Hierarchical ZnO nanorods on Si micropillar arrays for performance enhancement of piezoelectric nanogenerators.

Enhanced output power from a ZnO nanorod (NR)-based piezoelectric nanogenerator (PNG) is demonstrated by forming a heterojunction with Si micropillar (MP) array. The length of the SiMP array, which was fabricated by electrochemical etching, was increased systematically from 5 to 20 μm by controlling the etching time. Our structural and optical investigations showed that the ZnO NRs were grown hierarchically on the SiMPs, and their crystalline quality was similar regardless of the length of the underlying SiMPs. The peak output voltage from the ZnO NR-based PNG was greatly increased by ∼5.7 times, from 0.7 to 4.0 V, as the length of the SiMP arrays increased from 0 (flat substrate) to 20 μm. The enhancement mechanism was explained based on the series connection of the ZnO NRs regarded as a single source of piezoelectric potential by creating a heterojunction onto the SiMP arrays.