Where the resolution of flexographic printing using elastomeric stamps is typically limited to tens of micrometres, due to liquid instabilities and ink spreading issues during stamping, the researchers have developed a novel type of stamp material specifically engineered to work well with a number of commonly available electronic inks at near 1μm resolution.
The material had to have pores large enough to host colloidal ink particles when wetted with the electronic inks, yet not so large as to impact negatively the printed features. It had to be solvent resistant, mechanically compliant to establish uniform contact, yet durable.
The researchers found what they were looking for by growing vertically aligned carbon nanotubes (CNTs) on a lithographically patterned silicon substrate, creating repeatable microstructures, 99% porous. After various chemical surface treatments and a conformal pPFDA polymer coating, the stamps were fit for purpose, supporting the capillary-driven loading of ink and nanoscale contact-mediated ink transfer.
As a demonstration, the researchers then printed a number of micrometre-scale patterns of a variety of functional nanoparticle inks, including Ag, ZnO, WO3, and CdSe/ZnS, onto both rigid and compliant substrates. That was performed at a resolution and a printing speed (0.2 m/s) far superior to today's industrial printing technologies, they wrote in their paper, hinting that their process could make cheap mass-produced electronics a reality if translated into roll-to-roll tool fabrication.
In one example, the MIT team printed conductive networks of transparent electrodes, a key layer to connect to LEDs, LCDs, touch-screen panels, or solar cells to name a few applications. In a single flexographic printing step using the an honeycomb-patterned nanoporous stamp, the researchers were able to print a thin Ag honeycomb with a minimum linewidth of 3μm between adjacent holes, with a transparency of 94% (from 200 to 800nm) and a sheet resistance of 3.6 ohm/sq.