Diagnostic medicine relies heavily on X-ray imaging. Different methods – including fluoroscopy, radiography, and computer tomography (CT scans) – enable visualization of the human body’s internal structure.
But as useful as the technology is, it comes with associated risk linked to exposing the body to ionizing radiation. That’s why the medical imaging industry is forever looking for new ways to reduce the necessary imaging dose through the medium of sensitive, high-resolution detectors.
One rising resolution is to create high-sensitivity X-ray detectors using lead halide perovskites. Such materials consist of heavy elements like iodine and lead that bear a large X-ray scanning cross-section. Furthermore, perovskites offer lower cost and higher sensitivity than competing scintillator-based detector technology since they directly convert X-ray photons into electrical signals.
However, integrating perovskites into standard microelectronics has remained a challenge until now. Researchers in Switzerland have demonstrated that X-ray detectors – made of the perovskite methylammonium lead iodide (MAPbI3) on graphene – can be built using 3D aerosol jet printing, an affordable technique that deposits material with micron precision.
The sample devices they made exhibited a record sensitivity (2.2 × 108 μC/Gyair/cm2) when exposing 8 keV X-ray photons at low dose rates. That’s four times higher than today’s best-in-class detectors.
Group leader László Forró, a researcher from EPFL’s Laboratory of Physics of Complex Matter, said in a press statement:
By using photovoltaic perovskites with graphene, the response to X-rays has increased tremendously. If we would use these modules in X-ray imaging, the required X-ray dose for forming an image could be decreased by more than a thousand times, decreasing the health hazard of this high-energy ionizing radiation to humans.
How The Device Is Made
- When halide perovskites, like MAPbl3, are dissolved in a polar aprotic solvent, they form elongated crystallites.
- As this solution travels through the nozzle during aerosol jet printing, the crystallites grow and settle on the substrate as crystalline nanowires.
- The solvent is mostly evaporated once a droplet reaches the substrate, so the nanowires are nearly solvent-free. This reduces any spatter, allowing for the formation of the well-defined 3D structures required to construct an X-ray photodetector with high spatial resolution.
- The researchers 3D-printed the perovskite onto graphene to further improve the photodetection characteristics. The graphene amplifies the generated photocurrent.
They tested several architectures and operation modes, including diode, resistor, and heterojunction – the last mode was the optimal pixel design, with a MAPbI3 pillar printed on a graphene layer spanning the gold electrodes.
Lastly, the team made their perovskite-based optoelectronics stable by encapsulating the detector in a PDMS polymer after wire-bonding. This assembly method protects the active MAPbI3 layer in the device. The unit was stable for more than nine months, experiencing no degradation in performance.
The researchers conclude:
We are confident that this X-ray detector unit architecture is extremely promising for highly sensitive X-ray imaging. This approach could allow significant lowering of the radiation doses required for X-ray imaging, resulting in safer and more affordable CT imaging systems.
Their detector units will have to be assembled onto a larger surface area X-ray detector for medical imaging.
Meanwhile, a group of researchers from several different universities has developed a way to make lenses more precise using graphene microbubbles.
