Assembly of optical-scale dumbbells into dense photonic crystals
Authors: JD Forster, J Park, Manish Mittal, H Noh, CF Schreck, CS O'Hern, H Cao, EM Furst, and ER Dufresne
Publication: JD Forster, J Park, Manish Mittal, H Noh, CF Schreck, CS O'Hern, H Cao, EM Furst, and ER Dufresne. (2011) "Assembly of optical-scale dumbbells into dense photonic crystals." ACS Nano 5 (8), pp. 6695-6700.
Dumbbell-shaped colloidal particles can be made to assemble into crystals in the presence of an external AC electric field. These crystal structures can have long-range order, and they exhibit photonic band gaps and birefringence.
Colloids with anisotropic interactions are sometimes called "colloidal molecules." The entire phase space of self-assembly of such colloidal molecules has not been fully explored. However, there have been many numerical simulations. For dumbbell-shaped particles, such simulations indicate that the aspect ratio is very important for determining the phase behavior.
The self-assembly of dumbbells without external fields cannot produce large-scale structures, since such assembly can only take place in thin films.
Explanation of Experimental Methods
Monodisperse polymer dumbbells were synthesized using known methods described in other papers. The basic idea is to start with a monodisperse solution of colloidal polystyrene spheres and then swell them and induce additional polymerization twice. The methods were not the focus of this study. The dumbbell particles used have two lobes of the same size, diameter 270nm, and an overall length of 420nm. Thus the aspect ratio is about 1.58. AC electric fields were applied using gold electrodes near samples. Drying during crystallization experiments took place slowly, and in one direction.
Figure 1 shows a thin film of dumbbells which has crystallized upon drying, in contrast to the bulk which does not.
Particle crystallization can be enhanced by applying an external electric field. This helps to align the dumbbell particles, due to their anisotropic polarizability. Figure 2 shows this alignment with an external field as a function of time. Alignment of the particles manifests interesting properties including structural color and birefringence, which can be seen in Figure 2 due to the fact that the images were acquired using crossed polarizers. This alignment is reversible and rapidly disappears without the applied field.
Figure 3 shows a crystal of aligned dumbbell particles which has been created by drying a suspension of particles aligned by an external electric field (rather strong: 1040V/cm, 50kHz). The authors note that capillary forces drive particles toward a dense packing.
Numerical simulations were also carried out to explore the probability of crystallization as a function of dumbbell aspect ratio and their alignment due to the presence of the external electric field. It can be seen in Figure 4 that crystallization probability is very high for a certain range of aspect ratios aligned by electric field, while crystallization probability is almost zero without the field, when dumbbells are not aligned.
It has been shown that the crystallization of dumbbell-shaped colloidal particles can be brought about by applying an external electric field to align the particles. Such crystals have interesting properties, such as birefringence and structural color. In solution the ability to align the particles using a field could prove useful for creating switchable photonic crystals.
 JD Forster, J Park, Manish Mittal, H Noh, CF Schreck, CS O'Hern, H Cao, EM Furst, and ER Dufresne. (2011) "Assembly of optical-scale dumbbells into dense photonic crystals." ACS Nano 5 (8), pp. 6695-6700.