Scanning hard X-ray microscopy (SHXM) is used to image nanostructures because X-rays can resolve much finer details than visible light. Their penetrating power also allows access to deeper layers in a sample, which is useful for 3D tomographic imaging of structures such as biological cells, semiconducting chips, batteries and many other functional materials. But this high penetration also means that X-rays pass straight through conventional lenses without being bent or focused.

Apart from X-ray mirrors, which are limited in their convergence and that need to be mechanically polished, thus making them expensive, an alternative way to bend X-rays is to use crystals. To do this, researchers today make use of tailor-made artificial crystals consisting of different material layers to sharply focus X-rays. These crystals are known as multilayer Laue lenses (MLLs), named after the German physicist Max von Laue who discovered 100 years ago that crystal lattices diffract X-rays.

Two crossed MLLs

The new hard X-ray imaging technique developed by Hanfei Yan of the Brookhaven National Laboratory and colleagues has a spatial resolution of nearly 10 nm thanks to two crossed MLLs.

“We image the sample in multimode thanks to absorption-, phase- and fluorescence contrast,” explains Yan. “We raster scan the sample with respect to a nanobeam applied to its surface. While we do this, we record the excited fluorescence and transmitted signals using energy-dispersive and pixel-array 2D detectors, respectively, at each position on the sample. The former provides us with quantitative images of the constituent elements in the sample and the latter an electron density map of the sample.

The researchers characterized the focus size of the crossed MLLs using so-called ptychography reconstruction and conventional knife-edge scans. They determined the imaging resolution of the acquired fluorescence image using power spectrum density (PSD) analyses.

Imaging resolution is nearly as good as 10 nm

“With the knife-edge scans, we calculated a full-width-at-half-maximum (FWHM) focus size of 15.3 x 16.9 nm2 while the ptychography reconstruction produced a FWHM size of 13.9 x 12.3 nm2,” says Yan. “PSD analysis of a test pattern fluorescence image revealed the smallest detectable feature size down to 10.3 x 10.8 nm2.” These measurements all imply that the imaging resolution is nearly as good as 10 nm, he adds.

“We can use the technique to image a variety of samples,” he tells nanotechweb.org. “In our study, we imaged a test pattern fabricated by lithography, a nanoparticle array and an ionic ceramic-based membrane (used in solid oxide fuels cells) containing small grains. We could see the chemical composition of these materials as well as morphological variations. In the ionic membrane, we were also able to make out an emerging material phase.”

Towards sub-10 nm resolution

“The direct scanning image (that is, with no post-imaging processing and deconvolution) shows a resolution of around 12 nm,” he adds. “With ptychography, which is an inverse reconstruction technique that can then further enhance the resolution of an image, we found that we could clearly resolve a roughly 10 nm-sized gap between two nanocrystals. This indicates a resolution of better than even 10 nm in this special case.”

The technique can be used in situations in which electron microscopy is limited – for example to study local variations in 3D nanoparticle superlattices formed by self-assembly, he says. “We can also image trace metals present in extremely low concentrations in biological samples as well as investigate the connection between the physical and chemical properties of nanoparticle catalysts and their performance.

“We would like to emphasize that this technique is now ready for routine measurements and available to the scientific community in its present form. This represents a significant advance in itself, aside from a pure demonstration of resolution enhancement. In our view, it sets an important milestone in the development of high-resolution SHXM.”

The team, reporting its work in Nano Futures DOI: 10.1088/2399-1984/aab25d, says that it is now continuing to reduce the focus size in its technique and improve the nanofocusing optics.