Raman Anisotropy Measurements: An Effective Probe of Molecular Orientation in Conjugated Polymer Thin Films

Abstract

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 (∼ 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 μm) 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.

Alignment and Dynamic Manipulation of Conjugated Polymer Nanowires in Nematic Liquid Crystal Hosts

“…[25] f(u), the proportion of emitters at an angle, was represented by a normalized 1D Gaussian distribution with a maximum at u ¼ 0 and a standard deviation, s. Fitting for s gave an upper value of 248 for the spread in emissive b-phase molecular orientations about the wire axis. [27] This axial texturing was likely due to mechanical extension of the molecules during solution-assisted template pore filling and, after filling, during solvent evaporation along the pores.…”

“…However, the non-zero emission intensity measured at 908 to the wire long axis (VH) indicated that not all of the emissive molecules were perfectly aligned. The spread in molecular orientations relative to the nanowire long axis was estimated from the emission dichroic ratio according to: [25,26] DR…”

Probing the electrode-polymer interface in conjugated polymer devices with surface-enhanced Raman scattering

“…4,5 Optical spectroscopy techniques are facile tools for characterizing polymer-based devices [6][7][8] and can be used to probe non-pristine interfaces that are likely to occur in a production environment. 9 In particular, Raman scattering, where the vibrational modes of a molecule are mapped out through the inelastic scattering of light, is capable of elucidating subtle structural and chemical differences in molecular systems and has been successfully utilized to probe important changes 10,11 and degradation effects [12][13][14] in organic thin films and device structures. An extension of Raman scattering is surface-enhanced Raman scattering (SERS) where the plasmonic modes of nanostructured metal are used to enhance the Raman process by several orders of magnitude 15 enabling sensitivities reaching the detection of single molecules.…”

Electrically conducting polymer nanostructures confined in anodized aluminum oxide templates (AAO)

“…Indeed, orientation affects the intensity of the Raman scattered-light for a given polarization of the incident light [35]. Owing to the smaller pore diameter, uniaxial symmetry is more pronounced for polyaniline obtained in AAO of 45 nm than of 300 nm, so the polymer chains tend to have a preferred orientation (parallel to the AAO walls) [36] thus increasing the intensity of the band at 1167 cm -1 , as observed. To summarize, the spectra of PANI obtained in pores of 300 nm diameter present the same tendency than that of PANI bulk while spectra of PANI obtained in pores of 45 nm illustrate a confinement effect which is not seen in 300 nm.…”