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Ultrathin Films of Semiconducting Polymers on Water Dag W. Breiby,*,†,‡ Emil J. Samuelsen,‡ Oleg Konovalov,§ and Bernd Struth§ The Danish Polymer Centre, Risø National Laboratory, P.O. Box 49, 4000 Roskilde, Denmark, Department of Physics, Norwegian University of Science and Technology (NTNU), N-7491 Trondheim, Norway, and European Synchrotron Radiation Facility, B.P. 220, 38043 Grenoble, France Received December 18, 2003. In Final Form: March 8, 2004 Ultrathin films of polythiophene derivatives spread on water were studied by means of synchrotron radiation, using grazing incidence diffraction and (specular) reflectometry to obtain the molecular orientation in the films. The semicrystalline films were anisotropic, showing a strong tendency of orienting the crystalline a-axis perpendicularly to the water subphase. The crystalline domains extend essentially through the entire sample thickness, found to be 10-15 nm. A large expansion of the unit cell a-parameter was seen upon doping the films in situ. The reflectometry data were well-fitted by a model with a sinusoidal density variation being damped toward the water subphase. This indicates that the crystalline order was most developed at the polymer-air interface and deteriorated down toward the water, possibly due to the hydrophobicity of the alkyl side chains.
Introduction Semiconducting conjugated polymers have presently reached a sufficient level of sophistication to serve as the active, light-emitting medium in polymeric light-emitting diodes (PLED) and are also considered for other applications such as solar cells, lasers, and field effect transistors (FETs).1 Since the absorptivity and emissivity of the thiophene monomers are strongly anisotropic, it is important to get increased control of the molecular orientation of these materials. For example, by having highly oriented polymers in the PLEDs, the emitted light will be polarized.1 In general, self-organization in polymers is a topic of huge current interest, with considerable effort being done to make the polymer assume complex conformations and suprastructures.2 Poly(alkylthiophenes) (PATs) are promising and much studied conjugated polymers, whose semicrystalline structure is well-established.3 The crystalline regions have an orthorhombic unit cell. The a-axis (along the side chains) is 15-35 Å depending on the length of the alkyl chain. The π-stacking distance b is typically 3.8 Å, and c is about 7.8 Å along the direction of the main chain. The structure of PAT can be considered lamellar, formed by π-stacking, and with a serving as an interlayer vector. Many applications of conjugated polymers are based on the doped (conducting) state (e.g. conducting paints1) or the switching between the doped and undoped states (e.g. artificial muscles1). The optical absorption edge at about 2.4 eV for PATs is associated with π-π* transitions and is evidence for the semiconducting nature of these materials. Upon doping, additional features appear in the absorption spectra, revealing the presence of new states in the band gap.1 Structurewise, an expansion of the * To whom correspondence should be addressed. E-mail: dag.
[email protected]. † Risø National Laboratory. ‡ Norwegian University of Science and Technology. § European Synchrotron Radiation Facility. (1) Hadziioannou, G.; van Hutten, P. F.; Eds. Semiconducting Polymers: Chemistry, Physics and Engineering; Wiley: Weinheim, Germany, 2000. (2) Jones, W.; Rao, C. N. R.; Eds. Supramolecular Organization and Materials Design; Cambridge University Press: Cambridge, U.K., 2001. (3) Samuelsen, E. J.; Mårdalen, J.; Nalwa, H. S.; Eds. In Handbook of Organic Conductive Molecules and Polymers, Vol. 3; Wiley: New York, 1997; p 87.
crystalline a-axis and a contraction of the b-axis are reported upon doping of thick (bulk) PAT films.4 For solution-cast PAT films, it has been reported that the crystalline a-axis tends to orient perpendicularly to the substrate.3-6 Interestingly, spin-cast PAT films exhibit preferred orientation with the crystalline a-axis parallel to the substrate.6-8 Because solution casting is a much slower process than spin casting, the normal alignment of the side chains is believed to be thermodynamically preferred. The films studied previously have typically been quite thick, about 0.2-200 µm. However, by substituting the alkyl side chains with hydrophilic ones, Reitzel et al. obtained monolayer films on water.9 As they report, monolayers cannot be obtained from PAT solutions, because the films tend to break into small islands when the monolayer thickness is approached. The aim of the present work is to bridge the gap between the thick solution-cast films and the monolayers obtained for a modified polymer. Of particular interest was a further study of the self-organization observed in these polymers. Possible orientation effects of the substrate were sought minimized by deposition on liquid water, the Langmuir technique. As it turned out, homogeneous films of thickness down to about 10 nm could be obtained. Studies of molecular order in these films are of high relevance to applications within molecular electronics, as 10 nm is a suitable thickness for, e.g., FETs. Synchrotron X-ray diffraction and reflectometry were used to study these few-molecular-layer Langmuir films, revealing a tendency of self-organization with the a-axis orienting perpendicu(4) Prosa, T. J.; Winokur, M. J.; Moulton, J.; Smith, P.; Heeger, A. J. Synth. Met. 1993, 55, 370. (5) Samuelsen, E. J.; Aasmundtveit, K. E.; Breiby, D. W. In Electronic and Optical Properties of Conjugated Molecular Systems in Condensed Phases; Hotta, S., Ed.; Research Signpost: Trivandrum, India, in press. (6) Aasmundtveit, K. E.; Samuelsen, E. J.; Guldstein, M.; Steinsland, C.; Flornes, O.; Fagermo, C.; Seeberg, T. M.; Pettersson, L. A. A.; Ingana¨s, O.; Feidenhans’l, R.; Ferrer, S. Macromolecules 2000, 33, 3120. (7) Samuelsen, E. J.; Aasmundtveit, K. E.; Breiby, D. W. In Electronic and Optical Properties of Conjugated Molecular Systems in Condensed Phases; Hotta, S., Ed.; Research Signpost: Trivandrum, India, in press. (8) 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. (9) Reitzel, N.; Greve, D. R.; Kjær, K.; Howes, P. B.; Jayaraman, M.; Savoy, S.; McCullough, R. D.; McDewitt, J. T.; Bjørnholm, T. J. Am. Chem. Soc. 2000, 122, 5788.
10.1021/la036399x CCC: $27.50 © 2004 American Chemical Society Published on Web 04/17/2004
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Chart 1. Chemical Structure of the Studied Polymers: (a) Poly(alkylthiophenes); (b) Poly(oxyoctylphenylthiophene)
larly to the water subphase. Also in situ doping was studied, with results indicating well-ordered intercalation layers of the PF6 dopant. UV-vis absorption spectroscopy measurements showed that the ultrathin films had an absorption spectrum resembling that of the bulk polymer both in the doped and pristine state. Experimental Section Samples. Regioregular polyhexylthiophene (R-PHT) with >98% h-t couplings purchased from Aldrich, and FeCl3 polymerized polyoctylthiophene (POT) with ∼80% h-t couplings, kindly supplied by Neste OY, were used for the present study. A partly stereoregular PAT with dodecyl side chains (PDoDT), provided by K. Levon, and also the poly(thiophene) derivative poly(oxyoctylphenylthiophene) (POOPT), obtained from M. R. Andersson, were also studied; see Chart 1 for the chemical structure of these monomers. The Langmuir technique consists of dripping a few drops of low concentration (∼3 mg/mL) of the polymer dissolved in chloroform onto a purified and deionized water surface in a Teflon trough. The solution instantly spreads on the surface, and the solvent evaporates within a second, leaving an ultrathin, faintly reddish layer floating on the surface. Efforts failed to further reduce the film thickness by using more dilute solutions, as numerous “islands” with thickness similar to that of the films from the thicker solutions were then obtained. By slowly draining the water, the films could be transferred onto horizontal, submerged glass substrates for further studies. After long exposures (∼20 min) to the intense X-ray beam, beam damage could be observed as a thin straight colorless line on the floating film. To avoid experimental artifacts hereof, the samples were routinely translated laterally with respect to the beam. Doping. The salt NOPF6 dissolved in acetonitrile, with a concentration of about 40 mg/mL acetonitrile, was used for doping the films while floating on the water subphase. Acetonitrile is miscible with water and is a specific solvent that dissolves the salt, but not the polymer. About 1 mL of the water in the subphase was removed using a syringe and replaced with the dopant solution. Iodine is often used for studying the doping process in conjugated polymers,4 but for the present purpose, the strong optical absorption of iodine, and also its volatility giving subsequent dedoping, motivated the search for other doping agents. Sample Cell. For the in situ X-ray measurements of the Langmuir films, a trough of diameter 80 mm and about 4 mm depth was placed on the goniometer. Teflon was used because of its hydrophobicity, resulting in an upward water meniscus. Since the evaporation of water leads to a descending water surface, the sample height zs had to be adjusted periodically. This was done about every 15 min, by vertically scanning zs through the direct beam, while observing the direct beam hitting a few channels of the detector. This kind of scan results in a sigmoidal-shaped curve as a function of the sample height, as the intensity drops to a low value when the film and water meniscus start shadowing the beam. The optimal height was taken as the zs value corresponding to the inflection point of the scanned intensity. To slow the descending rate of the water level, caused by evaporation, the trough was connected to a closed, bigger-area reservoir and was covered with a visually transparent hood. To
Figure 1. Sketch of the experimental geometry. The linear detector is indicated in the detector plane (∆, γ) and is seen to intercept a Debye-Scherrer ring. For GID, the incidence angle Ri ∼ 0.13° was used with water substrates, whereas for reflectometry, Ri was scanned from 0 to about 5°. The Teflon trough with water and the floating film are also indicated. For clarity, the various slits and the sample hood are not drawn, and the sample-detector distance is shown unrealistically short. avoid undesired scattering, holes were cut for the in- and outgoing beams. For reducing the background, diffraction studies on ultrathin layers on liquid surfaces are often done in helium atmosphere, which requires a closed sample cell. Experiments with helium showed that the benefit of a somewhat reduced background was outweighed by disturbing diffraction signals introduced by the Kapton windows. This work was therefore done in ambient atmosphere. Studies Using Synchrotron Radiation.10 The films were studied at the undulator beam line ID10B (‘Troika II’) at the ESRF, using a wavelength λ ) 1.554 Å. Harmonics were removed using a double-mirror setup. Being low-divergent, the width and height of the incoming beam were defined by the last upstream slit. A 50 mm 1D position sensitive detector (PSD) oriented vertically was used for collecting the scattered intensity. The active detector area was divided electronically into 300 channels. The detector could be scanned both horizontally (angle ∆) and vertically (angle γ) with respect to the direct beam. The scattering angle 2θ is given by cos(2θ) ) cos γ cos ∆. For studying the films in situ on the (horizontal) water surface,11 the incoming beam must be tilted slightly down, which was achieved by rotating a single crystal about the incident beam while keeping it at Bragg reflection. To keep the sample in the proper position with respect to the incoming beam, it is necessary to shift the sample position both sideways and vertically when the incidence angle is changed. The Troika II beamline is specifically designed for this kind of experiment, enabling scanning of the incidence angle Ri in the range of interest. The geometry of the experiment is shown schematically in Figure 1. For the grazing incidence diffraction (GID) experiments,10,11 a Soller collimator was installed in front of the detector, giving a lateral resolution of about 1.4 mrad. The upstream slit had vertical and horizontal gaps of 0.05 and 1.0 mm, respectively. With this setup, the PSD was about 400 mm from the sample and covered a vertical angle of approximately 8°. An Ri ) 0.13° was employed for the experiments on water. To avoid the strong scattering (background) in the incidence plane of the beam, the out-of-plane GID measurements were done with a horizontal offset of ∆ ) 0.2°. In the reflectometry setup, the PSD was about 650 mm from the sample. The Soller collimator was replaced by a 470 mm evacuated flight tube, having vertical entrance and exit slits of 5 and 8 mm, respectively. The upstream slit was then used with a vertical gap of 0.1 mm and a horizontal gap of 5 mm. The linear detector was used for collecting the scattered intensity also for reflectometry. The background could thus conveniently be (10) Als-Nielsen, J.; McMorrow, D. Elements of Modern X-ray Physics, 1st ed.; Wiley: New York, 2001. (11) Als-Nielsen, J.; Jacquemain, D.; Kjær, K.; Leveiller, F.; Lahav, M.; Leiserowitz, L. Phys. Rep. Rev. Phys. Lett. 1994, 246, 252.
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Breiby et al. Table 1. Some Key X-ray Optical Parameters: the Absolute Electron Density G, Gr0, the Index of Refraction in Terms of δ, the Critical Angle of Reflection rc, and the Critical Wave Vector Transfer Qca F × 1029/m-3 Fr0 × 106/Å-2 δ × 106 Rc/deg Qc/Å-1
material PHT POT POOPT water glass (SiO2)
Figure 2. Optical transmission setup with the trough and the mirror system. The light path from the source to the detector is shown with block arrows. Also the primary reflection from the sample is indicated (stapled arrow). The stapled lines indicate the experimental well of the spectrophotometer. estimated, as the detector signal contained both the strong reflected beam, being confined to a few detector channels, and a diffuse background. UV-Vis Studies. A Varian Cary 5 photospectrometer was employed for the UV-vis measurements. This instrument is based on a differential double-beam approach. For measuring the transmission of the film, a homemade mirror system was placed in the sample beam. The sample was floating on water in a Teflon trough (diameter ∼ 50 mm) with a glass window in the bottom. The horizontal incoming light was deflected down through the film, the ∼4 mm of water, and the glass window. By using two more mirrors, the beam was redirected into the detector. The incidence angle (with respect to the film normal) was about 30°. The light was predominantly s-polarized and highly monochromatic. The wavelength was scanned from 350 to 1050 nm. See Figure 2 for a sketch of the setup. The reported data were obtained by dividing the measured signal T(λ) by a reference measurement T0(λ) done with no film present (water, glass, and mirrors only), thus giving a rough estimate of the absorption. Other optical effects, most notably that of the increased primary reflection, were neglected; an approximation often done in absorption studies. From the transmission spectra, the absorption Γ was calculated by Γ ) 1 - T(λ)/T0(λ).
Thin Film X-ray Techniques The index of refraction for X-rays, n ) 1 - δ, can be calculated from the material stoichiometry as the parameter δ is given by δ ) λ2/(2π)r0F.10,12 The parameter r0 ) 2.82 × 10-5 Å is the classical electron radius. δ is proportional to the electron density F and of the order 10-6. With the solid material thus being optically less dense than air, the incoming beam is totally reflected when the incidence angle Ri (with respect to the sample surface) is less than the critical angle of reflection Rc ) x2δ. For grazing Ri smaller than Rc, only the rapidly damped evanescent wave propagates into the sample, giving a short penetration depth Λi, typically less than 1 µm. The penetration depth increases rapidly for higher incidence angles.11 Some X-ray optical parameters for materials relevant to this study are given in Table 1. The GID technique exploits the low penetration depth into the substrate at small Ri, thereby enabling diffraction studies of thin sample layers, while keeping the scattering from the substrate at a minimum. Well-collimated beams are required, and synchrotron radiation is thus necessary to obtain a sufficient intensity. The incident beam, with wave vector ki, impinges on the substrate at the grazing angle Ri. The scattered wave vector kf determines the total (12) Wilson, A. J. C.; Ed. International Tables for Crystallography, Vol. C; Kluwer Academic: Dordrecht, The Netherlands, 1992.
3.6 3.4 3.5 3.34 8.4
10.2 9.6 9.9 9.42 23.7
3.9 3.7 3.8 3.62 9.1
0.16 0.16 0.16 0.148 0.24
0.023 0.023 0.023 0.021 0.034
a The absolute value for the electron density is also given. All parameters were estimated at λ ) 1.554 Å. For PHT and POT the mass densities 1.05 and 1.10 g/cm3 were used.3 POOPT was calculated with an assumed density of 1.10 g/cm3. Glass was taken as SiO2 with a density of 2.6 g/cm3. The uncertainty of the parameters is of the order of 10%, stemming largely from the polymer and glass densities.
scattering vector Q, Q ≡ kf - ki. The scattering vector can conveniently be decomposed into an in-plane component Qxy and an out-of-plane component Qz (with respect to the film plane). Sample anisotropy is observed as intensity variations along the Debye-Scherrer rings, observed by scanning the detector in ∆; cf. Figure 1. Reflectometry. Using (specular) reflectometry, information on the laterally averaged electron density profile F(z) is obtained, which allows parameters such as the film thickness, surface roughness, and internal structure to be deduced. Kiessig (thickness) oscillations require sufficiently homogeneous films within the scattering volume. In contrast to the GID measurements, the setup is symmetric, R ≡ Ri ) Rf, the latter angle being the exit angle of the detected beam. The scattering vector Q is thus always parallel to the film normal, and Qz ≡ |Q| ) 4π/λ sin R. It can be shown from Fresnel’s equations that for angles R . Rc, the reflected intensity RF for an abrupt interface falls off proportionally to Qz4.10 With rough interfaces, the falloff is even steeper. For describing the reflectivity mathematically, the formalism of optics in stratified media is employed. By discretizing the model profile of the electron density F(z) into a number of layers, the specular reflectivity can be calculated as a function of incidence angle.10,13,14 In practice, the fitting was done using the Parratt32 freeware written at the Hahn-Meitner Institut, and also some homewritten simulation routines. A common way of expressing interfacial roughness is via the error function erf, whose derivative is a Gaussian with width σ.10 We define
erfstep(z;h,σ) )
(
( ))
z h 1 + erf 2 x2σ
(1)
where the parameter h is the height of the step. A model for F(z) found to work well in the present study is given by
F(z;xi) ) erfstep(z,Ffilm,σ1) + erfstep(z - df,Fsubstrate -
sin
(
(
Ffilm,σ2) + A box z;zoffset,zoffset + Z
)(
)
a 2
)
2π(z - zoffset) 1 ‚ 1(2) a 1 + exp(-(z - z0)/w)
The two first terms give two steps, being the air-polymer (13) Born, M.; Wolf, E. Principles of Optics: Electromagnetic Theory of Propagation, Interference and Diffraction of Light, 7th ed.; Cambridge University Press: Cambridge, U.K., 1999. (14) Parratt, L. G. Phys. Rev. 1954, 95, 359.
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and the polymer-substrate interfaces, having roughness parameters σ1 and σ2, respectively. The distance between the steps is df, the film thickness. The last term in (2) gives a damped harmonic modulation of F, having amplitude A and wavelength a. zoffset serves to shift the modulation in z, and the last factor describes damping with width w from z ∼ z0. The integer Z ensures that the modulation is a multiple of half-modulation lengths a, obtained by using the box function
box(z; zmin, zmax) )
{
1, (z > zmin) ∪ (z < zmax ) (3) 0, elsewhere
Similar models are reported for modeling smectic liquid crystals.15 Note that full coherence is implicit in this model. The instrumental resolution can be included by convoluting the modeled electron density by a Gaussian with width ∆Qz. However, for simplicity and because the instrumental resolution was quite good, most of the fits presented here were obtained without this correction. Results A. The Films. The described procedure of spreading the dissolved polymer on water resulted in barely visible thin floating films, typically having a diameter of about 40-50 mm. By visual inspection, the films appeared homogeneous over areas of several square centimeters. The films from regioregular PHT tended to be slightly less homogeneous than the others, exhibiting some irregular patterns of darker stripes with a sub-millimeter width and lengths spanning several millimeters, probably being due to thickness variations. The films were structurally robust; by blowing gently with a pipet, they could be translated and even rotated on the water surface. Attempts to mechanically compress/ stretch the films showed that the films were elastic for small changes of the film area. Further compression or stretching led to visually observed darker, crinkled regions or cracks, respectively. That the films could be transferred to glass substrates is further evidence for their continuous structure. B. Grazing Incidence Diffraction. In diffraction, the strongest peaks reported for bulk film PATs are the 100, 200, 300 and 010 reflections. These peaks were also observed for the present ultrathin films. Diffraction patterns are shown in Figure 3 for the PAT samples. The peak positions observed for PHT, POT, and PDoDT correspond to interplanar distances of 16.5, 20.5, and 28 Å, respectively. The values are in good agreement with reported bulk film values3 and demonstrate the linear dependence of the a-parameter with the length of the side chains even for these ultrathin films. The average crystallite dimension in the vertical direction can be estimated from the observed peak widths. The widths were estimated to 0.86, 1.24, and 0.77° for PHT, POT, and PDoDT, respectively, with uncertainties of about 0.2°. By using the Scherrer formula,16 the widths give estimated average dimensions of the crystalline domains in the vertical direction of about 70-110 Å. These values are comparable to the film thickness; see later. Due to the considerable uncertainty in using the Scherrer formula, further interpretation of the widths was not attempted. For probing the anisotropy, the vertically oriented PSD was scanned in the ∆-direction, cf. Figure 1. The resulting intensity distribution for an R-PHT film is shown in Figure (15) Ocko, B. M.; Braslau, A.; Pershan, P. S.; Als-Nielsen, J.; Deutsch, M. Phys. Rev. Lett. 1986, 57, 94. (16) Kakudo, M.; Kasai, N. X-ray Diffraction by Polymers; Kodansha: Tokyo, 1972.
Figure 3. Grazing incidence diffraction of PDoDT, POT, and R-PHT films floating on water, the two latter vertically offset by 10 and 20, respectively. The horizontal angle ∆ was 0.2° (Qxy ≈ 0.02). Note the increasing scattering vector of the 100 Bragg peak with decreasing length of the alkyl side chains. The regioregular PHT exhibits a somewhat more pronounced peak than the other materials. The fitted solid lines are Gaussians with a linear background. The inset shows a typical example of the 010 π-π stacking peak, here for R-PHT. The curve was obtained by scanning the vertically oriented linear detector horizontally and summing over all detector channels, corresponding to Qz ∼ 0.1-0.5 Å-1. The thin solid line is a fitted background.
4. The azimuth angle χ is defined as the angle between the scattering vector Q and the unique axis n of the film normal. The intensity of a given Bragg reflection as a function of χ thus gives information about the orientational distribution function. All the films studied showed a large degree of anisotropy. A full width at half-maximum (fwhm) of the 100 peak as low as ∼12° was obtained for the regioregular PHT sample. For the other materials, the anisotropy was somewhat lower. The 100 Bragg peak is strongest when the scattering vector Q is directed along the unique axis, implying preferred orientation of the crystalline a-axis perpendicular to the film plane, i.e., as reported for thicker PAT films.3 Conversely, the (weak) 010 reflection, cf. Figure 3, could only be observed for Q (almost) parallel to the film plane. C. Reflectometry. Reflectometry data are presented in Figures 5, 7, and 9. The fitted curves were obtained using the outlined formalism, giving fitted density profiles F(z). The electron densities of Table 1 were used as starting values, and the fitted densities were reasonably close to the tabulated values. Fitted values with uncertainty estimates are given in Table 2. One may distinguish between two principal contributions to the detected intensity, namely, thickness (or “Kiessig”) fringes, and fringes due to the internal structure of the film. The relative intensities of these contributions are important for the interpretation of the observed curves. The strength of the Kiessig fringes depends on the contrast between the polymer layer and the substrate. As can be read from Table 1, the contrast for films on glass substrates is much more pronounced than on water. The polymerwater contrast is only 5-8%, and as a consequence the Kiessig fringes are barely visible in the figures. The Bragg signal from the crystalline periodicity is strong in the (off-specular) GID scattering; cf. Figures 3, 4, and 10. Also for reflectometry data on water (“low contrast”) the diffraction effects are substantial. Conversely, in the high-contrast (glass) case, the Bragg signal can at best be seen as a modification of the strong thickness fringes.
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Figure 4. Anisotropy in an R-PHT film floating on water. (a) Intensity map obtained by scanning the detector sideways (∆direction) for fixed γ. The blank central region had to be avoided due to the strong reflected beam. (b) Intensity curves obtained by numerically integrating along the lines indicated in the intensity map, showing one trace on and two traces off the 100 Bragg peak. In c, the result after subtracting the background traces recorded off the Bragg peak is shown. The line serves as a guide to the eye; the peak might be sharper.
Figure 5. Reflectometry data (filled circles) and theoretical fits (solid lines) obtained for thin films of POT and R-PHT, the latter offset by a factor 10-2. The arrows indicate the approximate positions of their respective 100 Bragg peaks as obtained from the GID measurements. Note the stronger falloff of the R-PHT film indicating its higher roughness.
POT. Reflectometry data obtained from POT films on water are given in Figures 5 and 7, with the corresponding density profiles in Figures 6 and 8, respectively. The airpolymer interface was fitted with a roughness parameter σ1 of about 4.5-5 Å. The fitted film thicknesses were ∼98 and ∼144 Å. The features near Qz ∼ 0.3 Å-1 in the reflectometry curves are due to the internal structure of the film, a Bragg-like signal interfering with the reflections from the interfaces. In both of the fitted models, this considerable internal structure can be seen, with the ordering being most pronounced at the polymer-air interface. The periodicity of the modulation was 20.8 ( 0.5 Å, which is in good agreement with the lattice parameter a reported for bulk POT,3 and also with the GID value; cf. Figure 3. One POT film was studied both as deposited on water (low contrast) and then after being transferred to glass (high contrast); see Figures 7 and 8. The fitted film thicknesses were in both cases near 150 Å, and also the internal structure of the film appeared much the same, indicating that the transfer to glass did not markedly change the film structure. One significant change is that the roughness toward the substrate is increased after transfer. In Figure 7, reflectometry data are shown for a POT film floating on water and for the same film after transfer
Figure 6. Fitted variation of the electron density F given as a function of depth z, for the reflectometry curves in Figure 5. (The R-PHT curve is offset by 3 × 106 Å-2.) Coincidentally, both polymer films had a thickness of about 100 Å. Internal structure is modeled as a harmonic variation. For POT, the oscillations decay toward the water subphase, whereas for R-PHT the amplitude is about constant. The air-polymer interface is rough. The contrast, i.e., difference in refraction index, between polymer and water is small.
Figure 7. Experimental and fitted reflectometry data for a POT film, studied first as deposited on water (offset by a factor 10-2) and then after transfer to glass. On glass, curves both with and without correction for the instrumental resolution are shown.
to glass. With the much higher contrast, the approximation of neglecting the instrumental resolution is no longer good. Upon convoluting the curve with a Gaussian of width ∆Qz
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Table 2. Approximate Parameter Values Obtained from Reflectometry (2) for the Various Fits of Polymer on Watera material
df/Å
σ1/Å-1
σ2/Å-1
Fr0 × 106/Å-2
a/Å
W/Å
z0/Å
R-PHT POT POOPT Water Glass
100 ( 15 98 ( 2, 144 ( 2 153 ( 8
7.2 ( 1 4.5 ( 0.2 (6.8 ( 0.2)b 3.2 ( 0.2
8.2 ( 4 2.0 ( 1 (4.4 ( 0.2)b 3.0 ( 1
9.6 ( 0.2 9.8 ( 0.2 10.0 ( 0.2 9.2 ( 0.1 21.5 ( 0.3
16 ( 2 20.8 ( 0.5 25.0 ( 1
20 ( 5 10 ( 3
15 ( 10 95 ( 10
a The uncertainty of the parameters was estimated from multiple fits resulting from different films and initial values. b Obtained for film transferred to glass.
Figure 8. Profiles obtained for the POT film shown in Figure 7, for the film as deposited on water and after transferring the film to glass (vertically offset by 5 × 106 Å-2). The fits for the film part of the signal (∼0-145 Å) are similar, indicating that the transfer has not markedly changed the film structure.
Figure 9. Reflectivity of a POOPT film. The inset shows the model profile giving the solid line.
) 0.018 Å-1 for Qz > 0.05 Å-1 a good fit was obtained. The main effect of including the resolution is that the deep and sharp minima of the thickness fringes are smeared. The error introduced by neglecting the resolution for the other films is thus small. R-PHT. As mentioned, the R-PHT films tended to be somewhat less homogeneous, thus giving weaker thickness fringes and a higher fitted roughness; see Figures 5 and 6. The fitted thickness was about 100 Å, with a considerable uncertainty due to the roughness. The 100 diffraction signal is expected at Qz ∼ 0.38 Å-1, as demonstrated also by the GID measurements; cf. Figures 3, 4, and 10. The R-PHT reflectometry scans were halted at about 0.32 Å-1 due to diminishing intensity, however, being sufficiently close to the Bragg peak that some modulation of the reflected intensity can be seen. Contrary to the case for the other materials, the fits suggest that for R-PHT the amplitude of the modulation stays constant through the entire film thickness.
Figure 10. GID curves from a floating R-PHT film before, immediately after, and about 2 h after adding NOPF6 to the subphase. With Qxy ∼ 0 (∆ offset by 0.2°), the abscissa axis gives Qz ∼ Q. After injecting the dopant, the 100 peak moved considerably toward a lower scattering angle, and the peak appears sharper.
POOPT. This material was chosen for study because its side chains contain an oxygen group, which might make the side chains less hydrophobic than the alkyl groups of the PATs. In Figure 9 the reflectometry curve of a POOPT film is shown. Visually the film appeared as “perfect”. It was realized only during the data analysis that the detector had been overloaded to saturation during the scan covering the 100 Bragg peak (for Qz ∈ (0.25, 0.30) Å-1), which adds some uncertainty to the absolute intensity level of both the Bragg peak and the subsequent scans. A fitted model is shown in the inset of Figure 9. The Kiessig contrast is low, and most of the observed features can be accounted for by a “double-layer” structure of total thickness ∼ 155 Å, involving an ordered (density modulated) upper surface layer and a disordered (nonmodulated) lower layer near the water. The modulation wavelength obtained, a ≈ 25 Å, is to be compared with a lattice parameter value 29 ( 2 Å obtained for POOPT using powder diffraction.17 The fitted model is qualitatively similar to those of POT, and it is thus not possible to see any effect of the presumably more hydrophilic side chains of POOPT. D. In Situ Doping. The dopant counterions often get intercalated into the polymers semicrystalline structure, and doping can then be seen by X-ray diffraction as a change of the lattice parameters. For bulk PAT films, an elongation of the a-axis accompanied by a contraction of the b-axis is reported.4 In Figure 10 the effect of the doping with NOPF6 of a floating R-PHT Langmuir film is seen as a downward shift of the 100 diffraction peak. For the undoped PHT sample, the 100 diffraction peak position was at Q ) 0.38 Å-1. A few minutes after the injection of dissolved NOPF6, the peak had moved to Q ) 0.33 Å-1, corresponding to an (17) Aasmundtveit, K. E.; Samuelsen, E. J.; Mammo, W.; Svensson, M.; Andersson, M. R.; Pettersson, L. A. A.; Ingana¨s, O. Macromolecules 2000, 33, 5481.
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Figure 11. Corrected optical absorption curves for a floating thin POT film before (solid line) and after (stapled line) injecting the dopant. The absorption maximum of the π-π* transition is seen to be reduced and moved to higher energy after doping, whereas the vibronic shoulder stays at ∼2.1 eV. The thick vertical line indicates the absorption onset. Note the comparably strong new absorption feature below the band gap, being a (bi-)polaron state, in the doped state. The features below ∼1.25 eV are experimental artifacts.
expansion of the a-parameter by ∼15%. The signal from the 010 was too weak for reliably discerning differences of the b-axis between the doped and pristine film, both showing a broad peak around Qxy ∼ 1.65 Å-1 (Qz ∼ 0). It appears from Figure 10 that the peak was more pronounced after doping. This might be interpreted as a well-ordered intercalation of the dopant ions into the structure, having a periodicity equal to that of the a-axis. It is thus reasonable to assume layers of PF6 ions between noninterdigitating side chains. This is an interesting contrast to reports on doping of less ordered bulk PATs, where the dopant ions have been found to have a fairly random position in the a-direction.4 A possible explanation is that the polymer strands are more entangled in bulk. When the dopant ions move into the structure, stress is induced. For the about five molecular layers studied here, the chains might more readily respond to this stress by relaxing into an ordered structure than in the bulk case where the stress supposedly remains unreleased. No attempts were made to use other dopant ions or to vary the concentration of ionsswhich could both be interesting issues for further study. The doping effect could also be observed visually, as the film changed its color from being faintly reddish to a hardly visible pale blue. A front of color change was observed to travel across the sample area after application of the dopant, showing the doping process propagating as the dopant ions diffused in the water subphase. The color change is associated with the altered electronic configuration of the polymer, with new states emerging in the band gap, making the polymer electrically conducting. When remeasuring after about 2 h, the a-parameter was slightly reduced, and the film was regaining its original color. This effect is likely to be due to dedoping. However, since conjugated polymers in the doped state are reactive, the possibility exists that the polymer was chemically degraded. The film was exposed to the beam for just a few minutes, so the effect is certainly not due to beam damage. To study the optical aspects of the doping effect more quantitatively, separate experiments were done using UV-vis spectroscopy, with a setup as described in the Experimental Section. The results are given in Figure 11 for a POT film on water, having thickness and homogeneity comparable to the other films reported here. Similar
Breiby et al.
Figure 12. Schematic illustration of a crystalline region in the semicrystalline film floating on water. For simplicity, three rather than the estimated five to seven repetitions along the a-axis are shown. The crystallites have a vertical dimension extending through most of the film thickness. The films have a high degree of preferred orientation, with the side chains (a-axis) tending to orient vertically. The ordering is most pronounced at the air-polymer interface. To the right, the corresponding electron density F is indicated: the dashed line is for the crystallites; the effective electron density given by the solid line appears after averaging due to less ordered regions in the film.
results were obtained using R-PHT. Compared with data for thicker bulk films,4 the signal for the pristine sample is weak and has smoothed features. However, despite the weak signal, there is a pronounced vibronic shoulder at about 2.1 eV, indicative of a high degree of order. After injecting the dopant, the π-π* absorption maximum was reduced and shifted to higher energy. The most important feature of Figure 11 is the appearance of a peak well below the band gap, centered around 1.5 eV, being the optical fingerprint of doping-induced states in conducting polymers.4 The vibronic shoulder stayed constant. The nonzero absorption below the band gap in the undoped state might be due to the neglected primary reflection; cf. Figure 2. Discussion Combining the results from the GID and the reflectometry studies, we suggest a picture of the averaged molecular structure, as indicated in Figure 12. PATs can be considered a special class of block copolymers, segregating to form lamellar structures of π-stacks separated by the side chains. The preferred orientation of the crystalline a-axis perpendicularly to the film normal reflects the uniaxial symmetry of the substrate-polymerair system. The oriented crystallites, presumably dispersed in an amorphous matrix, are of sufficient size in the layer normal direction to extend through most of the film thickness. The crystalline order is most pronounced at the air-polymer interface. The order toward the water is reduced, possibly due to the hydrophobicity of the aliphatic side chains. For intermediate depths, the arrangement is a compromise between an inherent tendency of ordering and the reluctance of the side chains to being submerged in water. The qualitatively different results obtained for R-PHT is probably due to the high stereoregularity promoting the crystalline packing tendency. The modulation wavelength a for the PAT films was found to be close to the bulk values of the crystalline a-parameter. The fitted modulation for the POOPT film was somewhat short but still compatible with the reported powder diffraction value when considering the experimental uncertainty. The
Semiconducting Polymer Ultrathin Films on Water
presence of oxygen in the side chains of POOPT does not seem to have any noticeable effect on the polymer-water interface. Even though the parameters of the model (2) are not fully independent, it is worth noting that df is essentially determined from the Kiessig fringes, as is a from the GID peak positions. The stoichiometry of the samples and substrates gives effective constraints to Fsubstrate and Ffilm. The interfacial roughnesses σ1, σ2 appear reasonable. The amplitude A must necessarily be >0; see below for a discussion of its magnitude. The few remaining parameters govern the internal structure of the film, under the qualitative constraint that the electron density profile F(z) should not have unphysically sharp features. The presented fits were obtained using a decaying sinusoidal density variation. Also other models were tried, notably including functions with a sinusoidally varying increasing electron density toward the water and also with the amplitude of the oscillations increasing/decreasing toward both interfaces. The combination of best fits and simplest model was obtained for the model (2), as presented in the figures. The amplitude of the electron density modulation is about 10%. From the idealized structure of the crystalline regions of PAT, having π-stacks with electron-rich sulfur atoms, a considerably higher modulation would be expected, as also indicated in Figure 12. Taking the amorphous regions into account, presumably having a homogeneous electron density, the oscillations corresponding to the “ideal” crystallinity are smeared. Analogous considerations were recently applied also for semicrystalline electrochemically prepared methyl- and decylsubstituted poly(3,4-ethylenedioxythiophene) (PEDOT) films.18 As specular reflectometry yields the laterally averaged electron density profile, it is in principle always possible to fit a layer model to the data. One should be careful not to oversimplify the interpretation of the fitted layers, as a proper understanding might include features such as a distribution of crystallite sizes and orientations. In accordance with these considerations, note that the modeled gradually decreasing crystallinity toward the water subphase might in fact represent a collection of crystallites of different sizes, with a tendency of gathering at the interface to air. For this reason, it is hard to give statements about the crystallinity itself, being of the order of 30% in bulk PATs. Also for the GID measurements, the weak signal and the low contrast make it hard to go beyond the qualitative statements that there is a considerable degree of crystallinity, and somewhat higher for R-PHT than for the other materials. The literature is scarce on reflectometry studies of systems with disorder going beyond the interlayer roughness as modeled by (1). For the present case of a distribution of crystallites in an amorphous matrix, no proper formalism exists. One complicating factor is the issue of coherence, which will increase with increasing order. As mentioned, full coherence was assumed throughout this work. The comparably large crystallite dimensions (18) Breiby, D. W.; Samuelsen, E. J.; Groenendaal, L. B.; Struth, B. J. Polym. Sci., Part B: Polym. Phys. 2003, 41, 945.
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combined with thin films make this a plausible approximation. One might suggest that using a Monte Carlo technique would be a more satisfying approach to this complexity, however, at the expense of a less transparent formalism. As the presented fits account for all the main features of the data, we judge that a more elaborate fitting scheme would not increase the understanding of the polymer structure. The Scherrer formula, used for estimating the crystallite dimensions from the GID peak widths, is based on the assumption of identical scattering planes. In the cases studied here, with exponentially decaying order, also the damping of the amplitude of the electron density modulation contributes to the measured peak width. The value obtained by the Scherrer formula thus represents an effective crystallite dimension, which is compatible with the reflectometry data. Conclusion Using the Langmuir technique of spreading dissolved polymers on a water subphase, ultrathin and apparently homogeneous films with areas of several square centimeters were obtained. Films of the semiconducting polymers R-PHT, POT, PDoDT, and POOPT were studied by means of synchrotron radiation. Using grazing incidence diffraction and reflectometry, structural information of the ultrathin films as deposited on water was obtained. Some of the films were subsequently transferred to glass for further studies. The thickness and structure after transfer were similar to those obtained on water. The R-PHT differed from the other materials in giving less homogeneous and more crystalline films with a higher degree of anisotropy. The films were observed to be semicrystalline, having crystallites with a strong tendency of orienting the side chains (the crystalline a-axis) perpendicularly to the water subphase, despite the side chains being hydrophobic. The anisotropy, and also the measured lattice parameters, corroborate the results from studies of bulk films. Upon doping an R-PHT film floating on water with NOPF6, a large expansion of the crystalline a-axis was measured. Indications were found for intercalation layers having the periodicity of the a-axis. Results from UV-vis absorption studies indicate that the photophysics of these ultrathin films is similar to that for thicker films reported previously, both in the pristine and the doped state. A model for the electron density exhibiting damped harmonic variation of the electron density toward the water subphase gave good fits to the experimental reflectometry data. This is interpreted as a well-developed crystalline order at the polymer-air interface deteriorating toward the water. The density modulation had a wavelength compatible with the bulk lattice parameter a. Film thicknesses of 100-150 Å were obtained, being comparable to the crystallite dimensions obtained from the diffraction data. Acknowledgment. The Norwegian Research Council is gratefully acknowledged for financial support of D.W.B.’s Ph.D. studies. LA036399X
