University of Houston researchers have developed a new stretchable and transparent electrical conductor, bringing the potential for a fully foldable cell phone, or a flat-screen television that can be folded and carried under your arm, closer to reality.

Such a material has to be transparent, flexible, and conductive. Some materials have two of the components, but until now, finding one with all three has remained difficult.

Zhifeng Ren, a physicist at the University of Houston and principal investigator at the Texas Center for Superconductivity, and his research associates at UH and Harvard University developed the solution: gold nanomesh electrodes.

The material also has potential applications for biomedical devices, said Ren, lead author on a paper in Nature Communications.

The researchers reported that gold nanomesh electrodes, produced by novel “grain boundary lithography,” increase resistance only slightly, even at a strain of 160 percent, or after 1,000 cycles at a strain of 50 percent.

The nanomesh — a network of fully interconnected gold nanowires — has good electrical conductivity and transparency, and has “ultrahigh stretchability,” according to the paper.

Gold nanomesh does not easily oxidize, which Ren said causes a sharp drop in electrical conductivity in silver and copper nanowires.

Grain boundary lithography

The grain boundary lithography involved a bilayer lift-off metallization process, which included an indium oxide mask layer and a silicon oxide sacrificial layer. It offers good control over the dimensions of the mesh structure.

Samsung demonstrated a cellphone with a bendable screen in October, and LG Electronics has introduced a curved cellphone that is available now in Asia. But neither is truly foldable or stretchable, instead curving slightly to better fit against the user’s face.

The work at the University of Houston was funded by the Department of Energy; the Harvard work was funded by a National Science Foundation grant.

Abstract of Nature Communications paper

Foldable photoelectronics and muscle-like transducers require highly stretchable and transparent electrical conductors. Some conducting oxides are transparent, but not stretchable. Carbon nanotube films, graphene sheets and metal-nanowire meshes can be both stretchable and transparent, but their electrical resistances increase steeply with strain <100%. Here we present highly stretchable and transparent Au nanomesh electrodes on elastomers made by grain boundary lithography. The change in sheet resistance of Au nanomeshes is modest with a one-time strain of ~160% (from ~21 Ω per square to ~67 Ω per square), or after 1,000 cycles at a strain of 50%. The good stretchability lies in two aspects: the stretched nanomesh undergoes instability and deflects out-of-plane, while the substrate stabilizes the rupture of Au wires, forming distributed slits. Larger ratio of mesh-size to wire-width also leads to better stretchability. The highly stretchable and transparent Au nanomesh electrodes are promising for applications in foldable photoelectronics and muscle-like transducers.