Oxide perovskite Ba₂AgIO₆ wafers for X-ray detection

Abstract

X-ray detection is of great significance in biomedical, nondestructive, and scientific research. Lead halide perovskites have recently emerged as one of the most promising materials for direct X-ray detection. However, the lead toxicity remains a worrisome concern for further commercial application. Great efforts have been made to search for lead-free perovskites with similar optoelectronic properties. Here, we present a lead-free oxide double perovskite material Ba₂AgIO₆ for X-ray detection. The lead-free, all-inorganic nature, as well as the high density of Ba₂AgIO₆, promises excellent prospects in X-ray applications. By employing the hydrothermal method, we successfully synthesized highly crystalline Ba₂AgIO₆ powder with pure phase. Furthermore, we prepared Ba₂AgIO₆ wafers through isostatic pressure and built X-ray detectors with Au/Ba₂AgIO₆ wafer/Au photoconductive structure. The as-prepared X-ray detectors showed a sensitivity of 18.9 μC/(Gy_air·cm²) at 5 V/mm, similar to commercial α-Se detectors showcasing their advantages for X-ray detection.

Key words： oxide double perovskite; lead-free; X-ray detection

1 Introduction

X-ray detection has attracted significant attention for various applications, including medical diagnostics, computed tomography, nondestructive inspection, and scientific research [1,2]. Generally, there are two methods to realize X-ray detection: indirect detection and direct detection. Compared with the indirect strategy, which converts X-ray to photons by scintillators [3], the direct X-ray detection, that converts X-ray radiation directly into the electrical response, is considered to have great potential due to its simple system and high spatial resolution [4,5]. Among X-ray detection materials used for direct detection, metal halide perovskites have recently shown huge prospects due to their high average atomic number, considerable mobility–lifetime product (μτ), high defect tolerance, and simple preparation process [6–8].

However, lead in metal halide perovskites is a significant obstacle to its application in X-ray detection. For example, to absorb 99% of the X-rays at 50 keV, MAPbBr₃, a promising X-ray detect material, requires a 2.3-mm-thick flat-panel containing a total Pb of 3836 g/m², much significantly higher than the EU RoHS regulation limit of 1000 ppm Pb [9]. Therefore, replacing the Pb with non-toxic metal in practical application is necessary [10–12]. Among prospective candidates, double perovskites have been proved as a highly suitablealternative. Cs₂AgBiBr₆ has been reported to achieve high sensitivity with 105 μC/(Gy_air·cm²) and a low detection limit with 36 nGy_air/s [13].

Nevertheless, the density of metal halide perovskites is relatively small for an ideal X-ray detector. For example, the density of MAPbBr₃ is 3.582 g/cm³, and the density of Cs₂AgBiBr₆ is 4.954 g/cm³, which is still lower than the density of CdTe (6.2 g/cm³), a material usually used in direct detection. Therefore, perovskite materials with combined characteristics of non-lead and high density are highly desired for X-ray detection.

Ba₂AgIO₆ has newly emerged as a lead-free oxide double perovskite with a cubic structure with the centers of metal-oxygen octahedron occupied by Ag⁺and I⁷⁺ [14], which was first reported by Volonakis et al. Ba₂AgIO₆ possesses great advantages in X-ray detection: 1) Large absorption coefficient. It is much denser than metal halide perovskites and slightly denser than CdTe (The density of Ba₂AgIO₆ is 6.411 g/cm³, the density of Cs₂AgBiBr₆ is 4.954 g/cm³, and the density of CdTe is 6.2 g/cm³). 2) Appropriate bandgap structure. It has a bandgap of 1.9 eV, making it ideal for X-ray detection [14] (Larger bandgap would lead to a decrease in conversion efficiency, and a smaller bandgap would lead to an increase in hot carriers). 3) Low ion migration oxides have a more stable crystal structure, which can effectively inhibit ion migration [15,16]. Thereby, we believe that Ba₂AgIO₆ and a series of oxide double perovskites represented by Ba₂AgIO₆ are very promising in applying X-ray detection.

Despite holding great promises, the optoelectronic applications of Ba₂AgIO₆ have not been widely researched. Thus, in this study, we focus on the sample quality improvement, and the application of oxide double perovskite Ba₂AgIO₆ for a sensitive X-ray detector. We synthesized high crystallinity Ba₂AgIO₆ powder by hydrothermal instead of a low-temperature solution process, prepared a Ba₂AgIO₆ wafer by isostatic pressure to satisfy the upscaling ability, and further built the direct X-ray detector with Au/Ba₂AgIO₆/Au structure. Thanks to the above suitable process and Ba₂AgIO₆ itself excellent nature, our detector’s X-ray sensitivity is 18.9 μC/(Gy_air·cm²) at 5 V bias (about 5 V/mm), which is comparable to commercial α-Se X-ray detectors (20 μC/(Gy_air·cm²)) [17].

2 Experimental section

Materials and chemicals: Barium hydroxide (Ba(OH)₂·8H₂O, 98%), silver oxide (Ag₂O, 99.99%), and periodic acid (HIO₄, 99.9%) were purchased from Alfa Aesar. Nitric acid (HNO₃, 65 wt%−68 wt% in water) and acetonitrile (CH₃CN, 99.8%) were procured from Sinopharm Chemical Reagent Co., Ltd., China. All materials and chemicals were used without further purification unless otherwise noted.

Synthesis of Ba₂AgIO₆ powder: Ba₂AgIO₆ powder was grown from mixed solvents in equal proportions of water and acetonitrile by hydrothermal method. 630.92 mg (2 mmol) Ba(OH)₂·8H₂O, 298.769 mg (1 mmol) AgIO₄, and 20 mL acetonitrile and 20 mL H₂O were first loaded into a PTFE container successively. The PTFE container was then heated to 180°C in a hydrothermal kettle for 10 h. After another 20 h of slow cooling, the Ba₂AgIO₆ powder was successfully synthesized. The obtained powder was rinsed with water three times to remove surface impurities and organic solvents and dried in an oven for subsequent preparation of the wafer.

Structural characterization: X-ray diffraction (XRD) experiments were performed by X'Pert3 Powder (PANalytical B.V using a Cu Kα rotating anode). X-ray fluorescence experiments were conducted by EAGLE III (EDAX Inc. using A1 Kα excitation)

Optical characterization: The UV-Vis absorption spectrum was measured by a UV-visible spectrophotometer (SolidSpec-3700, Shimadzu). Fluorescence spectra were examined by a transient/steady-state fluorescence spectrometer (FLS 920) with a 325 nm laser.

X-ray detector fabrication and characterization: The as-prepared powder was modeled into a pie shape by a compressor under 4 MPa pressure. Subsequently, it was loaded in an elastic mold made of latex and put into an isostatic pressing machine with an oil bath under 200 MPa for 5–10 min. After the oil on the mold is dried, the wafer was transferred into a thermal evaporation machine. 50 nm-thick gold with an area of 2 mm×5 mm was evaporated on both sides at a rate of 1 Å/s. For X-ray detector characterization, we used a gold anode X-ray tube (Newton Scientific M237) with a maximum output of 10 W as our X-ray source and used the Keithley 6571B semiconductor characterization system as a voltage source and a device for measuring current and voltage. We controlled the X-ray dose by changing the current of the X-ray tube from 140 to 10 μA with a constant working voltage of 50 keV. All the X-ray characterization was conducted in a lead box to minimize interference from ambient noise and light.

X-ray dose calibration: We calibrated the X-ray dose rate using the MagicMax ion chamber dosimeter from IBA Dosimetry. First, the X-ray tube is fixed in a specific position with a working distance of 10 cm. Then the dose rate was measured at 50 kV working voltage. We adjust the dose rate during this process by changing the working current of the X-ray tube from 140 to 10 μA. Finally, remove the dosimeter, and place the sample to be tested in this position. If you need to simulate other working scenarios, you can adjust the voltage as appropriate or add the aluminum plate for attenuation. The radiation dose rate of the X-ray tube is not the same as the actual absorbed dose rate of the sample, which is affected by many factors, such as distance, position (heel effect), air attenuation, sample area, and other factors. Therefore, we use Gy_air/s in the article, which is the absorbed dose rate obtained from the dosimeter, to refer to the actual absorbed dose rate of the sample.

3 Results and discussion

We focus on Ba₂AgIO₆ for potential X-ray detection. The absorption is such an important property in X-ray detection that we first calculate the absorption coefficients of Ba₂AgIO₆, MAPbBr₃, Cs₂AgBiBr_6, Si, and CdTe for photons in the range of 0.01 to 100 MeV using the photon cross-section database [18]. As shown in Fig. 1(a), the absorption coefficient of Ba₂AgIO₆ across the energy region we calculated is much higher than that of MAPbBr₃, and comparable to the value for CdTe. The large density of Ba₂AgIO₆ enables such high absorption coefficient. To determine the X-ray detector thickness, we further calculate the attenuation spectra of these materials for hard X-rays at 50 keV. For 50 keV X-ray photons, a 0.7 mm thickness of Ba₂AgIO₆ is sufficient to absorb 99% X-rays, while 2.3 mm is required for MAPbBr₃ (Fig. 1(b)) [19], indicating a higher X-ray attenuation coefficient of Ba₂AgIO₆.

Fig.1 (a) Absorption coefficients of Ba₂AgIO₆, Si, MAPbBr₃, Cs₂AgBiBr₆, and CdTe from soft X-rays to gamma rays. (b) Attenuation efficiencies of Ba₂AgIO₆, Si, MAPbBr₃, Cs₂AgBiBr₆, and CdTe at different thickness. (c) Crystal structure of Ba₂AgIO₆ and the fitted bandgap width (1.9 eV)

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In addition to the absorption coefficient, the electronic bandgap is also important for X-ray detection [20]. The moderate bandgap provides a higher current response while avoiding the noise generated by hot carriers (The smaller the energy band, the higher the current response, and the higher the hot carrier concentration). The bandgap of Ba₂AgIO₆ fits to be 1.9 eV (Fig. 1(c)), which is smaller than that of CsPbBr₃ (2.3 eV) and larger than that of Si (1.1 eV).

Fig.2 (a) X-ray diffraction (XRD) data of the powders synthesized by hydrothermal method and a low-temperature solution process (LTSP). (b) Photoluminescence spectrum of the powders synthesized by different methods. (c) Schematic of preparation of Ba₂AgIO₆ wafers by cold isostatic pressing

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Volonakis et al. prepared Ba₂AgIO₆ by mixing acetonitrile solution of AgIO₄ with the aqueous solution of Ba(OH)₂, and Ba₂AgIO₆ was directly precipitated from the solution [14]. However, the as-prepared Ba₂AgIO₆ powder shows inferior quality with broad photoluminescence, which is not ideal for a high-performance detector. The low-temperature solution process (LTSP) possesses the disadvantage of insufficient solvent mixing and fast precipitating.

Thus, we improved his method, using the mixed solvent hydrothermal method to prepare Ba₂AgIO₆ powder. This method adjusts the crystallization speed by controlling the temperature gradient and obtains high-quality powders [21]. In preparing Ba₂AgIO₆, it is necessary to add an appropriate amount of acetonitrile in the solvent due to the poor solubility of AgIO₄ in water. We adjusted the acetonitrile and water ratio to 1:1 after several tries to ensure a complete and uniform reaction.

As illustrated in Fig. 2(a), Ba₂AgIO₆ prepared by the improved hydrothermal method shows narrower XRD peak, indicating its higher crystallinity. The complete match between XRD patterns and calculated results indicates a pure phase of as-prepared Ba₂AgIO₆. Besides, we used X-ray fluorescence (XRF) measurement to verify the composition of its elements (Table S1), the content of Ba, Ag, and I match with 2:1:1. Moreover, we measured the photoluminescence spectrums of Ba₂AgIO₆ powder prepared by both methods (Fig. 2(b)). Ba₂AgIO₆ powder prepared by hydrothermal processes records narrow half peak width, and also confirms its high crystallinity.

Based on high-quality Ba₂AgIO₆ powder, we explored the preparation of the Ba₂AgIO₆ X-ray detector. It is still difficult to prepare the large-area and millimeter-thick X-ray detector. The traditional solution process leaves the pinholes in the film, which blocks the transmission of carries. In comparison, the whole process of isostatic-pressing is free of solution avoiding the drawback [7,22]. In addition, during the process preparation by solution process, the oxidizing AgIO₄ and alkaline Ba(OH)₂ reduce the selectivity of the substrate and hinder the preparation of flat-panel [23]. Thereby, we finally turned to an isostatic-pressing method for Ba₂AgIO₆ film fabrication, as shown in Fig. 2(c) [7].

We preloaded the powder mechanically at 4 MPa to ensure the size and shape of the wafer. Ball mill is needed for the raw material to ensure that the grain size is uniform and the wafer’s shape is regular. Otherwise, the wafer is liable to crack during further operation due to the pressure’s unevenness on it. Then, the repressed wafers were loaded into an elastic mold made of latex in an oil bath. The fabrication could be completed after applying 200 MPa pressure to the oil for 5–10 min.

We characterized the wafers’ surface by scanning electron microscope (SEM) image, based on obtaining the wafer. The top-view surface SEM image shows a compact, flat, and pinhole-free morphology at 10 μm scale bar (Fig. 2(a)), which is beneficial for minimizing leakage current for direct X-ray detection. As illustrated in Figs. S2(b) and S2(c), the powder’s grain size is about 20−50 nm, which is consistent with the grain size of the wafer we measured. The grain size barely changed after isostatic pressure, and we believe this is primarily because Ba₂AgIO₆ is an oxide semiconductor with stronger covalence and is challenging to grow during the process of isostatic pressure without annealing [24].

Fig.3 (a) Device structure of Ba₂AgIO₆ X-ray detector. (b) Response for devices prepared by hydrothermal method and mixed precipitation at 785 μGy_air/s X-ray dose rate at 5 V bias. (c) Response for devices prepared by hydrothermal method from 5499 to 785 μGy_air/s X-ray dose rate and 1 to 5 V bias. (d) X-ray sensitivity of devices prepared by hydrothermal method and mixed precipitation method at different bias voltages

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Finally, Ba₂AgIO₆ thick-wafers-based X-ray detectors with a photoconductive vertical structure were fabricated as Fig. 3(a). A 50 nm gold electrode is evaporated on both sides of the 1 mm thick wafer with a size of 2 mm×5 mm. After a voltage was applied to both ends of the electrode, X-rays shined from one side of the gold electrode (Fig. 3(a)). All tests were performed with a 50 keV X-ray tube (Newton Scientific M237). We adjust the dose rate of X-rays by changing the amount of current. We focus on the performance of the devices prepared by the hydrothermal method and mixed precipitation method.

As expected, the detectors’ performance prepared by the hydrothermal method is much higher than that prepared by a low-temperature solution process. The former one has a lower dark current (1.22 nA) and a higher photon response (3.06 nA), while the latter has a higher dark current (6.08 nA) and a lower photon response (0.88 nA) (Fig. 3(b)). Figure 3(c) shows the response pattern of the hydrothermal device with the X-ray dose rate from 5499 to 785 μGy_air/s by changing the current of the X-ray source from 140 to 20 μA at a constant DC bias of 5 V. It should be noted that the contribution of air ionization can be ignored because of the considerable difference between the response of air ionization and that of our device (Fig. S4). Here, we consider that the electrode area size is 0.1 cm². We can estimate the ray detection sensitivity under different bias voltages by linear fitting according to this data. The result is reported in Fig. 3(d). The X-ray detection sensitivity of the Ba₂AgIO₆ device is calculated to be about 4.05 μC/(Gy_air·cm²) at 1 V bias (about 1 V/mm) and 18.9 μC/(Gy_air·cm²) at 5 V bias (about 5 V/mm). We further compared the responses of wafers with different thicknesses (Table S2). According to Fig. 1(b), the 0.7 mm thickness of Ba₂AgIO₆ is sufficient to absorb 99% of X-rays. Therefore, the 0.5 mm-thick wafer cannot fully absorb X-rays, resulting in a significant loss of sensitivity. For a 2 mm-thick wafer device, photogenerated carriers cannot reach both ends of the electrode due to the poor charge transportability of our Ba₂AgIO₆ wafer, leading to the decrease of our device’s sensitivity. After considering factors such as X-ray absorption and charge transfer, We obtained the optimal X-ray sensitivity of 18.9 μC/(Gy_air·cm²) at 5 V of the device at 1 mm thickness. The sensitivity is very close to commercial α-Se X-ray detectors (20 μC/(Gy_air·cm²)) [17]. Its relatively good sensitivity and non-lead toxicity fully demonstrate its application in X-ray detection.

Further, we analyze the physical process of the Ba₂AgIO₆ X-ray detector. Because the X-ray energy is relatively low and the atomic number of Ba₂AgIO₆ is relatively high, the primary process of our device is the photoelectric process rather than the Compton scattering process. In detail, the process generates high-energy electrons that lose their energy subsequently in relaxation with producing more pairs of electron holes [24].

For our detector, we focus on its theoretical sensitivity and gain factor. According to the empirical formula ΔE = 1.43+2E_g [25], the energy loss for generating one electron-hole pair (ΔE) known as the ionization energy for Ba₂AgIO₆ can be evaluated to be 5.29 eV. Based on the ionization energy, we can calculate the theoretical sensitivity of the Ba₂AgIO₆ X-ray detector by

S_{\mathrm{0}}={\displaystyle \frac{{\displaystyle \frac{\phi }{X}}\overline{E}\beta }{\mathrm{\Delta }E}}e\eta ,

where {\displaystyle \phi /X} is the number of photons per unit of exposure, \overline{E} is the mean energy of X-ray photons, \beta is the energy absorption efficiency of X-ray, and \eta is the gain factor (\beta and \eta can be considered as 100% in calculating). The theoretical sensitivity of the Ba₂AgIO₆ X-ray detector is 1279 μC/(Gy_air·cm²).

As for the gain factor, which can also be called current collection efficiency, it can be calculated by G=I_{\mathrm{R}}/I_{\mathrm{P}} [8,26,27]. Here, I_{\mathrm{R}} is the signal current, and I_{\mathrm{P}} is the theoritical current. Finally, we got the gain factor under different dose rates and electric fields (Fig. S5). As the irradiation increases, the gain factor decreases because of the filling in the shallow defect by photonic carriers [28]. That phenomenon is widely observed in photoconductive photodetectors [29].

Fig.4 (a) X-ray diffraction (XRD) data of Ba₂AgIO₆ powder exposed in air for different time. (b) X-ray sensitivity of Ba₂AgIO₆ X-ray detector exposed in air for different time

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To quantitatively characterize the stability of our detectors, we recorded the stability of the wafers. The wafers were exposed to the air for 60 days, and changes in their XRD and X-ray sensitivity were mainly observed. We tested the XRD at the beginning, the 20 days, the 40 days, and the 60 days. As Fig. 4(a) shown, the XRD of the device had some tiny burrs on the 20 days. On the 40 days, these mixed peaks increased, and on the 60 days, the mixed peaks became more apparent, indicating that the material might have decomposed at this time. We speculate that this phenomenon comes from the unstable high price state of I⁷⁺.

Consistent with the XRD data, the sensitivity of the X-ray decreased with the increase of storage time. The inflection point of decreasing sensitivity appeared on the 20th day, and it finally dropped to 12.91 μC/(Gy_air·cm²) on the 60th day, which was 68% of the original value (Fig. 4(b)). In general, the device’s stability remains good in a short period, but due to the high-priced I⁷⁺, it is not suitable to be stored in the air for a long time. Thus, an appropriate packaging process needs to be considered.

4 Conclusions

In conclusion, we report a Ba₂AgIO₆ direct X-ray detector. We have improved the mixed precipitation process, synthesized high-quality Ba₂AgIO₆ powder using the mixed solvent hydrothermal. Compared with the powder synthesized by a low-temperature process, our powder possesses a higher purity and crystallinity. Then, a 1 cm diameter wafer was prepared by cold-isostatic-pressing. In this way, we can fabricate a large area X-ray plate detector to avoid the solution process’s pinholes. Finally, a simple X-ray detector was prepared by steaming electrodes on both ends of the wafer. Its response at a bias voltage of 5 V is as high as 18.9 μC/(Gy_air·cm²), comparable to the commercial α-Se X-ray detectors’ performance. This work shows its promise in the field of X-ray detection.

Acknowledgements

This work was financially supported by the National Postdoctoral Program for Innovative Talent (No. BX20200142), the National Natural Science Foundation of China (Grant Nos. 61725401, 5171101030, and 51761145048), the National Key R&D Program of China (Nos. 2016YFB0700702, 2016YFA0204000, and 2016YFB0201204), the HUST Key Innovation Team for Interdisciplinary Promotion (No. 2016JCTD111), and China Postdoctoral Science Foundation (Nos. 2020M62004075 and 2020M62005089). The authors from HUST thank the Analytical and Testing Center of HUST and the facility support of the Center for Nanoscale Characterization and Devices, WNLO. We also thank Prof. Guangzu Zhang for providing the isostatic pressing machine.

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