A new technique to see crystals like never before

Jointly published by the CNRS and the ESRF.

To date, two techniques have been used to analyse complex crystalline materials, each having  specific limitations. Classic "X-ray diffraction" can acquire information on atomic positions and irregularities in the sample without damaging the sample, but with a limited spatial resolution in the range of a micrometre. Transmission electron microscopy (TEM) exhibits a much better spatial resolution (0.1 nm), providing a "real" image of the crystal, but has the disadvantage that the sample is destroyed in the process.

The new technique developed by Virginie Chamard (CNRS) of the Institute Fresnel and her colleagues overcomes these limitations by combining the advantages of both methods: it produces high-resolution 3D images – down to a few tens of nanometres - without destroying the sample, and it provides as much information as conventional X-ray diffraction on the atomic structure. In addition, it enables analysis of very large areas, potentially of infinite dimensions.

The new technique uses a microscopic beam of X-rays produced by a synchrotron—for their work, researchers used ESRF beamline ID01—and focuses it on the sample. This lens-less microscopy technique, where computers replace the lens, exploits the coherence properties of the X-ray beam. As the beam scans across the sample, a detector captures the diffracted X-rays and acquires a series of "diffraction patterns". These are processed by a special algorithm generating a 3D image of the entire sample with a resolution smaller than the width of the X-ray beam.

The foundations of this technique called "ptychography" were laid in 1969 by German physicist Walter Hoppe who invented it to improve the resolution of electron microscopy. Virginie Chamard and her team spent three years developing the technique for the imaging of crystals.

This new microscopy approach could be crucial to our understanding of many mysteries both scientific and technological, for example the growth of mollusk-shells and the optical properties of semiconductors. It could even help in studies to improve the electrical performance of metal alloys. These examples all involve complex crystalline materials, for which the new technique was specifically developed.
 

Complementarity of ptychography with existing techniques. a) Scanning electron micrograph of lines patterned on a silicon-on-insulator substrate. The magnified region shows the area investigated during the ptychography scan. The translation direction tx is shown together with the two first illumination areas (white ellipses). b) The surface of the same region as seen by atomic force microscopy. c) Cross-sectional views of the 3D reconstructed silicon crystal lines. The density (upper image) and the "phase" (lower image) are shown separately, and give an internal view of the sample. The phase (in radians) represents the internal "crystal field" where atomic displacements are visualised like variation of the colour.

 
Reference
Three-dimensional high-resolution quantitative microscopy of extended crystals, P. Godard et al., Nature Communications, November 29, 2011.

Top image: Ptychography scan of lines patterned on a silicon-on-insulator substrate.