The X-ray detector was created from tin monosulphide (SnS) nanosheets. It measures just 10nm in thickness: approximately the length grown by human fingernails every 10 seconds. Before this study, the thinnest X-ray detectors measured between 20nm and 50nm.
SnS has previously shown great promise as a material for use in photovoltaics, field-effect transistors and catalysts. The scientists found that SnS nanosheets have other properties, such as high photon absorption coefficients, which make them excellent candidates for use as soft X-ray detectors. Their work found that they were more sensitive than another candidate material (metal halide perovskites) and had a faster response time than established detectors.
These features allowed the scientists to create detectors with high sensitivity and rapid response time.
Although much work remains to be done to explore the full potential of SnS-based X-ray detectors, the researchers believe it could be possible to use SnS nanosheets to build tools for real-time imaging of cellular processes.
"The SnS nanosheets respond very quickly, with milliseconds," said Professor Jacek Jasieniak of Monash University, senior author of a paper in Advanced Functional Materials reporting the work. "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."
X-rays can be crudely divided into two categories: 'hard' X-rays are the sort used by radiographers to scan the body for broken bones and other injuries and illnesses. 'Soft' X-rays have a lower photon energy and can be used to study wet proteins and living cells. Some of these measurements take place in the so-called '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, but access to this infrastructure is difficult and costly to secure. Recent advances in non-synchrotron soft X-ray laser sources may allow lower-cost, portable detection systems to be developed as an alternative to synchrotrons. However, this approach depends on soft X-ray detector materials sensitive to low-energy X-rays.
These SnS X-ray detectors happen to be tuneable for sensitivity across the sot X-ray spectrum.
The sensitivity and efficiency of SnS nanosheets depends greatly on their thickness and lateral dimensions, which are impossible to control through conventional fabrication methods. However, using a liquid metal-based exfoliation method, the researchers produced SnS nanosheets with large areas and controlled thickness; these are suitable for detecting soft X-ray photons in the water window. Their sensitivity can be further enhanced by a process of stacking the ultrathin layers.
They represent a major improvement in sensitivity and response time compared with existing direct soft X-ray detectors.
Speaking on the future of these ultrathin detectors, first author Dr Babar Shabbir of Monash University commented: "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."
