Crystalline structure-tunable, surface oxidation-suppressed Ni nanoparticles: Printable magnetic colloidal fluids for flexible electronics

In this study, we suggest the chemical methodology that allows for the facile controllability of phase transformation between face-centered cubic and hexagonal close-packed structures for Ni nanoparticles with a 0.4-2 nm thick shallow surface oxide layer, resulting in a maximum saturation magnetization of 33.2 emu g^-1. As a first proof-of-concept of the potential for the formation of flexible, printed magnetic devices on cost-effective polyethylene terephthalate (PET) and paper substrates, it is demonstrated that the resulting Ni nanoparticles, prepared in the form of magnetic fluids, are transformed into bulk-like patterned Ni architectures via air-brush printing and instant photonic annealing in a timescale of 10^-3 s, exhibiting highly flexible properties under the harsh conditions of 10 000 times repeated bending tests.