Ferroelectricity in Undoped ZnO Nanorods

The technological interest in developing novel functional nano-objects that exhibit new properties arising from their nanoscopic nature is undoubtful. Although bulk ZnO is considered to be non-ferroelectric, herein, the ferroelectric behavior of undoped and self-standing ZnO nanoparticles is reported for the first time. The nanoparticles (NPs) under study are synthesized by an organometallic approach. When dispersed in a PMMA dielectric matrix, clear polarization loops and antisymmetric switching currents are recorded for the NPs with the larger volume, in coherence with the significant polar response of these NPs in solution, measured by polarization-resolved second harmonic scattering experiments. Structural characterizations reveal that ferroelectricity is related to the appearance of a new hexagonal crystalline phase, its unit cell being swelled with respect to the well-known wurtzite ZnO cell. The volume fraction of this new phase, the polar SHS response, and the amount of ferroelectric polarization response are all linearly linked, underlining the consistency between these complementary physical characterizations.