Gold-nanoparticle dispersion in liquid-crystal composite forms tunable metamaterial

A new class of artificial, composite materials, often called ‘metamaterials,’ holds great promise for applications in optics, microwave engineering, acoustics, and thermal management. This technology enables construction of materials with prescribed macroscopic functionalities. In metamaterials, the individual components making up the composite structures are much smaller than the operating wavelength. Therefore, electromagnetic or acoustic waves propagating in these materials do not ‘see’ the individual components but only the averaged medium. The composite material thus behaves like a homogeneous material with well-defined macroscopic properties. What makes these materials so unique is their ability to control the macroscopic properties by engineering the individual elements in the composite.

Optics has been at the forefront of metamaterials research with the discovery and demonstration of exciting new phenomena such as negative refractive index and invisibility. The major challenge in this context is fabrication. For optical metamaterials, nanoscale fabrication is needed to achieve the required subwavelength-sized elements. The current state of the art almost entirely relies on top-down techniques such as electron-beam and focused ion-beam lithography. While these techniques have been very successful in demonstrating a wide variety of metamaterial structures, they inevitably limit the latter to 2D patterns over small areas. To fully exploit the potential of metamaterial technology, it is essential to be able to manufacture 3D bulk nanostructures in large volumes.

Figure 1. Atomic-force-microscope image of a composite sample with a metal volume fraction of 0.54. The humps correspond to layer deformations induced by the nanoparticles.

Noble-metal nanoparticles that exhibit a strong surface-plasmon resonance naturally lend themselves to development of metamaterials. The term ‘plasmon’ refers to excitation of the free-electron gas by an electromagnetic wave. Plasmons provide a unique mechanism to achieve extremely small-sized resonators, which makes plasmonic nanoparticles excellent building blocks for optical metamaterials. Furthermore, typical plasmonic nanoparticles, such as those made from gold or silver, are synthesized easily by wet-chemistry processes. The size and shape can also be controlled precisely to produce platelets, rods, cubes, and many other shapes. The dispersion of plasmonic nanoparticles in a medium that can be handled easily, such as a polymer solution that can be spin coated, would provide an ideal scalable metamaterial.

There is, however, one fundamental roadblock in this approach. Most of the desired metamaterial effects require high concentrations of metal nanoparticles. However, this is extremely difficult to achieve, because the nanoparticle solution becomes unstable and suffers from irreversible agglomeration because of the strong van der Waals interaction between the metal particles. In organic solvents, the maximum metal volume fraction that can be achieved is ∼0.1%. For metamaterial applications, a volume fraction of one to two orders of magnitude higher is required.

We have achieved very high metal-nanoparticle concentrations in a liquid-crystal matrix by taking advantage of the soft-matter interaction between the nanoparticles and the host molecules. We used 14nm gold nanoparticles in the liquid-crystalline compound 4-n octyl 4^′-cyanobiphenyl (8CB) in smectic A phase. The optical image of the composite as observed under a polarizing microscope was very similar to 8CB when the gold-nanoparticle concentration was low. However for a large metal volume fraction of 0.5, the image exhibited an intense green color, while the characteristic focal conical structure of the smectic A phase remained intact.

To confirm the absence of agglomeration, we directly probed the distribution of the nanoparticles in the liquid-crystal film by atomic-force microcopy (AFM). When the nanoparticle/liquid-crystal dispersion is in the form of a thin (<100nm) film on a silicon substrate, the nanoparticles cause raised, hump-like deformations, with each hump corresponding to an individual nanoparticle (see Figure 1). From the AFM image it is clear that there was no severe agglomeration of the particles and that they remain stabilized and well-dispersed in the smectic A phase. We also conducted extensive spectroscopy and the coupled plasmon resonance of the nanoparticles further confirmed that they were uniformly dispersed.¹

The stabilization mechanism can be understood by considering the structure of a smectic A liquid crystal.² In the smectic A phase, the liquid-crystal molecules align in layers, and it takes a substantial amount of energy to push the nanoparticles out of their layers. This effectively prevents nanoparticles embedded in different layers from aggregating. For nanoparticles in the same layer, the molecular layer must deform around the nanoparticles to accommodate them. This strong curvature of the molecular layer raises the free energy, that is, the liquid-crystal molecules prefer flat molecular layers. Therefore, even for two nanoparticles embedded in the same molecular layer, they cannot get too close to each other, because it requires a strongly distorted molecular layer. The same mechanism is known to keep the dislocations apart in the smectic A phase.

This work presents a novel approach to fabricate metamaterials with an extremely high metal volume fraction using soft-matter interactions. By dispersing plasmonic nanoparticles at high concentrations in a liquid-crystal matrix, it is possible to produce large volumes of metamaterials, opening doors to many practical applications of these novel composites. We will next investigate other types of particles (such as nanorods and nanoplatelets) in a liquid-crystal matrix and explore dynamic tuning of these composites.