Colloidal assemblies modified by ion irradiation

Spherical SiO₂ and ZnS colloidal particles show a dramatic anisotropic plastic deformation under 4 MeV Xe ion irradiation, that changes their shape into oblate ellipsoidal, with an aspect ratio that can be precisely controlled by the ion fluence. The 290 nm and 1.1 µm diameter colloids were deposited on a Si substrate and irradiated at 90 K, using fluences in the range 3×10¹³-8×10¹⁴cm^-2. The transverse particle diameter shows a linear increase with ion fluence, while the longitudinal diameter shrinks; the particle volume remains constant. Size aspect ratios up to 3.1 are achieved. Applications of the ion beam deformation technique are shown in studies of liquid crystalline colloidal ordering, self-assembled two-dimensional colloidal lithographic masks for thin-film deposition, and in tuning the optical properties of three-dimensional colloidal crystals.