# Presentation: 576853

## 2D & 3D Phase Behavior of Non-Spherical Colloidal Building Blocks

- 2m

- 1m

- Fig. 1.  3D structure of non-spherical mushroom cap-shaped colloids self-assembled layer by layer using heat-assisted convective assembly.  FFT in inset indicates crystal layers with hexagonal in-plane symmetry.  Orientation is confined to either cap-up or cap down configuration.

**Colloid-Based Materials Research Laboratory (CBMRL)**

*Chekesha M. Liddell**Cornell University, MS**&**E*

**Notes:**
Structural colors, not originating from pigments, but rather from the interaction of light with features having the same size as the wavelength, can be observed in many living systems including butterfly wings, spines of marine animals, peacock feathers, and the brilliant blue facial skin of primates.  Examination of the microscopic structure of these tissues reveals the attributes of a photonic crystal system— fine-scale periodic arrays which serve as color-specific mirrors.   The arrays promote partial photonic band gap effects, i.e., light of specific wavelength and direction cannot travel through the material and is thus reflected (analogous to the electronic band gap for electrons).  

My work addresses the central challenge in the photonic crystal field, to develop inexpensive fine-scale periodic materials with sizable PBGs or large partial PBGs (stopbands) at visible and near-IR frequencies.  Calculations indicate the materials characteristics favorable for this achievement include a sub-micron lattice constant, a high refractive index contrast (high-n) between periodic regions, low optical absorption, and low filling fraction of high-n material (ordered porous structure). The crystal structure and shape and complexity of the photonic crystal basis also greatly influence the capability of photonic crystals to exhibit PBGs, by removing symmetry induced degeneracy in the photonic bands.  For the past 15 years experimental achievements have been mainly limited to simple inverted face-centered cubic (FCC) structures templated from close-packed spheres due to [1]  the lack of availability of non-spherical particles with consistent size and shape (CV<5%) in a variety of chemistries and [2] the lack of processes for particle orientation control which would lead to complex topologies and new lattice structures. 

The Liddell group creates complex colloidal crystals from buildings blocks with diverse morphology [i.e., non-spherical, core-shell] and with functionality [i.e., fluorescent, magnetic].   Our experiments investigate the effects of colloid particle chemistry, shape, size, packing, orientation, and refractive index on photonic band structures and the resulting optical properties of photonic crystals with the ultimate goal of developing new functional (beyond passive waveguiding) photonic crystal structures. These photonic crystal materials have applications in technologies such as optical integrated circuits, efficient light sources, photocatalytic systems, and solar cells. 

Figure descriptions:
This research is notable because we have demonstrated several non-spherical colloidal particles for which convective assembly produces various types of order in colloidal films (for the first time, Fig. 1-2).  The particles closest pack in the region defined by the meniscus as liquid evaporates from a colloidal suspension.  3D colloidal crystals were fabricated using layer-by-layer (LBL) deposition of successive mushroom-cap colloidal monolayers (Fig. 1).  FFT (fast fourier transform) confirms hexagonal close-packed monolayers.

The phase structures examined using SEM and confocal microscopy are compared with thermodynamic models from Monte Carlo simulations (collaboration with F. Escobedo). Transitions between isotropic and ordered phases [Fig. 2(d)] are marked by changes in slope of the simulated equation of states curve [EOS, Fig. 6(a)].  Translational order is quantified using radial distribution functions for the particles’ centers of mass, and the orientational order by calculating order parameters.  For the case of asymmetric dimer particles with highly fused lobes, in experiment  [LBL convective assembly, Fig. 2(b)] and simulations [Fig. 2(d)] we observed positional order on a hexagonal lattice, but random particle orientation; while asymmetric dimer particles with low degree of fusion between lobes exhibited both positional and orientational order (not shown) also in agreement with Monte Carlo simulations.  We have begun to use square well attractive potential energy of interaction (short range and longer range) to model the effect of attractive capillary forces during convective assembly and have observed pronounced effects on the equation of state curves (compared to hard sphere system), such as extended transition regions as well as equilibrium between rectangular and distorted hexagonal 2D lattices for the non-spherical colloidal systems.

## Education & Outreach Exhibits

**Notes:**
[1] Nanolab is an interactive science center exhibit.