Multidimensional Characterization Approaches for Janus and Patchy Particles

Janus and, more broadly, patchy particles open up many opportunities as anisotropic components of tailored functional materials. Nevertheless, their promise brings with it significant challenges, in particular the characterization of their often rather complex morphologies. Improvements in this area are especially needed to support the development of large-scale processes to synthesize Janus and patchy particles, which, just like isotropic particles, tend to have broader morphological dispersity than when produced in the minute quantities often used for basic scientific studies.<br/>We have developed various scalable continuous flow processes to synthesize Janus-like particles by coating dielectric core particles with patches of as silver or gold. Due to the localized surface plasmon resonance of the metal nanostructures, their morphology has a direct influence on their optical properties, leading to resonances in the visible and near-infrared spectral regions. To improve the later and target the product particles towards applications, it is necessary to optimize the process. The target here is to narrow the distributions of coating thickness, coverage and density. However, this can only be possible, if these parameters can be routinely and accurately measured. In this contribution, we will demonstrate how we use three different characterization techniques to extract complementary information from our complex particle system.<br/>Firstly, we will show how automated analysis of scanning electron micrographs can be performed to investigate the metal patch yield (fraction of core particles possessing a patch), thickness and coverage. Here a significant challenge is the random orientation of the particles, meaning that statistical corrections need to be applied to account for possible “hidden” patches. Next, as an ensemble characterization technique we perform multiwavelength analytical ultracentrifugation. This technique has proven itself to be very powerful for the multidimensional characterization of anisotropic particles. Here we will show how the metal to core particle mass ratio can be determined and, taking the thickness distribution determined by microscopy, an estimate of the patch coverage distribution obtained. Finally, we will show how a new technique, single particle extinction and scattering, can be used to identify the patch yield (fraction of core particles possessing a patch) and reveal trends in patch coverage and density.