Aus researchers create the world's thinnest X-ray detector
Australian researchers have used tin monosulfide (SnS) nanosheets to create the thinnest X-ray detector ever recorded, potentially enabling real-time imaging of cellular biology. Their work has been described in the journal Advanced Functional Materials.
X-ray detectors are tools that allow energy transported by radiation to be recognised visually or electronically, like medical imaging or Geiger counters. While ‘hard’ X-rays are the kind used by hospitals to scan the body for broken bones and other illnesses, ‘soft’ X-rays have a lower photon energy and can be used to study wet proteins and living cells — a crucial component of cellular biology. Some of these measurements take place in the ‘water window’ — a region of the electromagnetic spectrum in which water is transparent to soft X-rays.
Soft X-ray detection can be conducted using a synchrotron like the Large Hadron Collider in Switzerland, but access to this type of hugely expensive infrastructure is difficult to secure. Recent advances in non-synchrotron soft X-ray laser sources may allow lower-cost, portable detection systems to be designed, providing an accessible alternative; but for this approach to work, we will need soft X-ray detector materials that are highly sensitive to low energy X-rays, provide excellent spatial resolution and are cost-effective.
Some existing soft X-ray detectors use an indirect mechanism, in which ionising radiation is converted into visible photons. This approach allows for multiple energy ranges and frame rates to be studied, but is difficult to prepare and offers limited resolutions. Direct detection methods are easier to prepare and offer better resolutions, because the detector material can be thinner than indirect approaches.
Good candidate materials need a high X-ray absorption coefficient, which is calculated using the atomic number of the absorbing atoms, X-ray incident energy, density and atomic mass of an atom. High atomic mass and low energy X-rays favour high absorption, and soft X-rays are more strongly absorbed in thin materials compared to hard X-rays. Nanocrystal films and ferromagnetic flakes have shown promise as certain types of soft X-ray detectors, but they are not well equipped to handle the water region. That’s where SnS nanosheets come in.
SnS has already shown great promise as a material for use in photovoltaics, field effect transistors and catalysis; now, members of the ARC Centre of Excellence in Exciton Science have shown that SnS nanosheets are also excellent candidates for use as soft X-ray detectors. The researchers found that SnS nanosheets possess high photon absorption coefficients, allowing them to be used in making ultrathin soft X-ray detectors with high sensitivity and a rapid response time. The materials were found to be even more sensitive than another emerging candidate (metal halide perovskites), boasting a faster response time than established detectors, and are tuneable for sensitivity across the soft X-ray region.
One of the lead authors, Dr Nasir Mahmood of RMIT University, said the sensitivity and efficiency of SnS nanosheets depends greatly on their thickness and lateral dimensions, which are not possible to control through traditional fabrication methods. Using a liquid metal-based exfoliation method allowed the researchers to produce high-quality, large-area sheets with controlled thickness, which can efficiently detect soft X-ray photons in the water region. The team’s X-ray detectors measure less than 10 nm thick — previously, the thinnest X-ray detectors created were between 20 and 50 nm — and their sensitivity can be further enhanced by a process of stacking the ultrathin layers.
The SnS nanosheets thus represent major improvements in sensitivity and response time compared to existing direct soft X-ray detectors. Considerable work remains to explore the full potential of the SnS X-ray detectors, but senior author Professor Jacek Jasieniak, from Monash University, believes it’s possible this could one day lead to real-time imaging of cellular processes.
“The SnS nanosheets respond very quickly, within milliseconds,” he said.
“You can scan something and get an image almost instantaneously. The sensing time dictates the time resolution. In principle, given the high sensitivity and high time resolution, you could be able to see things in real time.
“You might be able to use this to see cells as they interact. You’re not just producing a static image; you could see proteins and cells evolving and moving using X-rays.”
The researchers hope their findings will open new avenues for the development of next-generation, highly sensitive X-ray detectors based on ultrathin materials. First author Dr Babar Shabbir, from Monash University, concluded, “In the long run, to commercialise this, we need to test a many-pixel device. At this stage we don’t have the imaging system. But this provides us with a knowledge platform and a prototype.”
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