Melting and solid-solid transitions of two-dimensional crystals composed of Janus spheres

Colloidal model systems have been extensively used in the studies of various phase transitions, but melting and solid-solid transitions have rarely been explored in monolayer colloidal crystals with anisotropic attractions. Patchy colloidal particles have served as important model systems of atoms and molecules with anisotropic interactions. In this work, we study the melting and solid-solid transitions of two-dimensional crystals composed of Janus colloidal spheres using Langevin dynamics simulation. We discovered a first-order solid-solid transition from a single crystal with uniform stripes to a novel crystal with polycrystalline domains of stripes. The centers of masses of the particles maintain the morphology of a single crystal with long-range translational and bond-orientational orders, but particle orientations form polycrystalline domains of stripes. The stripe domains form by a strain-induced nucleation process via the collective rotation of particles. In addition to this solid-solid transition, the melting transition at a higher temperature follows a two-step Kosterlitz-Thouless-Halperin-Nelson-Young (KTHNY) scenario, similar to most isotropic particle systems.