The wonder crystal could yield imagers that are far more sensitive than commercial detectors
The crystalline material known as perovskite makes for a superefficient photovoltaic cell. Researchers are also exploring perovskites’ potential in transistors and LED lighting. But there’s yet another use for this wonder crystal, and it may be the most promising of all: as X-ray detectors.
Dozens of groups around the world are exploring this area, and major X-ray imaging manufacturers, including Samsung and Siemens, are considering perovskite for their next-generation machines. Compared with today’s X-ray imagers, detectors based on perovskite compounds are far more sensitive and use less power. And for certain applications, the materials can be tuned to emit color when irradiated. Lab prototypes of imagers that use perovskite have been demonstrated to be at least 100 times as efficient as their conventional counterparts.
“Interest in perovskite crystals for imaging emerged out of all the recent enthusiasm to get better solar panels,” says I. George Zubal, director of the nuclear medicine and computed tomography programs at the National Institute of Biomedical Imaging and Bioengineering (NIBIB), in Bethesda, Md. His program funds research into new imaging devices, procedures, and software, including groups looking at perovskite X-ray detection.
What makes perovskites so useful for X-ray detection is the same thing that makes them good for solar cells: They’re excellent at converting light into electrical charge. In a direct detector, X-ray photons are converted into electrons inside a semiconductor. In a scintillator imager, the X-ray photons are first converted into visible light, which is then converted into electrons by a photodiode array.
Conventional direct X-ray detectors have higher resolution than do scintillators, but they take longer to acquire an image. That’s because the semiconductor material they typically use—amorphous selenium—isn’t great at stopping X-rays. Scintillator imagers, on the other hand, are more sensitive than direct X-ray imagers—meaning you need fewer X-rays to create the image—but yield a lower-quality image.
Perovskites could be the answer to the main shortcomings of current X-ray imagers, says Zubal. “Perovskite stops a lot more of the X-rays [compared to amorphous selenium], and being a semiconductor it should give us higher-resolution images, showing the small structures of objects…. You’re also lowering the radiation dose to the patient, which is another main reason for the NIBIB’s enthusiasm.”
In one experiment, Xiaogang Liu’s group at the National University of Singapore started with a commercial flat-panel X-ray detector that used bulk scintillators of cesium iodide thallium. The group removed the CsI(TI) layer and replaced it with a layer of nanocrystals of cesium lead bromide—an inorganic perovskite—directly coating them onto photodiode arrays. When coupled with photomultiplier tubes, the resulting device had a detection limit that was just 1/400 that of medical X-ray machines, as the group reported in Nature last September. Several X-ray manufacturers are now testing nanocrystal scintillators using his group’s approach, Liu says.
Liu credits grad student Qiushui Chen for coming up with the idea of using perovskite nanocrystals in this way. “A lot of our recent work involves rare-earth materials, which is what conventional scintillators use,” Liu says. To form the perovskite layer, the researchers mixed the nanocrystals with liquid cyclohexane and then spin-coated the mixture onto a flexible substrate.
“We got a little bit lucky, because we discovered that the nanocrystals had to be deposited on the substrate through a solid-state process,” Liu says. “If the particles are dispersed in solution, it’s no good.”
Researchers have also demonstrated perovskites in direct X-ray detectors with vastly superior performance to that of commercial imagers. In general, says the NIBIB’s Zubal, direct X-ray detectors are “highly more desirable” than scintillators because they avoid the extra step of converting visible light into electrons. The projects that NIBIB is supporting involve direct detection.
Jinsong Huang and his group at the University of North Carolina at Chapel Hill have been studying direct X-ray detectors based on perovskites since 2014. (Huang also works on perovskite photovoltaics.) In one experiment, they coated methylammonium lead tribromide—a common perovskite compound—onto a regular X-ray detector that used amorphous silicon to convert the X-rays to electrons. The addition of the perovskite layer made it 3,000 times as sensitive.
“When you want extremely efficient and sensitive detectors, you need to count single photons, and that’s not easy,” Huang explains. “We showed that we can make materials that allow you to distinguish the signal from the noise.” Huang recently created a startup to commercialize radiation detectors based on his group’s work.
There are still a number of hurdles to cross before perovskite scintillators or direct X-ray imagers will be ready for market. A big obstacle is that some perovskites are sensitive to moisture. Liu has developed a method for coating each nanocrystal with silicon dioxide and is exploring other protective methods. Perovskite layers can also be encapsulated in glass, much like traditional solar cells are.
But in general, perovskite X-ray imagers won’t need to be quite as hardy as perovskite PVs or LEDs, because the environmental conditions they’ll face are more benign. Solar panels need to perform even after being exposed to the elements for 20 years, while LEDs are exposed to heat and, of course, light, both of which can degrade a perovskite compound. X-ray machines, by contrast, are typically used in climate-controlled settings. For that reason, Liu and Huang believe perovskite X-ray detectors will be commercialized much more quickly than other perovskite applications.
Huang predicts that perovskite detectors will open up new applications for X-rays, expanding what’s already a multibillion-dollar industry. More efficient imagers would draw less power, lending themselves to portable machines that run on batteries. Liu’s group has also demonstrated a variety of tunable, color-emitting perovskite nanocrystals. That work could lead to multicolor X-ray displays, which are impossible with today’s scintillator X-ray machines.
And because they use flexible substrates, perovskite imagers could conform to whatever’s being scanned; anyone who has experienced the discomfort of a mammogram will appreciate that feature. Faster, more sensitive imagers would also reduce the radiation from dental and medical X-rays and airport security scanners.
“Once we can make X-rays much safer, the market will change because you’ll be able to put the detectors everywhere,” Huang says.
Source: IEEE-Spectrum – Fulltext