Local Measurement of Janus Particle Cap Thickness

Janus particles have anisotropy in surface chemistry or composition that will effect dynamics and interactions with neighboring surfaces. One specific type of Janus particle is that consisting of a native micrometer-scale particle with a cap of gold, platinum, or another metal deposited with a typical thicknesses of ∼10 nm. A key characteristic of metal-capped Janus particles prepared with glancing angle deposition is the cap thickness. The nominal thickness is usually assumed to be uniform across the cap for modeling or interpretation of data, but the vapor deposition fabrication process likely does not produce such a cap because of the particle's curvature. These nonuniformities in the cap thickness may have a profound impact on Janus particle dynamics at equilibrium and in response to external fields. Herein, we summarize an experimental technique that utilizes focused ion beam slicing, image analysis, and results for the direct and local measure of cap thickness for 5 μm polystyrene spheres with a gold cap of nominal thicknesses of 10 or 20 nm. We found the cap varied in thickness continuously along the perimeter of the particle and also that the deposition rate, varying between 0.5 and 2.0 Å/s, did not significantly alter the way in which the thickness varied. These data support the hypothesis that cap thickness of a Janus sphere will vary across the gold surface contour, while demonstrating a feasible route for direct measurement of Janus particle cap thickness.