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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