## 4 Diamond-Based 1D Metal Oxide Heterojunction Classes
### 1D N- WO\({}_{3}\)/p-BDD-Related Optoelectronic Devices  
WO\({}_{3}\) is a low-cost metal oxide semiconductor with a wide band gap (2.7 eV), excellent electron transport and unintentionally n-type doping performance. WO\({}_{3}\) is considered as another candidate material for applications in electronics [7; 107]. 1D WO\({}_{3}\) nanostructures (such as NRs, NWs and nanoneedles) have been used in photocatalytic applications [108; 109], sensor switching devices [110], gas sensors [111; 112] and UV photodetectors [2]. Because of the excellent performance of 1D WO\({}_{3}\) and BDD, it is worth combining them in 1D n-WO\({}_{3}\)/p-BDD heterojunction to provide new applications in electronics, especially at higher temperatures. In 2017, [PERSON] et al. [34] studied the high-temperature electrical transport behavior of n-WO\({}_{3}\) NRs/p-BDD heterojunctions fabricated by the hydrothermal method. WO\({}_{3}\) NRs with a square feature of the top surface were deposited onto the BDD with perpendicular alignment to the substrate plane (Figure 21a-c). The p-n heterojunction demonstrated excellent thermal stability and rectified characteristics within the temperature range from room temperature to 290 \({}^{\circ}\)C. The turn-on voltages declined, and the rectified ratio was relatively high with the temperature increase. The ideality factor decreases with the increase in temperature (Figure 21b). Under the influence of high temperature, when the reverse voltage is large, both the forward current and the reverse leakage current increase (Figure 22). At elevated temperatures, the E\({}_{\text{F}}\) value of the heterojunction semiconductor usually moves close to the middle region of the bandgaps (shown in the energy band diagrams, Figure 21d,e) [113]. More intrinsic carriers are thermally excited, and it becomes easier for electrons (holes) to pass from the n-WO\({}_{3}\) conduction band (p-BDD valence band) to the p-BDD conduction band (n-WO\({}_{3}\) valence band) under the voltage applied, which leads to the decreasing turn-on voltage, increasing forward current, and slightly improved injection efficiency. At higher voltages and for temperatures above 200 \({}^{\circ}\)C, the traps are filled with more thermally excited carriers, and the injected current complies with the trap-free SCLC law, with an exponential power of about 2. The E\({}_{\text{t}}\) value of the characteristic trap energy obtained from the heterojunction is 50 meV (inset of Figure 22). The characteristic trap energy values measured for ZnO NRs/BDD heterojunction [24], amorphous polymer semiconductor [114] and organic heterojunction [115] are higher than the listed value. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Heterojunction Morphology** & **Synthesis Route** & **Applications (Properties)** & **Ref.** \\ \hline TiO\({}_{2}\) film/diamond & Sol-gel method & Photoelectrocatalytic activities & [102] \\ TiO\({}_{2}\) NRs/diamond & Hydrothermal method & Photocatalytic activities & [103] \\ TiO\({}_{2}\) NRs/diamond & Liquid phase deposition method & Photocatalytic activities & [38] \\ TiO\({}_{2}\) NRs/diamond & Liquid phase deposition method & Photoelectronic nanodevices & [39] \\ TiO\({}_{2}\) NRs/diamond & Liquid phase deposition method & Photocatalytic devices & [104] \\ TiO\({}_{2}\) NRs/diamond & Anodization method & Hybrid electrode & [105] \\ TiO\({}_{2}\) NRs/diamond & Anodization method & Supercapacitor or Energy Storage Devices & [106] \\ TiO\({}_{2}\) NRs/diamond & Anodization method & Supercapacitor & [70] \\ TiO\({}_{2}\) film/diamond & Radio Frequency sputtering & Photoelectrochemical performance & [41] \\ TiO\({}_{2}\) film/diamond & Sol-gel method & Photocatalytic activities & [37] \\ TiO\({}_{2}\) film/diamond & Sol-gel method & Hybrid electrode & [27] \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Heterojunction applications and properties of 1D n-TiO\({}_{2}\)/p-diamond. Figure 20: SEM morphology of BDD (**a**) BDD with 20 nm (**b**), 100 nm (**c**) and 500 nm (**d**), thick TiO\({}_{2}\) layer [41].