## 2 Heterojunctions with 1D Metal Oxides Semiconductors  
Since the discovery of carbon NTs by Professor [PERSON] in 1991 [42], 1D nanomaterials have quickly become one of the hotspots in the research of nanomaterials and functional devices. Carbon NTs have excellent physical and chemical properties, such as large surface-area-to-volume ratio, high mechanical strength and brilliant thermal conductivity, as well as good chemical stability. However, the use of presently existing growth technologies does not allow a readily available effective chiral control over the synthesis of carbon NTs [43], always resulting in a compound with both metallic and semiconducting properties. As such, it is impossible to obtain a completely pure semiconductor, which greatly limits the applications of optoelectronic devices. Because of the above limitation, researchers started with the development and investigation of other 1D semiconductor nanomaterials. Silicon (Si) is the most widely used semiconductor material for optoelectronics devices. Professor [PERSON] of Harvard University, as the leading expert in this field, for the first time has successfully prepared Si NWs [44] and employed them in photovoltaic sub-devices, biosensors, etc. [45; 46], showing the revealed nanomaterials and devices to have broad-range expectations in engineering. However, Si NWs also demonstrate some shortcomings, such as easy oxidation to form polycrystalline or crystal defects in air, which may have a prevailing effect on the electrical transmission characteristic. Due to this fact, researchers reoriented to the development and implementation of 1D nanometer semiconducting materials with less oxidizing ability and higher stability. In 2001, the discovery of metal oxide semiconductor nanobelts pushed the study of nanomaterials forward to a new challenge. Metal oxides as the prime candidates for new functional inorganic materials are finding many promising applications in aerospace, biological engineering, semiconductor electronics, functional ceramics, and other fields. The 1D metal oxide nanosystems belong to the most prominent examined systems due to their good crystal quality, low defect density, excellent charge-carrier mobility, and fast response [47], thereby forming a promising replacement for traditional silicon-based electronic and optical devices. The most recent studies have reported on the excellent optical-electronic performance of 1D metal oxides as well as new functional devices based on various substrate materials. [PERSON] et al. prepared a high-speed ultraviolet photoelectricity detector of ZnO-NWs Schottky barrier based on the surface-ionic-gate powered by tribo-nanometer generator [48]. [PERSON] et al. demonstrated a novel bipolar response in self-powered ZnO NWs/Sb\({}_{2}\)Se\({}_{3}\) heterojunction photodetector with adjustable polarity switching wavelength. As demonstrated in Figure 1, the output signal shows the change in the photocurrent polarity between shorter (405 nm-690 nm) and longer (760 nm-880 nm) wavelength regions [49]. [PERSON] et al. have recently obtained a UV-free white LED based on high-level Fe-doped p-ZnO NWs arrays on the n-GaN substrate [12]. [PERSON] et al. developed a real-time wearable UV-radiation monitor by exploiting the excellent properties of p-CuZnS/n-TiO\({}_{2}\) photodetector [50]. [PERSON] et al. proposed a feasible way to improve the hole doping in ZnO:N films with introduced beryllium and demonstrated strong near-band edge UV emission of the ZnO homojunction LED devices, which can be observed even at 400 K under continuous current injection [51]. [PERSON] et al. improved the performance of n-ZnO NRs/p-GaN LED with the use of transparent graphene electrode. The transparent graphene electrode was used as the current diffusion layer, showing better performance compared to that in the ITO analogs [52]. The photoelectrochemical self-powered photodetectors related to ZnO/CdS NWs were manufactured by [PERSON]'s group [53].