM tip increases slower in order to maintain a constant current through the stack and then it converges to a constant height as γ 2 becomes comparable to γ 1 due to the gradual decrease in π π coupling. As a matter of fact, for a distance d π < 3.7 Å, the calculated height for the STM tip is higher than the theoretical curve (with respect to the signal evaluated at 3.7 Å), while the tip becomes much closer to the top of the stack for d π > 3.7 Å. These variations are consistent with a higher electron conductance (large π π coupling) within the stack for small d π distances, and lower conductance (small π π coupling) for large d π distances. Hence, the precise height of the stack depends on the coupling between polymer layers, which determines the electron transport properties in the stacking direction. To avoid any interferences from the tip surface interaction in a constant current mode, we have evaluated the influence of the tunnel current on the calculated height for the case of one, two, and three adsorbed layers of PTP. Figure 10 gives the maximum values of the corrugation of the stacks where we considered a constant current over 3 orders of magnitude. Assuming a similar thickness of the π-electron cloud for polythiophene and graphite, the difference between curves represents the interlayer distance. First, we want to emphasize that the tip remains at a relatively large distance (>5 Å) from the graphite over a large range of currents. Hence, this result supports the absence of significant tip surface interactions that could influence STM contrasts. Since the structure of the stack is constant, the other curves confirm the validity of eq 2 in describing the exponential dependency of the tunneling barrier length on the overall transmission probability, and hence on the tunnel current. CONCLUSIONS In summary, we have demonstrated from DFT calculations and STM simulations that an improved π π coupling between stacked polymer layers broadens the contrasts of STM images and attenuates the fine atomic features associated with the structure of the topmost polymer layer. The tunneling transport within the stack is strongly influenced by the distance separating the polymer layers within the stack. Consequently, it appears very difficult to determine the structural properties (stack height, interlayer distance) of a thin-layer stack only on the basis of STM contrast measurements. For multiple-layer stacks, we have clearly identified two different tunneling regimes where one is assisted by π π coupling that enhances the tunneling current, whereas the other is a pure tunneling transport mechanism. This variation in STM contrasts suggests that the STM technique can be used to probe the presence of π-electron interactions in molecular stacks made of π-conjugated systems. AUTHOR INFORMATION Corresponding Author * alain.rochefort@polymtl.ca (A.R.), stephane.bedwani@ polymtl.ca (S.B.), alm@ornl.gov (A.L.-B.). ACKNOWLEDGMENT This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC). Computational resources were provided by the Reseau quebecois de calcul de haute performance (RQCHP), Compute Canada, and the National Center for Computational Sciences at Oak Ridge National Laboratory (ORNL) supported by the Office of Science dx.doi.org/ /jp204832q J. Phys. Chem. C 2011, 115,

8 of the U.S. Department of Energy under Contract No. DE-AC05-00OR A.L.-B. acknowledges the support from the Center for Nanophase Materials Sciences, sponsored at the ORNL by the Division of Scientific User Facilities, U.S. Department of Energy. Finally, S.B. is grateful to FQRNT for an international scholarship. REFERENCES (1) Torsi, L.; Dodabalapur, A.; Rothberg, L.; Fung, A.; Katz, H. Charge transport in oligothiophene field-effect transistors. Phys. Rev. B 1998, 57, (2) Dimitrakopoulos, C.; Malenfant, P. Organic thin film transistors for large area electronics. Adv. Mater. 2002, 14, (3) Liu, P.; Wu, Y.; Pan, H.; Li, Y.; Gardner, S.; Ong, B. S.; Zhu, S. Novel high-performance liquid-crystalline organic semiconductors for thin-film transistors. Chem. Mater. 2009, 21, (4) Huynh, W.; Dittmer, J.; Alivisatos, A. Hybrid nanorod-polymer solar cells. Science 2002, 295, (5) Kline, R. J.; McGehee, M. D. Morphology and charge transport in conjugated polymers. J. Macromol. Sci., Part C: Polym. Rev. 2006, 46, (6) van Bavel, S.; Sourty, E.; de With, G.; Veenstra, S.; Loos, J. Threedimensional nanoscale organization of polymer solar cells. J. Mater. Chem. 2009, 19, (7) Pei, J.; Yu, W.; Huang, W.; Heeger, A. A novel series of efficient thiophene-based light-emitting conjugated polymers and application in polymer light-emitting diodes. Macromolecules 2000, 33, (8) Jousselme, B.; Blanchard, P.; Levillain, E.; Delaunay, J.; Allain, M.; Richomme, P.; Rondeau, D.; Gallego-Planas, N.; Roncali, J. Crownannelated oligothiophenes as model compounds for molecular actuation. J. Am. Chem. Soc. 2003, 125, (9) Che, Y.; Datar, A.; Yang, X.; Naddo, T.; Zhao, J.; Zang, L. Enhancing one-dimensional charge transport through intermolecular pi-electron delocalization: Conductivity improvement for organic nanobelts. J. Am. Chem. Soc. 2007, 129, (10) Faleev, S.; Leonard, F. Electron transport through π-stacked molecular multilayers: Metal-semiconductor transition. Phys. Rev. B 2007, 76, (11) Keg, P.; Lohani, A.; Fichou, D.; Lam, Y. M.; Wu, Y.; Ong, B. S.; Mhaisalkar, S. G. Direct observation of alkyl chain interdigitation in conjugated polyquarterthiophene self-organized on graphite surfaces. Macromol. Rapid Commun. 2008, 29, (12) Lim, J. A.; Liu, F.; Ferdous, S.; Muthukumar, M.; Briseno, A. L. Polymer semiconductor crystals. Mater. Today 2010, 13, (13) Sirringhaus, H.; Brown, P.; Friend, R.; Nielsen, M.; Bechgaard, K.; Langeveld-Voss, B.; Spiering, A.; Janssen, R.; Meijer, E.; Herwig, P.; de Leeuw, D. Two-dimensional charge transport in self-organized, highmobility conjugated polymers. Nature 1999, 401, (14) Maillard, A.; Rochefort, A. Structural and electronic properties of poly(3-hexylthiophene)-stacked crystals. Phys. Rev. B: Condens. Matter Mater. Phys. 2009, 79, (15) van de Craats, A.; Warman, J.; Fechtenkotter, A.; Brand, J.; Harbison, M.; M ullen, K. Record charge carrier mobility in a roomtemperature discotic liquid-crystalline derivative of hexabenzocoronene. Adv. Mater. 1999, 11, (16) Gearba, R.; Lehmann, M.; Levin, J.; Ivanov, D.; Koch, M.; Barbera, J.; Debije, M.; Piris, J.; Geerts, Y. Tailoring discotic mesophases: Columnar order enforced with hydrogen bonds. Adv. Mater. 2003, 15, (17) Salleo, A.; Kline, R. J.; DeLongchamp, D. M.; Chabinyc, M. L. Microstructural characterization and charge transport in thin films of conjugated polymers. Adv. Mater. 2010, 22, (18) Brabec, C. J.; Heeney, M.; McCulloch, I.; Nelson, J. Influence of blend microstructure on bulk heterojunction organic photovoltaic performance. Chem. Soc. Rev. 2011, 40, (19) Mena-Osteritz, E.; Meyer, A.; Langeveld-Voss, B.; Janssen, R.; Meijer, E.; Bauerle, P. Two-dimensional crystals of poly(3-alkylthiophene)s: Direct visualization of polymer folds in submolecular resolution. Angew. Chem., Int. Ed. 2000, 39, (20) Gesquiere, A.; De Feyter, S.; De Schryver, F.; Schoonbeek, F.; van Esch, J.; Kellogg, R.; Feringa, B. Supramolecular pi-stacked assemblies of bis(urea)-substituted thiophene derivatives and their electronic properties probed with scanning tunneling microscopy and scanning tunneling spectroscopy. Nano Lett. 2001, 1, (21) Grevin, B.; Rannou, P.; Payerne, R.; Pron, A.; Travers, J. Scanning tunneling microscopy investigations of self-organized poly- (3-hexylthiophene) two-dimensional polycrystals. Adv. Mater. 2003, 15, (22) Crispin, X.; Cornil, J.; Friedlein, R.; Okudaira, K.; Lemaur, V.; Crispin, A.; Kestemont, G.; Lehmann, M.; Fahlman, M.; Lazzaroni, R.; Geerts, Y.; Wendin, G.; Ueno, N.; Bredas, J.; Salaneck, W. Electronic delocalization in discotic liquid crystals: A joint experimental and theoretical study. J. Am. Chem. Soc. 2004, 126, (23) J ackel, F.; Watson, M.; M ullen, K.; Rabe, J. Tunneling through nanographene stacks. Phys. Rev. B 2006, 73, (24) Scifo, L.; Dubois, M.; Brun, M.; Rannou, P.; Latil, S.; Rubio, A.; Grevin, B. Probing the electronic properties of self-organized poly(3- dodecylthiophene) monolayers by two-dimensional scanning tunneling spectroscopy imaging at the single chain scale. Nano Lett. 2006, 6, (25) Spano, F. C.; Clark, J.; Silva, C.; Friend, R. H. Determining exciton coherence from the photoluminescence spectral line shape in poly(3-hexylthiophene) thin films. J. Chem. Phys. 2009, 130, (26) Clark, J.; Silva, C.; Friend, R.; Spano, F. Role of Intermolecular coupling in the photophysics of disordered organic semiconductors: Aggregate emission in regioregular polythiophene. Phys. Rev. Lett. 2007, 98, 1 4. (27) Brinkmann, M.; Rannou, P. Molecular weight dependence of chain packing and semicrystalline structure in oriented films of regioregular poly(3-hexylthiophene) revealed by high-resolution transmission electron microscopy. Macromolecules 2009, 42, (28) Kayunkid, N.; Uttiya, S.; Brinkmann, M. Structural model of regioregular poly(3-hexylthiophene) obtained by electron diffraction analysis. Macromolecules 2010, 43, (29) Soler, J. M.; Artacho, E.; Gale, J.; Garca, A.; Junquera, J.; Ordejon, P.; Sanchez-Portal, D. The SIESTA method for ab initio order- Nmaterialssimulation.J. Phys.: Condens. Matter 2002, 14, (30) Salzner, U.; Pickup, P.; Poirier, R.; Lagowski, J. Accurate method for obtaining band gaps in conducting polymers using a DFT/hybrid approach. J. Phys. Chem. A 1998, 102, (31) Janta-Polczynski, B. A.; Cerda, J. I.; Ethier-Majcher, G.; Piyakis, K.; Rochefort, A. Parallel STM imaging of low dimensional nanostructures. J. Appl. Phys. 2008, 104, (32) Bedwani, S.; Guibault, F.; Rochefort, A. Nanoscale adaptive meshing for rapid STM imaging. J. Comput. Phys. 2008, 227, (33) B uttiker, M.; Imry, Y.; Landauer, R.; Pinhas, S. Generalized many-channel conductance formula with application to small rings. Phys. Rev. B 1985, 31, (34) Cerda, J.; Van Hove, M. A.; Sautet, P.; Salmeron, M. Efficient method for the simulation of STM images. I. Generalized Greenfunction formalism. Phys. Rev. B 1997, 56, (35) Cerda, J.; Soria, F. Accurate and transferable extended H uckeltype tight-binding parameters. Phys. Rev. B 2000, 61, (36) Dubois, M.; Latil, S.; Scifo, L.; Grevin, B.; Rubio, A. Scanning tunneling spectroscopy simulations of poly(3-dodecylthiophene) chains adsorbed on highly oriented pyrolytic graphite. J. Chem. Phys. 2006, 125, (37) Rochefort, A.; Wuest, J. D. Interaction of substituted aromatic compounds with graphene. Langmuir 2009, 25, (38) Rochefort, A.; Martel, R.; Avouris, P. Electrical switching in π-resonant 1D intermolecular channels. Nano Lett. 2002, 2, (39) Rochefort, A.; Boyer, P.; Nacer, B. Resonant tunneling transport in highly organized oligoacene assemblies. Org. Electron. 2007, 8, dx.doi.org/ /jp204832q J. Phys. Chem. C 2011, 115,

9 (40) Zhou, L.; Wang, X.-F.; Han, Q.; Wu, J.-C.; Li, Z.-Y. Crystallitic orientation effects on charge transport in polythiophene thin-film transistors. Appl. Phys. Lett. 2010, 2, (41) Grevin, B.; Rannou, P.; Payerne, R.; Pron, A.; Travers, J. Multiscale scanning tunneling microscopy imaging of self-organized regioregular poly(3-hexylthiophene) films. J. Chem. Phys. 2003, 118, (42) Sautet, P. Atomic adsorbate identification with the STM: a theoretical approach. Surf. Sci. 1997, 374, (43) Bocquet, M.-L.; Sautet, P. Molecular transparence and contrast with the STM: a theoretical comparison of carbon monoxide and ethylene. Surf. Sci. 1998, 415, dx.doi.org/ /jp204832q J. Phys. Chem. C 2011, 115,