Transparent electrodes are critical in solar cells and electronic displays. In solar cells, the electrodes let sunlight pass through them and be converted into electricity, thereby improving the solar cell’s efficiency. In displays, they let light out and become part of a seamless display.
For these types of tasks, engineers are looking for a transparent metal with high conductivity. The closest we come to that are metal oxides that collect electricity in a solar cell or to send electricity through for a display. In those cases, you need a conductive contact (like a metal), but you also need to be able to let light in (for solar cells) or out (for displays).
Metal is opaque, so the current techniques use metal oxides, most often indium tin oxide. It is conductive and—if thin enough—transparent to visible light, but it reflects IR. It is also a hard to find rare-earth metal. Because supplies of it are limited and expensive, researchers at Lawrence Livermore National Laboratory decided to use metal nanowire meshes that provide high transmissivity (due to the small diameters of the nanowires) and high electrical connectivity (due to the many contact points in the mesh) and using more common elements.
The nanowire arrays also have applications for optical metamaterials—composite materials usually made of metals and dielectrics—that have optical properties not found in nature. For example, all naturally occurring materials have a positive index of refraction. But metamaterials can be designed to have a negative index of refraction, which means that light passing through this material would go in the opposite direction from what one would normally see, and possibly create structures such as cloaking devices and perfect lenses.
Because the structure of optical metamaterials must be smaller than the wavelength at which they function, fabricating optical metamaterials operating at visible wavelengths requires features on the order of 100 nanometers or smaller. “We’ve demonstrated a scalable method to create metallic nanowire arrays and meshes over square-centimeter-areas with tunable sub-100 nanometer dimensions and geometries,” says LLNL materials scientist Anna Hiszpanski, principal investigator of the project. “We were able to attain comparable or smaller dimensions than what traditional nanofab techniques can produce and do it over a significantly larger area relevant for real-world applications.”
For transparent electrodes, having such small metal nanowire meshes is important because the small size lets more light pass through. Although the ordered nature of the arrays/meshes increases, the number of electrical contacts between nanowires increases conductivity.
“Using nanowires to increase the number of electrical interconnections between wires is highly desirable but difficult to do,” Hiszpanski says. “Relying on the self-assembly behavior of block co-polymers that other groups have demonstrated, we have met this challenge and created ordered metal nanowire meshes. The bottom-up approach we used to fabricate these meshes is inherently scalable to device-relevant areas.”
A common sample area using traditional nanofabrication techniques for metamaterials is 100 microns (square), but the team could create nanopatterns with more than centimeter (squared) areas—areas more than six orders of magnitude larger.
The next step is to increase the conductivity of the metal nanowire mesh.