Buckled colloidal crystals with nonspherical bases for two-dimensional slab photonic band gaps

Abstract

Theoretical modeling of the photonic band gap forming properties is reported for the buckled phase of anisotropic particles. These exist between the first and second particulate layers of confined colloidal suspensions. Inspired by the range of non-spherical mushroom-cap building blocks for self-assembly that have been synthesized using seeded emulsion-polymerization, we explore in particular the band structures as a function of toroid shape parameter. The parameter is adjusted to incrementally transform hemispheres to spheres. Additionally, corrugation heights that systematically modulate the slab photonic crystal unit cell from rectangular monolayer to square bilayer are investigated. Polarization independent gaps in the guided modes are determined for direct and inverted structures that exhibit bifurcation in the particle orientation perpendicular to the slab plane. Gaps in the guided modes are observed between the fourth and fifth, twelfth and thirteenth, as well as higher band locales as the particle morphology and lattice aspect ratio vary.

Phase Behavior of Bowl‐Shaped Colloids: Order and Dynamics in Plastic Crystals and Glasses

“…An anisotropic shape that recently has received increased interest is that of an indented sphere or bowl. The exact shape can differ from a sphere with a dimple in its surface, to a mushroom cap, and a bowl‐like shape . These anisotropic colloids can be obtained via polymerization‐induced buckling instability, the collapse of thin hollow spherical shells, or temperature controlled swelling with hydrocarbons .…”

“…This geometry of the molecules offers the possibility to form stacks and columns that are promising for the formation of defect‐free columnar phases or specialized antiferroelectric phases . Due to their larger size, bowl‐shaped nanoparticles and colloids are of interest as superhydrophobic and infrared blocking coatings, contrast agents for optical coherence tomography imaging, or superstructures with a full photonic bandgap . In addition, bowl‐shaped colloids are also of interest for real space studies of lock and key assembly between particles with complementary shapes and for the formation of complex crystalline and columnar phases …”

“…Ivell et al showed that indented colloids do not crystallize as easily as their spherical counterparts under slow sedimentation, as later explained by simulations due to kinetic trapping . In addition, crystallization studies presented by Liddell Watson and co‐workers were performed in spatial confinement and other works on indented colloids and bowls relied on electric fields to induce the particle assembly.…”

Phase behaviour and gravity-directed self assembly of hard convex spherical caps

“…Moreover, MCS particles have gained considerable attention due to the possibility of forming photonic band-gap materials with highly anisotropic Brillouin zones. 58,[63][64][65] The focus of the present report is to use Monte Carlos (MC) computer simulations to explore the competing effects of gravity and particle shape on crystal structures formed on both planar structureless walls and patterned substrates to study colloidal epitaxy 66 . A good understanding of crystallisation by sedimentation is necessary if it is to be a viable route for the production of novel structures from anisotropic particles.…”

Slab photonic crystals with dimer cylinder bases

“…For example, rotating a rounded triangular or trigonal cluster of circular cross section pores in a GaAs (dielectric contrast ϵ c 12.25) slab 9°from a lattice vector led to a 12.8% polarization-independent bandgap (Δω∕ω, gap-to-midgap ratio) [13]. Mushroom cap-shaped colloids in buckled crystal arrangements exhibited multiple polarization-independent bandgaps between band indices 4-5, 8-9, and 12-13 for the inverted case, depending on the specific shape parameters [14].…”