Direct Imaging of a Solid-Solid Phase Transition in Truncated Tetrahedral Colloids

Self-assembly of colloids crystal have been used to understand fundamental atomic dynamics such as disordering during melting and defect dynamics during applied stress. These types of systems have also been used to build meso-scale metamaterials with unqiue optical or mechanical responses. Although self-assembly of polyhedral colloids have predicted to form a variety of crystal structures, they have been challenging to fabricate chemically. Here, we use two-photon lithography to fabricate a subset of polyedral shapes (tetrahedrons and truncated tetrahedrons) that self-assembly on a 2D surface. We observe that truncated tetrahedrons assemble in a hexatic close-packed phase initally, and then transform into a quasi-diamond phase under additional graviational potential. Free energy calculations and Monte Carlo simulations reveal that supression of particle rotation is the main mechanism behind the initial hexatic phase, which is then unsupressed upon additional gravitational force. This mechanism is also confirmed by in-situ optical imaging of the phase transition at the single particle level. Our results demostrate the self-assembly of truncated tetrahedrons, with different assembly states controlled through particle rotation supression or activation by gravitational potentials.