Magnetically rewritable photonic ink based on superparamagnetic nanochains

One-dimensional photonic nanochains composed of periodically arranged superparamagnetic nanoparticles have been exploited as basic color units for a new type of photonic ink. The photonic response of ink can be rapidly and repetitively switched on and off by tuning the direction of external magnetic fields. Temporary bi-stability is realized in the photonic ink by introducing highly viscous solvents as a dispersant, which suppress the rotational movement of nanochains and allow them to stay temporarily aligned after the removal of external magnetic field and to retain their photonic response for a considerably long period. Increasing the viscosity of the dispersant enhances the bi-stability of photonic inks; for instance, the ink prepared in 95% glycerol solution is able to retain its color up to ten minutes. The length of the chains also plays an important role in their temporary bi-stability. By mixing chains with different photonic properties and lengths, a display has been developed which exhibits two distinct colors in response to the application and removal of external magnetic fields.