Vertically Stackable One-Dimensional p-n ZnO Homojunction Architectures for UV Sensing/Energy Harvesting Applications

Abstract

For wide-ranging applications in nanoscale optoelectronic and piezoelectric energy harvesting devices, one-dimensional ZnO nano-architecture and single crystalline doped ZnO having n-/p-type conduction are essential. Herein, catalyst-free ZnO sheet-like nanorods (SLNR) p-n homojunction in which the Lithium (Li) served as p-dopants and intrinsically n-type ZnO, respectively, have been synthesized by a controlled in-situ doping process via hydrothermal synthesis for fabricating efficient ultraviolet UV detectable photonic devices. In addition, a comprehensive investigation was carried out on Li-doped ZnO SLNRs to control the amphoteric electrical behavior of Li in ZnO host lattice. In such cases, Li occupies octahedral sites (Lii O) in the empty cages of the ZnO wurtzite structure and subsequently acts a donor because of the low formation energy, thereby, maintaining or improving the initial n-type behavior of the material. In contrast, the p-type doping by Li atoms occupying Zn site (LiZn) along the growth direction has been achieved by proper thermal activation energy. Further, to obtain n-/p- ZnO nanorods architecture and their vertically stacked p-n homojunctions, the nanorod diameters and morphologies of the ZnO SLNRs were adjusted dramatically by manipulating precursor concentration and growth kinetics. The well-aligned and clean interface between n-/p-ZnO SLNRs could enhance the electron-hole pair generation, transport, separation, thus improved the performance of the homojunction devices. The ZnO p-n homojunction structures have been successfully attained by continuous multi-step solution growth, representing the well-faceted hexagonal SLNR end planes of ZnO segments. The Li-doping effects which are occupied in interstitial site (Lii) or substitutional site (LiZn) in single crystalline ZnO SLNRs were carefully examined through the lattice spacing differences of lattice fringes in high-resolution transmission electron microscopy. An extensive analysis of the luminescence features was carried out in order to identify conduction type conversion from n-type to p-type in the Li-doped ZnO SLNRs. The appearance of acceptor-bound exciton emission, extremely enhancement of near-band-edge emission and the longer carrier life-time indicate that the Li dopants act as desirable shallow acceptor in the ZnO SLNRs. In addition, the SLNR-based p–n homojunctions exhibited distinct electrical features for their potential use as light emitting diodes and UV photovoltaics, thereby spurring progress in the development of practical optoelectronics. Figure 1