Fabrication of field-effect transistors and functional nanogenerators using hydrothermally grown ZnO nanowires

Abstract

The present work demonstrates the production of single crystalline ZnO nanowires (NWs) using the low temperature hydrothermal process and their integration as the active channel material and piezoelectric elements in single NW field-effect transistors (FETs) and functional nanogenerators (NGs), respectively. Even though hydrothermally grown ZnO NWs show high levels of excess free carriers ≫10¹⁸ cm⁻³, we show that an optimized thermal annealing process at just 450 °C in atmospheric air sufficiently reduces this level to around ∼3.7 × 10¹⁷ cm⁻³. The excess free carrier suppression is verified by assessing the field-effect transport behaviour in a single NW FET. The single device is found to exhibit good performance metrics, including low off-state current (pA range), high on-state current (in the 10 s of μA range) and moderate effective mobility (∼10 cm² V⁻¹ s⁻¹). The functional NGs are based on vertically grown ZnO NWs with ∼7 μm thick polydimethylsiloxane (PDMS) polymer matrix. We show that a NG incorporating annealed ZnO NWs can continuously generate higher output voltages and power compared to a reference device based on as-grown ZnO NWs. This included peak output voltage of ∼109 mV and an output power density of ∼16 μW cm⁻³. We envisage that this approach of thermal annealing may find practical applications in other areas of hydrothermal ZnO NW research, including high performance NW FETs and piezoelectric energy harvesters.