Transparent electrodes
Low-energy electron microscopy
Increasing interest in thinner and more flexible electronic devices has led researchers at the University of California, Los Angeles (UCLA) Henry Samueli School of Engineering and Applied Science to develop what they’re calling transparent electrodes. The components employ a thin polymer made from silver nanowires (AgNW).
Led by professor of materials science and engineering Qibing Pei, research authors Zhibin Yu, Qingwu Zhang, Lu Li, Qi Chen, Xiaofan Niu, Jun Liu, and Qibing Pei believe their discovery could prove to be a low-cost alternative to indium-doped tin-oxide (ITO) electrodes. These ITO-based components are common in LCDs, solar cells, touchscreens, and organic LED (OLED) displays. Target applications for the transparent electrodes include thin-film solar panels, wearable displays, and non-invasive biomedical devices.
Advantages
According to the transparent-electrode inventors, ITO technology is becoming more costly to manufacture while having the propensity for being both toxic and not flexible enough (delicate) for some applications. Based on AgNW, the transparent electrodes consist of stable, lower-cost non-toxic materials that are more manageable in the fabrication process.
Compared to ITO, the AgNW-polymer electrodes offer a lower sheet resistance, higher transparency, and significantly smoother surfaces. They are quite versatile, formed on a transparent substrate that’s less expensive than glass while having the ability to be flexible, rigid, or expandable depending on application requirements.
Of particular note, the electrodes (Fig. 1) exhibit shape memory, meaning designers can bend and form them into various common and unique shapes while they retain the ability to revert back to their original shape when necessary, even after numerous repetitions. According to the researchers, this only causes minimal if any damage to the device.
A Viable Electrode Material
Another team of researchers from UCLA Engineering, Sandia National Laboratories, and the Colorado School of Mines led by UCLA assistant professor of materials science and engineering Suneel Kodambaka have found that the electronic properties of graphene rely on its crystalline properties and the metal contact. An allotrope of carbon, graphene is a one-atom-thick sheet of graphite formed into a crystal lattice resembling a honeycomb.
Graphene’s extremely thin makeup and high carrier ability make it more than viable for use in low-power applications as well as a material for creating transparent electrodes. Based on the research findings, graphene-based electronics will depend on the substrate material’s orientation and tunable band gap. Devices based on graphene will require metal contacts and a working knowledge of the material’s electronic properties. For one, designers need to address the nature of the contact, be it Ohmic or Schottky, and electron transport at the metal-graphene interfaces.
To analyze and understand the impact of the metal substrate and the in-plane orientation of graphene, the researchers used palladium in a model substrate. Low-energy electron microscopy (Fig. 2) and density functional theory calculations revealed monolayer graphene on palladium exhibiting a work function varying significantly with domain orientation.
The researchers deduced that this is most likely a result of spatial variations in charge transfer at the graphene-palladium interface, i.e., the in-plane orientation plays an important role in the nature of contact. In conclusion, their results suggest that precise control over graphene orientation with respect to the metal contacts is critical for designing large-scale graphene electronics.