ARTICLE pubs.acs.org/Langmuir
Self-Templating Polythiophene Derivatives: Electronic Decoupling of Conjugated Strands through Staggered Packing Amandine Bocheux,†,‡ Ibtissam Tahar-Djebbar,‡ Celine Fiorini-Debuisschert,† Ludovic Douillard,† Fabrice Mathevet,‡ Andre-Jean Attias,*,‡ and Fabrice Charra*,† †
CEA/DSM/IRAMIS/Service de Physique et Chimie des Surfaces et Interfaces/Laboratoire de Nanophotonique CEA Saclay, 91191 Gif-sur-Yvette Cedex, France ‡ Laboratoire de Chimie des Polymeres-UMR 7610, Universite Pierre et Marie Curie, 4 Place Jussieu-case 185, 75252 Paris Cedex 05, France ABSTRACT: Whereas molecular electronics needs well-controlled 3D geometries for decoupling or interconnecting individual molecules, conjugated polymers form disordered structures when deposited on a substrate. We show that this trend can be overcome in polythiophene derivatives designed so as to exploit weak sulfur bromine interactions. A self-template effect follows, leading to staggered organizations of well-aligned electronically decoupled conjugated strands, as observed in situ by scanning tunneling microscopy and spectroscopy on graphite.
’ INTRODUCTION Molecular self-assembly constitutes the central bottom-up approach toward the manufacturing of nanostructures. Although the early molecular building blocks—the so-called tectons— were designed to spontaneously assemble as three-dimensional structures in the bulk of a solution, the self-assembly of monolayers steered by a surface, which permits long-range order and easier coupling with top-down systems, is now privileged. Still, it is often necessary to build 3D entities in order to obtain specific properties. For instance, the decoupling of molecular orbitals from the substrate may be at the origin of negative differential resistances or of photoexcited state confinements.1 More generally, the ability to decouple or interconnect individual conjugated moieties constitutes one main requirement for molecular-scale electronics. This is currently achieved by self-assembly on surfaces of 3D molecular architectures.2,3 Beside their optoelectronic properties exploited in many applications, ranging from light-emitting devices to thin film transistors and photovoltaics,4 6 organic π-conjugated polymers may also participate in molecular electronics as semiconducting wires. In this respect, regioregular poly(3-alkylthiophenes) (P3ATs) were the focus of most of the attention due to several particularly interesting assets. First, it has been shown that they favor molecular organization, especially as 2D monolayers on surfaces,7 as evidenced notably by scanning tunneling microscopy (STM) on highly oriented pyrolytic graphite (HOPG) in ultrahigh vacuum (UHV) as well as at the solution/substrate interface.8 10 Although the first layer actually forms reasonably ordered arrangements, upper layers of conjugated oligomers and polymers on diverse substrates form merely random patterns.11 14 Second, P3ATs can be chemically derivatized in order to tune their structural properties. In particular, it has been shown that r 2011 American Chemical Society
the introduction of a bromine atom at position 3 of a thiophene ring in a bithiophene derivative can lead to the development of noncovalent intramolecular sulfur bromine interactions.15 One can thus anticipate that the presence of such halogen substituents in more complex architectures such as P3ATs could induce original weak intermolecular interactions and finally lead to predictable 3D self-assemblies. In this paper, we compare by STM, in particular by applying a novel dual-bias image acquisition technique, the 3D self-assembly of two P3ATs homologous series having either pristine or brominated alkyl chains (Scheme 1) at the interface between HOPG and a solution. We show that the functionalization produces a self-template effect which leads to staggered organizations of well-aligned electronically decoupled conjugated strands.
’ RESULTS AND DISCUSSION As a typical example for m = 10, Figure 1a,b presents a comparison of STM images of classical (P3ATs) and brominated poly(3-alkylthiophenes) (P3ATBrs). In both cases, a first layer of polymers, with an intermediate apparent height of ∼2 Å compared with HOPG (intermediate pseudocolor, see palette), presents a well-organized two-dimensional polycrystalline structure covering most of the surface, consistent with previous observations made on nonbrominated P3ATs.9,10 Interstrand lateral distances were found to vary slightly from domain to domain in the same image. Typical distances are in the range 1.50 1.69 nm for P3HT, 1.56 1.87 nm for P3HTBr, Received: May 11, 2011 Revised: June 24, 2011 Published: July 01, 2011 10251
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Scheme 1. Chemical Structures of Regioregular HT-HT Poly(3-alkylthiophenes) and Poly(3-(ω-bromoalkyl)thiophenes)
Figure 1. Comparison of STM images of (a) P3DT on HOPG at 3.11 10 6 M in phenyloctane (IT = 50 pA, VT = 1520 mV, 109 109 nm2) and (b) P3DTBr on HOPG at 7.78 10 6 M in phenyloctane (IT = 50 pA, VT = 1440 mV, 109 109 nm2).
1.93 2.39 nm for P3DT, 2.17 2.63 nm for P3DTBr, and 2.98 3.28 nm for P3PDTBr. Considering symmetry, the observation of three, rather than six, possible orientations of monodomains shows that the adsorption of the P3ATs enforces the alignment of the polymer strands, that is, the main axis of the conjugated polymer backbone, along one of the two possible mirror-symmetry axes of the topmost atomic layer of HOPG.16 A comparison of the polymer orientation and HOPG lattice obtained by increasing the setpoint current shows that the polymer strands are oriented along Æ2,1,0æ axes of HOPG. A second layer can be observed (brighter pseudocolor, apparent height of ∼4 Å above HOPG) that is clearly organized in the case of the brominated P3ATs, in striking contrast with nonbrominated ones. For these latter ones, the second layers are constituted of isolated strands intersecting the underlying chains at random angles. On the contrary, the polymers bearing a Br atom clearly present a well-organized upper layer (Figures 1b and 2a), where polymer strands lie aligned following the same three HOPG directions as the first layer. Moreover, apart from some chains that remain isolated, there is a marked trend for secondlayer ones to aggregate together so as to form parallel bundles of various widths ranging from two to tens of well-aligned strands. They present a clear trend for alignment with respect to the underlying layer and with the same periodicity. Finally, a careful observation of the borders of the second-layer domains, e.g., for P3HTBr in Figure 2b (see also enlarged view in the inset), suggests that conjugated backbones are staggered, upper ones laying exactly in between lower ones. Increasing the concentration of solutions does not influence the respective organization of P3AT or P3ATBr derivatives but only the second layer coverage. For the highest concentrations (∼5.6 10 5 M), the inception of a third level was observed in the case of P3PDTBr (Figure 2c) and confirmed by the height profile (Figure 2d), where the apparent heights of the three steps
Figure 2. Topographic STM images on HOPG of (a) P3DTBr, at 7.78 10 6 M in phenyloctane (IT = 50 pA, VT = 1440 mV, 60 43 nm2), with some stacks of polymers and (b) P3HTBr, at 9.77 10 6 M in phenyloctane, with a small amount of second layer but clearly located between the chains of the first one (IT = 240 pA, VT = 1400 mV, 90 58 nm2) with, inset, its corresponding zoom (IT = 240 pA, VT = 1400 mV, 10 10 nm2). (c) Topographic STM image of P3PDTBr on HOPG (IT = 100 pA, VT = 1225 mV, 117 117 nm2), at 1.08 10 5 M in phenyloctane, showing a second layer organized with some brighter spots belonging to a third layer. The blue line corresponds to the topographic profile given in part d.
are similar. At the biases applied for images described above, 1440 mV, the second layer appears very bright and completely hides the underlying one. In order to analyze in more detail the relative electronic structure of the two superimposed layers, we performed spectroscopic measurements. Since it appeared that, at the solid/liquid interface, acquisitions of dI/dV (V) curves over full-range biases are not reliable enough, probably because of solution polarization, electrochemical reactions, or desorption phenomena leading to unstable tunneling junctions, we programmed an original dual-bias image acquisition mode. It was intended to exploit specifically the peculiar electron-tunneling properties through 3D systems for imaging the buried layer, as explained in what follows. If the applied bias is tuned so as to stay within the HOMO LUMO gap of the polymer, no states will be available for resonant tunneling and the polymer will then appear only with a weak contrast resulting from nonresonant tunneling. A large contrast appears when the bias is increased so as to reach a value which will allow resonant tunneling through either LUMO or HOMO. Yet, in 3D systems, the effective gap increases with the distance to the conducting substrate. This effect is attributed both to the narrowing of the frontier orbitals through a reduced coupling with the substrate and to the build-up of an electric field between the molecule and the substrate which results in the bias being only partly applied on the tip-molecule junction. This effect has been evidenced in the P3ATs through local I/V spectroscopy (STS) measurements.8 To confirm the possibility to exploit this phenomenon for imaging, we acquired differential conductance (dI/dV) maps at intermediate biases during constant-current mode STM imaging of our systems. The example of P3DTBr is reported in Figure 3a,b. It confirms a highly contrasted tunneling behavior between first layer and naked HOPG 10252
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Scheme 2. Intermolecular Interactions of Alkyl Side Chains in Lamellar Structures of Regioregular Polythiophenes on (a) End-to-End Side Chain Interaction and (b) Side Chain Interdigitation
Figure 3. (a) Image of P3DTBr on HOPG, at 7.80 10 5 M in phenyloctane (IT = 50 pA, VT = 1200 mV, 53 42 nm2) and its corresponding dI/dV lock-in map acquired synchronously and showed in part b. (c) Images of P3DT on HOPG, at 5.60 10 5 M in phenyloctane, scanned synchronously (IT = 70 pA, VT = 550 mV, 60 60 nm2 for the left one and IT = 70 pA, VT = 1385 mV, 60 60 nm2 for the right one). (d) Images of P3DTBr on HOPG, at 1.56 10 5 M in phenyloctane, scanned synchronously (IT = 50 pA, VT = 910 mV, 31 31 nm2 for the left one and IT = 50 pA, VT = 1365 mV, 31 31 nm2 for the right one).
and, above all, between the first and the second layer. Actually, we observe a well-resolved, systematic, and uniform increase of the differential resistance when the tip is located above second-layer polymer strands, which confirms that the corresponding applied bias matches the edge of a resonance, presumably toward the polymer HOMO level. More importantly, this spectroscopy observation demonstrates that the second layer conjugated cores are electronically decoupled from the substrate, with a reduced broadening and/or the build-up of a nonzero electrostatic potential difference with the substrate. From the experimental point of view, this indicates that the second layer will appear brighter at higher biases and, conversely, less contrasted at lower biases. Thus, our homemade equipment has been programmed to image at different biases during the forward and backward line scans. That is, the applied bias is changed twice per line acquisition. Parts c and d of Figure 3 expose the resulted dual-bias image pairs for P3DT and P3DTBr, respectively, with lower backward biases and higher forward ones. Typically, the lower bias is chosen above the resonant tunneling between HOMO levels of the first layer, whereas the larger one is resonant with the second layer. The precise values
have to be tuned to match each specific case depending on parameters such as the shape and the nature of the tip. Under such conditions, the second layer is imaged distinctly at higher biases, whereas it vanishes, leaving only the image of the first buried layer, at lower biases. The comparison between the two images acquired simultaneously, each one filtering either the first or the second layer, shows that bundles of second layer strands of P3DTBr are superimposed on a single domain with the same strand orientation. It also fully confirms the systematic staggering, with second-layer conjugated backbones lying in between first-layer ones. These results show that the improved 3D organization has to be attributed to the presence of the bromine atom. A pure steric effect must be ruled out, since variations in side chain lengths have no influence on microscopic order. Hence, the driving force of the improved upper layer organization is to be searched for in a weak noncovalent bonding. In particular, the bonding between sulfur and bromine in bithiophene must be considered.15 To this aim, a better understanding of polymer adsorption scheme is needed. The organization of regioregular P3ATs has been widely studied by different techniques such as X-ray diffraction, atomic force microscopy, and scanning tunneling microscopy on various substrates.17,18,8 10 X-ray diffraction analysis indicates that HT-P3ATs take the π-stacked structure with the end-to-end packing mode (Scheme 2a) in opposition with the interdigitation mode (Scheme 2b).19 In our case, let us first notice that the variations of the average distances between polymer strands from P3HT to P3DT, from P3HTBr to P3DTBr, and from P3DTBr to P3PDTBr correspond to 1.53 ( 0.18 Å per methylene, whatever the termination of the alkyl chain. This is larger than the methylene methylene distance along the alkyl chain (1.25 Å) and is thus not compatible with the alkyl chain interdigitation scheme. Also, the thiophene thiophene distance that we measure on high-resolution images as the one showed in Figure 4a, 3.75 Å, which agrees with previous molecular simulations, is shorter than required for alkyl chain packing (4.25 Å).10 We are thus led to consider an end-to-end model for the alkyl chains of P3ATs and P3ATBrs with a tilt angle θ between the alkyl chain and the conjugated backbone (Scheme 2a).19,20 The interstrand distance changes of 1.53 Å corresponds to 2 sin(θ) 1.25 Å, which is obtained for θ ∼ 52. This angle is hardly compatible with alkyl chains lying flat on the surface in the Groszek geometry, both because their orientation is different from the Æ100æ axis of HOPG but also because an in-plane thiophene alkyl bonding would have a different geometry.21,22 However, it is confirmed by intramolecular STM observations such as the one reported in Figure 4c, acquired with P3HT. Also, the distance between two adjacent chains, 4.6 Å, is not much larger than the optimum packing distance of alkanes (4.25 Å). Accounting for the measured interstrand distance, models can be built as illustrated in Figure 4b 10253
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they are systematically located above such a rail of bromine atoms. This staggered geometry allows the strongest interactions between sulfur and bromine across the layers. Hence, the bromine sulfur weak bondings15 are at the origin of a selftemplate effect, each layer guiding the self-assembly for the next one. The distances between strands closely match those measured on multilayer films drop-casted on glass or on mica (1.636 nm for P3HT and 2.388 nm for P3DT).24 26 Owing to the proximity of the structural parameters, the self-templating is very likely to propagate this staggered growth toward the bulk during the solution growth of multilayers, although thicker films are not directly observable by STM. In conclusion, the early stages of thin-film solution growth of highly stereoregioregular poly(3-alkylthiophenes) and of bromine-terminated poly(3-alkylthiophenes) have been compared by STM at the solution/substrate interface. The use of bromineterminated P3ATs has been demonstrated to induce a selftemplating effect which steers the relative positioning of the successive layers: the strands of the second layer adopt the same HOPG direction as the underlying layer with its conjugated backbone located in between the strands of the first one in a staggered geometry. The present observations demonstrate the interest of the weak bromine thiophene bonding in the 3D molecular self-assembly of this leading class of conjugated systems. This opens new opportunities in the design of conjugated molecular systems selforganizing in a specific 3D geometry. As evidenced by spectroscopic mapping and a specially developed technique of dual-bias imaging, the particular positioning of the second-layer strands obtained here exhibits the characteristic electronic response of a 3D system decoupled from the substrate. Such systems may be of interest in nanoelectronics or nanophotonics, for example, for the realization of negative differential resistances or the confinement of excitons at the epilayer.27,28
Figure 4. (a) Intramolecular resolution STM image of P3HT on HOPG (IT = 25 pA, VT = 430 mV, 6 6 nm2). (b) CPK model showing the arrangement of two P3DTBr chains including eight thiophene units. (c) Intramolecular resolution image of P3HT on HOPG (IT = 55 pA, VT = 1140 mV, 22 9 nm2). One cycle in two is visible in the thiophene backbone and the tilt angle between the thiophene rings and the alkyl side chains can be observed. (d) Images of P3DTBr on HOPG, at 7.80 10 6 M in phenyloctane, scanned synchronously (IT = 55 pA, VT = 910 mV, 27 27 nm2 for the left one and IT = 55 pA, VT = 1215 mV, 27 27 nm2 for the right one).
’ EXPERIMENTAL SECTION
in the case of P3DTBr. The nonperpendicular orientation of alkyl chains with respect to the polymer strand explains the alternation of the appearance of the interval between polymer strands on the high-resolution image Figure 4c, which presumably corresponds to the two different orientations imposed on both sides of each strand (Scheme 2a). The aforementioned dispersion of the interstrand distance can then be explained by the difference between the packing obtained when the chains of two neighboring strands have parallel orientation or not. The trend for nesting of Br atoms is also consistent with previous observations of 1-bromohexane deposited on HOPG and observed under high vacuum conditions.22,23 As a consequence of this geometry, the Br atoms of the first layer of P3ATBrs form a rail equidistant from conjugated backbones. In Figure 4d, the dual-bias scanning technique has also been used on a sample of P3DTBr. In the two images, bromine end-groups appear as bright protrusions equidistant from strands of the first layer. Notice that the alkyl chains are not visible with such imaging parameters. The second-layer strands appear in the second image, taken at higher bias, hiding the Br spots visible with the lowest bias. This image pair unambiguously shows that
Materials. Regioregular poly(3-alkylthiophenes) and poly(3-(ωbromoalkyl)thiophenes) were synthetized through a Grignard’s metathesis polymerization (GRIM), according to the McCullough route.29,30 The Grignard metathesis polymerization generates regioregular poly(3alkylthiophenes) with precise molecular weights and very narrow polydispersities. The ratio of head tail coupling to head head and tail tail couplings was estimated from the 1H NMR integration to be more than 98%, which fully concords with the results from this type of synthesis.31 The solid polymers were washed with methanol and hexane by using a Soxhlet extractor and then was dissolved by Soxhlet extraction with chloroform. Gel-permeation chromatography measurements based on polystyrene standards gave polydispersities of 1.1 (Mn ∼ 13 kg mol 1, Mw ∼ 14.30 kg mol 1) for P3HT, 1.3 (Mn ∼ 11 kg mol 1, Mw ∼ 14.30 kg mol 1) for P3HTBr, 1.2 (Mn ∼ 12 kg mol 1, Mw ∼ 14.40 kg mol 1) for P3DT, 1.14 (Mn ∼ 14.4 kg mol 1, Mw ∼ 16.41 kg mol 1) for P3DTBr, and 1.4 (Mn ∼ 19.90 kg mol 1, Mw ∼27.86 kg mol 1) for P3PDTBr. Comparison of GPC results of molecules with MALDI measurements highlighted that the GPC results overestimated the number-averaged molecular weight Mn by a factor of 1.2 2.3.32 As a consequence, the number-average degree of polymerization is estimated to be around 45 for P3HT, P3HTBr, P3DT, P3DTBr, and P3PDTBr and was finally determined by 1H NMR by integration of the resonances due to the chain end groups.33 10254
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Langmuir Methods. The gel-permeation chromatography measurements were carried out on an Agilent 1100 Series GPC equipped with a 300 mm 7.5 PLgel Mixed-D 5 μm 10 4 Å, a refractive-index detector, and diode array UV vis detector (DAD). The column temperature and the flow rate were fixed to 40 C and 1 mL min 1 in tetrahydrofuran (HPLC grade). The STM images were recorded under ambient conditions (∼300 K) with a homemade digital system by the immersion of a 250 μm mechanically cut tip of Pt/Ir (90/10) purchased from Goodfellow into a 3 μL droplet of phenyloctane containing the polymers at a concentration between 3.11 10 6 and 5.60 10 5 M, which was deposited on a freshly cleaved 10 mm 10 mm HOPG surface (Goodfellow). The scanning piezoelectric ceramic was calibrated by means of atomic resolution obtained on HOPG images in XY directions and with flame-annealed gold through the height of steps in the Z direction. In order to ensure a correct statistical representation of our measurements concerning the structural organization of the monolayers, several images were recorded at different locations of the samples for each polymer. All the images presented, except Figure 4c, were obtained at a quasiconstant current in the variable height mode (the bias voltage VT and the tunneling current IT are indicated in figure captions) and were corrected for the thermal drift by combining two successive images with downward and upward slow-scan directions. The acquisition of differential conductance (dI/dV) maps was based on an external analog phase-sensitive lock-in amplifier (EG&G Princeton Applied Research model 5209) detecting the tunneling-current response to a voltage modulation at 4 kHz with the amplitude of 100 mV (Hameg HM8030-6 function generator). The phase was set in order to eliminate the capacitance of the STM wiring, and the range of the lock-in was 10 mV. The acquisition of lock-in output was simultaneous with constant-current (height-mode) imaging, using an auxiliary analog input of the STM electronics. The scanning rate was 2.56 ms per pixel (256 pixels per line) and the lock-in integration time was set to 3 ms in the high-stability regime.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected] (A.-J.A.); fabrice.charra@ cea.fr (F.C.).
’ ACKNOWLEDGMENT This work has been partially supported by a Ph.D. joint funding from CEA and CNRS (A.B.). ’ REFERENCES (1) Gaudioso, J.; Lauhon, L. J.; Ho, W. Phys. Rev. Lett. 2000, 85, 1918–1921. (2) Soe, W.-H.; Manzano, C.; Renaud, N.; De Mendoza, P.; De Sarkar, A.; Ample, F.; Hliwa, M.; Echavarren, A. M.; Chandrasekhar, N.; Joachim, C. ACS Nano 2011, 5, 1436–1440. (3) Bleger, D.; Kreher, D.; Mathevet, F.; Attias, A.-J.; Arfaoui, I.; Metge, G.; Douillard, L.; Fiorini-Debuisschert, C.; Charra, F. Angew. Chem., Int. Ed. 2008, 47, 8412–8415. (4) Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackay, K.; Friend, R. H.; Burns, P. L.; Holmes, A. B. Nature 1990, 347, 539–541. (5) Garnier, F.; Hajlaoui, R.; Yassar, A.; Srivastava, P. Science 1994, 265, 1684–1686. (6) Halls, J. J. M.; Walsh, C. A.; Greenham, N. C.; Marseglia, E. A.; Friend, R. H.; Moratti, S. C.; Holmes, A. B. Nature 1995, 376, 498–500. (7) McCullough, R. D.; Lowe, R. S.; Jayaraman, M.; Anderson, D. L. J. Org. Chem. 1993, 58, 904–912.
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