Spectral reflectance switching of colloidal photonic crystal structure composed of positively charged TiO2 nanoparticles

We report the tunable photonic crystal behavior of highly concentrated, mildly charged TiO2 nanoparticle colloids under external electric field. Reflectance intensity change at the certain photonic bandgap and its irreversible switching characteristics are observed and discussed. Through computer aided numerical analysis, gradient spacing between charged particles is predicted under electric field, which is well coincidence with experimental results. Also, we report the experimental evidence that the charged nanoparticle can play the role as a major charge carrier in non-aqueous liquid. Its short range moving and stacking on electrode cause the capacitive current flow and charge accumulation like ions in liquid state.