## 3 Results and Discussion
### Leakage Current and X-Ray Sensing Studies  
Similarly to visible-light photodetectors, which need a low leakage current to have a high signal-to-noise ratio, semiconductor-based X-ray sensors must have low leakage currents to ensure their high sensitivity. Accordingly, before testing the X-ray radiation sensing capability of the SnO\({}_{2}\) NWs, in order to have insight into the leakage current of a fabricated sensor, the typical I-V characteristics of SnO\({}_{2}\) NWs were studied. Figure 4a-d shows the FE-SEM images of SnO\({}_{2}\) NWs with different densities of SnO\({}_{2}\) NWs from the highest to the lowest densities, respectively, and corresponding leakage currents versus applied voltages are shown in Figure 4e-h, respectively. As can be seen, as the density of the SnO\({}_{2}\) NWs decreases, the leakage current increases. Therefore, for practical applications, the use of highly dense SnO\({}_{2}\) NWs will result in a better sensing performance and will enhance the signal to noise ratio. In particular, the sample with the highest density of SnO\({}_{2}\) NWs showed the lowest leakage current. This sample had a very low dark leakage current; for example, for bias voltages of 1 and 5 V, the leakage currents were \(\sim\)0.25 and 0.9 pA respectively. These very low leakage currents enhance the signal to noise ratio for semiconductor radiation sensors. The leakage current of a semiconductor sensing material is strongly governed by the band gap energy and the structure of the sensing material. Large band gap materials, \(>\)1.5 eV, are needed to maintain a low intrinsic carrier concentration and a low leakage current during sensor operation at room temperature. Additionally, the amount of carrier trapping centers must be extremely low. For example, regions near and along the cracks show much higher leakage current than regions away from the cracks [41]. Therefore, the low current leakages obtained for the fabricated sensor demonstrate the high quality of the synthesized SnO\({}_{2}\) NWs in this study.  
To evaluate the X-ray sensing capability, the X-ray current response of the SnO\({}_{2}\) NWs sensor with respect to X-ray intensity was measured at room temperature, as shown in Figure 5. During the measurements, the current of the X-ray source was varied from 0 to 2 mA while the applied voltages for the X-ray generator and sensor were kept constant at 80 kV and 5 V, respectively. The signal intensity was strong as compared to dark current and it was quite linear, as shown in Figure 5. Based on these results, we have a meaningful response of the SnO\({}_{2}\) NWs to X-ray radiation. For example, for X-ray currents of 1 and 2 mA, the sensor currents were 65 and 120 pA respectively, demonstrating the near-linearity of the response of the SnO\({}_{2}\) NWs sensor.  
We also tested the conductivity of SnO\({}_{2}\) NWs. Figure 6 shows current passing through the SnO\({}_{2}\) NWs under a fixed voltage of 1 V over time. As can be seen, the SnO\({}_{2}\) NW sensor shows a stable signal over time, demonstrating its high stability for sensing applications.  
Figure 4: Scanning electron microscopy (SEM) images of SnO\({}_{2}\) NWs with different densities of NWs from (**a**), (**b**), (**c**) to (**d**) the highest density to the lowest density. (**e**)–(**h**) Corresponding leakage currents of SnO\({}_{2}\) NWs versus applied voltage.  
Figure 5: Dependence of the current response with respect to the intensity of the X-ray source under a constant applied voltage of 5 V.