This paper reports a straightforward approach in generating spheroid-like particles and also the orientational orders observed in the self-assembly of these particles. Nonspherical particles, such as spheroid-like particles, are useful in both fundamental studies and industrial applications due to the geometry impact that they bring to the bulk properties of various material systems. Developing processes to generate nonspherical particles is an ongoing quest to meet the need of using such particles in different applications. The approach reported here takes advantage of a controlled chemical etching process. Exposing the spherical silica particles partially to carbon tetrafluoride in a reactive ion plasma-etching chamber transformed the particles from spherical shape into spheroid-like shape. A simple model is proposed to predict the geometry of the resulting nonspherical particles. The shape and dimension of the nonspherical particles generated through such a process matched well with the prediction of the model. The assembly of these spheroid-like particles showed a unique orientational order associated with the alignment of their axes. This approach will help further studies on the fundamental properties of the nonspherical particles, such as packing, rheology, and optical interaction.
Tuning the plasma field in reactive ion etching generates different etching profile of nanoparticles. For nanoparticles in an isotropic plasma field, there will be uniform shrinkage of the particle sizes due to the isotropic etching, with the curvature of the particles unchanged after the etching. An anisotropic etching, on the other hand, provides rich opportunities to modify the shape of the particles with reduced dimensions. For a monolayer of silica nanoparticles on a flat substrate in a unidirectional plasma field, the reactive ion etching changed the shape of silica nanoparticles from spherical to spheroid-like geometry. The mathematical description of the final spheroid-like geometry was discussed and matched well with the experimental results. The surface curvature of the particles after etching remained the same for both the top and the bottom surfaces, while the overall shape transformed to spheroid-like geometry. Varying the etching time resulted in particles with different height to width ratios. The unique geometry of these non-spherical particles will impact fundament properties of such particles, such as packing and assembly. In the case of spheroid-like particles, packing of such particles into ordered structures will involve an orientational order, which is different from spherical nanoparticles that have no orientational order.
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