A Novel X-Ray Radiation Sensor Based on Networked SnO2 Nanowires

X-Ray radiation sensors that work at room temperature are in demand. In this study, a novel, low-cost real-time X-ray radiation sensor based on SnO2 nanowires (NWs) was designed and tested. Networked SnO2 NWs were produced via the vapor–liquid–solid technique. X-ray diffraction (XRD), transmission electron microscopy (TEM) and field emission scanning electron microscopy (SEM) analyses were used to explore the crystallinity and morphology of synthesized SnO2 NWs. The fabricated sensor was exposed to X-rays (80 kV, 0.0–2.00 mA) and the leakage current variations were recorded at room temperature. The SnO2 NWs sensor showed a high and relatively linear response with respect to the X-ray intensity. The X-ray sensing results show the potential of networked SnO2 NWs as novel X-ray sensors.


Introduction
Radiation can be broadly divided into two categories: uncharged radiation, including X-rays, gamma rays, and neutrons; charged radiation, including electrons, positrons (beta particles), and alpha particles [1]. Radiation sensors are able to transform the energy lost by an incident particle into an electrical signal, which is then processed by electronic techniques. Metal oxides (MOs) are among the most commonly used materials for advanced applications due to their availability, low cost, simple synthesis procedures and flexible morphology and composition [2]. Accordingly, many researchers have used these materials for different applications, ranging from catalysts [3] to sensors [4][5][6][7].
In particular, they can be used for high-energy radiation sensors. In fact, the radiations with a high energy like gamma rays and X-rays can alter the concentration of oxygen vacancies and also create point defects and other structural defects in MOs [8]. Therefore, metal oxides can be used for the detection of high-energy radiation [9].
Today, low-cost radiation sensors based on MOs operating at room temperature are sought after for applications in science, industry, medicine and security [10]. Additionally, unpredictable accidents may occur during the storage or transportation of radioactive materials, which highlights the needs for metal oxide radiation sensors.
A radiation sensor should have a high sensitivity, a linear performance and an on-line response, low levels of noise and high reliability [11]. Based on the above criteria, a number of MOs were suggested for gamma ray sensing [12,13]. For example, it has been reported that the electrical properties of TeO 2 and many other MOs greatly change after gamma ray bombardment [14]. Although X-rays can also affect the structural properties of MOs, there are few reports about X-ray radiation sensors based on MOs in the literature [15][16][17].
SnO 2 is a well-known n-type semiconducting metal oxide (Eg = 3.8 eV). It has a high mobility of electrons (160 cm 2 /Vs), high physical and chemical stability, a high availability and a low price [18]. Therefore, it has been used in many applications, including gas sensors [19], lithium ion batteries [20], photodetectors [21] and electrodes [22]. Therefore, different methods, such as the vapor-liquid-solid (VLS) technique [23,24], colloidal synthesis [25], chemical vapor deposition [26,27] and hydrothermal [28] have been used for the preparation of SnO 2 nanowires (NWs). Among them, the VLS method has advantages, such as simplicity, low price and the possibility of control over the SnO 2 NWs. Generally, the length of NWs depends on the growth time, and the NW's diameter is determined by the size of the metal catalyst droplets [29].
In this study, we prepared networked SnO 2 NWs and measured their capability to detect X-ray radiation at room temperature. Although networked NWs are very common for gas sensing studies [30,31], there is no report on their X-ray sensing capabilities. To the best of our knowledge, this is the first study reporting X-ray radiation sensing using networked SnO 2 NWs. From a sensing response and performance standpoint, networked NWs sensors are potential candidates for radiation sensors because their carrier lifetime is enhanced significantly due to the reduction in defect density on the surface passivated NWs [32]. The results obtained demonstrate the effective sensing capability of SnO 2 NWs as X-ray sensors. The sensing mechanism is explained in detail.

Synthesis of Networked SnO 2 NWs
We used the vapor-liquid-solid (VLS) technique for the selective growth of networked SnO 2 NWs, described in detail in earlier reports of the present authors [33][34][35]. To this end, tri-layered interdigitated electrodes comprising of Au (3 nm), Pt (100 nm) and Ti (100 nm) were deposited by a sputtering process on SiO 2 -grown Si substrates. The Au top layer served as a catalytic layer for the selective growth of SnO 2 NWs. Then, the substrates were put into a quartz-tube furnace, where an Al 2 O 3 crucible containing highly pure metallic Sn powders (Sigma-Aldrich, St. Louis, MO, USA, 99.9%) was placed. Afterwards, the furnace was then evacuated using a rotary pump down to a pressure of 1 × 10 −3 Torr and was heated (10 • C/min) to 900 • C in the presence of N 2 and O 2 gases with flow rates of 300 and 10 sccm, respectively. After keeping at this temperature for 15 min, networked SnO 2 NWs were successfully grown on the substrate.

Characterization
The morphology of the SnO 2 NWs was studied by field-emission scanning electron microscopy (FE-SEM, Hitachi-S-4200, Hitach, Ltd., Tokyo, Japan). Transmission electron microscopy (TEM) was also used to further investigate the morphology of the synthesized product. X-ray diffraction (XRD, Xpert MPD PRO, Philips, The Netherlands) with CuKα radiation (λ = 1.540 Å) was employed to study the phase and crystalline structure of the synthesized SnO 2 NWs.

X-Ray Sensing studies
An X-ray tube from DRGEM Co., Ltd, Gwangmyeong, Korea. with different currents was used to irradiate the fabricated sensor. The X-ray-induced current was measured by a high-resistance electrometer, a KEITHLEY 6517, versus bias applied voltage which was applied between the interdigitated electrodes, before and after exposure to X-rays. The fabricated sensor was held near the outlet of the X-ray tube where the X-ray intensity was highest. During the X-ray irradiation experiments, temperature and humidity were held constant. Figure 1a shows a digital photograph of the set up for sensing and Figure 1b schematically shows the X-ray sensing procedure.
Appl. Sci. 2019, 9, x FOR PEER REVIEW 3 of 10 the outlet of the X-ray tube where the X-ray intensity was highest. During the X-ray irradiation experiments, temperature and humidity were held constant. Figure 1a shows a digital photograph of the set up for sensing and Figure 1b schematically shows the X-ray sensing procedure.

Growth of Networked SnO2 NWs
SnO2 NWs were fabricated using the Au-catalyzed vapor-liquid-solid (VLS) growth method. In this process, ultrafine Au nanoparticles (NPs) were deposited on the Si substrate on which SnO2 NWs were grown. Sn vapors coming from the Sn source were first carried and condensed on the substrate and with the Au formed an alloy and liquefied. With a decrease in temperature and in the presence of flowing oxygen, the SnO2 crystals nucleated at the liquid-solid interface and further condensation/dissolution of Sn vapor increased the amount of SnO2 crystal precipitation from the alloy. The incoming Sn species preferred to diffuse to and condense at the existing solid-liquid interface because of the lower levels of energy involved with the crystal step growth as compared with secondary nucleation events in a finite volume. As a result, no new solid-liquid interfaces were formed, and the interface was pushed to form a SnO2 NW. After the system completely cooled, the alloy droplets solidified on the SnO2 NWs tips [36][37][38].

Growth of Networked SnO 2 NWs
SnO 2 NWs were fabricated using the Au-catalyzed vapor-liquid-solid (VLS) growth method. In this process, ultrafine Au nanoparticles (NPs) were deposited on the Si substrate on which SnO 2 NWs were grown. Sn vapors coming from the Sn source were first carried and condensed on the substrate and with the Au formed an alloy and liquefied. With a decrease in temperature and in the presence of flowing oxygen, the SnO 2 crystals nucleated at the liquid-solid interface and further condensation/dissolution of Sn vapor increased the amount of SnO 2 crystal precipitation from the alloy. The incoming Sn species preferred to diffuse to and condense at the existing solid-liquid interface because of the lower levels of energy involved with the crystal step growth as compared with secondary nucleation events in a finite volume. As a result, no new solid-liquid interfaces were formed, and the interface was pushed to form a SnO 2 NW. After the system completely cooled, the alloy droplets solidified on the SnO 2 NWs tips [36][37][38].  In Figure 3a-c, FE-SEM micrographs with different magnifications are provided. It is evident that NWs are grown selectively on interdigitated electrodes due to the catalytic activity of Au. In particular, Figure 3a indicates a plan-view low-magnified FE-SEM micrograph of networked SnO2 NWs, showing a selective growth of SnO2 NWs. Figure 3b shows a higher magnification image, where long SnO2 NWs with a relatively dense structure and a straight-line morphology can be seen. Figure  3c clearly shows the formation of ultrathin SnO2 NWs, where the diameter of the SnO2 NWs is estimated to be 50-60 nm.  In Figure 3a-c, FE-SEM micrographs with different magnifications are provided. It is evident that NWs are grown selectively on interdigitated electrodes due to the catalytic activity of Au. In particular, Figure 3a indicates a plan-view low-magnified FE-SEM micrograph of networked SnO 2 NWs, showing a selective growth of SnO 2 NWs. Figure 3b shows a higher magnification image, where long SnO 2 NWs with a relatively dense structure and a straight-line morphology can be seen. Figure 3c clearly shows the formation of ultrathin SnO 2 NWs, where the diameter of the SnO 2 NWs is estimated to be 50-60 nm. In Figure 3a-c, FE-SEM micrographs with different magnifications are provided. It is evident that NWs are grown selectively on interdigitated electrodes due to the catalytic activity of Au. In particular, Figure 3a indicates a plan-view low-magnified FE-SEM micrograph of networked SnO2 NWs, showing a selective growth of SnO2 NWs. Figure 3b shows a higher magnification image, where long SnO2 NWs with a relatively dense structure and a straight-line morphology can be seen. Figure  3c clearly shows the formation of ultrathin SnO2 NWs, where the diameter of the SnO2 NWs is estimated to be 50-60 nm.  To further examine the crystalline quality of synthesized NWs, TEM analysis was performed. A low-magnification bright-field image of a single NW is presented in Figure 3d. Obviously, the NW is very smooth at the surface and has a single-crystalline nature without any defects, including stacking faults or dislocations. The high-resolution lattice micrograph, presented in Figure 3e, again demonstrates the single-crystalline quality. The spacing between the parallel fringes is~0.340 nm, which can correspond to the (110) plane of SnO 2 [40].

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~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.

Sensing Mechanism
In semiconductor radiation sensor operation, incident radiation creates a charge pulse consisting of electrons and holes within the detector, which are then separated using an applied electric field and the current is sensed by an external circuit. Electromagnetic radiation can interact with a material via different mechanisms [1]: (i) elastic scattering, (ii) photo-electric absorption, (iii) Compton scattering, and (iv) pair production. However, the dominant mechanism and the result of the interaction strongly depend on the incident energy of the interacting X-ray beam and the composition of the sensor [42]. In the case of elastic scattering, the energy of the incident photon is not changed and this process does not contribute to the deposition of energy on the sensor. Photoelectric absorption is the ideal process for sensor operation, where the incident photon loses all of its energy upon incident with the sensor material to one of the orbital electrons of the atoms of the sensor material. Subsequently, this photoelectron loses its kinetic energy through Columbic interactions and creates plenty of electron-hole pairs, which are subsequently separated by the electric field, and a current is produced [41,43]. During Compton scattering, which is a collision between an incident photon and an orbital electron, both the energy and the direction of the photons change. In fact, some of the energy will be lost to the electrons of the sensing material. These electrons will then lose their energy through the generation of electron-hole pairs. In Compton scattering, a photon does not transfer all of its energy to an electron. Accordingly, the number of electron-hole pairs generated varies remarkably between different Compton events [1]. In the pair production process, a photon with high energy interacts within the Coulomb field of the nucleus and directly produces an electronpositron pair. Generally, the photoelectric effect dominates in the energy range to ~200 keV, the Compton effect to a few MeV and pair production above ~6 MeV [44].
The SnO2 NW is a semiconductor material which can directly convert ionization radiation into an electrical signal. In this study, X-ray irradiation was used to demonstrate the radiation sensing capability of SnO2 NW. When an X-ray traverses the sensor, it may partly or fully transfer its energy to electrons via the photoelectric effect or Compton scattering [45]. These primary energetic electrons may cause an ionization cascade until their energy is in equilibrium with the energy band gap of the media. This leads to the creation of several electron-hole pairs in the sensor. Signal intensity strongly depends on the collection efficiency of these electron-hole pairs. The collection probability is in competition with electron-hole recombination, which is related to carrier lifetime or defect density.

Conclusions
In this study, networked SnO2 NWs were fabricated by a VLS technique and were used for the detection of X-ray radiation at room temperature. XRD, TEM and FE-SEM characterizations approved the successful formation of NWs with the desired composition and morphology. The nonproportional response of the X-ray source was tested. The X-ray radiation sensing results show that

Sensing Mechanism
In semiconductor radiation sensor operation, incident radiation creates a charge pulse consisting of electrons and holes within the detector, which are then separated using an applied electric field and the current is sensed by an external circuit. Electromagnetic radiation can interact with a material via different mechanisms [1]: (i) elastic scattering, (ii) photo-electric absorption, (iii) Compton scattering, and (iv) pair production. However, the dominant mechanism and the result of the interaction strongly depend on the incident energy of the interacting X-ray beam and the composition of the sensor [42]. In the case of elastic scattering, the energy of the incident photon is not changed and this process does not contribute to the deposition of energy on the sensor. Photoelectric absorption is the ideal process for sensor operation, where the incident photon loses all of its energy upon incident with the sensor material to one of the orbital electrons of the atoms of the sensor material. Subsequently, this photoelectron loses its kinetic energy through Columbic interactions and creates plenty of electron-hole pairs, which are subsequently separated by the electric field, and a current is produced [41,43]. During Compton scattering, which is a collision between an incident photon and an orbital electron, both the energy and the direction of the photons change. In fact, some of the energy will be lost to the electrons of the sensing material. These electrons will then lose their energy through the generation of electron-hole pairs. In Compton scattering, a photon does not transfer all of its energy to an electron. Accordingly, the number of electron-hole pairs generated varies remarkably between different Compton events [1]. In the pair production process, a photon with high energy interacts within the Coulomb field of the nucleus and directly produces an electron-positron pair. Generally, the photoelectric effect dominates in the energy range to~200 keV, the Compton effect to a few MeV and pair production above~6 MeV [44].
The SnO 2 NW is a semiconductor material which can directly convert ionization radiation into an electrical signal. In this study, X-ray irradiation was used to demonstrate the radiation sensing capability of SnO 2 NW. When an X-ray traverses the sensor, it may partly or fully transfer its energy to electrons via the photoelectric effect or Compton scattering [45]. These primary energetic electrons may cause an ionization cascade until their energy is in equilibrium with the energy band gap of the media. This leads to the creation of several electron-hole pairs in the sensor. Signal intensity strongly depends on the collection efficiency of these electron-hole pairs. The collection probability is in competition with electron-hole recombination, which is related to carrier lifetime or defect density.

Conclusions
In this study, networked SnO 2 NWs were fabricated by a VLS technique and were used for the detection of X-ray radiation at room temperature. XRD, TEM and FE-SEM characterizations approved the successful formation of NWs with the desired composition and morphology. The non-proportional response of the X-ray source was tested. The X-ray radiation sensing results show that the SnO 2 NWs sensors have promising sensing capability, where low leakage current was observed for the SnO 2 NWs sensor. Another strategy to reduce the defect density of the oxide NWs is to perform high-temperature annealing [45]. Stabilization studies of these sensors will also be conducted in future work.