Raman anisotropy measurements: An effective probe of molecular orientation in conjugated polymer thin films

Understanding the molecular alignment of conjugated polymers within thin-film samples is essential for a complete picture of their optical and transport properties, and hence for the continued development of optoelectronic device applications. We report here on the efficacy of Raman anisotropy measurements as a probe of molecular orientation, presenting results for aligned polyfluorene nematic glass films. Comparison is made with the results of optical dichroism measurements performed on the same samples. We show that in many cases molecular orientation can be more directly characterized by Raman anisotropy, and that it can have a greater sensitivity to the degree of molecular orientation than conventional optical dichroism. The fact that the Raman measurements can be readily performed on the same thin films (similar to 100 nm thickness) that are required for optical dichroism means that there is no ambiguity in a direct comparison of results. This situation differs from that for standard X-ray diffraction measurements (these require film thicknesses of several mum) and electron diffraction or electron energy loss spectroscopy measurements (these require film thicknesses of 10 nm or less). The Raman data allow the angle (relative to the chain axis) for the optical dipole transition moment to be deduced from the dichroic ratio, and confirm the role that its off-axis component plays in limiting this ratio. The added fact that Raman anisotropy data can be collected in situ, in reflection geometry for standard device structures, and with microscopic resolution and chemical specificity makes the technique even more attractive as a non-invasive device probe.