## 5 Conclusions and Future Perspective  
In this review, we have summarized recent general findings for diamond-based metal oxide p-n heterojunctions, especially those that comprise three important types of 1D nanostructures studied in this photoelectric decade. The features of p-type BDD films and corresponding epitaxial growth relationships of 1D ZnO NRs on diamond facet have been  
Figure 21: (**a**) SEM morphology of the BDD substrate and (**b**) WO\({}_{3}\) NRs on the BDD substrate. (**c**) Schematic diagram of the n-WO\({}_{3}\) NRs/p-BDD heterojunction. (**d**) Room temperature and (**e**) higher-temperature energy band diagrams of the n-WO\({}_{3}\) NRs/p-BDD heterojunction [34].  
Figure 22: _I-V_ curve of the n-WO\({}_{3}\) NRs/p-BDD heterojunction operating from 20 \({}^{\circ}\)C to 290 \({}^{\circ}\)C. The top inset is the curve of log _I_-log \(V\), and the bottom inset is the relation of ln (\(l_{s}\)) vs. 1/\(k_{B}\)T [34].  
experimentally and theoretically predicted. High-temperature electrical transport behavior and NDR of n-ZnO NRs/p-BDD heterojunction have been presented in detail. 1D n-TiO\({}_{2}\)/p-BDD heterojunction-related optoelectronic applications, including photocatalysis and photoelectrochemical electrode, have been presented. The fabrication of 1D n-WO\({}_{3}\)/p-BDD heterojunction and its electronic behavior at high temperature are discussed.  
These above-mentioned reports provide a greatly expanded understanding of the background of electrical properties in diamond-based 1D metal oxides p-n heterojunctions. In the future, it is also desirable to find new fabrication routes and types of electrodes for a device to improve the device performance. For instance, by introducing a doping element into the metal oxide, one may increase the carrier concentration or fabricate composite 1D metal oxide with other nanomaterials to form complex structures. Optimization of the growth quality for BDD will raise the operating temperature of the device. By changing the electrode configuration of the heterojunction and controlling highly oriented nanosized 1D metal oxide and boron doping concentration of the BDD, we could construct a variety of heterojunction devices and optimize the preparation process. In order to better understanding the device tunneling physical mechanism, it becomes essential to simulate semiconductor theoretical and computational models for the development of diamond-based metal oxides p-n heterojunctions. This is not a comprehensive investigation, but rather a report on the most cited and unique explanations that look promising for the future. These may be only a small part of the possibilities, and the authors hope that this review will inspire further research activities to make more discoveries in the future.  
Writing--original draft preparation, Y.Y., S.D., and D.S.; writing--review & editing, D.S., Q.W., and C.L.; Supervision, D.S. All authors have read and agreed to the published version of the manuscript.  
This work was supported by the National Natural Science Foundation of China (Grant Nos. 11604133 and 11874174), the Natural Science Foundation of Shandong Province (Grant No. ZR2017 QA013), the Science and Technology Plan of Youth Innovation Team for Universities of Shandong Province (Grant No. 2019 KJJ019), the Introduction and Cultivation Plan of Youth Innovation Talents for Universities of Shandong Province, the Open Project of State Key Laboratory of Superhard Materials (Jilin University, Grant Nos. 201503 and 201612), the Initial Foundation for Doctor Program of Liaocheng University (Grant Nos. 318051612 and 318051610) and the Special Construction Project Fund for Shandong Province Taishan Scholars.  
We thank [PERSON] (Jilin University, China) for useful discussion.  
The authors declare no conflict of interest.