ZnO-Cu2O core-shell nanowires as stable and fast response photodetectors

In this work, we present all-​oxide p-​n junction core-​shell nanowires (NWs) as fast and stable self-​powered photodetectors. Hydrothermally grown n-​type ZnO NWs were conformal covered by different thicknesses (up to 420 nm) of p-​type copper oxide layers through metalorg. chem. vapor deposition (MOCVD)​. The ZnO NWs exhibit a single cryst. Wurtzite structure, preferentially grown along the [002] direction, and energy gap Eg = 3.24 eV. Depending on the deposition temp., the copper oxide shell exhibits either a cryst. cubic structure of pure Cu2O phase (MOCVD at 250 °C) or a cubic structure of Cu2O with the presence of CuO phase impurities (MOCVD at 300 °C)​, with energy gap of 2.48 eV. The elec. measurements indicate the formation of a p-​n junction after the deposition of the copper oxide layer. The core-​shell photodetectors present a photoresponsivity at 0 V bias voltage up to 7.7 μA​/W and time response ≤ 0.09 s, the fastest ever reported for oxide photodetectors in the visible range, and among the fastest including photodetectors with response limited to the UV region. The bare ZnO NWs have slow photoresponsivity, without recovery after the end of photo-​stimulation. The fast time response for the core-​shell structures is due to the presence of the p-​n junctions, which enables fast exciton sepn. and charge extn. Addnl., the suitable electronic structure of the ZnO-​Cu2O heterojunction enables self-​powering of the device at 0 V bias voltage. These results represent a significant advancement in the development of low-​cost, high efficiency and self-​powered photodetectors, highlighting the need of fine tuning the morphol., compn. and electronic properties of p-​n junctions to maximize device performances