NANO LETTERS
Relating Substitution to Single-Chain Conformation and Aggregation in Poly(p-phenylene Vinylene) Films
2003 Vol. 3, No. 9 1191-1196
M. Kemerink,*,† J. K. J. van Duren,‡ P. Jonkheijm,‡ W. F. Pasveer,†,§ P. M. Koenraad,† R. A. J. Janssen,‡ H. W. M. Salemink,† and J. H. Wolter† COBRA Inter-UniVersity Research Institute, Laboratory of Macromolecular and Organic Chemistry, and Dutch Polymer Institute, EindhoVen UniVersity of Technology, P.O. Box 513, 5600 MB EindhoVen (The Netherlands) Received May 16, 2003; Revised Manuscript Received July 14, 2003
ABSTRACT The morphology of films of PPV derivatives is studied with molecular (single chain) resolution by phase-imaging scanning force microscopy. It is found that the symmetry of substitution is directly related to surface morphology and aggregation behavior. The molecular resolution in the phase contrast is shown to result from van der Waals interaction between the conjugated backbone of the polymer chains and the metallic tip, and can quantitatively be described by a simple harmonic oscillator model.
The electrooptical properties of semiconducting conjugated polymer films depend critically on molecular structure, doping, and morphology.1-7 Whereas the former two can be relatively well controlled during synthesis, the latter is far from being well controlled or understood. In addition, most conjugated polymer films are highly inhomogeneous, and even in relatively well-ordered films the ordering generally occurs in domains of submicron dimensions.2-6 Here, we investigate the effect of substituents attached to a poly(p-phenylene vinylene) backbone on both the conformation of individual molecular chains and aggregation at the surface of conjugated polymer films. We found straight, aligned individual chains and strong aggregation for the symmetrically substituted bisOC10-PPV (poly(2,5-bis(3′,7′dimethyloctyloxy)-1,4-phenylenevinylene)). In contrast, spiraling chains and no aggregation were observed on the asymmetrically substituted OC1C10-PPV (also known as MDMO-PPV, poly(2-methoxy-5-(3′,7′dimethyloctyloxy)1,4-phenylenevinylene)). These results are found to be consistent with the results of molecular dynamics (MD) simulations. The rest of this paper will be organized as follows. First, the experimental results will be presented and discussed in terms of structure-morphology-property relations. Here, the phase signal will just be used as a method to visualize either individual molecular chains or aggregates. * Corresponding author. E-mail:
[email protected]. † COBRA Inter-University Research Institute. ‡ Laboratory of Macromolecular and Organic Chemistry. § Dutch Polymer Institute, Group Polymer Physics, Eindhoven Polymer Laboratories. 10.1021/nl034317j CCC: $25.00 Published on Web 08/05/2003
© 2003 American Chemical Society
Chart 1.
Structure of MDMO-PPV (left) and bisOC10-PPV (right)
In the second part, the observed contrast in the phase signal will be shown to result from the van der Waals interaction between the tip and the conjugated polymer. Nominally 100 nm thick polymer films were spin cast directly from a 0.1-0.5 wt % solution in chlorobenzene. Within this range of concentrations no differences in morphology were observed. The substrates consisted of a 100 nm thick Au electrode that was thermally evaporated under high vacuum conditions on cleaned glass substrates. MDMO- and bisOC10-PPV where synthesized via the Gilch route8 and had an Mw around 1×106 g/mol and an polydispersity of about 7 as was measured by gel permeation
Figure 1. Surface topography and phase images of MDMO-PPV (a-c) and bisOC10-PPV (d-f). (a, d) Surface topography (full height scale 5 nm) measured by TM-SFM. (b, e) Phase images, simultaneously measured with a, respectively. (d) The tip apex radius was 10 nm. (c, f) phase image, measured with a blunt tip on another spot of the same samples as a, b, respectively. d, e. The tip apex radius was 40 nm.
chromatography, using a GPC column that was calibrated with polystyrene. Layer thickness was determined using a Tencor P10 surface profiler. We have used a Digital D3100 scanning force microscope (SFM) in tapping-mode (TM) in combination with a homebuilt phase detection system to simultaneously measure topography and tip-sample interaction. The tip oscillation amplitude is used as feedback signal for the SFM controller, whereas the phase signal is used as a sensitive probe of the local forces between tip and sample. The latter enables detection of the van der Waals force between a single conjugated polymer molecule and the tip, as will be shown below. All SFM measurements were performed under ambient conditions using W-covered Si cantilevers (CSC12/ W, NT-MDT). Tip apex radii were determined by measuring a test grid (TGT01, NT-MDT). Figures 1a and b show typical examples of a conventional TM-SFM height image and of a simultaneously taken phase image, measured on an MDMO-PPV film with a nominal thickness of 100 nm. The typical lack of molecular resolution in the TM-SFM topography image is apparent. In contrast, the phase image clearly shows spiraling chains, i.e., the chains lay on the surface in a circular fashion, with a rather constant diameter of about 10 ( 2 nm.9 Because of the constancy of the observed edge width over the sample surface we interpret these structures as individual polymer chains, rather than as an arbitrary number of tightly packed chains. The measured width of about 4.8 ( 1 nm inevitably exceeds 1192
the 0.55 nm that is obtained from a molecular mechanics calculation for the width of the conjugated backbone of MDMO-PPV,10 because of the finite tip apex radius and the nature of the interaction. In the second part of the manuscript the line profile will be addressed in detail. It is surprising that the individual polymers in Figure 1b seem to form connected ring-shaped structures. The constant diameter of these rings can only be explained by an average bending angle in the polymer chain of around 8° at each monomer unit (360°/number of monomer units that fit in one circumference). The most likely candidate for the bending force is an interaction, either attractive or repulsive, between the aliphatic side chains of successive monomer units. This requires that the monomer units are predominantly in the syn conformation, i.e., most OC10 side chains point to one side of the polymer and, consequently, the OC1 chains to the other side. This explains the observed rings, but not their connectedness. A natural explanation for the latter can be given by assuming the presence of conformational or configurational defects, at which the prevailing orientation of the side chains is flipped from one side of the polymer chain to the other. Because the bending direction of the polymer is reversed at such a defect, a new (part of a) ring, connected to the previous one, starts at each defect, as illustrated in the inset of Figure 2. Complete rings will dominate the surface morphology when 360/Θ0 < 1/pdef, where Θ0 and pdef are the bending angle and the probability for a defect per monomer unit, respectively. Nano Lett., Vol. 3, No. 9, 2003
Figure 2. Simulation of the surface morphology of MDMO-PPV using the Monte Carlo model described in the text. The monomer length is 0.65 nm (ref 10), Θ0 ) 8°, pdef ) 0.01, 25 macromolecules which convey 1500 repeat units are simulated. The inset illustrates the proposed side chain orientation. At a conformational defect (thick arrow), the sense of spiralling (thin arrows) reverses, leading to a morphology that consists of connected rings.
A simple Monte Carlo (MC) model, based on the considerations above, is able to reproduce the characteristic features of Figure 1b, as is demonstrated in Figure 2. The MC model builds up chains by adding monomer units one by one to a randomly positioned and directed ‘seed’ monomer unit on a two-dimensional (2D) plane with periodic boundary conditions. The probability to have successive monomers in the syn-conformation is 1 - pdef, in which case the backbone is bent in-plane by an angle Θ0. Although the resulting morphologies are, in principle, 3D as chain crossings do occur, a 2D model is adequate, as the used conjugated polymers have a thickness of only a few Å.10 Therefore, multiple rings with an approximate diameter of 100 Å can be stacked while maintaining an almost 2D situation. The used value of Θ0 is chosen to match the circle diameters in experiment and simulation, as indicated above. Increasing pdef leads to an increase in the number of incomplete circles, which does not seem to be supported by the measurements. Decreasing pdef basically results in a reduction of the number of visible circles per chain as chains spiral longer before a defect occurs and a new ring is formed. This cannot be excluded on basis of the present experiments. Due to the roughness of the film on the length scale of the ring-shaped features, and the absence of a clear correlation between height and phase images, direct experimental confirmation of the tentative model outlined above is virtually impossible. Therefore, we performed the same measurements on a similar film of the symmetrically substituted polymer bisOC10-PPV. Compared to MDMO-PPV, the OC1 side chain is replaced by another OC10 chain, so any effect of the interaction between side chains on the conformation will be canceled by the symmetry of the polymer. From the absence of a net driving force for bending in a particular direction, a more or less straight molecular conformation is Nano Lett., Vol. 3, No. 9, 2003
Figure 3. Molecular structures as calculated by MD and subsequent energy minimization. The initial structure was annealed at 600 K to emulate the effect of the rapidly evaporating solvent.12 (a, b) MDMO-PPV and bisOC10-PPV, straight starting conformation. (c) MDMO-PPV, ring-shaped starting conformation with an average diameter of 9 nm. The total energy per monomer of the final conformations in panels a, b, and c is 25.3, 22.1, and 10.1 kcal/mol, respectively. All chains consist of 96 repeat units. For clarity, H atoms are not displayed.
expected for bisOC10-PPV, which is indeed observed, see Figure 1e. The strong spiraling observed on MDMO-PPV is now clearly absent, which confirms the importance of structural symmetry, as proposed above. The fact that the topography taken together with the phase image on bisOC10PPV (panel d) is very similar to that observed on MDMOPPV demonstrates that the topography is not at all a good measure of the morphology. Finally, it should be pointed out that only those parts of chains that lay directly at the surface are visible, preventing determination of the entire chain length. It is interesting to compare our results to the conformations found by Hu et al. on basis of a bond-fluctuation model.11 In ref 11 it is shown that a prototypical PPV derivative (poly(2-methoxy-5-(2′ethylhexyl)oxy-1,4-phenylenevinylene) or MEH-PPV) can form stable toroid-shaped structures, provided that the tetrahedral defect density is sufficiently low; at a defect density of 5% these structures are no longer formed. As the density of the most abundant conjugation breaking defect in Gilch MDMO-PPV, i.e., the TBB defect occurring at 1-2% of the monomer units,8 is a lot lower, our results seem to be consistent with ref 11 regarding the possible stability of ring- or toroid-shaped conformations. Simultaneously, our data show that also other factors, such as the symmetry of substitution, are important. Further confirmation of the interpretation given above comes from molecular dynamics simulations performed on isolated chains of either MDMO-PPV or bisOC10-PPV.10,12 Starting from a straight chain, we find that MDMO-PPV has a strong tendency to yield bent conformations, with (local) radii of curvature that compare favorably with the measured radii, see Figure 3a. For bisOC10-PPV this tendency is virtually absent, see Figure 3b. The stability of 1193
regular ring-shaped starting conformations was tested, and rings constructed with the observed dimensions are found to be stable and have an energy that is considerably lower than a straight conformation. Figures 3a and c show that attractive interactions between monomers in successive turns of the spiral are essential to stabilize the ‘natural’ bending of the conjugated backbone into a toroid. Interactions between monomers of different chains cannot be excluded (they are definitely present in the film) but are not needed to explain the occurrence of rings. As expected, rings of MDMO-PPV with the aliphatic OC10 chains in the all-syn conformation and pointing outward (Figure 3c) had a lower total energy per monomer (10.1 kcal/mol) than all-syn rings with the OC10 side chains pointing inward (11.1 kcal/mol) and rings with the OC10 side chains in the all-anti conformation (15.3 kcal/mol). Stable rings could also be formed from bisOC10-PPV, but the associated energy gain of 10.1 kcal/ mol per monomer is much less than for MDMO-PPV (15.2 kcal/mol). The lack of rings in the measured morphology of the former material possibly has to do with the absence of a tendency to start bending, as witnessed by Figure 3b, causing a time scale for ring formation that is large compared to other time scales involved in the spin casting procedure. Finally, it should be pointed out that from the present experiments it cannot be concluded at which moment the final conformation is reached. A striking feature of Figure 1e is the preferential orientation of the polymer chains along the lower-left to upperright diagonal. On larger scale images this alignment can be seen to occur within 50-200 nm domains. It is tempting to identify such domains as aggregates, i.e., regions in which a significant overlap between the wave functions of single chains enables the formation of one or more states that are delocalized over several chains.1 Although alignment of conjugated backbones over multiple monomer units is probably a prerequisite for aggregate formation, it may not be sufficient. To probe the coupling within these domains, we performed the same measurements on different areas of the same samples with a deliberately blunted tip, prepared by thermally evaporating 80 nm of Al on tip and cantilever, resulting in an apex radius of approximately 40 nm. For bisOC10-PPV, almost homogeneous domains with typical lateral dimensions of 100 nm are found, see Figure 1f. In contrast, for MDMO-PPV no domains are visible and only a faint version of Figure 1b is obtained, see panel c. The loss of detail is what one would expect for a measurement with intrinsically less resolution because of a blunt tip. The different morphology observed for the two polymers excludes the possibility that Figure 1f is a mere tip artifact. In that case, one would also expect all domains to have the same shape, reflecting shape and dimension of the tip apex. This is not the case. Having excluded the possibility of a tip artifact, interpretation of these domains as aggregates is the most logical option. This identification is confirmed by similar measurements, performed with the same tip, on regioregular P3HT (poly(3-hexylthiophene)) which material is well-known to show aggregation, see Figure 4. From a line shape analysis of X-ray diffraction measurements on a 1194
Figure 4. Phase image, measured with a blunt tip (apex radius 40 nm) on regioregular P3HT. Aggregates with typical dimensions of 10-50 nm are clearly visible.
similarly prepared film, a characteristic domain size of 1013 nm is found in ref 5, which agrees relatively well with the domain size of 10-50 nm that is found in the real-space image of Figure 4. Let us now compare our findings with the interpretation of mobility measurements on devices similar to those discussed here. Martens et al. have found that the energetic disorder in bisOC10-PPV is significantly lower than in MDMO-PPV.7 They tentatively assign this to a reduced spatial disorder in the former material, as a consequence of the reduction in conformational and configurational freedom. Our results univocally confirm that both the conformational order in individual chains, as well as the ordering of multiple chains are enhanced in bisOC10-PPV, as compared to MDMO-PPV. In the remaining part, the mechanism that leads to the molecular-scale contrast in the phase image will be discussed. To explain the observed contrast we adopted a simple but quantitative model to calculate the phase shift due to the van der Waals interaction of a metallic tip with apex radius Rt with a polarizable π-conjugated sphere of radius Rp located a distance dp below the surface of a homogeneous matrix.13,14 Starting from the equation of motion mz¨ + Rz˘ + kz ) A cos(ωt) + FzvdW(z), with m the effective tip mass, R the damping coefficient, k the cantilever spring constant, and A and ω the driving force amplitude and frequency, respectively, the shift in resonance frequency ω0 and the phase φ due to the z-component of the van der Waals force FvdW are obtained from
x
k-
ω′0 )
∂FzvdW ∂z m Nano Lett., Vol. 3, No. 9, 2003
Figure 5. (a) Cartoon of the phase-shift model outlined in the text. The lateral tip position is indicated by l. (b, c) Comparison of calculated (solid lines) and measured (symbols) phase shifts for single bisOC10-PPV molecules measured with a sharp tip (b) and the edge of a bisOC10-PPV aggregate measured with a blunt tip (c). The open and solid symbols are single line sections and their average, respectively. The experimental points in (b) are line-sections across single chains in Figure 1e that are well separated from neighboring chains. As the curves are symmetric, only one-half is shown.
φ ) arctan
(
Rω m(ω′02 - ω2)
)
where the van der Waals force along r is given by15,16
FrvdW )
[
32Rt3Rp3(r + Rt + Rp) H 3 (r(r + 2R )(r + 2R )(r + 2(R + R )))2 t p t p
]
The geometrical parameters are indicated in Figure 5a. The Hamaker constant H is estimated from16,17 H ) 4π2FtFpσ6 ≈ 4.8 × 10-20 J, where Ft () 5.9 × 1028 m-3, ref 18) and Fp () 8.1 × 1027 m-3 ) 1/(PPV unit cell volume), ref 10) are the number density of tip and polymer, respectively. , σ () 1.67 × 10-21 J, 0.34 nm, ref 18) are the Lennard-Jones potential parameters. The size of the polymer test sphere is chosen such that the conjugated part of a single repeat unit, (i.e., phenyl and vinyl groups) can be contained, Rp ) (PPV unit cell volume)1/3 ) 0.5 nm.10 Since m, R, and k can be obtained from the frequency response curve of the cantilever and Rt is known from measuring a test grid, the only free parameter in the model is the effective height of the tip above the polymer, a + dp, which basically determines the magnitude of the phase shift. In Figure 5b the calculated Nano Lett., Vol. 3, No. 9, 2003
phase shift due to a single conjugated element is compared with experiment. Apparently, the model gives an excellent description of the observed line profile. We believe that it is the interaction with the conjugated backbone of the polymer that leads to the observed contrast since π electrons are by far easier to delocalize, and thus have a higher polarizability, than the σ electrons in the aliphatic side chains. This view is confirmed by repeated measurements on samples that are kept in ambient conditions, in which the phase contrast is totally lost after about 1 day due to photooxidation of the vinyl double bond which destroys the extended conjugation of the polymer.19 Also on films of polystyrene, a phenyl-containing polymer without conjugation in the backbone, no reproducible phase contrast was observed. The reason that the molecular resolution was not obtained in earlier measurements probably lies in the used tips. They combine a small spring constant with a relatively high Q factor (≈200), making them very sensitive to weak external forces. Moreover, the metal coating is required to make the van der Waals interaction sufficiently large, as was concluded from the absence of molecular resolution in the phase image when nominally identical tips without the metal coating were used. The remarkable tip apex radius dependence of the phase 1195
signal measured on aggregated materials can now be understood as follows. When Rt is of the order of, or smaller than, the lateral intermolecular distance in the surface layer, simply the response (polarization) of the nearest single molecule is probed. However, when Rt is significantly larger than this distance, the collective response of the electrons under the tip is probed. Clearly, the collective response is different, i.e., larger, for electrons that are delocalized over several molecules in an aggregate, than for electrons that are confined to a single molecule. The larger and spatially homogeneous response of the aggregate will therefore dominate the smaller local variations due to individual chains, that, in addition, are smeared out due to the tip bluntness. In the absence of aggregation, only these smeared out local fluctuations are observed, as is the case for MDMO-PPV. The phase shift observed by scanning a blunt tip over an aggregate can now be approximated by convoluting a unit step with the blunt-tip response of a single molecule, as is shown in Figure 5c. Again, the agreement with the experimental curve is good. The possibility to probe either individual conjugated polymer chains or the collective behavior of multiple chains appears to be a unique feature of the technique demonstrated above. The high spatial resolution is ideal for unraveling the relations between molecular structure and morphology, as we have shown. Moreover, we believe that the ability to visualize regions of strong interchain coupling will prove to be crucial for the understanding of future measurements of the electrooptical properties on a truly local scale. In conclusion, we have studied the morphology of PPV derivatives with molecular resolution by phase-imaging SFM. We found that the symmetry of substitution is directly related to the surface morphology and aggregation behavior. Chains of the asymmetrically substituted MDMO- (or OC1C10-) PPV form rings of about 10 ( 2 nm in diameter, whereas the chains of the symmetrically substituted bisOC10-PPV show a linear orientation. Only the latter material is found to aggregate near the surface layer, which is consistent with the observed alignment of the individual chains at the surface. Finally, the phase contrast is shown to result from van der Waals interaction between the conjugated backbone of the polymer chains and the metallic tip, and can quantitatively be described by a simple harmonic oscillator model. Acknowledgment. We gratefully acknowledge Dr. H. F. M. Schoo for providing the bisOC10-PPV. The research of
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M.K has been made possible by the Royal Netherlands Academy of Arts and Sciences. References (1) Cornil, J.; Beljonne, D.; Calbert, J.-P.; Bre´das, J.-L. AdV. Mater. 2001, 13, 1053. (2) Nguyen, T.-Q.; Schwartz, B. J.; Schaller, R. D.; Johnson, J. C.; Lee, L. F.; Haber, L. H.; Saykally, R. J. J. Phys. Chem. B 2001, 105, 5153. (3) Tan, C. H.; Inigo, A. R.; Hsu, J.-H.; Fann, W.; Wei, P. K. J. Phys. Chem. Solids 2001, 62, 1643. (4) Blatchford, J. W.; Gustafson, T. L.; Epstein, A. J.; Vanden Bout, D. A.; Kerimo, J.; Higgins, D. A.; Barbara, P. F.; Fu, D.-K.; Swager, T. M.; MacDiarmid, A. G. Phys. ReV. B 1996, 54, 3683. (5) Sirringhaus, H.; Brown, P. J.; Friend, R. H.; Nielsen, M. M.; Bechgaard, K.; Langeveld-Voss, B. M. W.; Spiering, A. J. H.; Janssen, R. A. J.; Meijer, E. W.; Herwig, P.; de Leeuw, D. M. Nature 1999, 401, 685. (6) Hassenkam, T.; Greve, D. R.; Bjørnholm, T. AdV. Mater. 2001, 13, 631. (7) Martens, H. C. F.; Blom, P. W. M.; Schoo, H. F. M. Phys. ReV. B 2000, 61, 7489. (8) Becker, H.; Spreitzer, H.; Kreuder, W.; Kluge, E.; Schenk, H.; Parker, I.; Cao, Y. AdV. Mater. 2000, 12, 42. (9) As these rings show some similarity to a well-known artifact in phase images that results from improper feedback operation, usually due to too fast scanning, it should explicitly be pointed out that these rings are independent of scan parameters. In addition, the artifact rings center around topographic maxima, which the rings in Figure 1b do not. (10) HyperChem V7.04, Molecular Modeling System, Hypercube, Inc., using MM+ force fields. (11) Hu, D.; Yu, J.; Wong, K.; Bagchi, B.; Rossky, P. J.; Barbara, P. F. Nature 2000, 405, 1030. (12) Following Mehta, A.; Kumar, P.; Dadmun, M. D.; Zheng, J.; Dickson, R. M.; Thundat, T.; Sumpter, B. G.; Barnes, M. D. Nano Lett. 2003, 3, 603, each chain was heated from 300 to 600 K in 1 ps, followed by 3 ps at 600 K and 3 ps cooling-down to 300 K. Subsequently, an energy minimization step was performed. The energies mentioned in the text are per monomer and calculated after this last step. (13) Other interactions have also been considered. In particular, the Coulomb force between charge on the sample, either fixed or induced by the work function difference between tip and sample, and the metallic tip can be of equal strength as the van der Waals force. Nevertheless, this can be excluded since the bias dependence predicted by such models is not observed. (14) Bustamante, C.; Keller, D. Phys. Today 1995, 48, 32. (15) Hamaker, H. C. Physica 1937, IV, 1058. (16) Boer, E. A.; Bell, L. D.; Brongersma, M. L.; Atwater, H. A. J. Appl. Phys. 2001, 90, 2764. (17) Argento, C.; French, R. H. J. Appl. Phys. 1996, 80, 6081. (18) Kittel, C. In Introduction to solid-state physics, 6th ed.; Wiley: New York, 1986. (19) Sutherland, D. G. J.; Carlisle, J. A.; Elliker, P.; Fox, G.; Hagler, T. W.; Jimenez, I.; Lee, H. W.; Pakbaz, K.; Terminello, L. J.; Williams, S. C.; Himpsel, F. J.; Shuh, D. K.; Tong, W. M.; Jia, J. J.; Callcott, T. A.; Ederer, D. L. Appl. Phys. Lett. 1996, 68, 2046.
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Nano Lett., Vol. 3, No. 9, 2003
