(Nanowerk Spotlight) Medical imaging faces a persistent challenge: creating X-ray detectors that combine high sensitivity with practical usability. Current technology forces an uncomfortable trade-off between performance and practicality. Hospital X-ray machines typically use either scintillators that inefficiently convert X-rays to visible light before detection, or direct detectors containing toxic lead compounds. These systems often require complex manufacturing processes and produce rigid, limited-size devices that compromise between detection quality and practical implementation.
The search for better X-ray detection extends beyond hospitals into airport security, industrial quality control, and scientific research. Engineers have tried various approaches to improve detection technology, but face persistent challenges in creating uniform, large-area detectors that maintain high sensitivity without using harmful materials. Traditional semiconductor detectors need high-temperature processing, making large-scale production challenging, while alternative materials often suffer from poor charge transfer or inconsistent performance.
Researchers have now developed a novel solution using a precisely structured glass-like material. They created a metal-organic framework (MOF) glass by combining zinc atoms with organic molecules in a specific arrangement that can melt and reform into a smooth, continuous surface. The material’s structure consists of zinc ions coordinated with phosphate groups, forming one-dimensional chains with imidazolium molecules positioned between them.
Schematic showing the structure of [Zn(HPO4)(H2PO4)2](ImH2)2 (ZnPIm), including the repeating unit by linking two Zn centers and two HPO4 and its complete 1D chain by linking the Zn center to H2PO4, and the A-B layer packing shows two filling sites, the pocket and the inter-layer. Atom colors, Zn: cyan, P: orange, N: blue, C: gray, O: red. Hydrogen is omitted for clarity. (Image: Reprinted with permission by Wiley-VCH Verlag)
This molecular architecture, called ZnPIm, demonstrates remarkable electronic properties. The material exhibits a direct electrical bandgap of 2.88 electron volts and achieves a high charge carrier mobility of 1,174.81 square centimeters per volt-second – technical specifications that indicate excellent electrical performance. More importantly, when the researchers measured the mobility-lifetime product, a key indicator of how well charges move through the material, they found a value of 8.82 × 10−4 square centimeters per volt, among the highest reported for MOF materials.
The team synthesized ZnPIm through a straightforward process of grinding zinc oxide, phosphoric acid, and imidazole at room temperature. Unlike traditional semiconductor materials that require processing at hundreds or thousands of degrees, ZnPIm becomes liquid at just 158 degrees Celsius. This low melting point allows the material to be formed into large, perfectly smooth surfaces using simple manufacturing processes.
The molecular design proves crucial to the material’s performance. The researchers found that the valence band maximum – the highest energy level where electrons normally exist – localizes primarily on zinc and oxygen atoms. Meanwhile, the conduction band minimum – the lowest energy level where electrons can move freely – concentrates on the imidazolium molecules. This separation helps generate and transport electrical charges when X-rays strike the material.
When tested as an X-ray detector, ZnPIm showed exceptional sensitivity of 112.8 microCoulombs per Gray per square centimeter, with a detection limit of 0.41 microGray per second. These metrics surpass most reported MOF-based X-ray detectors. The material maintained about 95% of its initial performance during a 700-second continuous operation test under X-ray exposure at 400 microGray per second, demonstrating practical stability.
The researchers then explored several variations of their material by modifying the organic components. Surprisingly, they discovered that simpler molecular structures without benzene rings performed better at charge transport. The absence of benzene rings led to longer carrier-diffusion lengths, meaning electrical charges could travel further before recombining. This counterintuitive finding challenges the assumption that more complex molecular structures would improve performance.
The material’s thermal properties also impressed the researchers. It remained stable up to 200 degrees Celsius, showing no significant weight loss in thermogravimetric analysis. X-ray diffraction studies revealed that the material maintains its amorphous glass state after cooling from its melted form, ensuring consistent performance across the detector surface.
The team demonstrated the material’s practical potential by constructing a working X-ray imaging system. They successfully captured detailed images of various objects at a dose rate of 208 microGray per second, marking the first time MOF materials have achieved clear X-ray imaging rather than just detection.
Beyond its technical performance, ZnPIm offers practical advantages. Its atomic composition matches human tissue well, making it particularly suitable for medical dosimetry where accurate radiation measurement is essential. The absence of toxic heavy metals and the low-temperature processing requirements suggest potential for cost-effective, environmentally friendly manufacturing.
The development of this zinc-based glass detector opens new possibilities for X-ray technology. Its combination of high sensitivity, straightforward manufacturing, and stable performance addresses longstanding challenges in the field. As research continues, these materials might enable more widespread deployment of advanced X-ray systems, potentially improving both the safety and capability of medical imaging and security screening applications.
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2 (ZnPIm), including the repeating unit by linking two Zn centers and two HPO4 and its complete 1D chain by linking the Zn center to H2PO4, and the A-B layer packing shows two filling sites, the pocket and the inter-layer.](http://www.nanowerk.com/id66086_1.jpg)
