SWNT conductor orchestrates functionalization

Nanoelectronics systems must often be made with a wide variety of disparate materials, but the primary modes of manufacture – epitaxy or mechanical bonding – place stringent requirements on the ultimate scale or constituent materials of circuits. Researchers at the University of Illinois, Urbana-Champaign have developed a method for building twoand threedimensional circuits using a process akin to the printing press [Ahn et al., Science (2006) 314, 1754]. The highly scalable method can employ any material as a substrate, including glass and plastic, providing an attractive alternative to organic semiconductors for flexible electronics. Crucially, the method can use any mixture of component materials, in contrast to epitaxial methods. In the process, nanoscale semiconductor components such as GaN, GaAs, or Si nanowires or ribbons are first fabricated on a source substrate through standard lithographic procedures, with ohmic contacts formed by doping and annealing. These components are then lifted from the substrate by gentle van der Waals adhesion with an ‘inking pad’ made of polydimethylsiloxane, and then ‘stamped’ onto a target substrate prepared with a spin-cast prepolymer adhesive layer. The process can be repeated in the third dimension to create functional devices. The team reported high yields, >99% for Si and single-walled nanotube (SWNT) components. The room-temperature process reduces the thermal stresses that plague bonding approaches, and the finished circuits show little change as their temperature is cycled up to 90oC. Circuits manufactured on flexible substrates show no change in behavior under bending stress. John A. Rogers says that manufacturing prototypes in his lab can already print on glass substrates up to 30 cm by 40 cm with an accuracy of 0.5 μm; further scaling will be supported by a startup company, Semprius, and several patents. The group’s current focus is on flexible emissive displays, curved focal plane array imagers, and largearea solar cells. D. Jason Palmer A three-layer stack of GaN high-electron mobility transistors, Si metal oxide semiconductor field-effect transistors, and SWNT thin-film transistors on a flexible polyimide substrate. (Courtesy of John A. Rogers)