A team of researchers from Lund University have imaged the beam focus at the hard X-ray nanoprobe NanoMAX using a single nanowire as the detector. The result is an unprecedented ultrahigh-resolution 3D image of the 88 nm diameter focus revealing fine details of the beam.
“A basic way of describing an X-ray experiment is that a well-defined beam illuminates a sample and interact with it. Downstream from the sample, we place devices that can detect any signal that comes out on the other side. That may be a weaker X-ray beam, a scattered X-ray beam, electrons emitted by the sample, and so on. By comparing what we send in towards the sample and what comes out, we can learn about, for example, the structure, chemical or magnetic properties of the sample”, Jesper Walllentin, associate professor at the division of Synchrotron Radiation Research, said.
This requires exact ways of measuring the incoming and outgoing signals. Therefore, the development of detectors is a crucial element of X-ray science. In a recent project, researchers managed to use a tiny fibre known as a nanowire, with a diameter of only 60 nanometers, as the detector.
“We are excited to have made the smallest X-ray detection device yet, and that it worked so well after a long development time,” Jesper Wallentin, said. “We were able to make a high-resolution 3D image of the focus at NanoMAX. It has not been possible to do such high-resolution measurements directly in the focus before. The images themselves showed a lot of structure in the focus, with a number of peaks rather than a smooth flat field.”
When the nanowire is illuminated by X-rays, they excite electrons in the material and generate a current that can be recorded. A higher X-ray intensity generates a higher current. One of the challenges is that the X-rays need to travel a certain distance through the material to generate enough electrons since the X-rays are weakly absorbed. This can be achieved by mounting the nanowire so that the X-rays propagate along the nanowire axis. The nanowire is then scanned along and across the X-ray beam to achieve a 3D image of the focus.
“We feared that the intense X-rays would damage the nanowire device, but it showed surprising robustness,” Wallentin explained.
In the future, more than one nanowire and nanowires with a smaller diameter could be used.
“The long-term goal is to develop high-resolution X-ray detectors, but we still have a lot of work ahead of us to go from this single nanowire device to a full-fledged pixel detector system from arrays of nanowires,” Wallentin concluded. “We are currently focusing on improving the sensitivity of our device to be able to measure lower X-ray intensities. We will also make devices from even thinner nanowires, to explore the limits of the spatial resolution.”
This project was done in collaboration with Prof. Magnus Borgström at Solid State Physics, and the device processing was entirely done in the Lund Nano Lab at the Strategic Research Area NanoLund.
On December 4, 2020, at 09.00 Lert Chayanun, who is the first author of this study, defends his doctoral thesis “Nanowire devices for X-ray detection”.
Chayanun, L. Hrachowina, A. Björling, M. T. Borgström, and J. Wallentin, “Direct Three-Dimensional Imaging of an X-ray Nanofocus Using a Single 60 nm Diameter Nanowire Device” Nano Letters 20 (11), 8326 (2020) DOI: 10.1021/acs.nanolett.0c03477
Top image: X-ray Beam Induced Current measurements from the nanowire detector through the nano focus.