Electric Field Manipulation of Gold Nanorods: Characterization of Far-Field Alignment and Spatial Positioning Through Optical Response Imaging Techniques

Polymer nanocomposites offer extraordinary promise for the field of composite materials. If the unique properties found at the nanoscale can be harnessed for macroscale applications, the functional range of composite material can be significantly extended. Substantial advancements have been made in the design and manufacture of these composite materials, both in understanding the underlying physics as well as in developing fabrication techniques that ensure good dispersion of the nanophase. While good dispersion is critical, the next step is to move toward `micro\u27structures that may be able to take specific advantage of the nanoscale physical interactions. With respect to metallic nanoparticle polymer composites, current research is exploring the use of electric fields to spatially position nanoparticles in order to take advantage of optical and electrical properties resulting from plasmonic oscillations in addition to increasing the mechanical toughness of the polymer. The ability to create long range patterns in the nanophase will significantly enhance these applications. Previous studies have demonstrated the use of alternating AC electric fields to align and transport micron length gold nanowires by using darkfield optical microscopy to track position. Other studies have characterized the alignment of nanometer length gold nanorods by observing changes in the absorbance spectrum. A remaining challenge is to develop methods to quantify successful alignment so that electric fields, i.e. strength and frequency, associated with capacitor geometries, i.e. macroscale spacing and arrangement, can be easily established under a variety of conditions. Here total internal reflection microscopy (TIR) is investigated as a method of optically identifying alignment of gold nanorods when they are exposed to AC electric fields. Under the conditions of TIR, an evanescent wave is generated by the total reflection of incident light at a glass-water interface. This electromagnetic field decays rapidly from the interface, penetrating to a distance of 100 nm into the sample. The result is high resolution, localized illumination of sample features. In this work, the optical response of gold nanorods was observed and measured using polarized darkfield microscopy and TIR microscopy. For TIR, polarized 785 nm wavelength light, the frequency at which gold nanorods scatter light from their long axis, serves as the incident light. Under these TIR conditions, the evanescent wave generated by the reflection technique is additionally influenced by intensity changes due to the alignment of the nanorods. Nanorod alignment is characterized for several capacitor geometries as well as for electric fields applied at different strengths and frequencies using TIR and compared to the same observations using darkfield microscopy. Results demonstrate the ability of TIR to observe and quantify the polarization dependent optical response of the gold nanorods under p and s polarization. Depolarization of the light scattered by the nanorods is observed with TIR but not observed with darkfield microscopy, illustrating the sensitivity of the TIR measurement technique. Positive dielectrophoretic transport of gold nanorods in solution is observed both under TIR and darkfield microscopy and the observed nanorod behavior matches that simulated in a finite element model of the capacitor geometry. Finally, TIR is used to characterize nanorods included in a polyvinyl alcohol (PVA) thin-film linking the optical response of the film to nanorod concentration