(223c) Observing the Shape and Diffusion of Anisotropic Nanoparticles Near Interfaces

Engineered particles found in next generation solar cells, nano-electronics, photonic materials, and nano-sensors have experienced a rapid growth in research interest over the past decade. This is in large part due to improving techniques to control particle anisotropy in shape such as nano-rods, nano-plates, iso-hedras and nano-prisms, or to control anisotropy in material properties such as janus particles. Yet, direct force measurement methods including colloidal probe atomic force miscopy (AFM) and the surface forces apparatus, that provide fundamental insight in to the interaction forces between particle, are often limited to micron scale or larger surfaces with flat or simple curvature. The drive to examine smaller length and force scales, for example using AFM or total internal reflection microscopy, has shifted the field to sometimes focus on forces more relevant to assembly, but often with an emphasis on spherical particles. Thus, there is a need to see more quantitative methods capable of measuring interactions between anisotropic particles where the anisotropic nature of the particle is both more interesting and often critical to assembly.

We discuss the use of two new scattering methods to observes the size, shape and diffusion of label-free nanoparticles (e.g. Janus particles, ZnO nanorods, carbon nanotubes) near interfaces with nanometre resolution. The utility of conventional optical tools for probing these systems is limited by the proximity of an interface and the presence of particle anisotropy; yet it is the influence of these factors that makes such systems interesting, introducing asymmetric interfacial forces and separation-dependent hydrodynamic hindrance in each of the spatial modes of diffusion. We simultaneously record the spatially correlated scattering of multiple evanescent light sources by isolated anisotropic particles, and use this data to reconstruct instantaneous shape, positions and orientations at millisecond time-intervals. By observing diffusion in each spatial mode over time we are able to quantify each translational and rotational diffusion coefficient as a function of interfacial separation. Aside from fundamental applications, this approach will be particularly useful for understanding and tuning the self-assembly of films and other structures incorporating anisotropic nanoparticles.