Photothermal heating as a strategy to align anisotropic metal nanoparticles within polymeric films*

Thin polymer films containing aligned anisotropic metal nanoparticles are desirable for potential optical applications (e.g., polarizers and filters), as well as for fundamental research (e.g., micro-rheology). Experimentally, achieving alignment at dilute particle loading levels is challenging as reduced inter-particle coupling prevents self-organization or packing. Mechanical sample stretching (to several times its original length) or applications of a strong electric field during film casting are common approaches. However, both schemes result in polymeric systems that are fundamentally altered (with highly elongated chains) and thereby, limit the ability to probe the innate polymer dynamics. An alternative approach is to apply an electric field while simultaneously undergoing photothermal heating. In this case, application of light resonant with the surface plasmon of the nanoparticle results in localized heating, melting only the immediate surrounding polymer and enabling electric field driven reorientation. Because the environment within the small (< 1 μm) molten region is highly constrained and entangled, effects from fabrication on the polymer should be minimized.