Sub-diffraction limited morphology characterization in single noble metal nanoparticles and single conjugated polymer chains using optical microscopy techniques
MetadataShow full item record
At the nanoscale, materials exhibit special properties not present in the bulk, which may be exploited in diverse applications that include catalysis, sensing, and energy harvest and transfer. Due to their small size, nanoscale materials also present a characterization challenge, because optical microscopy techniques cannot resolve images of structural features smaller than finite lenses may focus visible light. Optical images of nanoparticles or single molecules show diffraction-limited spots with radii of approximately half the wavelength of the light used to interrogate them, and the underlying structure of the nanoscale object is not obvious to the eye. Fortunately, manipulation of excitation conditions and image processing techniques can tease out information about the morphology of nanomaterials investigated. The first example presented in this dissertation shows how an asymmetric excitation geometry and polarization spectroscopy elucidate the orientation of single silver triangular nanoprisms in the plane of an optical microscope’s stage. Characterizing this orientation using optical microscopy techniques opens possibilities for post-characterization nanoparticle functionalization and improved amplification of surface-enhanced spectroscopy signals. Electron microscopy may characterize single noble metal nanoparticles if one is unconcerned with those benefits, but electron microscopy investigations are more challenging for soft matter samples, so optical characterization becomes even more appealing for polymer studies. Bias-induced centroid (BIC) spectroscopy, correlated with polarization spectroscopy, reports not only on the distance over which highly ordered single poly[2-methoxy-5-(2’-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) chains transfer energy, but also that the rod-like structures these polymers are believed to adopt are likely to transfer this energy along their longitudinal axes. BIC relies on observable changes in the position of the fluorescence centroid, but when the bias-induced hole-injection partially quenching the fluorescence occurs symmetrically, the displacement of the fluorescence centroid is small, and defining the displacement direction becomes difficult. In this event, analysis of the ellipticity of the diffraction-limited images of the MEH-PPV fluorescence also supports the conclusion that the polymer transfers energy in the direction of the longitudinal axis of the rod-like structure. Taken together, these wide-field optical techniques allow simultaneous morphological characterization of many single nanoparticles or single polymer chains without appealing to scanning probe or electron microscopies, which can damage the sample or prevent post-characterization modification.