Shaping colloidal assemblies

An MeV ion beam penetrating an amorphous material causes rapid heating of a cylindrically shaped region around the ion track, which leads to anisotropic deformation: the irradiated material shrinks in the direction of the ion beam and expands perpendicular to it. This provides a unique way to tailor the shape of colloidal materials. Spherical silica particles are transformed into oblate ellipsoids, with anisotropy tuned by ion fluence. Ellipsoid suspensions can be used to test theories of colloidal phase transformations. Photonic crystals of ellipsoidal colloids can have enlarged optical band gaps. Deforming metal-silica or silica-metal core-shell particles leads to control over their surface plasmon resonance frequency. Two-dimensional colloidal crystals show complex deformation behavior, providing unique masks for nanolithography.