Spectroscopic Characterization of Crosslinked Polymer Thin Films by Energetic Helium Atoms through Stencils

Nanoparticles made from polymer provide a high degree of flexibility in their design and can accommodate additives that offer new functionality. Here, we demonstrate the formation of polystyrene particles by cross-linking a film through a stencil using a helium atom beam. The regions that are exposed to the beam are cross-linked and remain on the surface after a development step, allowing the user to control the size and shape by the patterns in the stencil. The particles can then be released into solution by dissolving a support layer on the substrate. We used a flow-coating approach to deposit uniform, 100 nm thick polystyrene films on glass slides with a polyvinyl acid (PVA) release layer and were able to add Nile Red fluorescent dye at various concentrations. We then studied the film emission spectra as a function of exposure dose and dye concentration by exposing the film through a large aperture and measuring the optical properties using a conventional plate reader. The emission prior to the development step shows some degradation due to radiation damage, typically around 25%, at the doses needed to cross-link the polymer. However, after development, the emission dropped by an additional 75%, indicating that most of the dye contained within the polymer is lost in this step. Hence, the formation of brighter particles relies on the ability to prevent the loss of dye during development. Fluorescent microscope images of the particles after release show that, even with these losses, they are bright structures that are easy to find, retain their shape, and do not agglomerate.