A Transparent Nano-Polycrystalline ZnWO4 Thin-Film Scintillator for High-Resolution X-ray Imaging.

Facile approaches for creating thin-film scintillators with high spatial resolutions and variable shapes are required to broaden the applicability of high-resolution X-ray imaging. In this study, a transparent nano-polycrystalline ZnWO4 thin-film scintillator was fabricated by thermal evaporation for high-resolution X-ray imaging. The scintillator is composed of nano-sized grains smaller than the optical wavelength range to minimize optical scattering. The high transparency of the scintillators affords a sufficiently high spatial resolution to resolve the 2 μm line and space patterns when used in a high-resolution X-ray imaging system with an effective pixel size of 650 nm. The thermal evaporation method is a convenient approach for depositing thin and uniform films on complex substrates. ZnWO4 thin-film scintillators with various shapes, such as pixelated and curved, were fabricated via thermal evaporation. The results show that the transparent nano-polycrystalline ZnWO4 thin-film scintillator deposited through thermal evaporation has a potential for use in various high-resolution X-ray imaging applications.

Introduction

High-resolution X-ray imaging is a promising technique in the field of X-ray microradiography for biological, archeological, material, and non-destructive analyses.[1−7] One of the most important parameters in high-resolution X-ray imaging is the spatial resolution[8−10] that is affected by the thickness of the scintillator because optical detectors with small effective pixel sizes are highly sensitive to the light-spread phenomenon.[11,12] To minimize light spread, several researchers have attempted to fabricate scintillators with several-micrometer-thin layers for high-resolution X-ray imaging.[8,13−15]

Powder screens and transparent films are typical forms of thin-layer scintillators. Powder-screen-type scintillators are commonly used because various scintillator shapes, such as pixelated and curved, can be fabricated easily using such scintillators.[16−18] Powder-screen scintillators are typically nanoparticle scintillators.[18−21] For powders with a particle size much larger than the optical wavelength range, however, the powder can significantly scatter the optical light generated by the scintillator, which reduces the spatial resolution of the resulting X-ray image.[19,22] However, the use of nanoparticle scintillators with particle sizes smaller than the optical wavelength range reduces the probability of optical light scattering, which increases the spatial resolution.[22−25]

Powder-screen scintillators are fundamentally limited by optical light diffusion.[8,24] To overcome this limitation, transparent film scintillators have been explored for high-resolution X-ray imaging.[26] Among relevant approaches, the liquid-phase epitaxial growth method is the most commonly employed technique.[27−30] Another approach is the one used by Kameshima et al., who produced a transparent Lu3Al5O12:Ce film scintillator on an undoped Lu3Al5O12 substrate using a solid-state diffusion technique.[31] However, this method is complicated and requires additional processes such as cutting and polishing to prepare a thin and uniform film.[31] In addition, neither the liquid-phase epitaxial growth method nor the solid-state diffusion technique can produce thin-film scintillators with variable shapes.

Therefore, in this study, a transparent thin-film scintillator composed of nano-polycrystalline zinc tungstate (ZnWO4) was fabricated on quartz glass using a thermal evaporation method with sintering. ZnWO4 has been extensively used as a material for X-ray scintillators.[32,33] The reported light yield of ZnWO4 (7000–9500 Ph/MeV)[34−36] is higher than that of the commercialized Bi4Ge3O12.[33] In contrast, ZnWO4 has lower emission intensity than recently reported inorganic scintillating and perovskite materials.[37,38] The perovskite scintillators are severely restricted due to their vulnerability to moisture and heat.[39,40] Furthermore, because ZnWO4 has a much lower melting point than other inorganic scintillating materials, it can be efficiently used in the thermal evaporation method.[41,42] Additionally, ZnWO4 has a high density (ρ = 7.87 g/cm3), short decay time, high stability, and low cost.[32,43−45] Since ZnWO4 has a high X-ray absorption capability,[44,46] it can be used in thin-film scintillators. There is a growing body of research on fabricating transparent ceramic thin films using thermal evaporation with sintering.[47−50] The thermal evaporation method is an efficient approach for conveniently depositing thin and uniform films on complex structures.[51−55] The main purpose of this study was to demonstrate that nano-polycrystalline ZnWO4 thin-film scintillators can be utilized in high-resolution X-ray imaging. In addition, the proposed method can be used to fabricate thin-film scintillators with various shapes, thereby demonstrating its potential for use in different high-resolution X-ray imaging applications.

Results and Discussion

ZnWO4 Thin Films Deposited via Thermal Evaporation

When the ZnWO4 powder fabricated by a solid-state reaction was irradiated by a 254 nm UV light, it emitted blue light (Figure a). Figure b shows the X-ray diffraction (XRD) pattern of the ZnWO4 powder. By comparing the peaks of the XRD pattern with the standard card (PDF ICDD card #01-078-0251), the powder was determined to be monoclinic crystalline ZnWO4.

Figure 1

(a) ZnWO4 powder irradiated with a 254 nm UV light and (b) the XRD spectrum of the ZnWO4 powder.

Bare ZnWO4 thin films were deposited through thermal evaporation using the crystallized ZnWO4 powder as the source material. Cross-sectional scanning electron microscopy (SEM) imaging revealed the grain-free structure of the bare ZnWO4 thin films (Figure a), which contained no cracks or pores.

Figure 2

(a) Cross-sectional SEM image of a bare ZnWO4 thin film obtained via focused ion beam milling. (b) EDX spectrum, (c) XRD pattern, and (d) AFM surface morphology (inset) and roughness of the bare ZnWO4 thin film.

Figure b shows the energy-dispersive X-ray (EDX) spectrum obtained for a bare ZnWO4 thin film; this spectrum shows peaks attributed to zinc, tungsten, and oxygen, as well as those attributed to silicon from the quartz glass. As shown in the XRD pattern in Figure c, no diffraction peaks were observed from the bare ZnWO4 thin film, indicating an amorphous structure. Figure d shows the surface roughness of the film analyzed by atomic force microscopy (AFM). The root-mean-square (RMS) surface roughness was measured to be 1.74 nm, indicating that the bare ZnWO4 thin film was highly uniform. The thin film was sintered at 700 °C and the surface roughness of the film was analyzed using AFM, and the RMS roughness was measured to be 8.90 nm (Figure S1). Since the bare ZnWO4 thin film has a highly uniform surface, the sintered thin film also has sufficient uniformity.

Sintered ZnWO4 Thin Films

Figure a shows photographs of the 3 μm-thick ZnWO4 thin films sintered at 600, 700, 800, and 900 °C with 254 nm UV irradiation. Figure b presents the XRD patterns of the films, which show that they crystallized into pure monoclinic ZnWO4. Figure c shows the transmission rate spectra of the films in the wavelength range of 200–800 nm. The most transparent samples were the films sintered at 600 and 700 °C, which exhibited transmission rates of 81 and 78%, respectively, at 480 nm. In contrast, the ZnWO4 thin films sintered at 800 and 900 °C exhibited transmission rates of 68 and 56%, respectively, at 480 nm.

Figure 3

(a) Photographs of ZnWO4 thin films sintered with 254 nm UV irradiation, (b) XRD patterns, and (c) transmission rate spectra of the ZnWO4 thin films sintered at (A) 600, (B) 700, (C) 800, and (D) 900 °C; panels (b) and (c) also show the results for bare quartz glass.

The structural morphologies determined through field emission scanning electron microscopy (FE-SEM) of the ZnWO4 thin films sintered at different temperatures are shown in Figure . After sintering, the amorphous film became crystallized. For the ZnWO4 thin film sintered at 900 °C, many pores and cracks on the scale of the wavelengths of optical light were formed (Figure d), which can reduce film transparency.[56,57] In other words, optical light scattering would occur if this film was used as a scintillator, which would reduce the spatial resolution of the resulting X-ray image.

Figure 4

SEM images of the ZnWO4 thin films sintered at (a) 600, (b) 700, (c) 800, and (d) 900 °C.

The ZnWO4 thin films sintered at 600, 700, and 800 °C were composed of polycrystalline structures. The average grain sizes measured by FE-SEM using the intercept technique were 92.6, 157.3, and 698.7 nm for the films sintered at 600, 700, and 800 °C, respectively. Thus, the grain size increased with increasing sintering temperature.

The differences between the transmission rates of the thin films can be explained based on Mie scattering theory. When the grain size is comparable to the wavelengths of optical light, Mie scattering occurs at the grain boundaries.[58,59] However, when the grain size is sufficiently smaller than the wavelengths of optical light, Mie scattering is suppressed, allowing optical light to penetrate the film.[59−62] Therefore, the ZnWO4 thin films sintered at 600 and 700 °C, with grains smaller than the optical wavelength range, were the most transparent.

The sintered ZnWO4 thin films were irradiated with X-ray and UV light to evaluate their luminescent properties (Figure ). For the thin films sintered at 700, 800, and 900 °C, relatively high-intensity light emission was obtained without any significant differences between samples because the amorphous films were all completely crystallized. The emission intensity of the ZnWO4 thin film sintered at 600 °C was lower than those of the films sintered at 700, 800, and 900 °C because in the former, the amorphous film did not completely change into a crystallized structure (Figure b). Thus, although the ZnWO4 thin film sintered at 600 °C had the best transparency, it would be impossible to utilize as a scintillator.

Figure 5

(a) Photoluminescence spectra under 325 nm UV irradiation and (b) emission intensities under X-ray irradiation of the ZnWO4 thin-film scintillators sintered at 600, 700, 800, and 900 °C (the error bars represent the standard deviations of 20 independent measurements).

The emission intensity of the film sintered at 800 °C was approximately 3.7% higher than that of the film sintered at 700 °C. However, since the latter had a smaller grain size, less optical light scattering would occur; thus, the spatial resolution, an important factor for high-resolution X-ray imaging, would increase. Hence, the film sintered at 700 °C with an average grain size of 157.3 nm was selected as the optimal.

Evaluation of High-Resolution X-ray Imaging Performance

A conventional nanoparticle screen scintillator was also manufactured, and its spatial resolution was compared with that of the nano-polycrystalline thin film in a high-resolution X-ray imaging system. The ZnWO4 nanoparticle screen scintillator was fabricated using a solid-state reaction and drop-casting, and the particle size was similar to the grain size of the optimal nano-polycrystalline ZnWO4 film.[22] The structures of the nano-polycrystalline film and nanoparticle screen are depicted in the SEM images in Figure . In the nanoparticle screen, the particles were separated by air gaps. The optical light generated from the scintillator became scattered due to the difference in the refractive index of the particles and the air in the aforementioned gaps. In the nano-polycrystalline film, however, there were no gaps, because the nano-grains were closely packed. Consequently, there was no difference in refractive index at the grain boundaries, and thus, optical light scattering was suppressed.[63]

Figure 6

Surface SEM images of the (a) nano-polycrystalline ZnWO4 film scintillator and (b) ZnWO4 nanoparticle screen scintillator. Cross-sectional SEM images of the (c) nano-polycrystalline ZnWO4 film scintillator and (d) ZnWO4 nanoparticle screen scintillator.

A schematic of the high-resolution X-ray imaging system is shown in Figure a. Figures b,c shows X-ray images of a 1500 mesh transmission electron microscopy (TEM) grid obtained from the high-resolution X-ray imaging system using the nanoparticle screen and nano-polycrystalline film scintillators, respectively. The modulation transfer function (MTF) graphs show that the spatial frequency of the nanoparticle screen and nano-polycrystalline film at the MTF value of 10% were 239 and 291 lp/mm, respectively (Figure S2a). Therefore, the spatial resolution provided by the nano-polycrystalline film was better than that of the nanoparticle screen scintillator. A high-resolution X-ray image of the JIMA RT RC-02 test pattern (JIMA, Chiyodaku, Japan) was developed using the nano-polycrystalline film (Figure d). The image confirms the better spatial resolution of the nano-polycrystalline film (Figure S2b), and the nano-polycrystalline transparent ZnWO4 thin-film scintillator can resolve 2 μm lines and spatial patterns.

Figure 7

(a) Schematic of the high-resolution X-ray imaging system. High-resolution X-ray images of a 1500 mesh TEM grid obtained using the (b) nanoparticle screen scintillator and (c) nano-polycrystalline film scintillator. (d) High-resolution X-ray image of the JIMA RT RC-02 test pattern developed using the nano-polycrystalline film scintillator.

The thermal evaporation method allowed for the fabrication of a pixelated scintillator (Figures a,b) and a curved scintillator (Figure c). The nano-polycrystalline ZnWO4 film scintillator was deposited on quartz substrates using a micro-patterning mask (Figure a). The thickness of the pixelated scintillator can be increased to prevent light spread, as shown in Figure b; this can increase the X-ray sensitivity due to minimized light loss.[64]Figure c shows the curved ZnWO4 thin-film scintillator under 254 nm UV irradiation. Curved scintillators are advantageous in that they can minimize distortion at the edge of X-ray images acquired via computed tomography (CT).[65,66] The curved transparent ZnWO4 thin-film scintillator fabricated in this study could be used effectively in micro-CT because it has a high spatial resolution for X-ray micro-images without any distortion at the edges.

Figure 8

(a) SEM image of the pixelated scintillator and (b) optical microscopy image of the pixelated scintillator under 254 nm UV irradiation. (c) Curved scintillator under 254 nm UV irradiation.

Conclusions

A nano-polycrystalline transparent ZnWO4 thin-film scintillator was fabricated by thermal evaporation and was successfully utilized in high-resolution X-ray imaging. Since it has an average grain size of 157.3 nm, which is less than the optical wavelength, optical light scattering is suppressed. The optimal 3 μm-thick ZnWO4 thin film exhibited high optical performance with a 480 nm transmission rate of 78%. The scintillator was analyzed using a high-resolution X-ray imaging system composed of a micro-focus X-ray tube, optical lens, and scientific complementary metal-oxide semiconductor (sCMOS) detector with an effective pixel size of 650 nm. An X-ray image of 2 μm line and space patterns was successfully resolved by the prepared scintillator. In addition, the demonstrated thermal evaporation method represents a convenient approach for fabricating thin-film scintillators with various shapes. Thus, the fabricated scintillators can be used in different high-resolution X-ray imaging applications. The demonstrated nano-polycrystalline transparent ZnWO4 thin-film scintillator is expected to be useful in X-ray micro-radiography.

Methods

Fabrication of ZnWO4 Thin Films

Zinc oxide (ZnO) and tungsten oxide (WO3) nanoparticles were purchased from Sigma-Aldrich (St. Louis, MO, USA). The nanoparticles were added to ethanol at a 1:1 atomic ratio between zinc and tungsten. The solution was mixed via vibrating ball milling and was dried in air at 60 °C; the resulting powders were heated in an electric furnace at 1000 °C for 5 h to induce a solid-state reaction.

The ZnWO4 powder fabricated by the solid-state reaction was used as the source material for thermal evaporation. A tungsten boat was used as the evaporating crucible, and deposition on quartz glass (B&C Tech, Daejeon, South Korea) was performed at approximately 10–6 Torr. The holder was rotated 25 cm away from the crucible. After deposition, the bare ZnWO4 thin films were sintered in an air furnace at 600, 700, 800, or 900 °C for 3 h.

Characterization

Cross-sectional samples of the bare ZnWO4 thin films were prepared using a focused ion beam (FIB; Helios 450 F1, FEI, Hillsboro, OR, USA) and were analyzed using an SEM microscope and an EDX detector equipped in FIB. The crystallinity of the bare ZnWO4 thin films was analyzed using XRD (D/MAX-2500, Rigaku, Tokyo, Japan) in the 2θ range of 10–70°. The surface structure was determined using an AFM instrument (XE70, Park systems, Suwon, Korea).

The structures of the sintered ZnWO4 thin films were analyzed using FE-SEM (FEI MAGELLAN 400, FEI, Hillsboro, OR, USA), and the crystallization and transparency of the thin films were analyzed using XRD and a UV–vis/NIR spectrophotometer (Lambda 1050, Perkin Elmer, Massachusetts, USA). The photoluminescence (PL) spectra were obtained on a PL spectrometer (RAMBOSS-Star, DONGWOO OddPTRON, Gyeonggi-do, Korea) equipped with a UV laser (325 nm He-Cd Laser, Kimmon, Tokyo, Japan).

High-Resolution X-ray Imaging

The fabricated films were incorporated into a high-resolution X-ray imaging system comprising a micro-focus X-ray tube (P030-24-12F100W, Petrick GmbH, Bad Blankenburg, Germany), optical lens, and a sCMOS detector (pico.edge 4.2, PCO, Kelheim, Germany). The operating voltage and current of the micro-focus X-ray tube were set to 50 kVp and 1 mA, respectively, and the focal spot size was 30–55 μm. A 10× optical lens was used to magnify the X-ray image on the scintillator film, and the sCMOS detector with a square pixel size of 6.5 μm × 6.5 μm was used to develop the image. The effective pixel size of this system coupled with the optical lens was 650 nm. The emission intensity of the scintillator was measured by averaging the pixel values of the X-ray images developed in the high-resolution X-ray imaging system. The MTF was analyzed using the Fourier transform of the ESF data obtained from the high-resolution X-ray image of an edge of the 0.5-cm-thick tungsten block for comparing spatial resolution.