Efficient X-ray luminescence imaging with ultrastable and eco-friendly copper(I)-iodide cluster microcubes

Scintillators are optical materials that emit low-energy ultraviolet and visible photons in response to ionizing radiation such as X-rays and gamma rays. This property makes scintillating materials useful for applications like non-destructive testing, X-ray astronomy, security inspection, and medical imaging.

In a recently published paper in Light Science & Applications, Professor Xiaowang Liu and Academician Wei Huang, along with their team from the Institute of Flexible Electronics at Northwestern Polytechnical University, introduced a novel class of X-ray scintillator based on monodisperse copper-iodine clusters. The synthesized microcubes demonstrated remarkable sensitivity to X-rays and exhibited excellent stability when exposed to humidity and X-ray irradiation. The uniform size distribution and superior scintillation performance of the copper-iodide cluster-based microcubes make them highly suitable for the production of large-area flexible scintillation films used in both static and dynamic X-ray imaging applications.

Traditional inorganic scintillators containing heavy metals offer excellent performance but face limitations in large-area and flexible X-ray detector development due to their high-temperature bulk crystal growth requirement. Moreover, commercially available scintillators like CsI:Tl and LaBr3:Ce are hygroscopic, adding complexity to device fabrication. Recent advancements in metal halide nanocrystals hold promise as a new class of solution-processable scintillators with improved performance. However, the challenge lies in developing efficient nano- and micro-scintillators with uniform morphology, environmentally-friendly composition, robust chemical stability, and integration into stretchable substrates for flexible X-ray detectors.

Copper-iodine cluster crystals, comprising inorganic cores and organic ligands, show the potential to be a new class of high-performance scintillators to address the challenges mentioned above due to the fact that: (i) These materials possess large effective atomic numbers, enabling strong X-ray blocking ability; (ii) The photoluminescence and semiconducting properties contribute to excellent X-ray conversion efficiency; (iii) Structural engineering enhances the lattice stability of Cu-I cluster scintillators, making them more resistant to moisture and facilitating rational crystal growth through wet-chemical processes.

In this report, the authors introduce the advancement of high-performance monodisperse microcube scintillators composed of copper iodide-(1-propyl-1,4-diazabicyclo[2.2.2]octan-1-ium)2 (Cu4I6(pr-ted)2). The crystal structure of Cu4I6(pr-ted)2 is presented in Fig. 1a. Through the utilization of the hot-injection method, microcubes with a uniform size distribution were successfully synthesized (Fig. 1b).

The optical characterization of the prepared microcubes revealed a luminescence peak at 535 nm, accompanied by a relatively low photoluminescence quantum yield (PLQY) of 40.6%. However, this limitation can be effectively addressed by subjecting the microcubes to annealing in a N2 atmosphere, thereby enhancing their crystallinity. Remarkably, following the annealing process, the PLQY experiences a substantial improvement, reaching an impressive value of 97.1%. Apart from the enhanced PLQY, the microcubes exhibit exceptional water resistance, surpassing that of both traditional scintillators and perovskite scintillators. Notably, when immersed in an aqueous solution, the luminescence intensity of the microcubes remains stable for an impressive duration of 18 h. This remarkable stability further reinforces the suitability of these microcubes for practical applications.

The radioluminescence characterization reavealed a emission peak aligning with the photoluminescence observed in Cu4I6(pr-ted)2 microcubes and showcases an exceptionally low X-ray detection limit (DL) of 22 nGyair s-1, along with outstanding radiation stability (Fig. 2a-c). The proposed scintillation mechanism is illustrated in Fig. 2d. Upon exposure to X-rays, the heavy elements like Cu and I, effectively absorbs high-energy photons, resulting in the generation of numerous energetic primary electrons. These primary electrons, in turn, give rise to secondary electrons through a combination of processes such as photoelectric absorption, Compton scattering, and pair formation. As high-energy secondary electrons move within the host lattice, they lose energy through interactions with the lattice and other electrons, producing many excitons. These excitons are then transformed into low-energy scintillation photons through radiative recombination in the 3CC states of optical exciton states.

The prepared Cu4I6(pr-ted)2 microcubes exhibit excellent scintillation performance, showcasing their immense potential in the realm of X-ray imaging. To further explore their practical applications, the microcubes were doped into polydimethylsiloxane (PDMS) as energy conversion fillers, resulting in the fabrication of a flexible X-ray scintillator screen. Leveraging this film, an X-ray imaging system was successfully constructed, enabling static and dynamic X-ray imaging of a mouse (Fig. 3). This achievement underscores the versatility and adaptability of Cu4I6(pr-ted)2 microcubes as a promising material for real-world applications in the field of X-ray imaging. Their incorporation into flexible scintillator films allows for the development of innovative and efficient imaging systems, opening doors to various biomedical and industrial applications.

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