Direct observation of phase transitions in truncated tetrahedral microparticles under quasi-2D confinement

Colloidal crystals are used to understand fundamentals of atomic
rearrangements in condensed matter and build complex metamaterials with
unique functionalities. Simulations predict a multitude of self-assembled
crystal structures from anisotropic colloids, but these shapes have been
challenging to fabricate. Here, we use two-photon lithography to fabricate
Archimedean truncated tetrahedrons and self-assemble them under quasi-2D
confinement. These particles self-assemble into a hexagonal phase under an
in-plane gravitational potential. Under additional gravitational
potential, the hexagonal phase transitions into a quasi-diamond two-unit
basis. In-situ imaging reveal this phase transition is initiated by an
out-of-plane rotation of a particle at a crystalline defect and causes a
chain reaction of neighboring particle rotations. Our results provide a
framework of studying different structures from hard-particle
self-assembly and demonstrates the ability to use confinement to induce
unusual phases.