Past exam of the MPhil in micro- and nanotechnology enterprise course of the University of Cambridge / 2021 / Paper 2 / Section B / 19 / iii / c / Solution

For the silver nanowire network, synthesize or source long, narrow wires, remove excess ligand without causing aggregation, formulate a stable coating ink and coat the flexible web by a scalable method such as slot-die coating or gravure printing. Dry below the substrate's damage temperature, then press or locally weld junctions and embed or cap the network if required. Define 100 µm stripes by masked coating, a printing-compatible patterned wetting layer, or post-coating laser ablation; provide robust end contacts and remove stray bridges between adjacent stripes. Roll processing offers throughput, but coffee-ring effect, wire alignment, random weak links and ablation damage require inline optical/electrical inspection. Verify that the local junction treatment improves resistance without simply darkening the whole film.

For the silver nanoparticle-derived grid, use a repeatable nanoimprint lithography to form a fine mesh inside the outer 25 mm by 100 µm stripe pattern. Fill the channels with a silver nanoparticle ink, remove excess from the transparent openings, and consolidate the metal by substrate-compatible chemical, photonic or low-temperature thermal sintering. Transfer or embed the grid into the polymer surface and add electrical pads. The quantitative design in part (b) sets target pitch, line width and thickness; the large-scale process must maintain those dimensions and a continuous metal cross section. A master enables repeated replication and deterministic open area, but tooling cost, incomplete filling, residual metal in windows, shrinkage cracks, imperfect sintering and adhesion are important limitations. Test the finished patterned stripes, rather than relying on a large unpatterned test coupon alone.