Conjugated polymers are promising for electro-optical applications like light emitting diodes, solar cells and lasers. These polymers are organic semi-conductors, which can be doped to yield conductivities approaching ordinary metals [1]. In order to fully exploit the properties of organic molecules, excellent control of the molecular orientation is required. To better understand the self-organisation observed in films of poly(alkylthiophenes), we have previously performed in situ studies of the drying process associated with solution casting [2]. In the present work, the Langmuir method was used in an attempt to obtain ultra-thin films. Regioregular poly(hexylthiophene) (R-PHT) with > 98% head-to-tail couplings, and poly(octylthiophene) (POT) with ~ 80% head-to-tail couplings were used. Closely analogous results were obtained for poly(dodecylthiophene) and other poly(thiophene) derivatives.

A few drops of low concentration (~ 1 mg/ml) solutions in chloroform were spread on a clean water surface in a shallow teflon trough. The films were studied with X-ray reflectivity and Grazing-Incidence Diffraction (GID) at the ID10B beamline. A vertical one-dimensional position sensitive detector was used for collecting the scattered intensity. In situ doping of the floating films was achieved by injecting NOPF6 dissolved in acetonitrile into the water subphase.

X-ray reflectivity obtained from a POT film floating on the water surface is shown in Figure 63. The fitted curve is obtained by applying a standard matrix formalism from the optical theory of stratified media, applied to the density profile shown in the inset of the figure. The refractive indices of water and polymer were determined from tabulated values. The air-polymer interface was found to be graded, with a (Gaussian) width ~ 5.3 Å. This particular film had a thickness of about 10 nm, corresponding to five repetition units of the semi-crystalline a-axis.

 

 

Fig. 63: Reflectivity data (green dots) obtained for a thin POT film floating on water versus scattering vector Q = 4sin/. The solid red line is the fit. The inset shows the employed scattering density profile. Inside the polymer film, there is a periodic oscillation being damped towards the water subphase.

 

The feature at scattering vector Q ~ 0.3 Å-1 is a result of the semi-crystalline structure of the film. The best fit was obtained for a model with an exponentially damped harmonic variation of the electron density, a model sometimes used for smectic liquid crystals. The period of the modulation, 20.8 Å, is compatible with the a - parameter of solid POT, which is 20.4 Å. The model implies a well-developed layering near the air-polymer interface, decaying towards less pronounced ordering at the polymer-water interface, in qualitative agreement with the aliphatic side chains being hydrophobic (Figure 64).

 

 

Fig. 64: a) The orthorhombic PAT unit cell, with a ~ 20.4 Å for POT, and c ~ 7.8 Å. In the b-direction (out of the paper plane), the aromatic rings form stacks. b) A sketch of the molecular ordering. c) The corresponding density profile.

 

GID gave complementary information to the laterally averaged density profile obtained from the specular reflectivity measurements. An important result was the observation of a large degree of anisotropy. The 100-reflection is strongest for the scattering vector perpendicular to the film plane, whereas the 010 reflection could only be observed for scattering vectors parallel to the film plane. This implies that the unit cell a-axis tends to be parallel to the film normal. From the width of the 100 diffraction peak, the average dimension of the ordered regions could be estimated to about 10 nm, in agreement with the reflectivity data.

Structural changes associated with the insulating-conducting phase transition in conjugated polymers have been reported previously for PATs, notably an expansion of the a-axis. A change of the 100 peak position following in situ doping of the floating films was indeed observed, indicating an increase by as much as 15-20%. The doping could also be observed visually, as the film changed its colour from being weakly reddish to a hardly visible pale blue. Dedoping takes place; after 2 hours the film is regaining its original colour accompanied by a contraction of the a-parameter.

In conclusion, we have demonstrated that the Langmuir technique can be used to obtain homogeneous ultrathin films consisting of a few molecular layers. A detailed model of the polymer conformation in these films accounting for all the observations has been established. These results are very interesting in view of the favourable molecular orientation and the thickness being appropriate for electronic logics nanoscale applications.

References
[1] G. Hadziioannou, P.F. van Hutten, (eds.), Semiconducting polymers: chemistry, physics and engineering, Wiley, Weinheim (2000).
[2] D.W. Breiby, E.J. Samuelsen and O. Konovalov, Synth. Met. 139, 361 (2003).

Principal Publication and Authors
D.W. Breiby (a), E.J. Samuelsen (b), O. Konovalov (c) and B. Struth (c), Langmuir 20, 4116 (2004).
(a) The Danish Polymer Centre, Risø National Laboratory (Denmark)
(b) Dept. of Physics, Norwegian University of Science and Technology (Norway)
(c) ESRF